Setup checklist to minimise distortion & excessive bandwidth of PSK signals
1. Tune the rig for normal output on CW
2. Change to the appropriate sideband setting (USB for PSK is standard)
3. Turn off the speech processor and leave it off
4. Set the audio control of the rig as you would for normal SSB operation
5. Set the PSK software to Transmit, with no data being transmitted
6. Adjust the audio output from the soundcard so that the RF power output from the rig is no more than 25% of the cw output. (e.g. 25 watts for a 100 watt rig) This will provide approx. 50W average output during QSO. There should be no ALC indication at this power level.
Although you might be able to set the power higher in step 6 with no ALC, this setup should ensure that a very clean signal is transmitted.
A few words of caution. If driving the microphone input of the rig, an attenuator between the soundcard output and the transceiver input may be needed. Distortion can also occur in the sound card at very low, or very high output levels. An attenuator can ensure that the soundcard output control is near the middle of its dynamic range, giving the cleanest audio signal to the transceiver.
Further information – Receiver settings & weak signal decoding
Be sure the receiver settings are not contributing to apparent distortion of incoming PSK signals. It’s possible to overdrive the front end of the receiver with a strong signal (you may need to reduce the RF gain) or the soundcard input may be being overdriven by the transceiver audio.
Another attenuator between the transceiver audio output and PC input may be needed. Adjust this attenuator to ensure the soundcard input control is also near the middle of its dynamic range.
Decoding weak signals:
If trying to decode a weak signal trace in the presence of other very strong signals, it may be best to reduce the receiver RF gain and or disable the AGC. Weak signals can be copied doing this, even in the presence of a local 20db over 9 signal.
Further, most modern rigs have a variety of DSP filters that can assist the isolation of a weak signal, whilst even use of older rig’s IF shift and notch filters can be of benefit.