Frequency agile RF filter for interference attenuation

This paper introduces a frequency agile RF filter for interference attenuation in a base station RF frontend. Digital and RF systems are combined in a feed-forward architecture. This architecture provides a frequency response with multiple tunable transfer function zeros. This feed-forward architecture is called the hybrid RF-DSP feed-forward filter. Measurements demonstrate successful interferer suppression for operation at several bands between 800 and 1800 MHz. The group delay mismatch between the feed-forward paths is reduced with a delay line to achieve between 32 to 24 dB of attenuation for interferers with bandwidths from 1 to 8 MHz. The digital filter used within the prototype had 32 complex taps, thereby providing the RF filter with 32 tunable transfer function zeros.