Trouble calibrating new RTL-SDR stick

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richardb
Posts: 1
Joined: Tue Jun 05, 2018 1:52 pm

Trouble calibrating new RTL-SDR stick

Post by richardb » Tue Jun 05, 2018 1:53 pm

Morning all, I am running through the process to calibrate a new RTL-SDR USB stick, and am having some issues. I am wondering if I might have a faulty stick. Here's what I have done.

At the moment, the stick is connected to a Raspi 3 running the latest version of the satnogs client. I have installed kalibrate, and a scan of the GSM900 band found a base station on channel 26. When I run the kalibrate program, I get this:

[email protected]:~/kalibrate-rtl $ src/kal -c 26
Found 1 device(s):
0: Generic RTL2832U OEM

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Exact sample rate is: 270833.002142 Hz
[R82XX] PLL not locked!
kal: Calculating clock frequency offset.
Using GSM-900 channel 26 (940.2MHz)
Tuned to 940.200000MHz (reported tuner error: 0Hz)
average [min, max] (range, stddev)
- 4.538kHz [-6783, -3469] (3314, 1324.203979)
overruns: 0
not found: 1281
average absolute error: 4.827 ppm

That's a huge standard deviation. So, I run it again and get this:

[email protected]:~/kalibrate-rtl $ src/kal -c 26
Found 1 device(s):
0: Generic RTL2832U OEM

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Exact sample rate is: 270833.002142 Hz
[R82XX] PLL not locked!
kal: Calculating clock frequency offset.
Using GSM-900 channel 26 (940.2MHz)
Tuned to 940.200000MHz (reported tuner error: 0Hz)
average [min, max] (range, stddev)
- 4.911kHz [-6818, -3483] (3335, 1309.166382)
overruns: 0
not found: 1124
average absolute error: 5.224 ppm

To my eye, this looks like there is something odd going on in here, with the RTL stick failing to lock onto the frequency properly. This is using the dipole antenna that came with the RTL-SDR stick, with the short antenna bits set for that frequency band.

Any thoughts?

rtlsdrblog
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Posts: 1731
Joined: Mon Nov 19, 2012 11:54 pm

Re: Trouble calibrating new RTL-SDR stick

Post by rtlsdrblog » Wed Jun 06, 2018 1:01 am

PLL not locked is not an issue, you'll normally see that at any frequency as the PLL tries to lock on a few frequencies before finding one that it can lock on.

Kalibrate doesn't seem to always work properly. If possible i'd recommend just manually calibrating it on a PC with a known signal on a program like SDR#. If it's a TCXO dongle the offset will usually be 0-2PPM anyway.

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