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

This is a simple keyer chip.It is designed to provide basic keyer functions for simple qrp rigs. It is particularly useful in Direct Conversion rigs, as it provides a means of generating side tone. Separate outputs for audio mute and transmitter keying provide sequenced audio muting and oscillator shift, allowing these to happen before and after the transmitter is keyed. This eliminates the need for R/C time constants in the auido mute switch(es) in the rig. source