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[Linrad] Re: Progress
- Subject: [Linrad] Re: Progress
- From: Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>
- Date: Sat, 18 Apr 2009 00:09:43 +0200
> You are correct. When I switch off the mast mounted stuff, the base line
> of the main spectrum display still only drops 5-7 dB, indicating that I
> don't have enough gain at the top.
OK. Try switching off or replacing with dummy loads at
all possible points and you will find out what to do about it:-)
> I am not using the MAP65-IQ program. I am in the Weak CW mode of LINRAD.
> I don't how the backward FFT relates to WCW mode. Right now, backward
> FFT is not selected.
OK. Then there is no quantization noise to worry about in Linrad.
You just have know that changing the fft1 gain from the default value
of 1000 might introduce quantization noise from the USB transfer.
> > Under normal circumstances, place "Floor" between 6 and 9.
> > (It is in units of bits in linear scale) The noise floor in the
> > main spectrum will then be around 20 dB.
> > Note that the SDR-IQ is a 16 bit device. The output shift it does
> > is a rounding and you have to be sure that the associated quantization
> > noise is very small.
> I am using SDR-14, so I don't know how SDR-IQ relates here.
Oooh! It is the same.
> > When you find 20 dB noise floor with an output shift of
> > 4 all is ok on the input side of fft1 (the SDR-IQ output)
> > ONLY if the first FFT amplitude is 1000 (or more)
> > In case you increase the output shift by 2 units and then
> > compensate by making the first FFT gain 4000 you will still
> > see the noise floor at 20 dB and a good "Floor" value at
> > the output of fft1, but you would have lost 2 bits at the
> > input and then the quantization noise caused by the output
> > shift in the SDR-IQ would no longer be negligible.
> I am understanding more of the concept of optimizing range and
> quantization error, but until I get my gain at the top fixed, I can't
> really worry about it.
Absolutely. One should adjust gain to fit the noise floor starting
from the antenna!
> I am now modeling my system using Agilent's ApCAD and the model doesn't
> show the same problem with gain at the top. I am modeling the SDR-14
> direct input at 33 dB noise figure and 0 gain. Is this reasonable?
I checked again. In 1 kHz bandwidth a signal on 144 MHz at -90 dBm produces
S/N = 24 dB. The noise floor is thus at (-90-24) -144 dBm which corresponds to
a noise figure of 30 dB for the direct input of the SDR-14 at 144 MHz.
Leif / SM5BSZ
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