The output of the Tx mixer is buffered by a 2N4401 emitter follower to drive the low impedance input side of a 10.7 MHz IF transformer. The transformer is returned to 75 meters by an additional, external 330 p cap. A second IF transformer is used to make a double tuned band pass filter, although for 75 meters.source

The circuit starts with a standard Collpits crystal oscillator. In order to provide a 600 Hz frequency shift, a trimmer cap and 2N7000 FET is added in series with the crystal. During transmit, the 2N7000 is turned on, effectively shorting out the trimmer cap and lowering the oscillator frequency. During receive, the ‘7000 is turned off, allowing the trimmer cap to raise the frequency. The trimmer cap is adjusted to give a shift of about 600 Hz. source

The circuit presented here is a method of using the standard PSK audio modulation from a PC sound card to transmit a PSK signal without the need for a SSB transmitter. Since the RF Power Amp runs in Class C mode, it is much more efficient than a linear amplifier normally used with PSK transmission. source