Robust FM Modulation Detector Using Signal Autocorrelation

Abstract

An FM receiver is unaware of the modulation level (frequency deviation) of the signal and has to make an estimate of it, or some reasonable time-average of it, and accordingly set the input filter's bandwidth. We calculate modulation by measuring the autocorrelation of the recovered audio signal instead of its peaks, and then applying a peakhold detector. Since FM noise can be modeled to be somewhat uncorrelated, we can expect to get an accurate estimate of signal power while rejecting noise power substantially if we measure a one-sample delayed autocorrelation estimate. Since the above measurement is alike a power measurement, we compute its square root, gain adjust it to obtain a cleaner peak measurement, and then track these clean peaks using a leaky integrator. This gives an estimate of modulation that subdues the effect of the noise.

Description

TECHNICAL FIELD OF THE INVENTION[0001]The technical field of this invention is frequency modulated radio communication.BACKGROUND OF THE INVENTION[0002]In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. The three key parameters of a periodic waveform are its amplitude, its phase and its frequency. Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used.[0003]The purpose of modulation is to convey a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passb...

AI Technical Summary

Benefits of technology

[0013]Modulation is calculated by measuring the autocorrelation of the recovered audio signal (the left channel and or the right channel), instead of its peaks, and then applying a peak hold detector. Since FM noise tends to be somewhat uncorrelated, this will result in a more accurate estimate of signal power while rejecting noise power substantially. Since the above measurement is similar to a power measurement, we compute its square root, adjust the gain to obtain a cleaner peak measurement, and then track these clean peaks using a leaky integrator. This gives an estimate of modulation while minimizing the effect of the noise.

Problems solved by technology

If the input filter's bandwidth is narrower than the required bandwidth, it will lead to distortion because necessary portions of the message spectrum have been clipped off.

However, if the RF signal level is weak, it will contribute to additional noise and thus degrade performance.

Estimation of modulation is a difficult problem.

The estimation is particularly hard when the RF signal level is weak, and when noise corrupts the signal.

However, under weak signal conditions, signal peaks get blanketed by noise peaks, so the peak-based approach overestimates the frequency deviation, thereby leading to a wider input filter than necessary and thus hurting SNR and SINAD.

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