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(a)eyeonix.com>
Sent: 2014/1/31 19:13
To: "Ralph A. Schmid, dk5ras" <ralph(a)schmid.xxx>
Cc: "osmocom-sdr(a)lists.osmocom.org" <osmocom-sdr(a)lists.osmocom.org>rg>;
"Richard Koch" <n1gp(a)hotmail.com>
Subject: Re: kalibrate-rtl results...
On Fri, Jan 31, 2014 at 12:47 PM, Ralph A. Schmid, dk5ras <ralph(a)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(a)lists.osmocom.org [mailto:osmocom-sdr-bounces@lists.osmocom.org]
On Behalf Of Nikos Balkanas
Sent: Thursday, January 30, 2014 7:35 PM
To: Richard Koch
Cc: osmocom-sdr(a)lists.osmocom.org
Subject: Re: kalibrate-rtl results...
On Thu, Jan 30, 2014 at 8:21 PM, Richard Koch <n1gp(a)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