de WAØTTN - PSK31 Tuning Tips

This page provides some tips on tuning your receiver for best reception of PSK31 signals.

The general problem with tuning PSK31 signals is that us old-timers are familiar wide-band SSB or narrow-band CW signal characteristics. PSK31 signals are not tuned by ear, but rather in a visual display. As a result, many operators interpret the visual frequency spectrum (aka, the "waterfall") display as the only means of discriminating PSK31 activity.

If you break away from the traditional tuning techniques employed for CW and SSB, you will find that PSK31 tuning can be greatly enhanced. The following tips demonstrate some of the techniques for optimizing PSK31 tuning.

The basic concepts for tuning PSK31 signals for optimum intelligibility are:

  1. Keeping the PSK31 signal in the center of the most sensitive audio region of your receiver.
  2. Limiting adjacent QRM without degrading the PSK31 signal you're monitoring.
The first point, keeping the signal in the receiver's best audio response area, is simply a matter of tuning your receiver so the PSK31 signal is somewhere around 1500 Hz. This is the area where most receivers have the best sensitivity.

The second point is where the tricks come into play. While most receivers have selectable filters, there are considerations in using narrow filters, and also other options for reducing nearby interference. For example, I have found that my narrow CW filter does not actually improve the S/N ratio of the PSK31 signal, but rather, because of (I suspect) ringing induced by the narrow filter it actually reduces the intelligibility of the signal. Therefore, I suggest that you use your receiver's filters to reduce the extraneous audio noise, but don't overdo it.

An additional feature that is often overlooked in tuning out ambient interference is the IF shift control. By varying the IF center frequency, you can further screen out offending signals that are still within the audio region of the receiver's passband. Low-frequency interference is typically responsible for problems in PSK31 reception. So, by shifting the IF passband up a bit, you can further eliminate unwanted energy from your audio spectrum. The following set of illustrations demonstrates this technique.

This image shows the normal wide-open audio spectrum. Note that there are strong signals both below and above that are overloading the PSK31 discriminator.

This image shows the audio spectrum when the narrow-band SSB filter has been switched in. Note that there are still strong signals on the low side of the passband that are swamping the AGC.

This image shows the audio spectrum when the IF Shift has been employed to eliminate the overloading signals on the low side of the passband. The signals on the high side of the passband are much lower amplitude and of little influence to the PSK31 signal discrimination.

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