FLDIGI Users Manual 4.0
Using the FLdigi Pseudo FSK (Rt. Channel) function to key a transmitter
Select the PseudoFSK check boxes.
FLdigi is now ready to generate a 1000 hertz tone burst signal on the right channel of the stereo audio out of your sound card.
This tone burst is on when the RTTY bit is on and off when the RTTY bit is off. The left channel will be the normal AFSK signal.
The following circuit may be used to take the FLdigi PSEUDO-FSK signal from the right channel of your SOUND CARD to key your transmitter's FSK input line. You may find it necessary to invert the sense of the keying signal.
L1 - Radio Shack has two items that may be used for this isolation transformer.
Catalog # 270-054, and Catalog # 273-1374
Attach an audio cable from the Rt. Channel out of the your computer's SOUND CARD to the input of this FSK INTERFACE CIRCUIT (input of L1).
Attach another cable from the output of this circuit to your Rig's Keying FSK Jack.
Every PSEUDO-FSK tone that is generated by FLdigi is rectified by this FULL WAVE VOLTAGE DOUBLER circuit. The resultant voltage turns the Q1 transistor on and "grounds" the collector.