1 KHz Synchronous Detector

Posted on May 27, 2012

This circuit employs a synchronous demodulator to separate a 1 KHz signal from noise and measures the amplitude of the 1 kHz signals once a second at about 60 microvolts per count then sends the measurements via an RS-232 interface for further processing or display. An LED on the board also lights when the measured signal exceeds a preset threshold. This experiment was started when I took an interest in receiving ELF wireless signals. It also has applictions in optics and high frequency RF, or for that matter, any place one needs to measure a tiny signal, of which the frequency and phase are known, in the presence of noise.

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With the addition of a preamplifier based on the LM324, the sensitivity of this circuit was easily extended to a sensitivity to 160 nanovolts per count. That an LM324 is used with little in the way of noise on the output testifies to the value of using this kind of detector. There are several sophisticated references on the web that describe how synchronous detectors work, so here I will only give a light overview and go into some specifics of this implementation. The idea is to multiply the input signal by the output of a local oscillator that is synchronized with the expected signal, and integrate the result. Imagine a square wave being fed into the signal input of the multiplier and a synchronized square wave being fed into the local oscillator input of the multiplier. If the local oscillator is synchronized such that they are perfectly matched in phase, then the output of the multiplier will be positive when the incoming signal and the local oscillator are positive and the output will also be positive when the inputs are negative (negative x negative = positive) this is actually a full wave rectifier when both signals are synchronized and in the proper phase with one another. The signals fed into the integrator charge the .047 uf capacitor in the integrator. After 999 cycles of the 1 kHz sampling signal, U3C is turned on and the capacitor is discharged with a constant current (1.8V/7.5K = 240 microamps), producing...

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