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
On 13/01/2020 17:57, Adrian Musceac wrote:
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)"
<muel...@kit.edu> 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="http://g4dbn.uk/";><small>g4dbn.uk</small></a>
