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

From: Christian Gagneraud
Subject: Re: [Discuss-gnuradio] Do RTL based devices need some sort of calibration?
Date: Thu, 02 Aug 2012 16:43:34 +0100
User-agent: Mozilla/5.0 (X11; Linux i686; rv:14.0) Gecko/20120713 Thunderbird/14.0

Thanks all for sharing your thoughts.
Indeed the RTL doesn't support arbitrary sampling rate (I was using 96KHz!).
I finally start reading the chips datasheet and the rtl-sdr source code, it's full of interesting information! ;) There's some formula for settings the sampling rate, for what i understood, if u want the real sampling rate to be what you ask for then u have to have (Fosc*10^22/rate) % 8 = 0
I couldn't find any information about minimum frequency.

So with 1.024MHz, frequencies are now in range.

Thanks again for your messages.


On 01/08/12 21:07, 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?


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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