[Discuss-gnuradio] rtl-sdr weirdness at lower frequencies (below 27.675MHz)

[Marcus D. Leech]

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...