Rtty Filter For 170 Hz Shift

00 Anyone interested in the reception of radio teletype traffic will appreciate the programmable audio filter described here, which may be fitted at the input of the RTTY converter. It improves the signal/-to -noise ratio, particularly of signals in the crowded short-wave bands. The circuit is based on programmable filter IC3-see Fig. 2. The special facet of this IC is that the resistors of the on- board RC filters are simulated by capacitors. This little-known technique is described in the January 1981, October 1982, and February 1983 issues of this magazine. The value of the capacitors, and therefore the pass-band frequency of the filter, is determined by the fre- quency of the clock at pin 8 of IC3. The clock frequency is made variable by passing a 10-MHz signal through a pro- grammable divider, IC,. The divisor may be set between 1 and 256 with the aid of switches Sla - Slh. Monostable IC2 converts the output pulses from the divider into a near- symmetrical signal, whi...