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PTT/FSK Circuits For RITTY/ Writelog/PSK31


Why bother?    The basic reason to add the optional PTT/FSK circuits to RITTY or Writelog/WinRTTY is to use narrow CW filters on radios that do not permit them to be used properly on LSB mode or , such as my Kenwood 940 or Icom 756. RITTY /WinRTTY can only operate on the final sound that comes from the radio; strong signals close by on LSB can ruin the sound of the desired signal as it can be much stronger. One way it hurts is to "capture" the AGC of the radio .. AGC is really automatic attenuator control and the strong adjacent signal causes the AGC to reduce the gain and drive the desired signal so low you will not even hear it and realize it is there. For PSK31, only the PTT is used so that VOX is not necessary. PSK31 must use "AFSK" audio, FSK will not work. However, your radio may be more capable of using narrow filters on SSB than the instruction manual shows ... see PSK31 Filters .

     CW filters operate at one or more of the IF frequencies and can remove these strong signals before the AGC stage. Some of the new top-end models of radio do DSP at the IF frequency; it requires very fast DSP processors and even then, a fairly low IF of perhaps 75 KHz is used; a CW filter at 8 MHz or 455 KHz may well be more effective.

Let's get started ..

     The first step is to obtain a RS 232 plug and shell from radio shack or cut a stock cable. You will need a female plug of the size to fit the com port you will use. For more on com ports, see:
You need to identify the following wires in a stock cable or wire up a plug as follows:

There are only three wires needed, the DTR, TxD, and Gnd wires. This port will be used as output only; no signals go into the computer from here. This means that a unique IRQ is not required in a DOS system; it can "share" an IRQ; I use the same one as the mouse. For Windows 3.1, you may have to try a few to find one it will allow. Windows 95 will probably not allow sharing. In all cases, you still need a unique com port address.

      The next step is a simple circuit which converts the RS-232 signals into the levels needed by the radio.

   There is very little power generated in these circuits, so small parts are OK. I used tiny plastic transistors and 1/4 watt resistors. The "ATT" box is for an attenutator if your software requires it. This circuit may give reversed FSK tones, it did on my Knwd 940. The easy solution is to reverse the emitter and base leads of the FSK transistor, so that the input line goes to the emitter (the pointed end) and the base (center) goes to ground. In the same vein, the amount of FSK shift (170, 425, or 850) may be set by a jumper buried inside the radio. My Kenwood was set to 425 Hz. Strangely, I made 10 contacts with 425 Hz shift and reversed tones before someone told me I had a little problem!

    BTW, reversed tones are really right side up! The convention is that they sound right in LSB, which is reversed .... which leads to the difference between manufacturers in the polarity convention. For my IC-756 and 706, the circuit as shown is the correct polarity. The correct polarity is what the radio requires to send tones such that the SPACE is the lower tone in the RF on-the-air and the MARK is the lower tone in the demodulated audio. For my IC-738 (no FSK), I use just the PTT signal to avoid using VOX.

  Ready to get a little more complicated?

     Perhaps you would like to keep your regular TNC all hooked up ?   The next circuit permits both RITTY/WinRTTY/PSK31 and the TNC to receive simultaneously and a switch that selects which (TNC or RITTY) will be allowed to transmit. A little study will show that all combinations of FSK/AFSK/PTT are permitted from both TNC and RITTY/WinRTTY/PSK31. This complete isolation of the transmit circuits is not required for all lines on some TNC's but it is probably easier to do it all and eliminate all possible interactions. Also, the center "OFF" position permits Phone operation without any potential parasitic effects from the TNC's (Thanks, Claude!).

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Questions? I will try! email me   n1rct@megalink.net  73 de Dick