Capacitive-based, closed-loop frequency control of substrate-integrated cavity tunable filters

Shahrokh Saeedi, William S. Wilson, Tyler R. Ashley, Hjalti H. Sigmarsson, Ju Seop Lee

Research output: Chapter in Book/Report/Conference proceedingConference contribution

3 Citations (Scopus)


In this paper, we present an integrated capacitive sensing technique that can be used to achieve closed-loop feedback control of variable capacitors. The technique is well suited for diaphragm-based actuators that can be used to tune the center frequency of microwave reconfigurable filters. A commercially available capacitance- to-digital converter is used to measure the capacitance of a custom-made monitoring capacitor. This capacitor is completely external to the filter structure and does not contribute to any added losses. A tunable, second-order, bandpass filter -using substrate-integrated, evanescent-mode cavity resonators- is created to demonstrate the concept. The frequency tuning is achieved using piezoelectric actuators to displace a flexible copper diaphragm that forms the top part of the loading capacitor. The monitoring capacitor consists of a second cavity that is mounted above the actuator forming a second air-filled, metalinsulator- metal capacitor. There exists a direct relationship between the monitoring capacitance and the loading capacitance in the evanescent-mode cavity. Therefore, by tracking the monitoring capacitance the center frequency of the filter can be monitored, which allows for direct in-situ closed-loop control of the filter. An algorithm for the tuning operation is presented, which includes an automatic calibration technique to initialize the controller. The effectiveness and repeatability of the technique is evaluated as the filter is tuned from 3.3 GHz to 3.7 GHz. Having stable feedback control integrated with this type of evanescentmode cavity filters, brings the technology one step closer to actual fielding.

Original languageEnglish
Title of host publicationProceedings - 2014 47th International Symposium on Microelectronics, IMAPS 2014
PublisherIMAPS-International Microelectronics and Packaging Society
Number of pages6
ISBN (Print)9780990902805
Publication statusPublished - 2014
Event47th International Symposium on Microelectronics: Future of Packaging, IMAPS 2014 - San Diego, United States
Duration: 2014 Oct 132014 Oct 16


Other47th International Symposium on Microelectronics: Future of Packaging, IMAPS 2014
Country/TerritoryUnited States
CitySan Diego


  • Automatic tuning
  • Capacitive sensor
  • Evanescent-mode cavity tunable filter
  • Feedback control

ASJC Scopus subject areas

  • Electrical and Electronic Engineering


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