Sampling and Nyquist/Antialiasing filters

[Magnus Danielson]

>>>>> "DRB" == Duane R Balvage <dbalvage at ptdcs2.ra.intel.com> writes:

 DRB> Hello all -
 DRB> Is there anyone out there who has any basic guidelines on how many dB
 DRB> down "aliasing" frequencies have to be so they are not a problem?

 DRB> I'm retrofitting my Prophet-2000 sampler with 2 switched-cap
 DRB> 6-pole filters in series (12 poles, 72 dB/oct) for the antialiasing
 DRB> filter. (The antialiasing filters in it now are a bit, uhh, conservative
 DRB> to say the least) Right now they sit at:

 DRB> 41Khz sample rate : 16Khz corner freq.
 DRB> 32Khz sample rate : 12Khz corner freq.
 DRB> 24Khz sample rate : 9Khz  corner freq.

Ugh!

Is this the reason people have considered oversampling? Certainly.

 DRB> They are using 4 pole input filters (I think - have to check the schems)
 DRB> so the slopes are a bit gradual, taking off some of the "sparkle" of
 DRB> the samples because they have to have their corner frequency set low 
 DRB> enough to avoid aliasing. Any suggestions???

The benefit of 48 db extra per octave allows us to move the cutoff
upwards. I made some quick calculations and assuming that we require
the same damping at the nyquist point, we can move it to

41 kHz	18.85 kHz
32 kHz	14.4 kHz
12 kHz	10.6 kHz

This using a very crude method (basically using a Bode-plot fashion of
view and let the nyquist frequency be the common intersection point) .

Cheers,
Magnus