[Elecraft] unequal power output per tone in FSK

[Alan Bloom]

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
> >