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

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Do RTL based devices need some sort of calibration?
Date: Wed, 01 Aug 2012 16:32:55 -0400
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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?


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

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