A constant-Q tunable combline bandpass filter using angular tuning technique

This paper presents the design and implementation of a high-Q bandpass filter using a tuning technique that maintains constant Q value over a relatively wide tuning range. The traditional technique for tuning combline filters is achieved by changing the gap between the post and the tuning disk. Such technique is known to yield a Q value that degrades considerably at the lower edge of the tuning range. The proposed angular tuning technique shows a 25% improvement in Q value at the lower edge of the tuning range, in comparison to what is typically achieved using the traditional tuning technique. Using the proposed angular tuning technique, a 1% bandwidth 2-pole filter is designed, fabricated and tested with a 430MHz tuning range at a center frequency of 3.6 GHz. The measured insertion loss is changing between 0.25 and 0.33dB. The filter is integrated with miniature piezoelectric motors, demonstrating almost a constant insertion loss over the tuning range.