Two-pole varactor-tuned helical resonator filters

Two-pole varactor-tuned helical resonator filters that operate in the 960 to 1215 MHz band have been designed and built. They exhibit group delays of less than 10 microsec, and can be constructed to have insertion losses of 3 dB or less. Two designs have been investigated, one where both sections of the filter were tuned with a single bias source, and the other where each section was tuned individually. Varactor diodes are sensitive to changes in input power level and they are saturated at rf power levels above 0.3 mW. Tuning speeds of 10 microsec are possible when care is taken to minimize the inductances in the bias circuits.