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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
