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From: | Marcus D. Leech |
Subject: | [Discuss-gnuradio] rtl-sdr weirdness at lower frequencies (below 27.675MHz) |
Date: | Thu, 07 Mar 2019 01:46:30 -0500 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
Tracked down a weird quirk in the rtl-sdr library this evening.I was having problems with lower frequencies working, but only at sample-rates >300e3. There's a piece of code that tries to optimize selection of IF frequency and filters based on sample-rate. For some sample-rates, a lower IF is used, presumably to give "headroom"
at the filter edges.The *problem* with this heuristic is that at lower input frequencies, the sum of IF + input-frequency is too low for the PLL to lock--seems like it runs into trouble below 27.675MHz, which means that you need to pick an IF that's high enough to guarantee that the PLL will lock.
In my tests, I modified the code to detect this case and arrange for a higher IF frequency. This seems to work OK, but I wonder if the maintainers of the rtl-sdr library (are there any anymore?) have considered this issue...
Now, this isn't strictly gnuradio related, but the likely people probably hang out here from time to time...
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