Variable Bandwidth Double-Coupled Double-Tuned Filter with Capacitive Couplings

In this paper, an adjustable bandwidth high IP3 (third-order intercept point) sub-octave double-coupled double-tuned filter is presented. The proposed filter consists of two capacitively coupled resonators. Its center frequency and bandwidth are tuned by the varactors with two voltages. The filter characteristics are shaped by two transmission zeros located in its lower and upper transition bands. The frequency of the lower transmission zero is tuned simultaneously with the center frequency, and the upper transmission zero frequency is independently controlled. The achieved center frequency tuning range is from 300 MHz to 446 MHz, and the bandwidth swing is from 35 MHz to 75 MHz. In the whole operating band, the IP3 of the filter varies from 22 dBm to 29 dBm, and the insertion loss is from 1.4 dB to 3.3 dB. The filter has been full-wave analyzed and optimized. The electric size of 0.05 × 0.058 λg2 makes the proposed filter the smallest among the filters compared. The obtained measurement results are in very good agreement with the simulations.