kalibrate-rtl results...

Nikos Balkanas nikos.balkanas at eyeonix.com
Fri Jan 31 11:00:36 UTC 2014


On Fri, Jan 31, 2014 at 12:47 PM, Ralph A. Schmid, dk5ras
<ralph at schmid.xxx>wrote:

> GSM does not hop around inside its channel, this is wrong, the frequency
> is +- some Hz accurate, at least in normal western European commercial GSM
> networks.
>
>
>

Are you sure about that? Check it out:

https://svn.berlin.ccc.de/projects/airprobe/wiki/A

Legend of first picture. If still in doubt that *most" GSM carriers are
hoppers, use rtl_power to scan your region ;-)

Nikos


> Ralph.
>
>
>
> *From:* osmocom-sdr-bounces at lists.osmocom.org [mailto:
> osmocom-sdr-bounces at lists.osmocom.org] *On Behalf Of *Nikos Balkanas
> *Sen**t:* Thursday, January 30, 2014 7:35 PM
> *To:* Richard Koch
> *Cc:* osmocom-sdr at lists.osmocom.org
> *Subject:* Re: kalibrate-rtl results...
>
>
>
>
>
>
>
> On Thu, Jan 30, 2014 at 8:21 PM, Richard Koch <n1gp at hotmail.com> wrote:
>
> I'm attempting to calibrate my RTL dongles. I notice they are
> all about 2-4khz off if I compare against a known broadcast
> station's frequency.
>
> I am trying out kalibrate-rtl and am getting varied results.
> I am in the US, so I did a 'kal -s EGSM  -g 20' and got the following:
>
>         chan: 991 (928.4MHz + 26.986kHz)        power: 448020.99
>         chan: 1002 (930.6MHz - 5.631kHz)        power: 320357.07
>         chan: 1003 (930.8MHz + 16.192kHz)       power: 109489.92
>
> Then 'kal -c CHANNELNUM -g 20'  for each channel number.
>
> I then get the results below which the average frequency error looks
> different for each channel. It is consistent though if I run them multiple
> times.
>
> I would have expected less frequency error since I know that it is really
> only
> about 4Khz off. Or am I not reading the results correctly?
>
>
>
> Results seem OK. Do not forget that most BSTs hop around in their allotted
> 20 Khz channel.
>
> So a result of +/- 20 Khz shouldn't surprise you. Best calibrate against
> "anchor" BSTs that are steady.
>
> And yes, recommendation is to calibrate against 3 BSTs and correct for the
> average or stddev if you wish...
>
> Nikos
>
>
>
> -Rick
>
> Using device 0: Generic RTL2832U OEM
> Found Fitipower FC0013 tuner
> Exact sample rate is: 270833.002142 Hz
> Setting gain: 100.0 dB
> kal: Calculating clock frequency offset.
> Using E-GSM-900 channel 991 (928.4MHz)
> average         [min, max]      (range, stddev)
> + 21.342kHz             [17837, 27433]  (9596, 3402.206299)
> overruns: 0
> not found: 199
> average absolute error: -22.988 ppm
>
> Using device 0: Generic RTL2832U OEM
> Found Fitipower FC0013 tuner
> Exact sample rate is: 270833.002142 Hz
> Setting gain: 100.0 dB
> kal: Calculating clock frequency offset.
> Using E-GSM-900 channel 1002 (930.6MHz)
> average         [min, max]      (range, stddev)
> - 11.529kHz             [-15293, -5679] (9614, 3640.780273)
> overruns: 0
> not found: 194
> average absolute error: 12.389 ppm
>
> Using device 0: Generic RTL2832U OEM
> Found Fitipower FC0013 tuner
> Exact sample rate is: 270833.002142 Hz
> Setting gain: 100.0 dB
> kal: Calculating clock frequency offset.
> Using E-GSM-900 channel 1003 (930.8MHz)
> average         [min, max]      (range, stddev)
> + 14.283kHz             [9462, 19098]   (9636, 4777.044922)
> overruns: 0
> not found: 126
> average absolute error: -15.345 ppm
>
>
>
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