as WSPR is a
constant envelope signal, overdriving the TX Audio would
not produce intermodulation but only audio harmonics. With
a fundamental near 1.5 kHz, the odd harmonics at 4.5 kHz
etc. will be too far outside the passband of an SSB
transmitter to cause noticable contamination.
However this may
not be the whole truth: If the clipping already takes
place in the digital domain (i.e. before the DAC and
antialias filter), we’d still have the same harmonics but
they could be aliased down with the samplerate. For
example, a clipped 1.51 kHz signal contains components at
7f (10.57 kHz) and 9f (13.59) kHz. With 12 kHz samplerate,
these could be folded back to 1.43 and 1.59 kHz, and thus
create weak unwanted sidebands near the desired output
frequency. BTW the very same effect can happen on the
receive side when overdriving the ADC.
25 April 2016
Had a report that
my LF WSPR2 signal was dirty with massive side bands
on Sunday. I had a
look at my setup today found Windows had decided to
up the audio level
from WSPR to maximum into the TS-590 and scoping
the output of the
pre amp and frequency doubler before my G3YXM amp
showed it looking
very ropey indeed.
I reset it and did
a couple of normal WSPR2 transmissions interspersed
with using the WSPR
TUNE function, whilst watching on the G3XXM and
G4WGT LF grabbers.
I am no expert whatsoever, but it doesn't look bad
now to me. Is this
an adequate test to make and to assume things are
now OK? Thanks.
part time LF grabber is