Extended tuning range for R820T

Oliver Jowett oliver.jowett at gmail.com
Wed Aug 27 23:28:54 CEST 2014

Hi list

I have some experimental changes that extend the effective tuning
range of a RTL2832U+R820T somewhat:

$ ./rtl_test -t
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7
16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1
43.4 43.9 44.5 48.0 49.6
Sampling at 2048000 S/s.
Benchmarking R820T PLL...
rtl2832_set_if_freq(): 14700005 Hz out of range for downconverter
(divisor would be ffdf5555)
rtl2832_set_if_freq(): 14999902 Hz out of range for downconverter
(divisor would be ffdeaab9)
R820T range: 13 to 1864 MHz

You can get these changes from https://github.com/mutability/rtl-sdr/
(you'll need to build from source yourself). There should be no
application changes needed, just tune as normal. (gqrx needs the "no
limits" option turned on)

These changes work by limiting the tuner to a range of frequencies
that it can reliably tune to, then allowing tuning beyond those bounds
by making the 2832's downconverter do the final bit of tuning. This
can add up to 14.4MHz to each end of the range. Also, the tuner is
switched to low-side mixing at the top of the range which gives a bit
more range there. The practical range is limited by the width of the
IF filter and aliasing effects at the extreme edges of the
downconverter's range.

I've been able to pick up broadcast AM and amateur CW/SSB down to
around 15.5MHz without too much trouble.

I'd be interested to know how this works for others. Also.. these
changes are likely to have broken offset tuning, direct sampling mods,
and tuners other than the R820T, as it touches all those areas but I
only have an unmodified R820T to test against. If you have different
hardware and are willing to spend some time testing then please let me
know. I expect that the range of the other tuners can be extended in
the same way with not much trouble.


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