Is the output of a Lock In amplifier a signal with single frequency?

The Lock In amplifier uses Phase Sensitive Detection to "lock" to the signal using reference frequency. It is basically auto-correlation. The output of the Lock In amplifier is typically an X signal, the in-phase, and a Y signal, the out-of-phase.

My question: If the Lock In 'locks' to the reference frequency, say 200 Hz, then does this mean the output signal has only this frequency? Experimentally, I find that this is not true, as the output has lots of frequencies, only limited by measurement time and the low pass filter settings of the Lock In.

ali8 said:

The Lock In amplifier uses Phase Sensitive Detection to "lock" to the signal using reference frequency. It is basically auto-correlation. The output of the Lock In amplifier is typically an X signal, the in-phase, and a Y signal, the out-of-phase.

My question: If the Lock In 'locks' to the reference frequency, say 200 Hz, then does this mean the output signal has only this frequency? Experimentally, I find that this is not true, as the output has lots of frequencies, only limited by measurement time and the low pass filter settings of the Lock In.

Click to expand...

The lock-in amplifier is also named synchronous detector. In fact it can have two outputs: as the lock-in amplifier, the output exactly follows the input signal frequency (and can generate also harmonics).
As a synchronous detector, it operates as a multiplier-correlator, so the output is a DC or low-frequency signal with an amplitude proportional to the input signal amplitude. This synchronous detector operation allows to narrow the noise bandwidth, so it can "detect" input signal with a level deep in noise. Often signals with a level 60 dB under noise power can be detected.

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And yes, I forgot the phase function, that between the input and reference signal. I used sync detectors in my radio-telescope radiometers, where the phase is maintained at one value peaking the output.

Let me ask the question in another way. Assume I am using the Lock In with a reference frequency f0=200 Hz. Also assume the Lock In low pass filter is adjust so as its cut off freq. fc >> f0 Hz. I bias a resistor and measure the output. Experimentally, we will see the 1/f noise in the output, which ranges from 1/t, where t is the total measurement time, to fc.

What I don't get is how you get this signal that spans all that frequency range while you're supposed to have a signal of a nearly single freq. f0.

Like jiripolivka I would describe a lock-in amplifier as synchronous demodulator. Input of frequency fc is mapped to DC, signal components around fc to low frequencies. You'll see output up to fc if you don't apply a filter. But in a usual lock-in application, only the spectral components near DC will be processed.