Agilent EEsof EDA
Spectral Domain System Simulation Identifies
RF/Microwave Design Problems Early
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Spectral Domain System Simulation
Identifies RF/Microwave
Design Problems Early
By Rulon VanDyke
Eagleware Corporation
espite advances in simulation technology, radio frequency (RF) and
microwave system and architecture
design is still being done in spreadsheets,
math packages, custom software tools and
digital signal processor (DSP) based time
domain tools. This approach is error prone
and often leads to multiple design iterations. Each iteration costs time and money.
Design intervals are shrinking and quality
expectations are rising. Companies are
forced to use this approach because of the
lack of integrated software that supports
top-down RF design.
Eagleware Corporation’s new SPECTRASYS design module incorporates a new
approach to RF/microwave system analysis: ▲ Figure 1. Base station receiver optimization and yield
spectral domain simulation. This approach
analysis.
allows engineers to specify entire spectrums, including measured data for frequency sources. SPECTRASYS is completely faster calculations by making narrow-band
integrated into the GENESYS software suite, assumptions, thereby ignoring important specunifying the entire RF system design process by tral components. Real-time tuning or optimizamaintaining tight coupling between develop- tion based on harmonic constraints are comment stages from concept to production.
pletely impractical (see Figure 1).
The spectral domain engine offered in SPECAdvantages of spectral domain engine
TRASYS brings to the table simulation soluIn traditional system simulation methods, tions not possible with other techniques. Some
spectrums are all but ignored. In cases where of these solutions include user-defined paths;
power spectrum simulation is supported, the level diagrams; full-node spectrums at any
spectrum produced is typically the result of a node; measurement and path independent
Fourier transform from time domain analysis schematics; origin identification and travel path
data. While informative, this type of analysis of each spectral component; viewing phase of
takes millions of sample points to see an audio- any spectral component; analysis of parallel
frequency sideband on a X- or K-band carrier. paths; and directional power flow.
Analyzing a constrained bandwidth spectrum
for a static schematic can take tens of seconds or User defined paths
even minutes. Other applications arrive at
Many RF systems are composed of multiple
D
72 · APPLIED MICROWAVE & WIRELESS
▲ Figure 2. Composite spectrum output from cascaded
amplifiers due to three broadband input signals.
paths. For instance, a receiver will have a main path for
the received signal but will also have an local oscillator
(LO) path from the LO to the receiver input or transmitter output.
Traditional software requires a schematic to be created for each path, even though many of the stages may be
the same. Synchronizing these schematics becomes a
problem, and one simple change may need to be made to
multiple schematics. With SPECTRASYS, only one
schematic needs to be created for the system which can
have any arbitrary topology and only the path needs to
be specified instead of creating a new schematic. These
paths are as simple as a beginning and end node.
Multiple paths can be analyzed for a single schematic.
Level diagrams
Level diagrams show measurements at each node
along a designated path. Level diagrams have been used
by RF/microwave engineers for decades but have not
appeared in commercial software until now. In traditional Software: Level diagrams must be manually created (if they can be created at all), which typically
require users to write equations representing the signals
at each node. With SPECTRASYS, however, level diagrams can be easily added for any path in the schematic. More than 30 channelized RF measurements can be
added to these diagrams. Node numbers and schematic
symbols are automatically drawn at the bottom of the
level diagram. Schematic element properties can be
changed directly from the level diagram (see Figure 3).
Full-node spectrums
When an RF printed wiring board is powered up in
the laboratory, the designer can take a spectrum analyz74 · APPLIED MICROWAVE & WIRELESS
er and, theoretically, can probe any node on the board.
We can immediately examine any part of the spectrum:
main signal, intermods, harmonics and broadband
noise. Many times, the design cannot be completed until
full node spectrums are characterized. Unfortunately,
the initial examination of full node spectrums typically
occurs for the first time in the laboratory and not
through the simulation software.
Traditional time-based simulations use the concept
of an input and output for every RF system model. A
time signal is placed at the model input and the output
is determined and passed on to the input of the next
model. This process is repeated for the entire block diagram. Signals traveling backwards through a receiver
front end, like LO leakage will not accounted for and the
complete full-node spectrums cannot be seen. By contrast, SPECTRASYS is not limited to dedicated model
inputs and outputs. All sources and their created products, such as intermods, harmonics and broadband
noise, are propagated through each model in all directions. As a result, complete full spectrums appear at
every node (see Figure 2).
Measurement and path independent schematics
All RF printed wiring boards are created from a single schematic. In traditional software, multiple schematics typically have to be created, since measurement specific information (i.e., test measurement icons) is contained in the schematic. Because the signals only flow
from input to output, additional schematics must be created to represent reverse paths. For example, a new
schematic must be created to examine LO leakage. This
approach is error prone and a single change will need to
be propagated through several schematics.
With SPECTRASYS, only one schematic is needed to
represent the complete printed wiring board simulation.
Path and measurement definition information is not
kept within the schematic. System verification is simpler because circuit implementations only need to be
substituted into a single schematic.
Spectral component origin identification and travel path
Most designs go through the “Where did that spur
come from?” phase once the RF board is first powered
up in the laboratory. This is the wrong time to find these
problems. Traditional software uses output spectrum
based on a fast fourier transform (FFT). The time
sequence contains NO origin and path identification
information for spectral components. In SPECTRASYS:,
all spectral components propagate to every node in the
system. Origin identification, path information and
power level are attached to every component. From this
identification information, designers can quickly see the
cause of the spurious energy and the path it took to
arrive at the destination. The problem can then be easily mitigated before artwork (Figure 2).
View phase of spectral components
There are many cases where looking at the
phase of spectral components in an internal node
to the system would be very beneficial. For
example, in an error cancellation circuit, it
would be very useful for the designer to see the
exact phase shift of the identified spectral components and how this changes as the circuit is
tuned or modified.
The time-based FFT engines used in traditional software means that all individual spectral component information is unavailable. In
SPECTRASYS, spectral component phase is
available at every node for every component. A
marker and a graph are all that is needed to
examine this phase.
Parallel paths can be compared and analyzed
Circuit topologies are becoming more complex. RF architecture and system design tools
need to help designers determine correct cascad- ▲ Figure 3. A 2 GHz receiver with receive-level diagram and coned lineups for parallel paths. Defining paths in
ducted emissions.
traditional time domain simulators is nonexistent. Looking at parallel paths is impossible if
you cannot define them. Spreadsheets require the defin- nent; viewing phase of any spectral component; analysis
ition of all paths and links between common stages, of parallel paths; and directional power flow. SPECwhich is both time-consuming and error-prone.
TRASYS allows the RF designer to quickly identify and
SPECTRASYS, however, supports arbitrary topolo- resolve potential problems before they become probgies. Multiple paths can be placed on a single table. lems. Time and money are saved if designers can identiMeasurements for all paths having common stages will fy as many architecture and implementations problems
be automatically taken into consideration. No links to as early as possible in the design cycle. Finding problems
common points or stages need to be established.
in the laboratory is costly and time-consuming.
■
Total power can be seen traveling in all directions
RF engineers know that power flows in all directions
at any node on their circuit board. For example, we
know that an LO will be conducted to the input of
receiver or the output of a transmitter. The receiver LO
will travel in the opposite direction through the front
end as would the received signal.
In traditional software, power flow is from input to
output so a reverse traveling signal does not make a lot
of sense. The spectrum output only represents forward
flowing power. SPECTRASYS has the ability to display
power flowing in all directions entering a node. All
power traces can be displayed on the same graph using
a unique trace color for each direction. With a quick
glance the user knows the travel direction of all spectral
components.
Conclusion
SPECTRASYS has these unique advantages not
found in other RF simulation software: user-defined
paths; level diagrams; full-node spectrums at any node;
measurement and path independent schematics; origin
identification and travel path of each spectral compo-
Author information
Rulon VanDyke is the lead engineer in systems simulation at Eagleware Corporation. He received both a
bachelor of science degree and master of science degree
in electrical engineering from Brigham Young
University in 1990. For 10 years, he designed first-, second- and third-generation digital cellular transceivers
and base stations for AT&T Bell Labs and Lucent
Technologies. In 2001, he joined Eagleware to develop
SPECTRASYS. He may be reached via E-mail:
[email protected].
For more information, contact:
Eagleware Corporation
635 Pinnacle Court
Norcross, GA 30071
Tel: 678-291-0995
Fax: 678-291-0971
E-mail: [email protected]
Internet: www.eagleware.com
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Revised: March 27, 2008
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© Agilent Technologies, Inc. 2008
Printed in USA, June 01, 2002
5989-9284EN