On Feb 20, 2008, at 2/20 10:30 AM, Bill Turner wrote:
> Everyone has great things to say about the DX capabilities of PSK, but
> as long as it is regarded as a "low-power" mode I don't think it will
> catch on as a DX mode. The problem seems to be the accursed waterfall
> that everyone likes to use. If you fire up in the PSK "band" with the
> legal limit, you swamp out everyohne's waterfall and you are a Very
> Bad
> Person.
To set the record straight -- if your rig has been adjusted properly,
you don't swamp anyone. Unless you are exceeding their receiver's
dynamic range, but that is not a mode-specific problem.
In fact, at a given separation (say 100 Hz away), there is much less
QRM from a PSK31 rig than from an FSK rig, since there is only one
set of single keying sidebands in PSK31 when you don't overdrive the
audio.
To reduce RTTY keying sidebands in cocoaModem, I had placed a pretty
much brick wall bandpass filter around the AFSK signal at about the
third keying sideband of an RTTY signal. But that is an exception
rather than the rule; pure FSK rigs cannot do that and have to depend
on waveshaping of the mark/space transitions to try to achieve a
clean signal. I don't think many AFSK software has a transmit filter.
Unlike RTTY, the bits in PSK31 are modulated with a raised cosine
window. You can see this from clean PSK31 signals that are idling --
all you see are the two "rails." A clean PSK31 signal has other
keying sidebands than these two carriers when idling.
When modulated with random bits instead of an idle pattern, the next
keying sideband reaches about -34 dB. But you can (and cocoaModem
does) use a filter to attenuate this and further sidebands by 10 dB
and the receiving end will not even notice it.
However, if you were to overdrive the audio, the PSK31 keying
sidebands will approach the square wave shape of FSK's keying
sidebands for an FSK rise and fall time that is not properly controlled.
For example, a good "clean" PSK31 idling signal has IMD (IMD pretty
much measures the signal outside of the primary keying sideband) of
about -28 to -30 dB, and if you try really hard, you can push that to
-40 dB. The -30 dB figure is easy to achieve, the -40 dB figure is
harder to achieve since most transmitters have no better than -35 dB
of IMD when running full power (take a look at the QST reviews).
If you achieve IMD that is clean to -40 dB, extraneous keying
sidebands from a kW station is down to the region of a QRP PSK31
station. Someone running a dirty PSK31 signal at 100 watts is
likely to be doing much more harm than a clean 1 kW station.
The importance is not how much power, but how the power is
distributed. I have actually measured PSK31 signals at -9 dB IMD;
that is worse than an unwindowed square wave -- because the audio was
driven so hard the top of the bits were not flat. These are the
PSK31 signals that appear as wide as an RTTY signal.
When N8S (Swains Island) operated PSK31, I had no trouble getting
through with 50 watts, in the midst of the California kilowatts.
73
Chen, W7AY
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