Dick - sometimes the IMD readings vary a lot, and the reading can be
confused by other strong stations in your receiver passband. So you have to
be careful about taking the reading.

In general, -13 dB is the worst that a PSK31 signal can be. This indicates
100% compression of the audio. Sometimes you will see worse readings, due
either to problems other than overdrive, or other nearby signals. Soundcard
output levels are about 20 dB higher than the microphone input level of most
rigs. A 20 dB. attenuator between the soundcard and the mic input helps a
lot (two resistors).

PSK signals are filtered by the transmit software, to limit their bandwidth.
If you overdrive the transmitter and cause limiting or compression of the
signal (such as showing any ALC), that compression causes spectral
regrowth -- i.e. the signal gets wider because of it. Some people like to
see all the meters up high. However in this case, the amount of power
dedicated to the main signal slightly decreases, because more power is going
into the (unnecessary) 3rd, 5th, etc sidebands. So this actually hurts the
strength of the desired signal a little.

Signals with IMD of -21 or so are reasonably clean, and you can ocasionally
see signals with IMD near to -30 -- a very good quality signal.

Unfortunately the majority of PSK signals that I have seen have an IMD close
to -13 dB.

-- Tom





"Dick, AA5VU" wrote in message
...
I am new to PSK31 and have noticed stations giving out IMD reports.

How do you know what's good or bad? I have been printing stations with
solid copy but the IMD meter only shows -12 and sometimes -5.

Dick - AA5VU