Microwave Office®
Application Note
INTRODUCTION
The X-band frequency range has been designated for critical military and
public safety applications such as satellite communications, radar, terrestrial
communications and networking, and space communications. It is important to
ensure that these signals deliver quality, reliable, and secure communications.
This application note describes the design and realization of a complex X-band
transmission analyzer for use in real-time material testing.
The purpose of this analyzer is to gather complex-valued X-band transmission
coefficients at high update rates of greater than 100,000 measurements
per second. This note describes how manufacturing costs were minimized by
integrating the many RF components in the device onto a single printed circuit
board (PCB), how coupling issues between the RX and TX paths caused by the
requirement for high dynamic range were addressed, and how EM simulatorbased tuning was used for the numerous distributed elements on the board to
ensure optimal performance.
Resultant X-band
transmission analyzer.
End-to-end Design
and Realization of an
X-band Transmission
Analyzer
THE DESIGN FLOW
The design team at the Vienna University of Technology was tasked with designing from
scratch and realizing this X-band transmission analyzer. The design flow involved the
design and optimization of several breeds of circuits, including critical elements such as
bias-T and microstrip filters, all of which were designed using AWR’s circuit, system, and
EM analysis software within the single, integrated AWR Design Environment™.
The PCB layout was done entirely within AWR’s Microwave Office® circuit design
software. Additionally, AWR’s Visual System Simulator™ (VSS) communication system
design software was used to find an optimal RX chain and to estimate the phase locked
loop’s (PLL) phase noise properties. The PLL’s loop filter characteristics were optimized
using the exact same schematic that was also used for carrying out the PCB layout.
Finally, AWR’s AXIEM® 3D planar EM software was utilized for simulating the varied
distributed element circuits, as well as to tackle shielding issues in the final PCB design.
(Figure 1 shows all relevant RF-circuits that were investigated during this project in order
to design the resulting final, outstanding overall system.)
Figure 1: PCB topologies investigated
during the design flow.
Because all AWR’s technologies are integrated into a single design environment, the
team was able to easily reuse structures and circuits from system models down to
PCB layout structures. For example, EM simulations of the actual PCB traces could be
checked against VSS models to see whether the designed shielding was sufficient. This
approach enabled the designers to reuse highly optimized sub circuits in the final PCB
design (Figures 2 and 3).
Figure 2: Final design layout shown within the AWR software environment.
Figure 3: Final manufactured prototype.
THE BIAS-T
Microwave Office software enables users to readily tune circuits based upon EM
simulations. In this case, the design required that the X-band RF link, the high-speed
serial bus, and the DC power supply be combined onto the same cable, thus requiring
a bias-T for each and every X-band cable interface.
In order to reduce assembly complexity and manufacturing costs, the circuit
was realized using distributed elements. A classic radial stub approach was
determined to be the best, considering tradeoffs between PCB real estate,
circuit performance, and board complexity. For reasons emerging from signal
post-processing, two different-sized radial stubs were used to ensure that the
circuit would perform consistently over about 30 percent bandwidth (Figure 4).
Using two unequal stubs made it possible to achieve constant low RF leakage
over a broad frequency range, however, this architecture was more challenging to design because placing resonant structures in close proximity resulted
in significant coupling effects. Typically this is less of a problem when dealing
with multiple structures, all operating at the same frequency. Yet in this case,
the coupling effects tended to disrupt the phase relations if not all length ratios
were close to optimum.
LF +DC
λ/4 @ flow
low RF stub
λ/4 @ flow
RF
λ/4 @ fhi
high RF stub
λ/4 @ fhi
combined RF + LF +DC
LF - block
It was clear to the team that this issue could not be resolved using closed form
approximations or simple models. For this reason, AXIEM and the Microwave Office
optimizers were the weapons of choice. Numerous runs were necessary to find a
proper solution and to answer questions like “what’s the required clearance to the
surrounding ground?” In the end, the same circuit was reused several times within
the overall design without any issues.
The inner workings of the circuit could
be understood using the current density
derived by AXIEM (Figure 5). The current
density’s maximum, which is equal to the
phase center of the RF reflect, shifted
with respect to the excitation frequency.
This resulted in a frequency-independent
virtual RF open exactly at the branch line
intersection with the RF path. This was
observed as current density minimum
at this position. Therefore, the branch
Figure 5: Bias-T standing wave current
density.
line was virtually “invisible” for the RF
signal. All the RF leakage measurements
Figure 4: Bias-T dimensions.
as well as the simulations were combined, as shown in Figure 6. The final circuit
indeed shows the desired flat frequency response in leakage over a broad band and
at constant low leakage.
0
Bias-T RF leakage (dB)
-10
-20
-30
-40
Figure 6: Measured data vs. simulation.
-50
measured - both stubs
- only high stub
- only low stub
- no stub
-60
-70
8
9
10
Axiem
11
Model Based
12
13
14
Frequency (GHz)
THE MICROSTRIP FILTER
The microstrip filter design was carried out by deriving an equivalent lumped circuit
from the design’s specifications. This representation can easily be reformulated in
terms of characteristic impedances, which is the starting point for any distributed filter
design. After that, it is up to the designer to choose a transmission line topology that
1) can realize all the desired impedances, 2) enables a compact setup, and 3) can be
manufactured with an available PCB process. In this situation, the Microwave Office
software environment assisted the design team with various tools like TX-Line® and
simple transmission line models available for many topologies. It further enabled the
designers to quickly gauge whether a certain substrate, topology, and tolerance mix
could work. These decisions are critical to the overall success of the design and must
be carefully considered.
The challenge with designing the microstrip filter for this project was that the
substrate needed to be quite thin in order to achieve a compact design in a 50Ω
microstrip stack up. It was also problematic for the PCB manufacturer to deal
with very narrow coupling gaps. The design team needed to find an alternative to
the classic edge-coupled microstrip design. The resultant design utilized two lines
with higher characteristic impedance in parallel instead of a single line, which
resulted in a larger range of achievable characteristic impedances. Figure 7 shows
the two filters in stop-band and pass-band excitation. Even though the design was
complex due to the extensive use of circuit parameters, the simulation power as
Figure 7: X-band filters (standing wave
current density).
well as the hierarchical design capabilities of the Microwave Office software made
it possible to run the same optimization routines on both circuits. This not only
reduced the design team’s time and effort, but also meant that the two blocks
were exchangeable in the final design.
CONCLUSION
This application note illustrates a complete design flow for the end-to-end design
and realization of an X-band transmission analyzer. The ability to not only design
and optimize several different circuits on a single PCB but also to work through
many design iterations and verification steps at different abstraction layers was
critical to achieving the project’s ambitious performance goals. Keeping design
changes and parameter variations consistent through all abstraction layers was
a challenging task that was made possible and ultimately successful with the
help of the AWR Design Environment that offered complete integration of all the
constituent design and verification steps.
Final prototype of the X-band
transmission analyzer.
AWR would like to thank Norbert
Leder, Dipl.-Ing., lead engineer from
Vienna University of Technology to his
contributions to this application note.
Dr. Holger Arthaber
Head of Microwave Engineering,
Project leader
holger.arthaber@tuwien.ac.at
Dipl.-Ing. Norbert Leder,
System architect, Lead engineer
norbert.leder@tuwien.ac.at
Copyright © 2013 AWR Corporation. All rights reserved. AWR, Microwave Office, AXIEM, and TX-Line are registered
trademarks and the AWR logo, AWR Design Environment, and Visual System Simulator are trademarks of AWR
Corporation. Other product and company names listed are trademarks or trade names of their respective companies.
AN-TUV-MWO-2013.2.14
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