I decided that rather than depend on a hand-waving argument I would go ahead and do a Mathcad simulation to calculate the effect of 1 dB of tone skew (unequal mark and space tone amplitudes) on the transmitted FSK spectrum. Basically, I found that it made no significant difference.
A PDF of the Mathcad file is at: ftp://ftp.elecraft.com/tmp/FSK_tone_skew.pdf The equations are on the first page and the spectrum graphs are on the second. (On my browser I have to copy the part of the above URL after the "ftp://"; part and paste it into the browser window to get FTP to work.) Alan N1AL On Tue, 2011-01-11 at 13:10 -0800, Alan Bloom wrote: > 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
