Signal Extraction
Phase-sensitive detection instruments extract small alternating signals from environments where the noise level is orders of magnitude higher than the signal. An instrumentation setup uses a lock-in amplifier to multiply the input signal by a reference signal and pass the product through a low-pass filter. This process effectively rejects any noise that is not at the exact frequency and phase of the reference signal, allowing sub-microvolt signals to be measured.
Reference Synchronization
Synchronizing the detection circuitry to the excitation source is essential for maintaining phase alignment. The lock-in amplifier utilizes a phase-locked loop to track the frequency and phase of the reference input, which may come from an external function generator or an internal oscillator. Any phase drift between the reference and the input signal will result in a measurement error, making high-stability phase tracking crucial for accurate measurement.
Noise Rejection
Extremely narrow equivalent noise bandwidths are achievable by adjusting the time constant of the low-pass filter on the output stage. The lock-in amplifier provides several orders of magnitude of noise reduction by filtering out out-of-phase and off-frequency noise. This high selectivity is particularly useful in optical systems, where it enables the detection of weak modulated light signals against a large, unmodulated ambient background.
However, a longer time constant increases the measurement settling time, which requires a careful tradeoff between noise rejection and response speed when designing automated test sweeps.
Sensor Calibration
Metrological validation of the measurement system requires calibrating the gain and phase accuracy of the instrument. Adjusting the lock-in amplifier involves applying a known reference voltage at a specific frequency to verify the amplitude response and phase offset.
