[Linrad] Re: Sampling and FFT SDR-IQ

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Thomas,

> I am still working with linrad and the SDR-IQ and trying to understand
> things.  Is this is off topic?
Of course not. Anything that helps to understand how things work is OK:-)

> In an earlier email, you wrote:
> 
> > Now, if you want 4000 to 21000 Hz on a screen that is e.g. 1100 pixels
> > wide you would want a bin separation of (21000-4000)/1100 = 15.4545 
> > Hz/pixel. Linrad can only compute FFT in sizes that are powers of two 
> > so you would have to use an FFT size of 2048 or more. You may choose 
> > larger if you like. The sampling rate that you would have to set 
> > would be:
> >  FFT size     Sampling rate
> >   2048            31.65 kHz
> >   4096            63.30 kHz
> >   8192           126.60 kHz
> > I would suggest a sampling speed of about 126 kHz
> 
> How do I get from (21000-4000)/1100 = 15.4545 to a sampling rate of 2048
> or greater.  Is this because in the SDR produces I and Q and I need 2
> bins per pixel?
With 15.45 Hz per pixel and also 15.45 Hz per fft bin you will get a spectrum
with a resolution of 30 Hz (with a sine squared window.) That will be optimum
in some situations, but if you want to look at really weak signals that are
close to very strong signals you might want a sine to power 8 window and
then you might want to have two bins for each pixel.

Now, with one bin for each pixel you would need 1100 bins for the frequency
range that you want to display. Linrad can not compute 1100 bin FFTs. The
smallest one would be 2048. If you would choose 1024, the maximum frequency
coverage would be 1024*15-45 Hz = 15.82 kHz, but you would not be able to use
all of that. There has to be filters that prevent aliasing from resampling
so the useful bandwidth might be something like 14 kHz only. You said you want
17 kHz bandwidth so it will be necessary to use 2048 FFTs (or larger.)

If you sample at 31.65 kHz, the dynamic range will not be particularly good.
The SDR-IQ only transfers 16 bit over the USB. At 126 kHz you would get 6 dB
more dynamic range.

When sampling at 126 kHz you might ask for a bandwidth of 60 Hz and
display 2 bins for each pixel. It would be a bad idea for low order
windows, but with higher order windows you might like the result.
It all depends on what you are looking for.

By the way, I completed "newcomer install instructions for Ubuntu 10.4"
yesterday. One important thing is that "Visual Effects" is enabled by
default for all users. One has to disable it individually for each
user....
http://www.sm5bsz.com/linuxdsp/install/cd-dists.htm

73

Leif / SM5BSZ


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