[Elecraft] JT65A output power variations.

[Richard Ferch]

Julian, G4ILO wrote:
> 
> Hmm. As I wrote in my blog a while back (
> http://www.g4ilo.com/2009/10/embarrassing-moments-in-ham-radio-1.html ) I
> once inadvertently did an experiment that proved the crystal filtering does
> not do a perfect job of removing the harmonics of a distorted RTTY signal.

If the primary tones fed into the audio input of the radio are 
accompanied by other unrelated frequencies (e.g. hum and noise), or are 
laden with harmonics that fall within the filter bandpass (e.g. 
harmonics of a 500 Hz tone), then a 2.7 kHz transmit filter may not be 
able to do much about it. That is one reason why many people use "high 
tones" for RTTY (2125/2295 Hz), to ensure that even the second harmonics 
are outside the filter bandpass.

When I suggested that single-tone FSK/MFSK signals are not subject to 
IMD, I was of course oversimplifying. The FSK modulation itself 
introduces sidebands, and excessive compression can cause IMD that 
broadens those sidebands. However, the JT65A signaling rate is very low 
compared to the occupied bandwidth (2.7 Hz vs. 177.6 Hz, if I have 
understood the documentation correctly), so the effect of IMD is 
probably not noticeable in most cases. I suspect (without hard evidence, 
of course) that JT65A is much more forgiving than PSK31, i.e. that 
producing a splattered JT65A signal is much harder to do than producing 
a splattered PSK31 signal. RTTY would fall somewhere in between.

73,
Rich VE3KI