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

From: Christian Gagneraud
Subject: [Discuss-gnuradio] Do RTL based devices need some sort of calibration?
Date: Wed, 01 Aug 2012 21:07:45 +0100
User-agent: Mozilla/5.0 (X11; Linux i686; rv:13.0) Gecko/20120601 Thunderbird/13.0

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?


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

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