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From: | Neil |
Subject: | Re: FT8 with GNU radio and the PlutoSDR on the 2 meters band |
Date: | Mon, 13 Jan 2020 20:35:58 +0000 |
User-agent: | Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.9.1 |
OK, agreed that the envelope in even a sharply-switched FSK is
sort-of constant at baseband (except during tone startup, when it
goes from zero to full amplitude in the first half-cycle), but
with the sharp transitions, some energy goes into the modulation
sidebands created by those transitions. The level of the main tone
during a sharp transition must drop as energy is being put into
the sidebands generated by the abrupt frequency change. With a
nonlinear amplifier well into saturation, the level of the energy
away from the main tone will be boosted, so there is distortion of
the relative amplitudes of the tone and sidebands. In a linear
amplifier, the changes in level of the main tones during frequency
transitions will be reproduced faithfully and the overall
bandwidth will remain as per design. Also when operating close to
the edge of a filter passband, a fair amount of the energy in
those sidebands is removed during tone transitions. The amplitude
variations only happen around the time of a tone transition, so it
depends on how you define the granularity of your amplitude
measurements. I should run a proper test on the GFSK version. Anyway, none of this really matters unless you happen to be in
line of sight to a 30kW ERP station on 144MHz and you are trying
to resolve a signal 400Hz away that is 80dB weaker. The change to GFSK is *hugely* welcome as it has reduced the
occupied bandwidth of FT8 signals (even those using over-driven or
not-very-linear amplifiers) to a much more neighbourly level. Nice work by the OP. Neil G4DBN Average amplitude spectrum of an old-style FT8 signal at
baseband. Spectrum after going through an SDR transmitter. Blue is a
15-second average of the envelope, yellow is instantaneous. Some
of the sidebands have gone missing, so there is amplitude
variation, at least very briefly during tone transitions.
On 13/01/2020 19:48, Ron Economos
wrote:
-- Neil <a href="" class="moz-txt-link-rfc2396E" href="http://g4dbn.uk/">"http://g4dbn.uk/"><small>g4dbn.uk</small></a> |
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