Filters for PSK31
By Dick Stevens, N1RCT
PSK31 has built-in DSP algorithms that reduce random white noise near the set receive frequency. However, strong signals nearby (digital and CW) can still interfere with the desired PSK signal, which must be received in SSB, with USB preferable. There are basically three methods possible ... (1) Using the radio passband controls, (2) using a narrow CW filter on SSB (8 MHZ /455KHz), and (3) using a DSP box. I have shown what happens in the following pictures, taken using Spectrogram, a fine freeware program. These were taken from my IC 756, one of the more complicated to set up. The soundcard is set up exactly as if I were listening with the PSK31 program, but not actually running, as Spectrogram needs the use of the soundcard. All the pictures following were made with following setup:
First, here is what the raw, unfiltered sound looks like:
There is a strong Pactor signal about 500 Hz below my desired listening frequency of 1200 Hz. This has a horrible sound and I probably would not be aware of a PSK signal nearby by ear alone.
Most radios have a passband contol or even a dual one, which can quiet the desired range quite a bit to the ear. Here is what the optimal adjustment is for my IC 756:
While it sounds a bit better, it does little for the higher frequency noise, but is still a big improvement. If I turned the knob too far, it would have wiped out the desired signal also.
Next, let's look at what happens when I try to use my FL-232 350 Hz RTTY
filter on PSK, by telling the IC 756 it is really the FL-223 SSB filter. While
it filters well, it is far off the desired frequency range and the desired PSK31
signal is lost:
The trick to getting the narrow filter moved to the desired range is to use the passband control again ... On the IC756, it is the outer control that moves it .. but only down to about 1200 Hz, which is why I chose 1200 for the operating frequency for PSK. Now it is centered and very quiet compared to the passband alone.
The noise spikes are from the PSK signal itself, which was very strong and I had the volume a bit high. Turning the sound card volume down until the main signal is barely yellow is a good starting point for setting the sound card drive level but makes for dull pictures!
Lastly, here is what my Timewave 599zx set for CW, 1200 center freq, and 50 Hz bandwidth looks like:
This shows the remarkable performance of the Timewave on audio signals, but a strong signal nearby could desensitize the radio via the AGC, resulting in a weak signal. So the best performance is obtained with the combination of all three methods.
Your rig will have different possibilities but Spectrogram will let you see exactly what is happening. Trust your ears also ... the better you can hear the signal, the easier it will be to find a PSK31 signal. The general method can also be used to be sure your RTTY filter is actually centered on 2210 Hz (checked Kenwood 940, IC 756, IC 706/2 .. all were off)) and using the bandpass controls to find the best setting. 73 de Dick