Abstract :
A comparison is made of the output spectra comprising signal and low-frequency noise when a sinusoidal signal plus noise is applied to several types of detector. As typical of the incoherent type of detector the common square-law detector and the so-called linear detector have been studied. The coherent type is represented by the homodyne and commutator detectors. The comparison is made for input noise from several common types of filter. The signal/noise ratio at the output is calculated in terms of that at the input, and curves showing this relationship for the various types of detector are given, both for the case when no further bandwidth limitation is introduced other than the filtering out of the high-frequency components, and also for that when the detector is followed by a narrow-band low-pass filter. It is shown that, provided certain precautions are taken, a very considerable gain in signal/noise ratio may be obtained by using the coherent detector when the input signal/noise ratio is somewhat less than unity. The actual amount of this gain for various circumstances may be read directly from the curves given in the text. This condition has not been of much practical importance until recently, but has now found many applications involving the detection of signals much below noise level by means of post-detector integration. The values of output signal/noise ratio for the various detectors are given for a range of values of the input signal/noise ratio in a series of figures, and the behaviour for large and small values of the latter is compared in Tables 1 and 2.
