Highly-selective transmission-measuring equipment for communication circuits

A circuit is described which permits a highly-selective transmission-measuring equipment to be designed to meet a specified accuracy of measurement; it is suggested that ±0.25 db is quite readily obtainable. The basic principles of the method are (a) the direct demodulation of the test signal by means of an identical frequency obtained from an oscillator synchronized to the test tone, (b) discrimination against unwanted line signals obtained by means of low-pass filters, (c) the elimination of the effect of the phase difference between the test and demodulating tones by Barber´s two-path method, and (d) the use of an envelope-modulated test signal.