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

[Christian Gagneraud]

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

--
Christian Gagneraud,
Embedded systems engineer.
Techworks Marine
1 Harbour road
Dun Laoghaire
Co. Dublin
Ireland
Tel: + 353 (0) 1 236 5990
Web: http://www.techworks.ie/