Abstract:An X-band combline tunable bandpass filter is proposed. Through adjusting the bias voltage of the loaded varactor, the central frequency of the filter is tuned. According to the constant bandwidth requirements on coupling coefficients and external Q factor, the initial dimension parameters of the combline filter are determined and then optimized with an electromagnetic field simulator. The completed X-band voltage controlled tunable filter is 16 mm?20 mm?21 mm in size and has a controlled voltage range of 7.6-15.4 V by which the center frequency of the filter is tuned continuously within the range between 8 GHz to 10 GHz. The passband width of the filter is 15%, the return losses are below -10 dB and the rectangle coefficient is less than 2.8 within the tuning frequency range.