Cognitive and encrypted communications, part 2 : A new approach to active frequency-agile filters and validation results for an agile bandpass yopology in SiGe-BiCMOS

A new theory for a 2^nd-order frequency-agile filter is introduced in this paper. The center frequency of the filter is proportional to the gain of a feedback amplifier and thus can be tuned over a wide frequency range. This new theory is thereafter generalized to the n^th-class leading to a center frequency proportional to (A)^n/2. The resulting filters make use of the minimum passive elements for 2^nd-order filters: two capacitors. Simulation results of band pass agile filters in current mode and made from second-generation current controlled conveyors (CCCIIQ) in 0.25¿m SiGe BiCMOS technology are given for n=1 and n=2. These simulation results along with results of measurements carried out on the fabricated filters entirely confirm the new approach. They also highlight the improvements to be expected for cognitive and encrypted communications.