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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 30And 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). JMThe 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. _______________________________________________ Discuss-gnuradio mailing list address@hidden https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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