Consistently 10kHz off when tuning

[irl]

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,

[ha7ilm]

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 (...)

[ha7ilm]

Now we have a ppm option in config_webrx.py.

[nickoe]

@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.

[ha7ilm]

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.