[Discuss-gnuradio] R820T frequency tuning issue

As briefly mentioned on the #gnuradio IRC mailing list, I am facing an issue with tuning a R820T
DVB-T dongle at the GPS carrier frequency. I had qualified the various dongles at 1.5 GHz, and had
not considered that the conclusions from this measurement would not apply at 1.57 GHz. This issue
only exists when using a R820T and *not* with a E4000 dongle (which works fine at all mentioned
frequencies, and whose datasets allow for processing GPS signals).

I have completed the following experiment:
* a frequency synthesizer (Rohde & Schwartz SMA100) is set to 1570 MHz, output power of -10 dBm,
lab-made antenna (ie, dangling wire)
* R820T DVB-T located next to the synthesizer
* either running rtl_sdr -f 1570000000 -n 100000 file.bin and plotting the resulting log in GNU/Octave
(f=fopen('file.bin');d=fread(f,inf,'uchar');plot(abs(fftshift(fft(d))))) ...
* or in a simple gnuradio-companion flowgraph a gr-osmosdr source connected to the WX GUI FFT sink,
in both cases sampling at 2048 kHz

In the latter case I cannot get any signal, hinting at an unlocked or misconfigured PLL frequency. In the
former case (using rtl_sdr), I get for example http://jmfriedt.sequanux.org/rtlsdr.png in which the
carrier was shifted by 100 kHz steps below 1.57 GHz when recording each curve.

Unfortunately I don't know how to tackle the issue other than by filing this bug report.
gnuradio and gnuradio-companion are 3.7.3
gr-osmosdr is v0.1.1-1-g37b09bc5 (0.1.2git)

The receiver is R820T+RTL2832U, sampling at 2048 kHz.

Thanks, JM

--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 Besancon, France

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