A tunable 0.6 GHz – 1.7 GHz bandpass filter with a constant bandwidth using switchable varactor-tuned resonators

This work reports on a tunable second-order bandpass filter with a constant bandwidth using switchable varactor-tuned resonators. A wide center frequency tuning range of 600 MHz to 1711 MHz is obtained using p-i-n diodes to switch in and out λ/4 or λ/2 resonators for low-band or high-band modes, without increasing the tuning capacitance range of the varactors. A combination of electric and magnetic coupling is utilized to realize a near constant absolute bandwidth across the tuning range. The filter is fabricated on a Duroid substrate with ε_r =2.2 and h=0.787 mm. When the p-i-n diodes are on, the center frequency is tuned form 1070 MHz to 600 MHz while maintaining a 3-dB bandwidth of 114±10 MHz, insertion loss of less than 3.3 dB, and return loss of better than 15 dB. The IIP₃ and P_1dB are better 18.6 dBm and 12 dBm, respectively. When the p-i-n diodes are off, the center frequency is tuned form 1711 MHz to 1070 MHz (and potentially down to 1000 MHz) while maintaining a 3-dB bandwidth of 88±8 MHz, insertion loss of less than 4 dB, and return loss of better than 13 dB. The IIP₃ and P_1dB are better 13.5 dBm and 10 dBm, respectively. The rejection level at 200-MHz offset frequency from the passband center frequency is better than 22 dB across the entire tuning range. To our knowledge, this planar bandpass filter exhibits the widest tuning range with a near-constant bandwidth.