Am Sonntag, den 15.07.2012, 01:22 +0200 schrieb Francesco Gugliuzza:
2012/7/14 Michael Karcher
<osmosdr(a)mkarcher.dialup.fu-berlin.de>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