Application:

Filters

Software:

NI AWR Design Environment

Microwave Office

Company Profile Quasonix designs, develops, and manufactures high performance aeronautical telemetry

products and is a recognized industry leader for spectrally efficient modulations such as

SOQPSK-TG and Multi-h (ARTM) CPM. The Quasonix line of advanced products includes

multi-mode telemetry transmitters, multi-mode, multi-symbol trellis telemetry

demodulators, complete multi-mode telemetry receivers, and rack-mount receiver

analyzers. More and more, flight-test engineers are discovering that Quasonix products

regularly outperform and outclass all other equipment on the market.

The Design Challenge RF/microwave design computer-aided-engineering (CAE) tools have existed for many

years and are used extensively by engineers to design linear and nonlinear RF/microwave

circuits. These design efforts have been supported by linear S-parameter models for

passives and compact nonlinear models and load-pull power device data for active

devices, as provided by many component manufacturers.

At application frequencies above 1 GHz, the component parasitics such as series

inductance in capacitors and shunt capacitance in inductors, as well as substrate-

dependent component parasitics, significantly impact the actual circuit performance.

If these component parasitics are not included in the simulation, the accuracy of the

simulation results will be significantly degraded. Consequently, extensive prototype

board-level bench tuning and rework is typically required to achieve the desired results.

Often a second or third board iteration (or spin) is needed to achieve acceptable

performance. This is costly, not only in terms of board fabrication costs, but in

engineering time, design productivity, lost calendar time, and missed time-to-market

windows for new products.

Success Story

Quasonix Uses NI AWR Software and Modelithics to Achieve First-Pass Success With a Transmitter‘s Harmonic Filter Design

‘‘ Instead of relying

on multiple prototypes,

our designers were

able to use Microwave

Office combined with

Modelithics models to

optimize a harmonic filter

for peak performance.

This approach eliminated

several PCB spins, which

shortened the circuit

development cycle one to

two months.’’– Ted Longshore

Sr. RF Designer

Quasonix

quasonix.com

Quasonix transmitter product line samples.

ni.com/awr

The Solution The Quasonix product development team successfully designed a low-pass harmonic filter in a single pass using NI AWR Design

Environment, specifically Microwave Office circuit design software, and Modelithics® Global Microwave Models™ included in the

Modelithics CLR Library.

Starting with ideal lumped element capacitors and inductors and including the interconnecting microstrip lines, the 2.2 to 2.4 GHz

harmonic filter in Figure 1 was designed for low S11 in the passband and high rejection at the second harmonic and above.

The simulated response (Figure 2) from Microwave Office shows insertion loss less than 0.05 dB, second harmonic rejection of 49 dB,

and minimum 47 dB rejection up to 20 GHz. The unrealistically low passband insertion loss is due to the fact that the ideal components

utilized in this simulation differ from actual components at microwave frequencies.

The discrete portion of the filter was redesigned and optimized using Modelithics component models which include parasitics and shunt

pad capacitance. The results of this simulation indicate higher passband insertion loss as expected from the realistic component models

plus flybacks caused by the parasitic resonances in the capacitors and inductors. A distributed radial stub filter was added and optimized

using AXIEM co-simulation to reduce the flybacks.

Figure 1: Ideal harmonic filter schematic including transmission line interconnects.

Figure 2: Simulation of 2.4 GHz harmonic filter, including transmission line interconnects.

©2017 National Instruments. All rights reserved. AWR, AWR Design Environment, AXIEM, Microwave Office, National Instruments, NI, and ni.com are trademarks of National Instruments. Other product and company names listed are trademarks or trade names of their respective companies. SS-M-QUSX-2017.9.29

The completed PCB layout for the combined discrete plus distributed low-pass harmonic filter is shown in Figure 3. The gold outline

indicates where shielding will be added to reduce flybacks.

A comparison of the simulated and measured performance of the above filter (Figure 4) shows good agreement. The passband insertion

loss and stopband rejection were accurately simulated by Microwave Office, especially given the difference in inductor model versus

actual inductor body size as noted previously.

Why NI AWR Design EnvironmentUsing Microwave Office to simulate circuit performance using component models produced results that were accurate enough to realize

design goals on a first pass PCB fabrication. The filter example previously described demonstrated very good agreement in the passband

and second harmonic cutoff, as well as reasonable agreement through the stopband.

RF circuit development is typically accomplished by prototyping the individual circuit components and individually characterizing and

optimizing them via tuning on the bench; resulting in multiple iterations of the design before acceptable performance is achieved.

NI AWR Design Environment using Modelithics component models provided the capability to accurately simulate microwave circuits.

Instead of relying on multiple prototypes, this enabled the designers to utilize Microwave Office to optimize the combined circuitry for

peak performance. This approach eliminated several PCB spins, which shortened the circuit development cycle one to two months.

Thank you to Ted Longshore of Quasonix and

Larry Dunleavy of Modelithics for their

technical article in High Frequency Electronics

August 2017 issue titled “Using Component

Models to Achieve First Pass Success - A

Transmitter Case Study: Part 1, Harmonic

Filter Design” that inspired the creation of

this success story.

Figure 4: 2.4 GHz harmonic filter simulation results vs. measured data.

Figure 3: Discrete LC plus radial stub harmonic filter.