Disabling RTL2832 AGC

Steve Markgraf Steve.Markgraf at student.hs-rm.de
Sat Jul 7 11:57:33 UTC 2012


Hi,

On 07.07.2012 13:42, Francesco Gugliuzza wrote:
> Hi Leif! Nice work, and I'm glad to hear my post inspired you, even 
> if
> my code didn't work at all!

Your code was changing settings of the RF- and IF-AGC, which do nothing
more than modulating the AGC_RF and AGC_IF outputs of the RTL2832, 
which
are connected to the GAIN0/GAIN1 inputs of the E4000. But since we're
using the serial interface gain control mode, enabling the RF/IF-AGC
won't have any effect, because the GAIN0/1 inputs are ignored.

> I'm now setting 0x05 (0000 0101) instead of the original 0x25 (0010
> 0101) or your 0xd5 (1101 0101), to avoid setting unknown bits, and it
> seems to have the same effect (I see an about 10 dB noise floor drop
> when on minimum gain with no signal).

That's also what I observed, clearing bit 5 is enough to disable the 
AGC.
I also noticed that both DAGC gain registers (page 1, 0x12 and page 0,
0x17) have no effect once bit 5 in the settings register is cleared.
The control logic still seems to operate though, which can be observed
by reading register 0x05 in page 3.

Regards,
Steve




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