RTL2832U FIR filter

Michael Karcher osmosdr at mkarcher.dialup.fu-berlin.de
Sun Jul 15 07:16:20 UTC 2012


Am Sonntag, den 15.07.2012, 01:22 +0200 schrieb Francesco Gugliuzza:
> 2012/7/14 Michael Karcher <osmosdr at mkarcher.dialup.fu-berlin.de>:
> > I furthermore found out that this is not the only filtering going on in
> > the RTL2832 chip in SDR mode. There seems to be a second anti-aliasing
> > filter before the DAC, making signals disappear that are at frequencies
> > above 0.7 times the sample rate (in the baseband), even if the
> > programmable FIR filter lets them pass.
> That's probably an effect of the E4000 output filter. You can see the
> filter bandwidth being set in the tuner initalization code.

Good point, but actually, I am sure it is not an effect of the tuner
output filter. My USB stick contains a fc0012 tuner, which is always set
to 6MHz bandwidth. The bandwidth of the suspected anti-alias filter
changes clearly when I change the sample rate, though, on the other
hand, if I manipulate the FIR coefficients to contain sharp dips in the
range of hundred kHz, the dip position does not change when I change the
sample rate. This makes me cofident that the suspected anti-alias filter
has to be located in or after the resampling unit.

Regards,
  Michael Karcher






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