The spectrum shown is a direct capture from the Elecraft P3.
The leftmost two "signals" are most certainly as described - they
were copied directly with MMTTY using the default profile in RTTY-REV
(USB) for the lowest frequency pair (LSB "Image") and RTTY (LSB) for
the middle tone pair ("normal" signal). The rightmost "tone" has the
same "offset" (approximately 1580 Hz) relative to the mark tone as the
"inverted" mark (leftmost tone). That would make the right hand tone
pair the *third* harmonic of the modulating frequency and the shift 510
Hz (3 x 170 Hz). Interestingly, the LSB suppression is *at best* 25 dB
(-110 dBm for the image vs -85 dBm for Mark) and the third harmonic is
down only 11 dB (-96 dBm vs. -85 dBm). One can also see the *fifth*
harmonic products as the two pips to the right of the third harmonic
- only about 31 dB below the Mark.
In any case, the analysis is still the same - there are at least
three problems:
1) Insufficient lower sideband suppression - one can even see the
suppressed carrier position as a "notch" in the phase noise
"hump" at approximately 24.9249 MHz
2) audio clipping and (third, fifth) harmonic generation
3) the use of a very low tone pair - approximately 790/620 Hz Mark/Space
- which easily allows the third harmonic at approximately 2370/1860
Hz and fifth harmonic at 3970/3060 (24.9289/24.9280 MHz) to pass
what filtering exists (it can't be much given considering the poor
LSB suppression).
Of course, this is *exactly* what one would expect if an inexperienced
RTTY operator were to use AFSK and follow the instructions of certain
AFSK interface vendors to operate with all knobs/sliders to the max.
73,
... Joe, W4TV
On 3/13/2014 8:27 PM, Kai wrote:
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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