There are no amplitude variations. Take a look at the figure from
Daniel's link. What's being filtered is the frequency
transitions.
The time-constant length (attack/sustain/decay times) is the
root of the original problem with ALC of course. Is there a way
to store the averaged envelope and remember it in a variable
used to set the gain, so you don't have to recalibrate every 15
seconds with each new transmission, or syllabically with SSB
voice?
GFSK will work fine through a non-linear amplifier, but you
lose the benefits of the new smooth transitions. The amplitude
variations are only there during start, end and transitions.
The old way used to look like this in the frequency domain.
That startup splat is more than 2kHz wide. This is audio direct
from WSJT-X to Spectrum Lab within the same PC.
I don't have an image of the new GFSK, but it was much cleaner
when I checked. The FSK transitions in this are done at zero
crossing, but even so, that represents a step change in slope
and generates a spread of frequencies.
Neil
On 13/01/2020 19:05, Daniel Estévez
wrote:
Hi Neil,
As far as I know the new GFSK of FT8 and FT4 are still constant
envelope, so they are tolerant to non-linear amplification (but don't
read this as "immune to all sorts of terrible clipping and distortion").
See the bottom of page 4 in
https://physics.princeton.edu/pulsar/k1jt/FT4_Protocol.pdf
You raise a fair point that nowadays with SDR it is often enough to
adjust the gain in an open-loop fashion, by monitoring the output power
and changing the gain until an appropriate level is found. This can
often be done once in a set an forget fashion. I do that for my QO-100
groundstation.
However it's true that the gain of PAs can vary somewhat with
temperature and frequency, so sometimes some sort of close-loop
adjustment of gain (of an appropriately large time constant and
everything to prevent distortion) would be better.
Best,
Daniel.
--
Neil
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