Demodulation method and circuit for discretely modulated ac signals

A method of demodulating a modulated ac signal of predetermined frequency that has been amplitude-modulated by discrete changes between first and second amplitudes, comprising the steps of:

• bandpass filtering said modulated ac signal through a filter having a bandpass centered at said predetermined frequency to produce bandpass-filtered components of said modulated ac signal;
• detecting variations in the amplitude envelope of said bandpass-filtered components of said modulated ac signal by applying said components to a signal input of a switching circuit and operating said switching circuit in response to said modulated ac signal prior to the bandpass filtering thereof, to selectively conduct said bandpass-filtered components to a signal output of said switching circuit at a frequency and phase relative to said modulated ac signal, that is effective to cause a rectification-like detection of said bandpass-filtered components;
• and
• low-pass filtering the detected bandpass-filtered signal components of said modulated ac signal as they appear at said signal output of said switching circuit to pass a signal that varies in accordance with said discrete changes between said first and second amplitudes of said modulated ac signal.

A method of demodulating a modulated ac signal that has been frequency modulated by discrete changes between a first predetermined frequency and a second frequency, comprising the steps of:

• bandpass filtering said modulated ac signal through a filter having a bandpass centered at said first predetermined frequency to produce bandpass-filtered components of said modulated ac signal;
• detecting variations in the amplitude envelope of said bandpass-filtered components of said modulated ac signal due to said discrete changes between said first and second frequencies by applying said components to a signal input of a switching circuit and operating said switching circuit in response to said modulated ac signal prior to the bandpass filtering thereof to selectively conduct said bandpass-filtered components to a signal output of said switching circuit at a frequency and phase relative to said modulated ac signal, that is effective to cause a rectification-like detection of said bandpass-filtered components;
• and
• low-pass filtering the detected bandpass-filtered signal components of said modulated ac signal as they appear at said signal output of said switching circuit to pass a signal that varies in accordance with said discrete changes between said first predetermined frequency and said second frequency of said modulated ac signal.

A method of demodulating a modulated ac signal of predetermined frequency that has been phase-modulated by discrete changes between first and second relative phases of said predetermined frequency, comprising the steps of:

• bandpass filtering said modulated ac signal through a filter having a bandpass centered at said predetermined frequency to produce bandpass-filtered components of said modulated ac signal;
• detecting variations in the amplitude envelope of said bandpass-filtered components of said modulated ac signal due to said discrete changes between said first and second relative phases by applying said components to a signal input of a switching circuit and operating said switching circuit in response to said modulated ac signal prior to the bandpass filtering thereof, to selectively conduct said bandpass-filtered components to a signal output of said switching circuit at a frequency and phase relative to said modulated ac signal that is effective to cause a rectification-like detection of said bandpass-filtered components;
• and
• low-pass filtering the detected bandpass-filtered signal components of said modulated ac signal as they appear at said signal output of said switching circuit to pass a signal that varies in accordance with said discrete changes between said first and second relative phases of said modulated ac signal.