Synchronous Modulator and Demodulator
Recovering small signals in the presence of noise is the application most suited to the synchronous modulator and demodulator technique. If a DC voltage or low-frequency AC signal is modulated by a higher frequency carrier (also known as “reference”) the input signal is converted to an AC signal – the carrier frequency sidebands. Demodulating the signal uses a similar technique (also known as “phase sensitive detector”) resulting in the recovered sidebands.
A low- pass filter is used to remove carrier artifacts and out-of-band noise.
Random noise generated in the amplifier integrates to zero if a very long integration time (a very low freq LPF) is used.
It is entirely practical to recover a small signal buried in 40dB of random (white) noise with a synchronous detector. This is sometimes also called a lock -in amplifier”.
The sync modulation/demodulation switches SW1 & SW2 are usually good analog switches. To prevent DC offset, it is important to maintain a precise 50% duty
cycle in the reference generator and switches. (Created by Neil P. Albaugh, TI – Tucson)
Synchronous Modulator and Demodulator circuit
Online Simulation of the Synchronous Modulator and Demodulator Circuit
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Michael Koltai
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