Do RTL based devices need some sort of calibration?

GNU Radio
1

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

2

On 08/01/2012 04:07 PM, Christian G. wrote:

as well a scaling issue. When I’m looking at AIS signal i should see 2
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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

3

On Wed, Aug 1, 2012 at 4:07 PM, Christian G. [email protected]
wrote:

scaling issue. When I’m looking at AIS signal i should see 2 blobs 25Khz
it using some special technique, or does that simply mean that this
particular device is just non functional?

Thanks,
Chris

Chris,
I’ve only done very simple things with my RTL-SDR device, but it
looked fine to me. My first thought is to check to make sure you are
actually using the sampling rate that you think you are using. Can you
pull in a signal of a known bandwidth to make sure it’s showing up as
the bandwidth you actually expect?

Tom

4

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! :wink:
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.

Chris

On 01/08/12 21:07, Christian G. wrote:

well a scaling issue. When I’m looking at AIS signal i should see 2
calibrate it using some special technique, or does that simply mean that
this particular device is just non functional?

Thanks,
Chris

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