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

[Tom Rondeau]

On Wed, Aug 1, 2012 at 4:07 PM, Christian Gagneraud <address@hidden> 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?
>
> 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