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IN this note on lock-in-amplifier, I see that there is external which we phase lock the internal reference to. Then the internal reference is used for phase sensitive detection.

My first question is why not using the external reference for phase sensitive detection directly?

My second question is if the input signal of interest have fixed frequency(of interest) but drifting phase. What will happen?

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  • \$\begingroup\$ In the wiki, it is said that lock in amplifier can be used to measure the phase shift itself. \$\endgroup\$ – Rickyim Apr 15 '18 at 6:27
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When a lock-in amplifier is used for demodulation, the form of the internal reference used is important.

If the internal reference is a square wave, the PSD will be sensitive to the input and its harmonics. A sine reference is needed to be sensitive to only the fundamental signal itself.

The internal reference may be a two-phase sine and cosine, so that it can detect arbitrary phase angles.

Using an internal reference for the detection allows the external reference to be any signal of the correct frequency, square, pulse, sine, triangle, without the user having to provide the correct amplitude and waveshape as well.

With a single PSD lock-in, varying the phase of the signal would result in a sine-wave variation of output.

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