Hey everyone, I recently started playing around with RTL-dongles and the OsmocomSDR project.
I read (on this list) about people mentioning a second FIR anti-aliasing filter being included in the RTL2832U (besides the programmable symmetric FIR filter). >> 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. >> After sampling, the samples are filtered using a symmetric FIR >> filter (I reported how that filter is programmed around a year ago on >> this list), then resampled to four times the user ADC. At that stage, a >> second FIR anti-alias filter which seems ot be fixed (or running on >> default coefficients all the time) kicks in before down-sampling to the >> requested sample rate. >> 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. I am wondering if somebody has more detailed information about this. Are the coefficients really fixed or is there any way to (possibly) modify/change them? Where exactly does this filter come into play? Or can you possibly think of a way to "bypass" this filter? Thanks in advance for any help.
