I think there is something wrong with your analysis, and with your spectrum
display.
The widths you claim for the signals are not consistent with the spectrum width
you show
if the width of the center pair are to be 170 Hz.
The pair of signals to the right of the center pair are about 3 times as wide as
the center
pair, not two. If, as you claim, the center pair are a 170 Hz shift, then the
ones to the
right must be near 510 Hz shift, not 340 Hz, so it is not a second harmonic.
Perhaps you should investigate your spectrum analysis software and measurement
technique,
as it does not look robust.
-Kai, KE4PT
On 3/13/2014 4:51 PM, Joe Subich, W4TV wrote:
Lest anyone misunderstand the problem with AFSK and why I am still
so adamant that the only way to generate RTTY is FSK, take a look at
the spectrum capture I placed on the web site this afternoon:
http://www.microham-usa.com/appnotes/DX_AFSK.jpg
It shows a well known DXpeditioner who is obviously over driving the
the microphone preamplifier in his rig. The center signal is the
"normal" FSK, to the left by 1565 Hz is an *inverted* copy of the
FSK (perfectly readable in RTTY-REV) and to the right by 1565 Hz
is a *second harmonic* (340 Hz shift) copy of the signal.
This demonstrates *three* of the things to avoid with AFSK:
1) *never* overdrive the mic input of the rig - always use a
"Line" level (typically, the PACKET, DATA or ACCESSORY)
input.
2) *always* use the lowest possible audio drive level to avoid
clipping/distortion
3) Use *high tones* - keep the audio frequency *above 1500 Hz*
so the second harmonic stands a chance of being rejected by
the IF filter (if the rig actually has one these days).
Again, *USE FSK* it is far less likely to exhibit the garbage shown
here. This isn't a case of hum from a defective interface - it is
quite simply the fingerprint of an operator who does not know how
to properly set-up AFSK and who probably has no spectrum analyzer
to be ensure proper configuration.
73,
... Joe, W4TV
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