On Aug 5, 2007, at 4:35 PM, Art Searle W2NRA wrote:
> A strange thing happened in the Tara Grid Dip contest. I was
> tuning down in frequency when I came across a signal that was 350
> Hz shift. I set my shift to 350 Hz and could copy him fine. Later
> I came across a shift that was 240 Hz and the signal was reversed.
> A little further down the band I found 6F75A transmitting with a
> normal signal calling CQ. I opened up my bandwidth and could see 3
> pairs of RTTY tones with 3 different shift. You could see all 3
> signals in sync as 6F75A transmitted. Anyone ever see anything
> like this before?
Quite often, unfortunately.
A double-spaced shift is usually in the correct polarity (not
reversed) and is caused by someone severely overdriving an AFSK signal.
For example, if your nominal audio mark is at 1000 Hz and space is
1170 Hz, then their second harmonics are 2000 Hz and 2340 Hz. The
good pair has 170 Hz shift and the bad pair has 340 Hz shift.
This don't usually happen (or at least the bad signal pair is more
attenuated) if you have chosen a higher tone pair like 2125/2295 Hz
since the second harmonic is outside of the transmit filter of most
amateur SSB rigs.
The case of the reversed (inverted) signal often comes from an AFSK
signal through an SSB transmitter that doesn't have good balance in
the balanced modulator -- i.e., the transmitter behaves more like a
DSB-SC transmitter. So, if you transmit say 2125 Hz for Mark and
2295 Hz for space using LSB, then there is a USB image of the mark at
2125 *above* the suppressed carrier and a USB image of the space at
2295 Hz above the suppressed carrier.
Basically, you have another what looks like an RTTY signal that is
(2125+2295) Hz = 4420 Hz *up* the band from the "real" signal and it
is inverted.
(If the station is using USB AFSK, then the inverted image is 4420 Hz
down the band.)
I haven't seen a case where a 170 Hz shift turned into 240 Hz -- not
sure what mechanism caused that.
73
Chen, W7AY
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