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Re: Linrad Gain and Noise Floor
- Subject: Re: Linrad Gain and Noise Floor
- From: Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>
- Date: Sat, 29 Dec 2007 10:23:37 +0100
> >If you make a graph of noise floor level as measured
> >by Linrad vs gain you will find a straight line with
> >a knee where the noise floor goes to zero.
> > .
> > .
> I tried this today both with the preamp power on and off,
> though I think you intended it to be off.
> Within the limited accuracy of my measurements, each change of 1 in
> the RCF Shift gives a 6dB change in the Linrad Noise Floor.
> With preamp off, noise floor runs from 39 to -2 as RCF Shift runs
> from 0 to 7. With preamp on, noise floor runs from 54 to 13 as
> RCF Shift runs from 0 to 7.
OK. Presumably you would have to set the SDR-IQ gain to its minimum
and disconnect the cable to the transverter to see the knee.
> The whole noise floor is Red when the noise floor is 54, regardless of
> the setting of the 2 selective limiters, presumably because of my
> value of 4000 for Selective Limiter Max Level.
Yes. 4000 is a rather low value and if you do not place the noise
floor low enough already modestly strong stations will be considered
strong ones even after you clicked on them and that would destroy
the clever blanker in case you use 16 bit arithmetics.
Actually the reason for having a narrower bandwidth in fft1 is
that it allows a larger Max Level because the fft1 to fft2
bandwidth ratio is then smaller.
> No evidence there of any knee, so presumably I am above it.
> However I did notice a "funny" in the main spectrum scale when I used
> the arrows on the right to move the spectrum up to show below 0 on the
> Instead of nice scale increments in fives, in one case I got a scale
> 24, 19, 14, 9, 4, -0, -5, -10
> and in another slightly different scale position:
> 35, 30, 25, 20, 14, 9, 4, -0, -5.
> Sorry to keep finding these little things!
I think this is already fixed in 02-41. (It was an old
bug that has been present many years.)
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