INTRODUCTIONThe 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 simulator-

based tuning was used for the numerous distributed elements on the board to

ensure optimal performance.

Microwave Offi ce®

Application Note

End-to-end Design and Realization of an X-band Transmission Analyzer

Resultant X-band transmission analyzer.Resultant X-band transmission analyzer.

THE DESIGN FLOWThe design team at the Vienna University of Technology was tasked with designing from

scratch and realizing this X-band transmission analyzer . The design fl ow involved the

design and optimization of several breeds of circuits, including critical elements such as

bias-T and microstrip fi lters, 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 Offi ce® circuit design

software. Additionally, AWR’s Visual System Simulator™ (VSS) communication system

design software was used to fi nd an optimal RX chain and to estimate the phase locked

loop’s (PLL) phase noise properties. The PLL’s loop fi lter 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 fi nal PCB design.

(Figure 1 shows all relevant RF-circuits that were investigated during this project in order

to design the resulting fi nal, outstanding overall system.)

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 suffi cient. This

approach enabled the designers to reuse highly optimized sub circuits in the fi nal PCB

design (Figures 2 and 3).

Figure 1: PCB topologies investigated during the design fl ow.

Figure 3: Final manufactured prototype.

Figure 1: PCB topologies investigated during the design fl ow.

Figure 3: Final manufactured prototype.

Figure 2: Final design layout shown within the AWR software environment.

Figure 5: Bias-T standing wave current density.

THE BIAS-TMicrowave Offi ce 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 challeng-

ing 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.

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 Offi ce

optimizers were the weapons of choice. Numerous runs were necessary to fi nd 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 refl ect, 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

line was virtually “invisible” for the RF

signal. All the RF leakage measurements

Figure 4: Bias-T dimensions.

LF +DC

λ/4 @ fhi

high RF stub

λ/4 @ flow

low RF stub

RF

LF - block

combined RF + LF +DC

λ/4 @ flow λ/4 @ fhi

Figure 5: Bias-T standing wave current

as well as the simulations were combined, as shown in Figure 6. The fi nal circuit

indeed shows the desired fl at frequency response in leakage over a broad band and

at constant low leakage.

THE MICROSTRIP FILTERThe microstrip fi lter design was carried out by deriving an equivalent lumped circuit

from the design’s specifi cations. This representation can easily be reformulated in

terms of characteristic impedances, which is the starting point for any distributed fi lter

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 Offi ce

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 fi lter 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 fi nd 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 fi lters 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).

Figure 6: Measured data vs. simulation.

8 9 10 11 12 13 14-70

-60

-50

-40

-30

-20

-10

0

measured - both stubs Axiem Model Based - only high stub - only low stub - no stub

Bias

-T R

F le

akag

e (d

B)

Frequency (GHz)

Final prototype of the X-band transmission analyzer.

Copyright © 2013 AWR Corporation. All rights reserved. AWR, Microwave Offi ce, 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

AWR Corporation | www.awrcorp.com info@awrcorp.com | +1 (310) 726-3000

well as the hierarchical design capabilities of the Microwave Offi ce 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 fi nal design.

CONCLUSIONThis 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.

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