Echo Protection for Multifrequency Receiver
Original Publication Date: 1983-Aug-01
Included in the Prior Art Database: 2005-Feb-07
The echo protection circuit shown in the drawing is to be used in multifrequency receivers (MFRs) for protecting the receivers against echo, without degrading voice protection. The circuit is shown in Fig. 1A, and the timing is shown in Fig. 1B. The multifrequency (MF) signal is comprised of two frequencies A and B which are separated by filters. Frequencies A are the low range frequencies, and frequencies B are the high range frequencies. The echo protection circuit has two operating modes: normal and hold. In the normal mode, the frequency B signal is applied to a single-wave rectifier comprising amplifier A1 and diode D which provides a voltage VC3 with a time constant T3 = (R31$R32/R31+R32) $ C3 and an amplitude which is equal to two-thirds of the envelope amplitude of the MF signal.