Re: [Discuss-gnuradio] Do RTL based devices need some sort of calibration?

[Marcus D. Leech]

On 08/01/2012 04:07 PM, Christian Gagneraud wrote:
> Hi all,
>
> I've been using a FCD for a couple of weeks now and I decided to get a 
> go with a rtl-sdr compatible dongle. I bought one from osmocom and I'm 
> having trouble to get useful data out of it.
>
> Based on the USB IDs, this is a "Generic RTL2832U (e.g. hama nano)".
>
> Basically my problem is that the frequencies are wrong, first there's 
> a big offset (that can be fixed using the freq. corr. ppm), but i have 
> as well a scaling issue. When I'm looking at AIS signal i should see 2 
> blobs 25Khz apart of the center frequency (162MHz), but when i'm using 
> the RTL source the 2 blobs are separated by ca. 20KHz, instead of 50KHz.
>
> As a quick test, i set up 2 osmosdr sources (one with dev set to fcd=0 
> and the other rtl=0, they have the same freq and rate settings) and I 
> plotted their FFT. The FCD is working as expected, but for the RTL, 
> the 2 blobs are way closer the the central frequency.
>
> So my question is does this device need some extra special settings?  
> Is there any documentation about all the settings i can use? Should I 
> calibrate it using some special technique, or does that simply mean 
> that this particular device is just non functional?
>
> Thanks,
> Chris
Are you perhaps trying to run your RTL device at some weird sample 
rate?  The device really only supports sample rates from 1Msps to about 
2.8Msps, and only a few of those rates have actually been verified to 
work correctly.

My suspicion is that you're trying to run it at a rate it doesn't really 
support.  Try 1.0Msps   or 1.024Msps and see if that clears up your
  frequencies-aren't-right issue.



--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org