Phase‐Sensitive Detection. Part I: Phase, Gates, Phase‐Sensitive Detectors, Mixers, and the Rotating Frame

摘要

AbstractThe fundamental principles of phase, semiconductor materials, and transformers are presented. The use of diodes and transformers in the construction and operation of product detectors is then described. It is shown that the output of a phase‐sensitive detector “PSD” is a direct‐current “d‐c” voltage when the two inputs have the same frequency, and that the polarity of this voltage depends on the phase difference between the inputs. It is also shown that when the two input frequencies are different, the same device can act as a non‐linear mixer, because its output consists of the difference and the sum of the input frequencies. In an NMR spectrometer, a PSD acts as the heart of the receiver. One of its inputs is the NMR signal from the probe, and the other is a reference frequency obtained from the transmitter. The filtered PSD output is the difference between the two input frequencies, and is referred to as the free induction decay, or FID. It is shown that the reference to the PSD is conceptually analogous to the “rotating frame”. Finally, it is explained that the basic limitation of a single PSD is its inability to determine whether an NMR signal is of a higher or lower frequency