Open irl opened 9 years ago
Most likely this issue is caused by the inaccuracy of the crystal oscillator on the RTL-SDR. It can be calibrated (for example, with your own reference signal source, or the kalibrate-rtl utility). If you know the PPM value, you can add it to config_webrx.py:
start_rtl_command="rtl_sdr -p PPM_value (...)
However, this inaccuracy may be dependent on actual temperature, that's why people are modding RTL-SDR sticks with TCXO-s.
How to calibrate with kalibrate-rtl:
~/Temp/kalibrate-rtl $ kal -s GSM900 -e 0
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
kal: Scanning for GSM-900 base stations.
GSM-900:
chan: 11 (937.2MHz - 8.013kHz) power: 208040.92
8013 / 937200000 * 1000000 = 8.55 round(8.55)=9
Then you write:
start_rtl_command="rtl_sdr -p 9 (...)
Now we have a ppm
option in config_webrx.py
.
@ha7ilm I think this issue can also be closed, since this is not strictly a problem with openwebrx and that you can now use the ppm option.
Yeah. I've started to write an article on the wiki about this problem: https://github.com/simonyiszk/openwebrx/wiki/Correcting-the-frequency-offset
I'll close the issue when I'll have finished it.
I've noticed that when tuning, the reported value is 10kHz lower than the value I'm transmitting on with the radio.
My config is:
DSP/RX settings
dsp_plugin="csdr" fft_fps=9 fft_size=8192 samp_rate = 2000000 center_freq = 145000000 rf_gain = 5
Thanks,