ST600 USER GUIDE
This manual can also be used with ST300 or ST200 running Zscan V 4.4.8
*Getting started page 11
pg. 1
Zmetrix
1195 Spring Creek Pl., Suite B, Springville, UT 84663801-489-8708 www.zmetrix.com
Copyright
Copyright 2010 by Zmetrix, Inc. This manual and the hardware described in it are
copyrighted, with all rights reserved. The manual may not be copied in whole or in part for
any use without prior written consent of Zmetrix.
Disclaimer
The following paragraph does not apply in any state or country where such statements are
not agreeable with local law:
Even though Zmetrix has reviewed its documentation, Zmetrix provides this publication
“as is” without warranty of any kind, express or implied, including but not limited to, the
implied warranties of merchantability or fitness for a particular purpose. This
documentation is subject to change without notice, and should not be construed as a
commitment or representation by Zmetrix.
This publication may contain inaccuracies or typographical errors. Zmetrix will
periodically update the material for inclusion in new editions. Changes and improvements
to the products and / or programs described in this manual may be made at any time.
pg. 2
ST600 Warranty, Installation, Service, and Repair Policies
Warranty:
Zmetrix warrants the ST600 controlled impedance test system to be free from defects in material and
workmanship for two years from the date of original purchase. During the warranty period, Zmetrix will
repair, or at it’s option, replace any defective component(s) without charge for parts or labor.
The Zmetrix warranty applies only to products manufactured by Zmetrix. Accessories and items not
manufactured by Zmetrix are covered by the warranty of the original equipment manufacturer. Product
defects caused by misuse, accidents, or user modification are not covered by this warranty. No other
warranties are expressed or implied. Zmetrix is not responsible for consequential damages.
Installation:
Installation of ST600 systems is not a technical or time consuming process. The ST600 is a USB device, and
installation is primarily a software function. The ST600 comes ready to take measurements.
Training in the use of the ST600 system is generally done by phone and with the use of a series of training
videos (.wmv or mp4 format) which can be installed on the test computer and viewed as needed.
These videos cover all aspects of the ST600, including test program generation, taking measurements, saving
and reporting data, calibration, and ESD safe operation of the system. Help is always available via phone
seven days/week
Service:
Service of ST600 systems is available directly from the factory in Utah. Typical time to repair is just a few days
if the system is sent overnight.
Real Time Diagnostics:
The ST600 continually generates a file names TDR.csv. This file contains a “snapshot” of the system
electronics of the last measurement. In the event of a serious malfunction, e-mail that file to
www.zmetrix.com and our engineers will be able to determine if the system has problem.
A Note on Repairs:
The typical repair for a TDR system is due to an ESD event originating at one of the input/output connectors.
As part of the ST600 design, these events are dramatically reduced when compared to our competitors.
Additionally, those same design features dramatically reduce the cost of a repair due to ESD. The ST600 is
warranted for 2 years. After warranty, these repairs typically cost less than $1,500 U.S.
pg. 3
The ST600
The ST600 has four connections on the front panel. They are, from left to right, the two differential probe
ports, single ended one and single ended two.
Communication light
Differential ports
Single ended port #1
Single ended port #2
The USB port connects to a standard USB peripheral cable connection. The ST600 is compatible with USB
version 2.0
The ST600 system is configured with four measurement ports so that the user can take either single ended or
differential measurements without having to change cable connections. The single ended and differential
probes can be left connected to the system continuously.
The differential probe ports are the 2 left ports on the instrument. Connect the 100 ohm differential probe
(ACCST-602) via two SMA cables. These ports are used to make differential measurements, and can be
connected in either order.
There are two single ended ports. Looking at the ST 600, the 2 nd from the right is Single ended port one. The
port on the extreme right is single ended two. Either port can be selected for single ended measurements.
The second single ended measurement port can accommodate a probe with different pins spacing, ie, 0.050”
for measuring small pads.
During test operation, lights above the ports in use are illuminated to identify the ports currently in use.
The Zmetrix Zscan™ software™ is designed for use within the Microsoft Windows environment. Windows XP
through Windows 10 is assumed, running on an IBM-PC compatible computer with USB2 ports. The ST600 is
controlled by keyboard and mouse. A foot switch is provided for “hands off” operation.
Note: The ST600 is completely portable and requires no A/C power as long as the computer batteries are
charged and the USB ports of the host computer can supply the power required.
There is no wrist strap port on the ST600. The supplied wrist strap should be plugged into any anti static mat
that might be employed.
pg. 4
ST600 Specifications
Controlled Impedance Range
10 – 200 Ω Single Ended
10 - 200 Ω Differential
Measurement Accuracy
1% of reading over the entire range
Impedance Tolerance Range
0.1%-99.9%
Vertical Display Ranges (Ω)
1, 2, 5,10,20,50 Ω/Division
Horizontal Display Ranges
Infinitely adjustable to 30 inches
Testable trace lengths
Maximum 30 inches
Minimum 1.5 inches
Horizontal Distance Units
inches, feet, millimeters, meters
Horizontal Time Units
Nano seconds, Picoseconds
Test Method
Time Domain Reflectometer (TDR
measurement using PC software control)
Reflected Pulse Risetime
≥ 75ps
System Bandwidth
7 Ghz
Calibration Method
Ratiometric measurement to calibrated
internal reference
Series Loss Compensation
0-5Ω/inch
Vp Compensation
.33-0.99
Channels
4, 2 single ended and 1 differential
Inputs
USB 2.0 peripheral port
Test probe channels – 4 (SMA)
Anti-static wrist strap cord connection – 4mm
pg. 5
CONTROLS AND CONNECTORS
SMA connectors
The front panel SMA connectors should be connected to Zmetrix microstrip probes
using Zmetrix cables.
CAUTION: Tighten with care. Use a torque wrench set to 5 lbf inches
It is nearly impossible to get the SMA connectors tight enough with the fingers.
Connectors that are not tight combined with the stiffness of the test cables can create
erratic readings.
USB2 Interface to the PC
The host computer must have available USB2 ports.
Any newer computer will have them. Most older computers can have a USB2 card added.
If your computer is not equipped with USB2 ports please consult your IT person.
IMPORTANT! Install Zscan and USB2 driver software prior to connecting ST600 to the
host computer via the USB cable
ANTI STATIC WRIST STRAP
While the ST600 is not overly sensitive to ESD, it is a good policy to wear the wrist strap when
operating the system. The wrist strap can be plugged into any anti-static mat that you might
be using.
pg. 6
PROBES and CABLES
Probes and cables should be considered to be consumables. Like tires on cars, they are required for
operation, but at some point they will need to be replaced.
CABLES
The cables supplied with the ST600 are premium high bandwidth cables selected for the best
electrical performance. Cables do wear out with repeated flexing over thousands of tests, test results
can become erratic, therefore it is extremely important that the cables be electrically checked
periodically. This is simple procedure takes only seconds and will save hours of grief.
The cables are expected to provide trouble free performance for hundreds of hours.
PROBES.
The ST600 is supplied with one single ended probe and one differential probe. The interior of the
probe Is a microstrip made of Rogers™ material and the spring loaded probe pins have been
selected for optimal performance. Standard pin separation is 0.10”. Different pin spacing and/or
configurations are available on special order. Unless you require a different pin spacing,these are
the only probes you will need. You can measure over the full impedance range with full
accuracy. No matching probes are required888
Please do not attempt to repair the probe…it is a consumable! Measurements
made with repaired or modified probes will not be guaranteed by Zmetrix.
Impedance measurements are polarity sensitive. The ground pin is the pin soldered to the
copper side.
This is the Ground Side
pg. 7
Section 1 Overview
Controlled Impedance Testing
1-1 The ST600 system
The ST600 Controlled Impedance Test setup provides low cost automated measurements of the characteristic
impedance of PCBs and coupons. The system consists of any PC computer with USB2 ports, an ST600 TDR and
a printer for producing reports. It should only be used on unpowered pcb boards and/or coupons.
S E probe
Diff probe
The Zmetrix ST600 Controlled Impedance Test System is measurement system running on a PC with
™Windows XP through Windows 10. The system was specifically designed to easily and rapidly test
impedance coupons having single end and differential traces by production personnel. Connection to
the trace is via special high bandwidth cables and special probes utilizing ™Rogers material. The
ST600 is controlled via keyboard and mouse using standard PC protocols. A USB footswitch is
shipped standard with the system for hands-off testing.
pg. 8
1-2 TEST FILES
All of the test parameters for testing a controlled impedance track are stored in a test file. Once the
file has been created, the operator needs only to select that file to begin testing. Once a coupon has
been tested, the test results are saved in a .TDS file, the saved data also includes the test waveform,
So that future opening of this file will produce a copy of the original test.. These files can be shared
and opened by anyone running Zscan software on their PC.
1-3 DATA ANALYSIS
Test results are displayed in classic TDR fashion with impedance plotted against distance. Pass/Fail
is determined by a fully adjustable pass/fail window with the option of failing any incursion of the trace
into the fail region or on an average basis. The impedance of the entire track is displayed along with
SD, min, max and average values. Test data including and parameters and the test waveform are
saves as TDS files. The data may be printed out in a customizable report in PDF format. The ST600
also exports file in three different formats for use with Excel. A delimited data file is also available for
export for use in statistical analysis.
1-4 ST600 features
USER INTERFACE
Via Microsoft Windows XP through Windows 10
POWER REQUIREMENTS
The ST600 requires no AC power. It is powered by the host computer via USB
DISPLAY
Linear display of impedance against distance
True differential impedance measurement (not 2 separate single ended measurements)
End of probe marker showing exact test launch point.
Simple pass/fail test results displayed in waveform window.
STATISTICAL PROCESS CONTROL
THE ST600 provides a file enabling the user to log continuously to a delimited file. This file can be
easily exported for statistical process uses.
pg. 9
SECTION 2-- INSTALLATION AND SET UP
System Requirements
The ST00 will run on any PC computer including laptops. (Caution, some of the newer laptops are
supplied with USB ports which do not have enough power available)
The ST600 will run on any Windows operating system from XP to Windows 10
The ST600 does not require AC power
There is a single USB2 port connecting the ST600. All other functions i.e. ,printer, footswitch, etc are
connected to the host computer.
Note: When using footswitch for the first time you may need to reboot the computer to make sure
that it “sees” the footswitch and installs the drivers.
2.1 UNPACKING
The ST600 is shipped in custom packaging. Please report any obvious damage to the carrier.
Note: We recommended saving the packaging for return of the instrument for annual re-certification.
The packaging should contain:
1 ST600 TDR
3 Probe Cables
1 USB Interface Cable
1 Single Ended Probe
1 Differential Probe
1 Footswitch
2 - 50 ohm semi-rigid standards
1 Torque Wrench
1 Anti-static Wrist Strap and Cable
2-2 Controls and Connectors
SMA Connectors
The front panel SMA connectors should be connected to Zmetrix cables only.
The connectors should be tightened with the supplied torque wrench.
USB Interface to the PC
Use the supplied USB cable
Foot Switch
The footswitch should be plugged into any available USB2 port on the host computer.
Wrist Strap
The ST600 does not have a socket to accommodate a wrist strap, The ST600 does not require a
wrist strap Connect the supplied wrist strap to your anti static test surface if you employ one.
Connecting to a Power A/C Source The ST600 does not require A/C power
pg. 10
Getting Started
pg. 11
2.6
Getting Started
Installing the ST600 software:
Go to www.zmetrix.com and click on Download Software. Once you have downloaded, install on the
computer.
The Zscan software will run on any version of Windows™from XP through Windows 10™.
Depending upon the computer/software pairing, Windows may make installation challenging. The
first thing you will notice is this alert. Select the small arrow to the right of the discard button and
select keep.
Once the software is installed,click on the Zmetrix icon. After a few seconds the red communication
light on the front panel should light up. If it doesn’t, the computer has not found the system drivers.
Go to Device Manager and locate the Zmetrix hardware (it will be under USB devices). Click on
update drivers. If you have a choice, direct the system to C:/program files (x86)/Zmetrix Inc/Zscan.
A screen icon is automatically produced and should be on the screen.
2-4 Connecting the ST600 to the computer
There is one connection only….the supplied USB cable
2-5 Running the program
Double click on the ZMetrix icon and you should see this screen:
The Test Screen
pg. 12
The screen colors may be different. This may be a good time to select the colors you prefer.
Click on Utilities
The Utilities function has four sections:
Preferences in which all of the system preferences are selected
Default Test Definitions Test Edit Screen defaults can be set
Security System password can be created
Hardware: Cables, probes are checked and setup. Calibration can be reset Start X-Axis
and End of probe marker can be selected.
The recommended procedure is to start with hardware setup;
 Check the cables for electrical length using EOC
 Find the end of the probes and select end of probe marker.
Select starting the test at the end of probe. (Start X-axis from end of probe)
 Next set up test editor defaults
 Next choose all of your preferences, screen colors, printer fonts,automatic test advance and
automatic test data logging, you can turn on cursors set directories and select a folder for the
output file.
 Finally you can password the system if desired.
Preferences:
pg. 13
Auto Advance
In the testing process, once you have datalogged, auto advance will advance to the next test.
There are three choices: None, after test and after pass. We suggest selecting After Pass.
Using this setup allows you several tries to make a good reading.
Autolog
Selecting Autolog will log the test automatically
Show Cursors
Two selectable cursors are available. The cursors are a screen tool used to help make measurements.
Example:
The cursors can be used to set test region.
The cursors can also be used to measure propagation times.
The cursor colors are selected in the Color Set utility
REPORT ATTRIBUTES
The report fonts, font size and column width are selected in this screen
COLORS
Set the screen colors to your preferences
pg. 14
SET DIRECTORIES
In this pop-up you can select the directories
SET OUTPUT FILE *** Very important file
The output file contains all of the diagnostic info for system. Every time you test, that file is
overwritten with the latest system information. This file is labeled tdr.csv and you should store it in a
directory that is easily accessible (how about diagnostics). If you encounter problems and call
Zmetrix for assistance , this the file Zmetrix that customer service wants to see. With this file we can
determine the “health” of the system (calibration, static damage, etc) within minutes.
SOUNDS
Turns system sounds on/off (pass/fail)
AUTO EXPORT
Creates a delimited data file which can be exported
AUTO SAVE
SKIP REPORT SUMMARY
pg. 15
DEFAULT TEST DEF
The Default Test Def function allows you to set certain parameters which are not likely to change,
thereby saving test programming time, ie, Trace Window will be set for a value which will be the
optimum for the length of coupon that you use. Unless you change coupon length, you should not
have to change this setting. The same logic applies to Starting Position and Ending Position. Set
the Starting Position with a value of ½ the Trace Window value and set the Ending Position for the
same value plus 20%.
If you always use Pass on Average, you can default to this setting
pg. 16
TOOLBAR
pg. 17
The Tool Bar
Clicking on
brings up
accomplished by clicking on File, then New
Clicking on
Clicking on
New job screen. The same thing could have been
opens the folder holding the test files (.TDS)
saves the current job
Clicking on
Logs the current coupon data. The same thing could have been
accomplished by clicking on Actions then Log Coupon Data
VIEW
Clicking on View brings up this popup allowing you to view or hide status bar and tool bar
Clicking on:
Toggles “live mode” and single test
Displays version of Zscan in use
pg. 18
A GUIDE TO ZSCAN™ SCREENS, POPUPS and PULLDOWNS
The Zscan software is intuitive and easy to use when you have an idea of where to find things. Please take a few
moments to familiarize yourself with the location of the various functions.
This is the Test List Screen
Right clicking with the cursor in the Test List Screen will bring up this popup. You can select Columns, Export and do a
Virtual Retest
Clicking on Select Columns brings up this screen:
You can select the columns and the order in the Test List Pane
pg. 19
This is the datalog pane.
Right clicking with the cusor in the Datalog Pane will bring this popup
:
You can Print, Edit the Coupon, select columns Delete Coupon, Export, and Create a report
This is the main test screen
The test parameters are listed
Impedance value and Pass or Fail are shown. Pass/Fail region
Verticle screen sensitivity is selected by right clicking by right clicking on the OHMS column
pg. 20
ACTIONS
pg. 21
ACTIONS
Clicking on Actions on the tool bar displays this dropdown menu.
Customer Info Clicking on Customer info allows you to enter your customer data
Perform test is exactly what it implies. Depress the footswitch or left click with the cursor in the Test
screen or click on Actions/Perform Test or hit Enter any of these actions will initiate a single test
Live mode.
In live mode the ST600 tests continuously. Recommended mode for faster testing,
and when trying to make test connections with extremely small pads.
In this mode the instrument
is running in a mode similar to Tektronix or Agilent
pg. 22
Insert trace def Opens test editor. Test editor can be opened by right clicking with the cursor in the
testlist pane, or hitting F4
Log coupon data Logs the data of the current coupon
Create Report Generates a .PDF report using current datalog data
Set Next Serial # Allows you to set a new serial number Serial numbers are automatically sequenced. This
allows the user to set the next serial number
pg. 23
UTILITIES FUNCTION
pg. 24
Utilities
PREFERENCES
Auto Advance
Provides three choices, None, After Test and After Pass
Recommended setting is After Pass. This provides the opportunity to retest if
necessary before advancing.
The test sequence will be advanced to the next test upon datalogging
Show Cursors
Selects or deselects cursors
Report Preferences
pg. 25
The report can be customized by selecting the font of your choice and selecting the font size. Column widths can be
widened or narrowed by clicking on a field name selecting the field width in the window below.
Colors Clicking on colors allows you to change all the screen colors
SET DIRECTORIES
This popup allows the user to select the directories, for datalogging and reports
SET OUTPUT FILE This file is one of the most important files in the system
The output file contains all of the diagnostic info for system. Every time you test, that file is
overwritten with the latest system information. This file is labeled tdr.csv and you should store it in a
directory that is easily accessible (how about diagnostics). If you encounter problems and call
Zmetrix for assistance , this the file Zmetrix customer service wants to see. With this file we can
determine the “health” of the system (calibration, static damage,etc) within minutes
pg. 26
Sounds
Set pass fail sounds
Auto Export
Creates a delimited data file in .clf format for external use.
Skip Report Summary This option truncates the report, leaving the summary info out.
SET DEFAULT DEFINITIONS
Setting the defaults is one of the first things you should do. This feature allows you to preprogram a large
percentage of each test because some parameters rarely change. Setting the defaults can bring programming
down to inserting, Name,Target Impedance, and Layer…everything else is defaulted!
To get into the Set Default screen, click on ACTIONS, then PREFERENCES and then DEFAULT TEST DEF
You will see the screen below:
When you are primarily testing coupons, there are several parameters which are always the same.
pg. 27
Assuming your company uses one standard length of coupon (5 inches for example) We can safely set the
Trace Window the location of the pass/fail window…These parameters will probably always be the same. If
there is a need to change them it is easily accomplished. All defaults can be overwritten.
TARGET IMPEDANCE We suggest a setting of 50 ohms as a default because is the most common impedance
value.
UNITS Should be set to your normal measurement values which in the case of coupons is usually inches
OHMS PER DIVISION The system defaults this value to 10 ohms per division. We suggest leaving at this value
unless you have a need to change. (One reason for another setting would be when comparing screenshots
with a Tektronix…usually more Y axis sensitivity)
TRACE WINDOW The proper setting of the trace window will result in the displayed impedance curve being
shown in its entirety and the beginning of the curve or “launch point” being on the left side of the screen
resulting in the best possible display.
EXAMPLE: Assume a coupon length of 5 inches. When you do an End Of Cable (EOC) check, the system
remembers the end and places it at the left hand side of the screen. After you perform an EOC check, you
should attach the probe and do a Actions/find probe function This will ensure that the “launch” is on left part
of the screen.
It is critical that the impedance curve show the entire waveform, so the setting for the TRACE WINDOW To
should be set at the actual length of the coupon plus 0.5 to 1 inch.
The trace should look similar to the one below
Note that the end of the trace is visible
pg. 28
MEASUREMENTS REGION AS A PERCENTAGE RANGE
The IPC and many large PCB users have suggested that the test zone (PASS/FAIL region) be set so that the
beginning of the region is at the 50% of waveform point ( so for a 5 inch coupon, that setting would be one
half of 5 inches or 2.5 inches). They also suggest that about 20% of the waveform should be included in the
pass/fail decision. On a 5 inch coupon, 20% is one inch, if the beginning of the pass/fail region is at 2.5 inches,
the right hand edge of this test area should be set at 3.5 inches.
Clicking on measurement Region as a percentage will result in Starting and ending positions being in percent
(defaults at 25% to 75%...we recommend 50 % to 70%)
OR
You can click on the Measurements Region as a percentage Range and it’s all done automatically.
Once you have set the defaults for your test situation, the only thing you need to enter is target impedance,
layer number and whether the trace is differential
Keep in mind that once you have set these defaults you may not have to change them….a huge saving in test
programming time
pg. 29
HARDWARE
pg. 30
HARDWARE
Start X-Axis From
There are 2 choices, End of cable and end of probe. Select End of
Probe The only time to select End of Cable is when you are
measuring a standard. (airline or semi-rigid)
Single Point Calibration
.
Your Zmetrix TDR was calibrated to a specification much more stringent than the published
specification (± 1% across the measurement range). It should not need recalibration until it is checked
by the factory in a year. However, human nature is such that if you run a job and the instrument
doesn’t come up with the expected impedance results, you wonder if there might be something wrong
with the TDR.
YOU CANNOT CALIBRATE THIS INSTRUMENT. You can verify that it is within its specifications.
There’s an easy way to check. Click on Utilities, Hardware, Single Point Calibrate to see this
screen:
pg. 31
For single ended measurements, click on EOC and follow directions. Next, click on Verify and you should see a
trace at the left side of the screen at about the 50 ohm position and going to the top of the screen
This assures you that the system is working properly.
Next, attach one of the supplied reference standards** to the cable and click on verify:
pg. 32
The reading in the Measured Impedance window should be very close to the reading on the reference
standard tag. Don’t be concerned if the reading is not exact.
.
** The reference standards supplied have been checked against standards traceable to NIST..They will
typically read very close to 50 ohms and they are extremely stable over time
Never use a PCB as a standard. It will change over time.
Checking the calibration for differential measurements:
Open calibration screen and set channel to Differential- A
Click on EOC and follow directions
Attach one of the supplied reference standards to each of the differential
Reset Calibration
pg. 33
In this mode you can reset to factory calibration.
SECURITY
pg. 34
SECURITY
pg. 35
User name and password is entered in this screen
WINDOW
Clicking on Window initiates this popup:
pg. 36
This feature allows you to view more than one test at a time. Note that the 2 tests are different
Clicking on New Window adds another window.
The windows can be cascaded
Or Tiled
pg. 37
Clicking on:
Initiates Board Details screen
Initial setup
pg. 38
Cable and probe check
Prior to using the system for the first time and when you have changed cables you should check the cables for
their electrical length and check the probes to find the electrical end of the probe tip. This procedure ensures
that everything is working properly and that the ST600 has measured the probing system. You do not need to
perform these tests every time the system is used.
IMPORTANT NOTE: In this mode, we are not doing a calibration, we are only checking cable length and end
of probe. This will be re-labeled in the next software release.
Click on Utilities/Hardware/Calibrate
Clicking on Single Point Calibration will bring up this screen:
To check cables, click on eoc (End Of Cable check)
Clicking on eoc (End of Cable) will initiate this popup. In this function the ST600 is finding the electrical end of
the cable. Remove any probes or reference standards that you may have previously attached. Next step is to
verify.
pg. 39
Click on Verify. You should observe a trace on the screen similar to the one above
It is not necessary to click on save
NOTE: Disregard numbers in Calibration Constant and measure impedance boxes These numbers are
irrelevant for this procedure
You have determined the cable length for single ended measurements. Now, repeat the procedure for
differential by selecting Differential A and repeating the process.
Clicking on Verify should bring up this screen:
Note that the trace is beginning close to the 100 ohm mark
Next, attach all of the probes. Click on Utilities/Hardware/Find End of Probe.. If you want the End of Probe
marker on the screen click on Show End of Probe (Advisable)
pg. 40
Repeat for differential. Next click on Start X-Axis From and select End of Probe
The only time you would select End of Cable is when you want to check the reference
standards
Click on Verify:
The value in the Measured Impedance window should be close to the SUM of the tagged values on the
reference standards:
Adding the 2 values together 49.82 + 49.58 = 99.4…..very close to the reading in the Measured Impedance
window
If you have tried to calibrate the system and entered numbers in some of the boxes in the calibrate
screen you may have affected the calibration. You can reset the calibration to the original factory
settings by clicking on Utilities, Hardware, Reset Calibration
pg. 41
Click on Reset to bring the system back to original Factory Calibration
If this does not bring the unit into calibration please call Zmetrix
The Zmetrix TDR cannot be calibrated by the user. Calibration can be verified and
“tweaked” if required. Before trying to calibrate please contact the factory.
There is a built-in diagnostic called tdr.csv. This a file called output file. This file is saved in a folder
selected by you in the original set up. With this file Zmetrix can determine the condition of the TDR
Single point calibration
Please keep in mind that the ST600 cannot be calibrated in the field.
The accuracy of the ST600 is 1% anywhere in the measurement range (20-200 ohms), however, if
you discover that the system is not meeting this spec (possibly due to a ESD hit), you may be able to
bring the system back in calibration at a single point (either 50 ohms single ended or 100 ohms
differential. You can use the secondary standards that you received with the instrument (the 2 semi
rigid coaxes.
A rule of thumb regarding standards is that the standard should have an accuracy of at least 10 times
that of the value to be measured. Semi rigid coax will have an accuracy of 0.1% which 10 times
better than 1%, so that will suffice. To make this single point calibration, click on
Utilities/Hardware/Single Point Calibration.
Click on Single Point Calibration to bring up this screen:
pg. 42
Connect an airline or semi-rigid standard to Single Ended Port one. Enter the value of the standard
into the box labeled known impedance. In the box box labeled Length of Airline, enter the value 6.
Click on calibrate. Note the reading in the Measured Impedance box. Click Calibrate 4 or 5 times
and SAVE. If you now measure the semi rigid it should measure the standard within a digit or two.
Don’t go by the value in the Box labeled Measured Impedance. Create a test file and measure the
secondary standard. The reading should be very close the value on the tag of the standard.
The system is now accurate at this value, however it is possible that you may have effected other
calibration points slightly.
It would be a good idea to e-mail the”TDR” file to Zmetrix so that our service department can take a
look at the last measurement made to ensure that the system has not been damaged.
A word about ESD damage. ESD can make the instrument totally inoperable, and that would be
observed as soon as the damage occurred. The ST600 has built-in immunity from ESD strikes, so
that is unlikely. However, the ESD strike could do some damage to the test head which could affect
the accuracy.
Go to page 99 for addition ESD information.
A set of semi-rigid standards is available from Zmetrix. The set includes 8 NIST traceable standards
(two 25 ohm, two 50 ohm, two 75 ohm and two 100 ohm).
pg. 43
CREATING TEST PROGRAMS
CREATING A SINGLE TEST PROGRAM
pg. 44
Right click the mouse with the cursor in the test list pane and click on Insert or click on ACTIONS and
click on Insert Trace Def or hit INS to bring up the Test Editor
The Test Editor Screen is where you create test programs
Test Name What do you want to call this test (typical 50 ohms single,etc)
Target Impedance The impedance you are hoping to measure
Layer Optional info for use in reports
Starting Position This is the leading edge (left edge) of the pass/Fail region
Ending Position This is the right hand edge of the pass/fail region
Trace Window How long is your coupon? Trace window should be set so that the entire trace is visible.
Channel Select single ended or Differential measurement
pg. 45
Use Aggressive Deconvolution Special function discussed in a later section
Pass on Average. In this mode, the crosshatch area is replaced with a rectangle. This is a less
stringent, but valid test generally resulting in higher yields.
Measurement Region as a Percentage of Range Automatically selects pass/fail region based on
length of trace
Allowable Error Test tolerance, defaults to ±10% Can be set to your requirements.
Loss Compensation Compensates for “skin effect” when measuring narrow traces ( 3 mils or less)
this is covered in another section.
Propogation Velocity. Defaults to .66. Radio waves travel at the speed of light in air (186,000
miles/sec) In FR4 the speed of transmission is roughly 2/3rds, hence a default setting of .66 Vp is
normally left at the default of .66, however there can be instances where it is desirable to lengthen or
shorten the trace. ( to match previous test results) This can be accomplished by changing the setting
0 to .99. Small changes will make a big difference. This will not change the measured impedance
value as long as the pass/fail is properly set
To perform a test:
Enter all the parameters; Test Name, Target Impedance, ,Layer (opt),Trace Window, Starting Position
and Ending Position. The system defaults Tolerance to +/- 10%, so unless you want to change, leave
as is.
Place the cursor on the Test Name in the Test LIST pane and left click the Place the probe on the
trace you want to test and left click the mouse to highlight the test. The Test Name will be blue
highlighted.
Place probe on trace to be tested (observing polarity)
With the cursor anywhere in the test pane, left click the mouse. The system should have taken one
test which should be displayed on the screen. You can also initiate a test by hitting ENTER. save the
test result, right click and test test data will be displayed in the test list pane.
Your Test Screen should look similar to the screen below: (next page)
pg. 46
The right side or “end” of the trace should be visible, exiting to the top of the screen. If you do not see
the end of the trace, enter a larger number in the Trace Window box in the Test Editor.
Adjust the Pass/Fail window for optimal position (IPC recommends a starting position at the 50%
point of the waveform and the pass/fail window width to equal about 20% of the trace length, ie, a
trace 5 inches in length would have a pass/fail region width of one inch, (20% of 5 inches). The width
and position can be adjusted by “dragging the window with the cursor in the middle of the crosshatch
area. The width can be adjusted by grabbing an edge of the crosshatch area and dragging it to widen
or shorten the window. There is a quicker, easier way to set the pass/fail region. Drag the right hand
cursor so that it intersects with the end of the trace hitting the top of the screen. Drag the left cursor to
intersect with 00 on the x-axis. With the cursor anywhere in the cross hatch, right click to bring up
Click on Snap Test to % of Cursors
Right clicking with the cursor in the crosshatch area will save the test window parameters to the Test
Editor.
pg. 47
USING EXISTING TEST FILES AS TEMPLATES
You can create a new test file by opening an existing test file.
Start by opening a saved file that is similar to the one you want to create. Go into test editor and make any
changes you wish , ie, test names, target impedance, etc. Delete the data in the datalog pane, Add or delete
tracks then save under the new program name.
SELECTION OF PASS/FAIL MODE
The ST600 defaults to absolute test mode. That can be changed to pass on average by clicking the Pass on
Average box in the edit screen.
Pass/fail set in absolute mode. In the screen below, the average is within tolerance, but the curve is touching
the line resulting in a fail. For demonstration purposes we have tested a 75 ohm trace with the tolerance set
at 9%. The maximum allowable reading is 81.75 ohms. The value tested is 80.76…well within tolerance, but
the test failed because the trace is touching the crosshatch area
ABSOLUTE MODE
pg. 48
AVERAGE MODE
The identical waveform tested in the average mode results in a pass because PASS is based on the average
impedance value in the test zone. A rectangle has replaced the crosshatch.
DATALOG PANE
This is the datalog portion of the screen. All of the test results are displayed here.
Clicking on a datalog Result will bring up the saved test on the screen displaying the results of the original test.
Either the most recent test or from a saved file.
Right clicking on any test brings up this popup
pg. 49
Tests can be deleted,printed or exported in a .csv format for spreadsheet use and you can select
which columns we want to display
Create a report showing all the tests, only the passed tests , or only the failed tests.
Sort by Time/Date , Serial Number or Layer
How much data do you wish to display?
Select columns and move with the left/right arrows. Select their position with shift up/shift
down
pg. 50
Click on the columns you wish to display in the datalog pane
Note that the trace now goes almost completely across the screen. The longer trace that you see is the 50
ohm single ended probe. This indicates that everything is working.
If you do not see this, the system is not working properly.
In this part of the system, we are not calibrating. We are determining cable length and verifying that the
system is working. Do not be concerned with the reading in the measured impedance box and DO NOT enter
data in known impedance.
Next, Click on differential and repeat the sequence, attaching the differential probe. The screen should now
look like this:
Note that the trace is at 100 ohms and goes across the screen. The system is working properly.
Note: Do not click on the RESET button. This resets the calibration of the system.
pg. 51
Next, click on Actions, End of probe, find single, Repeat for differential
Now that the cables and probes have been checked and setup click on Actions/Insert Trace
Def
The screen should look similar to the screen below
Notice that there is nothing in the testlist pane or the datalog pane. The main screen has the default test
parameters and there is a pass/fail zone with white cursors at each side of the zone..
Next click on Actions/Insert Trace def The Test Editor screen will appear. Fill in test data. A good start would
be a 50 ohm test.
Once you have all the data filled in click on Save.
If you click on the newly created test you will notice a more complete description of the test at the bottom of
the test pane. You will also notice that the test is now highlighted.
pg. 52
SETTING DIRECTORIES
Now would be a good time to select where you want to store your files. Click on ACTIONS,
PREFERENCES, SET DIRECTORIES to see this popup:
pg. 53
CREATING A MULTI TRACK TEST PROGRAM
You’ve programmed a single test. Most coupons have more than one track, so now let’s program a coupon.
The following is an example of typical test program generation for a coupon.
This coupon has six tracks to be measured, 5 single ended and one differential. The quickest and
most efficient way to create a coupon test program is to make use of copy, paste and edit functions.
Start by right clicking with the cursor in the test definition pane to bring up the test editor. Identify the
first track however you wish, for example, 75 ohms layer 1. ( You can test the tracks in any
sequence, however it seems logical to start at one side of the tracks and sequence through to the
other side) In this example we are starting with the 75 ohm, layer one as the first test, 50 ohms layer
1 next.
In the edit screen:

Set target value at 75 ohms.

Set Starting Position at 2 This will be changed

Set ending position at 3 ( This will be changed

Set Trace window at 5 (because the track lengths are 4 ½ “)

The system defaults to absolute Pass/Fail…leave it there for now

The Tolerance defaults to ±10%. If you want to change it, now is the time.

Leave Loss Compensation and Vp in the default value
Click on Save
The first test is now programmed. Notice that it appears at the top of the test definition Pane
pg. 54
Next step is to validate this test and make any adjustments, so probe the appropriate pads and test.
IMPORTANT: You must highlight the test by clicking on it before you can copy, edit, delete,
etc
When you click on the test it will be highlighted with light blue
Here is the result of the test we just created
Next, we want to properly set the Pass/Fail zone.
Setting the PASS/FAIL Zone
Take a measurement and drag the right cursor to the end of the trace where the trace exits the
screen. Drag the other cursor to the 00 location on the x-axis. Next place the cursor anywhere in the
cross hatch are and right click.
This popup should appear. Click on Snap to % of Cursors
pg. 55
This popup should appear
Set the left hand cursor at the left screen edge and the right cursor at the end of the trace.
The program will default to 50% to 70%. We recommend keeping this setting, however you
can set to your choice.
The remainder of the tracks can now be programed . The easiest and fastest method is to
copy and edit, so click on the 75 ohm test to highlight, right click and copy. Paste 5 copies
below the 75 ohm test. You now have six identical tests. The pass/fail will be set the same for
all of the copy/pasted tracks. That’s OK, because it will close to the proper setting and you can
“tweak” if yo wish.
Starting with the next test in the sequence (50 ohms), click on the definition to bring up the
test editor and insert the changes (Test name,target impedance,etc) leave everything else as
programmed for the 75 ohm test….they are the same. Click on save and do the same for each
remaining test.
pg. 56
When you edit the differential pair, remember to click on differential in the edit screen.
Select Differential-A
You have now programmed a 6 track coupon and the test list should look like the one below.
Here are all six tracks programmed (plus some datalog)
Test each track and edit for any changes that may be necessary (maybe a repositioning of the test
window) .
YOU ARE NOW READY TO TEST
pg. 57
TESTING
pg. 58
TESTING
A test is initiated by left clicking with the cursor in the test screen or by pressing the outer most switch on the
footswitch. If the track tests PASS, the corresponding test in the test pane will display a green pass button:
A failed test will display a red “button”
Right clicking the mouse with the cursor in the test area will save the test.
the test description will also change in color to green for pass and red for fail.
When you have sequenced through all the tests a popup will show the test results and ask for operator name,
job number and starting serial number. Subsequent tests can be automatically saved if you choose.
The serial numbers are automatically sequenced.
CURSORS
Two selectable cursors are available. The cursors are a screen tool used to help make measurements.
Example:
The cursors can be used to set test region.
The cursors can also be used to measure propagation times.
pg. 59
The cursor colors are selected in the Color Set utility
Testing coupons
Prior to testing boards please ensure that the appropriate anti-static methods are observed.
A test is initiated by highlighting the trace definition of the trace then left clicking the mouse or by
pressing the outermost portion of the footswitch. A test can also be initiated by hitting ENTER
To save the test, right click the mouse or press in the innermost portion of the footswitch.
Place the Microstrip probe tips on the coupon pads to be tested (be sure you have observed the
correct polarity of signal and ground connect) and initiate a test. The impedance waveform will be
displayed and the impedance value and a Pass or Fail will be displayed.
Note: If a track to be tested is on an outer layer (typically backside) be sure that the layer is not in
contact with the test surface. This would result in an erroneous reading
Execute the test by left clicking the mouse or hit “Enter” or use the foot switch for “hands-free”
operation . Then right click to save the data in the datalog pane below the test screen which will
automatically advance to the next test (assuming auto advance is selected)
If the test fails the system will not automatically advance to the next test. Click on next test to
highlight it and continue testing.
pg. 60
If you have selected Autolog in utilities\preferences the system will automatically log the data and
advance to the first test on the coupon and will increment the serial number by one.
If the last track on the coupon fails, you must click on Actions\Log Coupon data to continue
The trace definition will turn to green on pass, red on fail.
As you test and save, data will accumulate in the datalog pane. Saving the program by clicking on
File\Save as will create a .tds file. The .TDS file contains all of the test information, including the test
waveform. Name the .TDS file and place into a folder of your choice. You can open this file at any
time to observe the test results. You can also send the file to another computer which can open the
file as long as Zscan software is installed ( ST600 does need to be installed on the receiving
computer).
Testing Hints
If you get this message, and the red comm light is on you probably have another instance of Zscan running.
If you are trying to test a differential trace and the test is failing badly, make sure you have clicked on
differential in the screen editor
Be sure you are observing probe polarity
If you have selected cursors, but you don’t see them, check the color setup…they could have defaulted to a
color not visible with your chosen background (ie, black on black)
pg. 61
DATALOG
This is the datalog pane
Passed tests are printed in green
Failed tests are printed in red
A click on any test will display the results of that test
A right click anywhere in the datalog pane will result in this popup
Clicking on select Columns will bring up this screen
Select the columns you wish to display in the Datalog Pane using the arrows and asjust their position with Shift Up and
Shift Down Then click on OK
pg. 62
GENERATING REPORTS
The ST600 can generate several types of reports:
A quick report generated by right clicking with the cursor in the datalog pane
If you click on Print Preview, you will generate a quick report like this:
Page 1
pg. 63
Page 2
This report can be printed
You can also generate a report by clicking on Actions/Create Report
pg. 64
The report created is a 3 page (minimum…could be more depending upon the amount of test
data) report in PDF format
Page 1 summary
Page 2 test criteria
pg. 65
Page 3 test data
The ST600 software also exports to Excel in .csv format
Clicking on export brings up this popup
Data is exported in three different schemes (all in .csv format):
1.
Summary …Typically, all the data needed
2.
Brief….A truncated version
3.
Complete…ALL the test data
pg. 66
This is summary export report
Same data as above in a brief format
pg. 67
Same data as a above in complete format
Clicking on EXPORT will save the datalog data in .csv format. This can be displayed in Excel. Clicking on EXPORT2 will
save the datalog data in a .csv file with slightly different Export
pg. 68
Export format
Export 2 format
SELECT COLUMNS Allows you to select the datalog columns you wish to display
REPORT ON Allows you to select whether you want a report on ALL coupons, only the Passed coupons, or only
the Failed coupons.
SORT BY Allows you to sort coupons by serial number or by time and date.
A saved datalog file can be opened and each test can be examined by clicking on the test.
pg. 69
LOOKING AT SAVED DATALOG DATA
You can look at data that was previously tested and data logged:
1. Open the saved file
2. Click on the trace that you want to examine
3. If you want to print out a specific trace waveform, click on the trace, then with the cursor
in the crosshatch area, right click which will bring up:
Click on print to print out waveform of the selected test:
pg. 70
HOT KEYS and CURSORS
The ST600 has several ”shortcut” keys:
Ctrl Y = CUT
Ctrl C = COPY
Ctrl V = PASTE
F4 = Opens test editor
ENTER = initiates a TEST
SHIFT/ENTER = record a test (datalog)
INSERT = Insert a new test
DEL = Deletes a test definition or deletes a datalogged test group (coupon)
Ctrl Z = To undo previous deletions
To verify the calibration in differential mode, same sequence as for single ended. With no probe
attached, click on eoc and save. Attach a standard to each cable and click on Verify
The value should be close to the sum of the values of the two standards.
NOTE: Pushing RESET will bring back the original factory settings.
CAUTION: Doing a calibration function should be a last resort. The ST600 is inherently
stable. Any sizable shift in calibration could indicate ESD damage. Please check with the
factory before proceeding’
pg. 71
A word about the semi-rigid reference standards supplied with the ST600
These semi-rigid coax cables have been calibrated against NIST traceable laboratory airlines .We
recommend storing them in an environment where they will not be bent. Semi-rigid coax is an
excellent choice for this application. Coupons do not make suitable standards because they do not
possess the stabilty and will change over time and conditions.
pg. 72
SPECIAL FUNCTIONS
The following special functions are typically not used in day to day testing. Virtual retest
and Aggressive Deconvolution are Zmetrix proprietary , not available in any other TDR:
Virtual Retest
Vp
Aggressive Deconvolution
Short Traces
Loss Compensation
Propogation Time
pg. 73
VIRTUAL RETEST
If you have just completed a test a group of coupons and the test results were below expected, you can do a
“virtual retest” of the lot. Or you can take an archived file and do the same.
In the case of a newly tested lot it is advisable to save the file. Once you have saved the file, decide which
tracks you that want to retest with changes in the test parameters.
You can change:
Test tolerance
Location and width of the pass/fail window
Pass on absolute or average
Usually , the failures will appear on one or two tracks on the coupon, and many times the failure is by a slight
amount,
We tested a coupon with 5 tracks. All coupons exhibited the same failures; the bottom 50 ohm and 75 ohm
tracks failed. We changed the pass mode to pass on average and performed a virtual retest. The before and
after results are shown below:
This is the data logged test data prior to Virtual Retest
pg. 74
Here is the pass/fail data after doing a Virtual retest changing to pass on average on the failed tracks.
To perform a virtual retest, select the track you want to test, right click to open Edit screen. Next make the parameter
changes and right clight click on the highlighted test definition to invoke this popup:
Clicking on Virtual Retest brings up the screen below
Clicking on yes will do a virtual retest of the track in ALL coupons
pg. 75
The following describes the power and utility of this function:
Assume that after testing a batch of 25 coupons, your yield was not as high as expected. What if you
had set up the test differently, perhaps located the pass/fail window in a different place on the
waveform. Or maybe if you had tested in Average pass instead of Absolute more coupons would
have passed. You can now do the “what ifs”. You can change the aforementioned parameters and
do a “virtual retest”. Because all of the test data is stored, we look at it again with the new test set
up.
If we like the results we can keep them and the changes in the test program are kept. If there are no
advantages, don’t save.
This a valid methodology, the data has not been changed, we’ve just looked at it with a slightly
different set of rules
Vp
The Velocity of Propagation is the speed at which electrical signals travel along a printed circuit trace
or a cable. Vp is given as a percentage of the Speed of Light (186,000 miles/sec)
The speed at which the signal travels is dependent on the dielectric of the medium in which it travels.
For simplicity sake, the ST600 assumes a velocity of propagation of 66% of the speed of light. The
ST200 calculates distance or time from a time based on a default value of Vp of 0.66. This value is
valid for most PCBs.
This will result in apparent errors when measuring airlines which have a Vp of 98-99% and they will appear
shorter than their actual length. This can be remedied by Changing Vp in the Criteria Editor.
In most instances: LEAVE Vp IN DEFAULT SETTING
pg. 76
AGGRESSIVE DECONVOLUTION
Aggressive deconvolutrion may be a new test term to you. The ST600 utilizes Digital Signal Processing and
deconvolution is an integral part of the system. The aggressive mode is invoked when there are special circumstances
like an extremely short trace or a trace with abrupt changes in impedance.
For a quick definition, go to www.wikipedia.org. Deconvolution is used in Radio Astronomy to sharpen blurred images.
It’s one of the things that makes the ST600 the superior instrument that it is.
The picture below is of a special coupon used to demonstrate the effectiveness of this feature. The trace starts out at
seventy ohms, then abruptly changes to one hundred ohms, then abruptly to fifty ohms, and back to seventy ohms.
Special stepped impedance coupon shown in actual size.
While this coupon does not represent a situation you are likely to encounter, it does show the effectiveness
of this feature.
pg. 77
This is standard deconvolution. 70Ω is a low, 100Ω reads low 85Ω, 50 reads 57Ω
The screenshot below shows the measurement of the same coupon,same instrument, same setup. The only
change is that Aggressive Deconvolution was invoked
Tested with aggressive deconvolution 70Ω reads 69.79, 100Ω reads 94.03Ω, and 50Ω reads 51.60Ω
Notice how the curve more closely represents the actual expected impedance. The 70Ω plateau is quite flat,
the 100Ω and 50Ω are somewhat flat….they are only about inch long on the coupon.
pg. 78
MEASURING SHORT TRACES
END OF PROBE MARKER
Note that the entire trace is less than one inch in length
This blue vertical line shows the end of the probe tip
END OF PROBE
The importance of End of Probe is: When observing the waveform “launch” you don’t really know what part of the
displayed trace corresponds the probe tip. This is extremely important when measuring short traces.
In the screenshot below, notice the thin blue vertical line at about 0.1” That point is the probe tip.
pg. 79
Measuring short traces
This is the trace we have measured
The ST600 can measure short traces (with some restrictions). The traces should have an impedance of
approximately 50 ohms. Other impedances on short trace can be measured but may require a different
probe. This is a short piece of flex. You can see that the trace is about one inch in length
pg. 80
The vertical blue line is the probe tip
In order to ensure an accurate measurement we need to
know the starting point of the measurement with accuracy. This is accomplished using
“ACTIONS/END OF PROBE/FIND END OF PROBE
In the screen shot below, we have placed the white left hand cursor almost on the blue end of probe
marker.
Note the screen dimensions. This is one inch
While we don’t advertise measurements this short , this exercise clearly proves that it can be done.
pg. 81
LOSS COMPENSATION
Caution!! Applying loss compensation improperly may invalidate test results
A sloping wave form can be due to a number of causes. As the trace width narrows the effective cross section area is
reduced increasing the DC resistance. This combined with “skin effect” can create series loss. This loss can be
compensated for by adjusting the slope of the waveform (if we know the loss/inch).
One way to check for series loss is to test the track from both ends. If series loss is present, the waveform will look the
same with the curve rising from left to right on the display.
APPLYING LOSS COMPENSATION
Click on trace test definition to highlight and click on EDIT. In the lower right hand corner of the edit screen you will find
the loss compensation.
Click on USER
Set the pass fail window for 1 inch and observe the impedance value at the left hand edge of the window. Compare it to
the impedance at the right hand edge of the window. The difference is loss per inch expressed in ohms. Plug this
number into the Ohm/Inch Window and select a starting point for the compensation to be applied to the curve.
Alternatively, you can observe the waveform and estimate the upward change in the value over one inch. Determine
the point on the waveform where the slope begins and put this number in the Start at box.
The result will be a display which shows the original uncompensated curve overlaid with the compensated curve
displayed in a color of your choice. Using the trial and error approach, you may have to try a few different settings
The original waveform shows a constant rise beginning at about 2.4”. Loss compensation has been
applied (1.5ohms/inch, beginning at 2.4”). The compensated portion of the wave form is shown in yellow. Pass/fail is
based on the compensated waveform.
UNLESS YOU ARE ABSOLUTELY SURE THAT YOU HAVE SERIES LOSS
LEAVE LOSS COMPENSATION IN THE DEFAULT MODE.
pg. 82
Propagation time
The propagation time is displayed here
Note that the delta time is always twice the propagation time.
The time displayed is the time between the cursors.
pg. 83
Testing the precision cables on the Zmetrix ST600
Over a period of time, with average usage, the precision cables supplied with the ST600 will
deteriorate and this will ultimately affect measurements made. This application note is intended to
help owners of the ST600 to check on the condition of the flexible test cables and decide when to
replace.
Equipment required
ST600
SMA barrel connector
“Cable test” test file (click to download)
Procedure
Load the “Cable test” file and open it. The test file should set up the ST600 test parameters as shown
in the diagram below.
ST600 Cable test file parameters
Choose a channel on the ST600 on which to run the tests. Ideally, this will be a channel on which the
original cable is attached and in good condition.
Edit the test file to run on this channel.
Connect the barrel connector to the cable on the test channel.
SMA barrel connector
pg. 84
Connect the cable under test to the barrel connector.
Run the test. The resulting waveform should be similar to that shown below.
If the test trace enters the red shaded areas — indicating that the impedance of the cable has
changed by more than ±5% at some point — then replace the cable with a new Zmetrix supplied
cable, Zmetrix Part No.
pg. 85
Measuring differential pairs with no grounds
Occasionally, you may have the need to measure a differential pair with no ground.
The structure looks like the one below:
With a signal of equal positive and negative going potential a virtual ground exists midway between
the two traces.
Measuring this structure is quite simple; program the ST600 to measure the expected impedance, but
insolate the ground pins of the test probe from the measurement.
pg. 86
GENERAL IMPEDANCE AND TDR INFO
pg. 87
Characteristic impedance
As the speed of electronic devices increases the electrical properties of the circuits carrying signals
between them become more important.
At the high frequencies and clock rates of modern digital circuitry, cables and PCB traces increasingly
need to be considered as transmission lines. In the case of PCB traces the series resistance and
parallel conductance of the transmission lines are largely negligible. √The most significant circuit
parameters governing performance are the distributed series inductance and parallel capacitance of
the conductors.
These parameters give rise to the characteristic impedance
Barring any dissipative effects such as dielectric “leakage” and conductor resistance,
the characteristic impedance of a transmission line is equal to the square root of the ratio of
the line's inductance per unit length divided by the line's capacitance per unit length:
Barring any dissipative effects such as dielectric “leakage” and conductor resistance,
the characteristic impedance of a transmission line is equal to the square root of the
ratio of the line's inductance per unit length divided by the line's capacitance per unit
length:
.
Where L is in henrys per unit length, C is in farads per unit length and Zo is in ohms.
Consider an electrical signal travelling through a conductor such as a PCB trace.
When the signal encounters a change of impedance (Zo) arising from a change in material or
geometry, part of the signal will be reflected and part transmitted.
These reflections are likely to cause aberrations on the signal which may degrade circuit
performance.
To optimize performance of high speed circuits it is desirable to match the input and output
impedances of devices with the characteristic impedance of the interconnections.
pg. 88
Controlled Impedance Measurements
Signal integrity in electronic circuits is becoming more important to designers as clock speeds
and data rates move ever upward. The continual trend towards higher speed circuits is finding
its way into the full spectrum of electronic devices.
They can now be found in everything from cell phones to space vehicles.
One of the factors in designing quality signals in electronics is the impedance of the pc
board/trace signal path. Most designers now use software modeling tools to predict the
effects of trace width, pc board material, and signal path geometries in the pc board itself. A
successful circuit design then requires that the pc board after manufacture complies closely
with the design parameters used in the software tools.
PC board manufacturers are being asked now by their customers to provide data as to their
compliance with these design parameters. One method for testing compliance, is the use of a
controlled impedance test system to measure the pc trace impedance for either a test trace on
the board, or traces on a representative coupon located on the fabrication panel.
The ST600 is designed to provide manufacturers with a complete application solution for
controlled impedance measurements. The system is configured for both single ended and
differential impedance measurements, and has everything the user needs to gather data,
store, and report data to customers. Measurement integrity is assured through a NIST
traceable calibration path.
pg. 89
Time Domain Reflectometry
A time-domain reflectometer (TDR) is an electronic instrument used to characterize and locate
faults in metallic cables (for example, twisted wire pairs, coaxial cables).[1] It can also be used to
locate discontinuities in a connector, printed circuit board, or any other electrical path.
A TDR transmits a short rise time pulse along the conductor. If the conductor is of a uniform
impedance and properly terminated, the entire transmitted pulse will be absorbed in the far-end
termination and no signal will be reflected toward the TDR.
Any impedance discontinuities will cause some of the incident signal to be sent back towards the
source. This is similar in principle to radar.
Increases in the impedance create a reflection that reinforces the original pulse while decreases in
the impedance create a reflection that opposes the original pulse.
The resulting reflected pulse that is measured at the output/input to the TDR is displayed or plotted as
a function of time and, because the speed of signal propagation is almost constant for a given
transmission medium, can be read as a function of cable or trace length.
Because of this sensitivity to impedance variations, a TDR may be used to verify trace impedance
characteristics, and discontinuities on a PCB.
Simplified TDR-cable system equivalent circuit
pg. 90
As a step wave propagates down a cable any increase in characteristic impedance causes a reflected
voltage which adds to the initial step. This is shown, for example, in the previous diagram by the step
up from the 50to 75cable. A decrease in impedance would cause a reflection in the opposite
sense which would show as a downward step in the TDR trace. As long as the characteristic
impedance remains constant the trace remains level.An open circuit at the end of the cable reflects
100% of the voltage in a large upward step. If the end of the cable were to be short circuited a 100%
reflection in the opposite direction would occur, returning the trace to the zero voltage level
pg. 91
Single ended impedance testing
Normal controlled impedance structures comprise a single signal conductor and a ground plane or
planes. Impedance measurements made between the signal and ground are sometimes referred to
as single-ended. The single-ended transmission line is probably the commonest way to connect two
devices. In this case a single
conductor connects the source of one device to the load of another device.
The circuit below is an example of a single-ended transmission line.
The reference (ground) plane provides the signal return path. This is an example of an unbalanced
line. The signal and return lines differ in geometry — the cross-section of the signal conductor is
different from that of the return ground plane conductor.
The impedance of a PCB trace will be determined by its inductive and capacitive reactance,
resistance and conductance. These will be a function of the physical
dimensions of the trace (e.g. trace width and thickness) and the dielectric constant of the PCB
substrate material and dielectric thickness. PCB impedances will typically range.
Single-ended transmission lines on PCBs
In practice, a single-ended PCB transmission line typically consists of a line conductor trace, one or
more reference planes and a dielectric material. Two configurations are
commonly used, microstrip and stripline. The simplest configuration, the surface (or exposed)
microstrip, shown in the diagram below, consists of a signal line, the top and
sides exposed to air, on the surface of a board of dielectric constant Er and referenced to a power or
ground plane. Surface microstrip can be implemented by etching one
surface of double-sided PCB material.
pg. 92
The diagram above shows the characteristic microstrip impedance attributes:
the microstrip impedance references a single plane the impedance trace is often on an outer layer.
The trace and plane(s), form the controlled impedance. The PCB will frequently be multi-layer in
fabrication and the controlled impedance can be constructed in several ways. However, whichever
method is used the value of the impedance will be determined by its physical construction and
electrical characteristics of the dielectric material:
The width W, W1 and thickness T of the signal trace
The height H of core or pre-preg material either side of the trace
The configuration of trace and planes
The dielectric constant Er of the core and pre-preg material
The stripline (shown in profile below) typically consists of a line conductor trace sandwiched between
two reference planes and a dielectric material.
The transmission line, i.e. the trace and planes, form the controlled impedance. The value of the
impedance will be determined by its physical construction and electrical
characteristics of the dielectric material: The width and thickness of the signal trace
The dielectric constant and height of the core or prepreg material either side of the trace
The configuration of trace and planes Electromagnetic waves in a vacuum travel at the speed of
light. The propagation velocity in a material is primarily dependent on the Er of the material (it’s
roughly inversely proportional to the square root of the dielectric constant of the material). Assuming a
value of about 4 for dielectric constant for G-10 or FR-4 (the actual value will depend on frequency
and the glass to resin ratio) the propagation velocity will approximate to half the speed of light.
pg. 93
Differential impedance testing
A more complex type of controlled impedance structure is that of a differential pair of signal
conductors. The lines are driven as a pair with one line transmitting a signal waveform of the opposite
polarity to the other. At the receiving end one signal is subtracted from the other so noise induced in
both lines is cancelled out. This method is commonly employed to achieve high noise immunity in
high speed digital and analog
systems.
The characteristic impedance of a differential pair is affected not only by the impedance of each line
of the pair to ground but also by the interaction of the signal lines with each other.
The ST600 includes the facility to make differential impedance measurements in which the
impedance between conductors is tested as well as the impedance between
conductors and ground. The differential configuration (often referred to as a balanced
line) is used when better noise immunity and improved timing are required. In differential mode the
signal and its logical complement are applied to the load.
The balanced line thus has two signal conductors and an associated reference plane or planes as in
the equivalent single-ended (unbalanced) case. Fields generated in the
balanced line will tend to cancel each other, so EMI and RFI will be lower than with the unbalanced
line. External noise will be "common-moded out" as it will be equally sensed by both signal lines.
pg. 94
Differential transmission lines on PCBs
A typical differential configuration, the edge-coupled surface microstrip is shown below
In this construction the gap S between the traces defines the coupling factor and hence the
differential impedance. The etch factor, plating density and undercut will make this
construction simple to manufacture, but with a wider tolerance due to the extra processing required
on external layers The stripline construction is also used in differential pair configurations. A typical
construction, the edge-coupled symmetric stripline is shown below
The edge-coupled symmetric stripline is the simplest differential construction possible, though it may
prove difficult to maintain the signal traces exactly in the centered position with any offset from the
centered position reducing the impedance.
The Differential Test
Normal controlled impedance structures comprise a single signal conductor and a ground plane or
reference plane.
Impedance measurements made between the signal and ground are sometimes referred to as singleended.
Where higher performance (e.g. greater noise immunity and better EMC performance) is required a
more complex type of controlled impedance structure is employed — that of a differential pair of
signal conductors. The lines are driven as a pair with one line transmitting a waveform of opposite
polarity to the other. At the receiving end one signal is subtracted from the other
so any noise induced in both lines is cancelled out. This is a technique commonly used to increase
noise immunity (and reduce signal skew in bus circuitry) in high speed digital and analog systems.
pg. 95
The characteristic impedance of a differential pair, therefore, is affected not only by the impedance of
each line of the pair to ground but also by the interaction of the signal lines with each other.
Test coupons
The impedance of a PCB trace depends on a number of factors, including the dimensions of the trace
(i.e. trace width and thickness) and the thickness and dielectric constant of the laminate and pre-preg
material. It's common practice for board manufacturers to check controlled impedance build integrity
initially by building engineering lots to verify copper weight, line widths and dielectric thickness and
constant before beginning volume production. Even in production, manufacturers perform 100%
controlled impedance testing on controlled impedance boards.
However, it is not uncommon for the actual PCB traces to be inaccessible for testing. In addition,
traces may be too short for accurate measurement and will probably include
branches and vias which would also make exact impedance measurements difficult. Adding extra
pads and vias for test purposes would almost certainly adversely affect the
performance of a controlled impedance trace as well as occupying valuable board space. PCB testing
is therefore normally performed, not on the PCB itself, but on one or two test coupons integrated into
the PCB panel. The coupon will be of the same layer and trace construction as the main PCB and will
include traces with precisely the same impedance as those on the PCB, so testing the coupon will
give a high degree of confidence that the board impedances will be correct
pg. 96
ESD
Know your enemy!
ESD is a killer of TDRs….every TDR. The ST600 is less sensitive to ESD damage than any other TDR, but it is
possible to damage the measurement section or “test head”. If you’ve ever lost a test head on a Polar,
Tektronix or Agilent you know that replacement/repair is costly.
The good news is that the maximum cost of repair of an ESD damaged ST600 is $1,500 if the unit is out of
warranty.
With some precautions like wearing the wrist strap and being mindful of the things that
generate ESD you can save youself the inconvenience.
Electrostatic Discharge, or ESD, is a single-event, rapid transfer of electrostatic charge between
two objects, usually resulting when two objects at different potentials come into direct contact with
each other. ESD can also occur when a high electrostatic field develops between two objects in
close proximity. ESD is one of the major causes of device failures in the semiconductor industry.
Electrostatic charge build-up occurs as a result of an imbalance of electrons on the surface of a
material. Such a charge build-up develops an electric field that has measurable effects on other
objects at a distance. The process of electron transfer as a result of two objects coming into contact
with each other and then separating is known as 'triboelectric charging'.
Examples of Static Generation
Typical Voltage Levels
Means of Generation 10-25% RH 65-90% RH
 Walking across carpet 35,000V 1,500V
 Walking across vinyl tile 12,000V 250V
 Worker at bench 6,000V 100V
 Poly bag picked up from bench 20,000V 1,200V
 Chair with urethane foam 18,000V 1,500V
 Removing packing tape
pg. 97
 Pulling slip sheets out from panels
ESD controls may be classified into three major categories: 1) prevention of static charge build-up;
2) safe dissipation of any charge build-up; and 3) improvements in the ESD robustness of the
product. The second category is very closely related to, and may sometimes even be
indistinguishable from, the first category. Safe dissipation of accumulated charge involves the
provision of a suitable electrical path that will allow charges to flow to ground. No ESD control
program will be successful without a sound policy on grounding. As much as possible, there should
only be one common ground for the entire factory.
Everything in the production line, from equipment to work tables to cabinets and racks, should be
connected to this common ground. If the factory uses conductive flooring, then this should also be
connected at regular intervals to this common ground. Having a single or common ground will ensure
that everything in the production floor will remain at the same potential. Any charge build-up will
immediately be dissipated by a good grounding system. The use of properly grounded wrist and foot
straps or conductive shoes will also fall under this category, since these will bring any charge buildup on personnel to the common ground.
Fig. 2. Examples of personnel grounding accessories:
wrist strap, sole grounder, and conductive shoes
For additional information
*Reference: www.esda.org
The preceding information is offered to make you aware of the potential for problems.
We don’t anticipate that you will have problems, but we do suggest that operators wear the wrist
strap when operating the ST600 as a precautionary measure..
This assumes that you have already performed an EOC check for both single ended and differential
and have the proper probes attached and you have also executed Actions/find the end of probe for
both single ended and differential
Click on Actions/preferences/set X-axis to and select Probe
pg. 98
Launch Point
NIST
pg. 99