karn@allegra.UUCP (01/22/84)
Relayed from TAPR: ANNOUNCELMENT, ANNOUNCEMENT, ... (CONTINUED) Well, we were just a bit premature with the modification indicated on our last installment. There's nothing wrong with the mod: it's exactly what's needed. Unfortunately, after you make this simple mod the demodulator will begin putting out garbage. This is due to another undocumented feature: pin 7 of the 2211 should be pulled to +12 volts instead of +5 volts, as is currently the case. The result of increasing the voltage is to increase the loop gain and the hysteresis in the data comparator. This in turn prevents the output data from containing double pulses, which is what you will see if you improve the loop filter design by replacing C20 (C25 on Beta boards). The solution is simple, but now we're getting into a slightly more complicated modification: you need to route +12 volts to the pullup on pin 7 rather than +5. Next, you have to break the connection between the data out (pin 7) and the jumper connection leading to pin 27 of the HDLC chip. Using a VN10 (like the one used to switch the PTT on the kit board), use the modulator output to switch the state of the line leading to this jumper connection. Attach the drain of this FET to the lead going to the jumper, attach the source to the nearby ground, and use the output coming from pin 7 of the 2211 to switch the gate. One final thing: you will need to pull up the drain end of the FET with about 5 k to +5 volts. If you do all all these things, the demodulator works unbelievably: We've been regularly running 1800 baud on good links here, and at 1200 baud the thing is just about unstoppable. 73 Dan