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Re: [Discuss-gnuradio] R820T frequency tuning issue

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] R820T frequency tuning issue
Date: Sat, 28 Mar 2015 11:06:59 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

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 
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).


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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