The effect of the skew (difference in amplitude of the mark and space tones) is to cause an undesired amplitude modulation in addition to the desired frequency modulation of the FSK. If the skew is due to a constant slope in the frequency response, then the shape of the modulation is the same for the AM and the FM.
1 dB of skew is equivalent to about 10% AM modulation. I haven't calculated the Bessel functions, but it seems clear that the sidebands from 10% AM modulation are much weaker than the sidebands of the 170-Hz deviation FSK (which is a modulation index much greater than 1 for 45-baud RTTY). > A rough guess turned out to be relatively easy and the bottom line is > that the far off QRM is only about 10 dB down from a phase > NON-continuous case of FSK generation, if the mark and space signals > are different by 1 dB. That sounds about right. If the FSK signal is unfiltered (instantaneous transitions between mark and space) then so will be the AM modulation. If you slow down the FSK transitions, then the AM transitions will slow down by the same amount. For any wave shape the AM sidebands are always much less than the FM sidebands. I don't believe 1 dB of skew would cause any significant additional QRM to nearby channels. As for affecting the received bit error rate, 1 dB of skew should be no worse than reducing power by 1 dB. Less than that if the transmitter is tuned for constant average power rather than constant peak power. Still, it would be worthwhile to eliminate the skew. I understand Wayne has that on his list. Alan N1AL On Mon, 2011-01-10 at 21:44 -0800, Kok Chen wrote: > On Jan 10, 2011, at 3:35 PM, Wayne Burdick wrote: > > > The effect on transmission bandwidth is negligible, and it's also > extremely unlikely to affect copy. > > > Hank W6SX had written to ask me for a rough guess of transmission > bandwidth when there is a 1 dB difference in level between mark and > space carriers. > > A rough guess turned out to be relatively easy and the bottom line is > that the far off QRM is only about 10 dB down from a phase > NON-continuous case of FSK generation, if the mark and space signals > are different by 1 dB. Basically, about 10 dB down from the first > plot in this web page: > > http://homepage.mac.com/chen/Technical/FSK/Sidebands/sidebands.html > > The ideal FSK phase continuous signal should look like the second plot > on that page. > > So, forget about phase noise! Your FSK keying sidebands are going to > be a much bigger problem for your nearby neighbors :-). > > I think that a judicious choice of tone pairs could help, but it is a > band-aid, not a technical solution. > > 73 > Chen, W7AY > > ______________________________________________________________ > 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 ______________________________________________________________ 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
