A codan for A.M. receivers

Carrier-Operated-Devices-Anti-Noise for a.m. receivers are discussed. The paper is limited to the form of CODAN in which the wanted carrier is selected out of the noise by means of a narrow-band filter and caused to operate a relay which opens the audio circuit. The simple form, with narrow- and wide-band filters at the receiver intermediate frequency, is usually unsatisfactory because of transmitter and/or receiver frequency drift. The principles of operation of the modified form, in which a double-mixing process is used to overcome frequency drift, are found to differ from those of the simple Codan. It is shown that the weaknesses of this modified Codan can be overcome by the addition of a delay line. A practical Codan unit with a band-pass delay line is described and some performance figures are given.