Re: [Discuss-gnuradio] R820T frequency tuning issue

[Marcus D. Leech]

On 03/28/2015 10:56 AM, jmfriedt wrote:
> well I have been *extremely* lucky then because I have been running 48 hours of 
> monitoring GPS
> signals by recording one second every 5 minutes 
> (http://jmfriedt.sequanux.org/gps.avi, X axis
> is the PRN number and Y axis is the frequency offset) and have not lost a 
> single dataset, while all my
> attempts for about a week at locking the R820T on a 1.57 GHz synthesizer (-35 
> dBm connected
> directly to the antenna input of the dongle) have yielded either total lack of 
> PLL locking
> or a PLL locking for a second at best. I suspect a subtle timing issue 
> (graphical display
> of the WxGUI FFT ?) of gr-companion wrt rtl_sdr but have unfortunately not been 
> able to
> locate the issue.
>
> I also wonder then why the osmosdr web page cites the R820T as operating up to 
> 1776 MHz
> (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?


> > > 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
> > 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.
> >
> >
> >
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