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.
