Digital FM demodulator and method with enhanced resolution
AssigneeBEI Electronics, Inc.
InventorTimothy R. Hilby
Life of this patent
Estimating the term…
Abstract
Digital FM demodulator and method in which zero-crossings of a frequency modulated input signal are detected, a binary pulse is provided for each of the zero-crossings, and the binary pulse stream is filtered digitally to provide enhanced resolution of the incremental phase (frequency) of the input signal over a period which is asynchronous with the input signal. In the disclosed embodiments, the digital filtering is done with comb decimation filters, with a higher order filter being employed where greater resolution is desired.
Claims16
In a system for demodulating an FM input signal having a frequency f.sub.i :
- a zero-crossing detector operating synchronously at a sampling frequency f.sub.s greater than f.sub.i and a sampling period T for detecting zero crossings of the input signal and generating a pulse for each zero-crossing detected;
- means responsive to the pulses from the zero-crossing detector for generating a binary pulse stream containing a predetermined number of binary pulses in response to each pulse from the zero-crossing detector;
- and
- means for digitally filtering the binary pulse stream to provide an output signal corresponding to the average frequency and incremental phase of the input signal over a sampling period nT, where n>1 and the sampling period nT is asynchronous with the zero-crossings of the input signal.
The system of claim I wherein the means for digitally filtering the binary pulse stream includes:
- an up/down counter having an up counting input to which the binary pulse stream is applied;
- a divide-by-p counter for dividing down pulses occurring at the frequency f.sub.s to pulses occurring at a frequency f.sub.s /p, where p is a positive integer;
- means for applying the pulses from the divide-by-p counter to a down counting input of the up/down counter;
- a reset-accumulator for adding and accumulating the output of the up/down counter during each sampling period nT and resetting at the end of each sampling period;
- and
- a back-differencer responsive to the output of the reset-accumulator for providing an output signal corresponding to the difference between outputs of the reset-accumulator in successive sampling periods.
In a method of demodulating an FM input signal having a frequency f.sub.i, the steps of:
- synchronously detecting zero-crossings of the input signal in a zero-crossing detector by sampling the signal at a sampling frequency f.sub.s greater than f.sub.i and a sampling period T and generating a pulse for each zero-crossing detected;
- generating a binary pulse stream containing a predetermined number of binary pulses in response to each pulse from the zero-crossing detector;
- and
- digitally filtering the binary pulse stream to provide an output signal corresponding to the average frequency and incremental phase of the input signal over a sampling period nT, where n>1 and the sampling period nT is asynchronous with the zero-crossings of the input signal.
Dependencies are inferred from the claim text.