FW: kalibrate-rtl results...

Ralph A. Schmid, dk5ras ralph at schmid.xxx
Fri Jan 31 11:29:54 UTC 2014


 

I am sure, a BCCH is a BCCH, this does not hop at all, and the frequency
hopping carriers do not hop some KHz, but in a sequence over a fixed scheme
of normal ARFCNs. There is no free frequency choice in GSM, hopping or not.

 

Ralph.

 

 

From: Nikos Balkanas [mailto:nikos.balkanas at eyeonix.com] 
Sent: Friday, January 31, 2014 12:01 PM
To: Ralph A. Schmid, dk5ras
Cc: Richard Koch; osmocom-sdr at lists.osmocom.org
Subject: Re: kalibrate-rtl results...

 

 

 

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
Sent: 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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