The issue is not IMD, it is sidebands from the transition between the
two frequencies. If that is not done carefully, the sidebands can cause
a wide signal.
Carl Moreschi N4PY
58 Hogwood Rd
Louisburg, NC 27549
www.n4py.com
On 9/23/2014 2:48 PM, ken.d.brown@hawaiiantel.net wrote:
There could only be intermodulation distortion caused by the combination
of those spectral components (the mark tone, the space tone and the
sidebands) if they were being generated simultaneously. I'm not so sure
that is the case. The mark tone is only there during mark, the space
tone is only there during space, and I'm pretty sure the sidebands are
only there during the transitions. The upper sideband tone occurs in the
transition when the FSK tone is shifting from the low frequency to the
high frequency, and the lower sideband tone occurs during the transition
when the FSK tone is shifting from the higher frequency to the lower
frequency. Yes there are multiple tones, more than just the mark and
space tones, but at any instant there is only one. I'd have to be a lot
better at the math of Fourier transforms to know this for sure, but that
is the way I think it is.
DE N6KB
On 2014-09-22 18:49, Jim Brown wrote:
On Mon,9/22/2014 8:38 PM, ken.d.brown@hawaiiantel.net wrote:
You're right about the transients and the sidebands they create. Keyed CW
creates sidebands too. I don't think those sideband components could be
accurately called intermodulation products though.
Hi Ken,
I'm not calling the sidebands produced by the transientsIM products. What I'm
pointing out is that those sideband components are amplified by the output
stage and linear amp, if there is one, and those amp stages generate IM from
the combination of those sidebands and the carrier. It's exactly the same
mechanism as with CW keying, and with SSB.
Those CW spectra in the ARRL reports are the result of the combination of the
sidebands, the IM produced by the amplifiers excited by the combination of
carrier and sidebands, and the phase noise. Indeed, I've been urging ARRL to
use pink noise to drive the mic preamp to produce SSB modulation spectra
comparable to the CW spectra, because they combine all three mechanisms.
73, Jim K9YC
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