Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band

[Daniel Estévez]

El 13/1/20 a las 19:47, Neil escribió:
> Alternative description of Automatic Level Control now we are in the
> world of SDR:
> 
> "ALC: An ancient and terrible use of amplitude feedback control by
> boat-anchor radios which had insufficient control over their modulation
> sources and had excessive gain.  A cause of envelope distortion,
> intermodulation products and general awfulness.  A thing which should be
> left firmly in the 1960s"
> 
> FT8 (8-tone FSK with 6.25 Hz spacing and 6.25 symbols per second) was,
> until recently a constant-amplitude mode, with all of the wideband
> clicks at start and end of transmissions and wideband splats at
> inter-symbol boundaries that entailed.  Now it uses Gaussian FSK so the
> amplitude element matters during start, stop and tone transitions,
> reintroducing linearity as a requirement of the transmit chain.  As it
> has a known steady-state amplitude, all that is required is constant
> linear gain.  Closing a control loop which looks at an averaged envelope
> of the signal to set the gain of an earlier stage to achieve a certain
> output level at run time is a recipe for reintroducing some of the
> nasties that have been removed by the move to GFSK. Calibrating the gain
> versus output level would achieve the same effect but without that
> unnecessary control loop.

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.