# R820T frequency tuning issue **URL:** \ **Category:** GNU Radio\ **Created:** [February 27, 2015, 12:28pm UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369 "2015-02-27T12:28:52Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [February 27, 2015, 12:28pm UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369/1 "2015-02-27T12:28:52Z")
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](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 ---
**Author:** ![jmfriedt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7feea3/32.png) [@jmfriedt](https://www.ruby-forum.com/u/jmfriedt)\ **Post date:** [February 27, 2015, 1:26pm UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369/2 "2015-02-27T13:26:20Z")
> As briefly mentioned on the #gnuradio IRC mailing list, I am facing an issue > with tuning a R820T > ^^^^^^^^^^^^ > channel […] > 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))))) … > ^ > d=d(1:2:end)+i\*d(2:2:end) sorry for the mistakes. JM – JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France ---
**Author:** ![jmfriedt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7feea3/32.png) [@jmfriedt](https://www.ruby-forum.com/u/jmfriedt)\ **Post date:** [March 28, 2015, 3:26pm UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369/3 "2015-03-28T15:26:33Z")
As a quick followup to my previous post, I confirm that the R820T is well suited for GPS signal decoding. I was not expecting the huge frequency offset (\> 100 kHz at 1.57 GHz) and was not searching far enough from the expected carrier frequency during the acquisition phase. However these results were acquired by runnig rtl\_sdr -s … -f … while gnuradio-companion running gr-osmosdr is unable to lock at 1.57 GHz and barely locks (for a few seconds !) at 1.56 GHz. I have analyzed the various version of librtlsdr (used by rtl\_sdr and by gr-oscmocom) as well as the set\_frequency functions and am unable to track any difference between the two codes (so far). JM – JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France ---
**Author:** ![jmfriedt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7feea3/32.png) [@jmfriedt](https://www.ruby-forum.com/u/jmfriedt)\ **Post date:** [March 28, 2015, 3:49pm UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369/4 "2015-03-28T15:49:35Z")
On 03/28/2015 10:23 AM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > The R820T is generally flaky above about 1550MHz. Maybe in your rtl\_sdr tests, you just got lucky. gr-osmosdr uses the _SAME_ underlying library as rtl\_sdr, and at the level of Gnu Radio, all of the low-level stuff like “is the PLL locked” is utterly invisible to the Gnu Radio layer. ---
**Author:** ![jmfriedt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7feea3/32.png) [@jmfriedt](https://www.ruby-forum.com/u/jmfriedt)\ **Post date:** [March 29, 2015, 10:45am UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369/5 "2015-03-29T10:45:06Z")
On 03/28/2015 11:29 AM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Sorry for the trouble, will make sure now the issue is indeed solved, and let > the measurement run until > monday morning. > > Thanks, JM Your results are consistent with what I’ve heard about the R820T above 1500MHz or so. It starts to become “deaf”, and then as you move higher, the PLL won’t lock at all. ---
**Author:** ![jmfriedt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7feea3/32.png) [@jmfriedt](https://www.ruby-forum.com/u/jmfriedt)\ **Post date:** [March 29, 2015, 10:14am UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369/6 "2015-03-29T10:14:36Z")
On 03/28/2015 10:56 AM, jmfriedt wrote: > ([Making sure you're not a bot!](http://sdr.osmocom.org/trac/wiki/rtl-sdr)) when the datasheet mentions a 1 GHz > upper limit. > > I should mention that the above statement has been tested on a random sampling > of all 10 dongles > I have bought for teaching, and conclusions have held reliably whichever > peripheral was used > (all bought from the same supplier, probably same batch, all running the same > R820T frontend, all > with the same very poor 60-80 ppm offset). > > Thanks for the reply, JM > So, if you just use something like osmocom\_fft -f 1575.420e6 -g 30 And feed it a low-level signal-generator CW signal, you’re saying that it doesn’t work? ---
**Author:** ![jmfriedt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7feea3/32.png) [@jmfriedt](https://www.ruby-forum.com/u/jmfriedt)\ **Post date:** [March 29, 2015, 10:52am UTC](https://www.ruby-forum.com/t/r820t-frequency-tuning-issue/234369/7 "2015-03-29T10:52:59Z")
> Your results are consistent with what I’ve heard about the R820T above > 1500MHz or so. It starts to become > “deaf”, and then as you move higher, the PLL won’t lock at all. I loose 10 dB sensitivity from 1.50 GHz to 1.575 GHz ! At 1.5 GHz the R820T was more sensitive than the E4k, but now I’ll have to review this statement at 1.57 GHz ! Thanks for the help, JM – JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France