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

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.