Quadrature demodulation, homodyne detection, lock-in detection - what's the difference?

1. How does quadrature demodulation work?
2. How does homodyne detection work?
3. How does lock-in detection work?
4. What is the purpose of lock-in amplifier?

How does quadrature demodulation work?

Quadrature demodulation uses two reference signals separated by 90° of phase, along with two multipliers and two low-pass filters, to generate I and Q demodulated waveforms.

How does homodyne detection work?

A homodyne detector directly measures the phase dependent quadrature of the quantum state's electric field. Therefore homodyne detection enables continuous variable quantum key distribution schemes with classical and non-classical states [1-4]. It also allows phase estimations at the Heisenberg limit [5, 6].

How does lock-in detection work?

They employ a homodyne detection scheme and low-pass filtering to measure a signal's amplitude and phase relative to a periodic reference. A lock-in measurement extracts signals in a defined frequency band around the reference frequency, efficiently rejecting all other frequency components.

What is the purpose of lock-in amplifier?

Lock-in amplifiers are used to detect and measure very small AC signalsall the way down to a few nanovolts. Accurate measurements may be made even when the small signal is obscured by noise sources many thousands of times larger.