Application of lock-in amplifier in TDLAS technology

2023-01-11 10:44


As technology advances, the phenomena people want to understand are becoming more and more refined, and the signals they want to measure are becoming weaker and weaker. Thanks to phase lock-in amplifiers, we can now extract modulated signals with known carriers from extremely noisy environments, even when the noise is orders of magnitude stronger than the signal.

Imagine how we can extract the signal we need from the noise clutter? The most intuitive linear amplifier can amplify a very small signal, but also amplify the noise. However, if the filter bandwidth is too large, the noise near the signal frequency will still be output together, even masking the signal.

The lock-in amplifier applies the technology of phase-sensitive detection (PSD), which can measure very weak continuous periodic signals. Compared to noise, continuous periodic signals have a fixed frequency and phase, and the phase-sensitive detection technique addresses this characteristic by using a reference signal with the same frequency to extract the periodic signal of the target, which can greatly reduce the effect of noise.

In practice, we need to generate a reference signal with a specific frequency ωr as a carrier (using a function generator or other methods), and use this signal to modulate the target signal so that the target signal has the same frequency. We can express the modulated signal as Vsig sin(ωrt+Θsig), where Vsig is the signal strength and Θsig is the phase. Meanwhile, the lock-in amplifier generates an internal local oscillation signal Vloc sin(ωLt+Θloc) based on the input reference signal. Multiplying these two signals, we will obtain two AC signals with frequencies of (ωr+ωL) and (ωr-ωL), respectively.

  

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Since ωr = ωL, this signal passes through a low-pass filter and we get a DC signal of

  

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To obtain a stable signal, the phase difference between Θsig and Θloc must always be the same. This requires a phase-lock loop in the lock-in amplifier to dynamically lock the external reference signal, ensuring that the internal signal has the same frequency as the external reference signal (i.e., ωr=ωL) and maintaining a fixed Θsig-Θloc phase difference.

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