Tunable 4-Pole Noncontiguous 0.7–2.1-GHz Bandpass Filters Based on Dual Zero-Value Couplings

This paper presents tunable 2- and 4-pole bandpass filters with a wide tuning range based on dual zero-value couplings. The zero-value coupling is achieved using a coupled line with two coupling paths. The proposed filter also has bandwidth tunability. Frequency and bandwidth tunings are achieved using silicon varactor diodes. Three types of tunable filters are designed and built on a substrate with ε r = 3.55 and h = 25 mil, two filters with two separate frequency bands (2- and 4-pole: Filter A) and one filter with contiguous frequency tuning (4-pole: Filter B). The 2-pole filter results in an insertion loss of 2.9-4.7 dB with a tuning range of 0.75-1.09 and 1.70-2.18 GHz. Filter A with a 4-pole response results in an insertion loss 3.7-7.9 dB and a -1-dB bandwidth of 68-87 MHz with a tuning ranges of 0.77-1.10 and 1.70-2.10 GHz. Filter B shows an insertion loss of 4.4-6.6 dB and a -1-dB bandwidth of 39-68 MHz with a tuning range of 0.70-1.44 GHz. The application areas are in wideband tunable filters requiring contiguous and noncontiguous frequency coverage with bandwidth tunability in separate bands.