[Linrad] Re: Sampling and FFT SDR-IQ

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

Hi Thomas,

> When I change the par files, I often get a waterfall display with large
> negative frequencies across the top.  I have a hard time getting Fmin at
> the top left and Fmax at the top right.
> 
> I have
> xM_CIC2 [7]
> M_CIC5 [25]
> M_RCF [3]

This generates a sampling frequency of 126.984 kHz.
To receive from zero to 127 kHz you would have to set the center
frequency to 0.063492 MHz 

> points per pixel [1]
> pixels per xpoint [0]
> first xpoint [258]
> xpoints [1024]
> freq [0.025000000372529029846191406250]

> When I start xlinrad and press B, I get
> 1st point -36.4846 KHz,  last point -28.5714KHz.

The fft size is 8192. your frequency range is 25-63.49 to 25+63.49 kHz=
-38.49 to 88.49 kHz. To get your scale starting at 4 kHz you need to set
the first pixel to (4000+38490)/15 = 2833
If you keep the center frequency 0.025 kHz you should set:
first xpoint [2833] to see from 4 kHz and up.

You could also set the center frequency to 60 kHz and
first xpoint [479] to see from 4 kHz and up.

Do not set the frequency to 64 kHz. At 4 kHz signals would then 
become attenuated by the RCF filter. The useful bandwidth is 
not 127 kHz but about 85% of that so you need to leave about
10 kHz unuset at both spectrum ends.

I suggest you try the arrow boxes. When you click inwards, you
will see more spectrum inside the window. Outwards will move 
a fraction of what you see outside the window. If you make the window
width small, the amount of spectrum you move outside will become small.

> What determines the frequency displayed across the top?
The frequency scale on top is the antenna frequency. It is determined
by the center frequency C from the freq/att window and by the offset 
in bins from (N/2)*w 
F=C+(B-N/2)*w
F=freq on top
B=FFT bin number
N=FFT size 
w=bin width
C=center frequency
w=S/N 
S=sampling frequency. 


73

Leif


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