原创 the typical Lock-in Amplifier

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In the signal channel the input signal, including noise, is amplified by an adjustable-gain, AC-coupled amplifier, in order to match it more closely to the optimum input signal range of the PSD. Instruments are usually fitted with high impedance inputs for voltage measurement. Many also incorporate low impedance inputs for better noise matching to current sources, although in some cases the best results are obtained through the use of a separate external preamplifier ^[16].

The performance of the PSD is usually improved if the bandwidth of the noise voltages reaching it is reduced from that of the full frequency range of the instrument. To achieve this, the signal is passed through some form of filter, which may be simply a band rejection filter centered at the power line frequency and/or its second harmonic to reject line frequency pick-up, or alternatively a more sophisticated tracking bandpass filter centered at the reference frequency ^[7-9]

It has been shown that proper operation of the PSD (phase-sensitive detector) requires the generation of a precision reference signal within the instrument. When a high level, stable and noise free reference input is provided, this is a simple task. However there are many instances where the available reference is far from perfect or symmetrical, and in these case a well designed reference channel module is very important^[16,21].

The internally generated reference is passed through a phase-shifter, which is used to compensate for phase differences that may have been introduced between signal and reference inputs, before being applied to the PSD ^{[16, 21]}

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^{From Bachelor's Paper}