signal purifier for SSB telegraphy receiver (P. von Berg)

42 P. von Berg The output of Nl wilt be logic 1 if a signal between 750. 850 Hz is applied to the inputs of the tone decoders. As long as this situation continues, N2 will pass the output of an external oscillator to the clock input of counter IC4. This counter moves one step for each pulse. After 256 clock pulses its output 09 will become logic 1, causing bistable FF1 tochangestate:outputQofthis flip-flop will then become 1. Conse- quently, output Q of FF1 becomes 0, gate N3 is blocked and the counter resets. The counter does not react to pulses at its input as long as the reset input is held at a logic 1. The fact that outputs 09 of 1C4 and Q of FF1 are logic 1 indicates that an AF signal was applied to the inputs of the two tone decoders 256 pulses before. As loon as the AF signal ceases the output of N1 (0) triggers MMV2. Output Q of this monostable then becomes 1 and causes bistable FF2 to change state. The consequent logic 0 at output Q of FF2 then allows IC5 to start counti...