kalibrate-rtl results...

Jiao Xianjun putaoshu at gmail.com
Fri Jan 31 11:27:14 UTC 2014


Though GSM spec has frequency hopping option, it can be switched on or off by operator. Yes, most operator switch it on.

I think You too are talking different thing. GSM doesn't hops inside each 200khz channel. It hops among multiple 200khz channel.

There is an exception: The downlink broadcasting carrier which carries FCCH SCH BCCH etc doesn't hop, and accordingly uplink RACH carrier doesn't hop either. These two uplink and downlink carrier frequency have a fixed 45MHz spacing if I recall correctly.

I think most of calibration programs use downlink broadcasting carrier to calibrate the dongle, because it doesn't hop and is easy to  be captured.

You may also try my matlab  calibration tool:

https://github.com/JiaoXianjun/multi-rtl-sdr-calibration




-----Original Message-----
From: "Nikos Balkanas" <nikos.balkanas at eyeonix.com>
Sent: ‎2014/‎1/‎31 19:13
To: "Ralph A. Schmid, dk5ras" <ralph at schmid.xxx>
Cc: "osmocom-sdr at lists.osmocom.org" <osmocom-sdr at lists.osmocom.org>; "Richard Koch" <n1gp at hotmail.com>
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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