An inexpensive lock-in amplifier suitable for cw NMR and cw NQR spectrometers has been built and is described in this paper. The heart of the instrument is the phase-sensitive detector and MOSFETs 3N138 are used as a series-shunt switch for this part. The input to the signal channel is a low noise pre-amplifier that can be included if needed or bypassed if input signal levels are high, with the help of a switch at the front panel. The reference channel uses the general purpose IC's μA 741 and MC 1458 for most of the functions. Phase shift between the signal and reference channels can be varied using digital techniques and the IC 74121 has been used for achieving this. The performance of the instrument was tested on a cw NMR spectrometer and a S/N ratio of about 100 was obtained with the standard sample of glycerine at room temperature.
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