Forming Narrowband Filters at a Fixed Sample Rate with Polyphase Down and Up Sampling Filters

Abstract

There are signal processing scenarios for which a narrowband signal, such as a pilot tone, is to be extracted from a wide bandwidth signal while preserving the sample rate of the wideband signal. A FIR filter that extracts this signal has many taps due to the large ratio of sample rate to the filter's transition bandwidth. We significantly reduce the workload required to accomplish the filtering by operating the filter at a reduced sample rate. We use a polyphase filter as a preprocessor to reduce the sample rate and then as a post processor to return the sample rate to its original value. This method proves to require considerably less resources than the alternate scheme using an iterated FIR filter.