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.
Neil G4DBN
Hi Marcus,
I'm not a hardware guy, but I think it's a circuit that monitors voltage from the final amplification stage and adjusts gain of previous stages if the maximum level is exceeded. Present in all classic SSB transceivers. Perhaps somebody else can explain better.
Adrian
On January 13, 2020 5:09:12 PM UTC, "Müller, Marcus (CEL)" <address@hidden> wrote:Hey Adrian, that's nice! Generally: you said you couldn't implement an ALC: what's an ALC? Best regards, Marcus Mon, 2020-01-13 at 16:40 +0000, Adrian Musceac wrote:Hi, I wrote a report about combining WSJTX and GNU radio with a PlutoSDR transceiver to achieve FT8 contacts on the 144 MHz amateur radio band. The short summary is that I achieved contacts at over 800 km and heard stations as far as 1000 km last month in excellent tropo propagation conditions with 20 Watts of output power. I thought you might be interested in reading this, but it's too long to paste here. The report can be read here: http://qradiolink.org/ft8-plutosdr-2meters.html Adrian YO8RZZ
-- Neil <a href="" class="moz-txt-link-rfc2396E" href="http://g4dbn.uk/">"http://g4dbn.uk/"><small>g4dbn.uk</small></a>