Perfect pair for filter tuning: CST Studio Suite® and R&S®ZNB

CST® Filter Designer 3D, which is part of the CST Studio Suite® simulation software from Dassault Systèmes, is a complete synthesis tool for filters and diplexers. It provides a range of solutions throughout the design process of coupled resonator filters and even beyond with its VNA based tuning capability. CST Filter Designer 3D assists in tuning filter hardware, significantly reducing the time for optimizing a device to meet the expected specifications. An attractive instrument to obtain fast data reading for realtime filter tuning is the R&S®ZNB vector network analyzer from Rohde & Schwarz.

VNA tuning setup with CST Filter Designer 3D and R&S®ZNB
Fig. 1: VNA tuning setup with CST Filter Designer 3D and R&S®ZNB.

Your task

4G/5G base station filters/diplexers are typically challenging DUTs. They require high out-of-band rejection. Transmission notches, also called nulls, can be as deep as 80 dB and more. The dynamic range and speed of the R&S®ZNB vector network analyzer together with CST Studio Suite® from Dassault Systèmes is a perfect match for this scenario.

R&S®ZNB dynamic range (at 10 Hz IF bandwidth).
Fig. 2: R&S®ZNB dynamic range (at 10 Hz IF bandwidth).

Rohde & Schwarz solution

For real-world tuning, CST Studio Suite® supports USB, socket and Ethernet connections for a wide range of Rohde & Schwarz vector network analyzers. The R&S®ZNB is the leading vector network analyzer for production environments with a focus on speed, dynamic range and ease of operation. It combines high measurement accuracy with exceptional speed – better than 5 μs per point. This enables short sweep times, e.g. 4 ms for 401 points. Furthermore, the R&S®ZNB vector network analyzer combines high power handling capacity with high sensitivity and low trace noise. The R&S®ZNB base unit provides typically 140 dB dynamic range (at 10 Hz IF bandwidth), which outperforms other, comparable products on the market. Optional dynamic range enhancements deliver a dynamic range as high as 150 dB (real dynamic range without receivers going into compression at low transmission coefficients).

Fully integrated and easy to use
There are four steps to tune a filter:

  • Calibrate the R&S®ZNB vector network analyzer and perform S-parameter measurements on the DUT
  • Establish a remote connection between CST Filter Designer 3D running on a PC and the R&S®ZNB vector network analyzer. The software automatically reads the measured data in realtime
  • Based on the selected filter topology, the software extracts a coupling matrix from the measured S-parameters
  • Error bars indicate to what extent couplings and selfresonances are detuned. This provides the necessary information to optimize the filter hardware through its tuning mechanisms such as screws, variable capacitors and varactor diodes.

The filter model was designed with CST Filter Designer 3D and simulated with CST Studio Suite®. Given an accurate manufacturing process, the same result will be obtained from real measurements with the R&S®ZNB when the hardware is tuned to match the simulated coupling matrix.

The CST Filter Designer 3D environment is shown in the screenshot below, with the measured S-parameters read from the R&S®ZNB. The error bars indicate to what extent each matrix element deviates from the optimal tuning.

Filter with numbered resonators.
Fig. 3: Filter with numbered resonators.


Combline coaxial resonator filter
The filter has a 6th order response with two transmission zeros just above the passband. The filter topology consists of two cascaded triplet sections. Figure 3 shows the device with the resonator tuning screws marked with red numbers.

CST Filter Designer 3D user interface showing the coupling matrix
Fig. 4: CST Filter Designer 3D user interface showing the coupling matrix.
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Trademark notice/Copyright
SIMULIA and CST Studio Suite are commercial trademarks or registered trademarks of Dassault Systèmes, a French “société européenne” (Versailles Commercial Register # B 322 306 440), or its subsidiaries in the U.S. and/or other countries. All other trademarks are owned by their respective owners. Use of any Dassault Systèmes or its subsidiaries trademarks is subject to their express written approval. Products and services names may be trademarks or service marks of Dassault Systèmes or its subsidiaries.

Images copyright © 2019 Dassault Systèmes Simulia Corp.

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