Thanks for the correction Neil, I was sure FT8 was RRC filtered FSK, now I know better. Like Daniel though, I'm not sure why GFSK would require more linearity than FSK with abrupt phase transitions. I've never used GFSK, but I have used GMSK which is quite similar and it didn't require very high linearity.
Adrian On January 13, 2020 7:23:38 PM UTC, Neil <[email protected]> wrote: >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 ><a href="http://g4dbn.uk/";><small>g4dbn.uk</small></a>
