User requires data with bandwidth 1.0MHz, but needs to eliminate significant noise in the region 1..10MHz.
For practical reasons, the minimum possible output data rate is requested (2MSPS), that makes it essential to eliminate higher frequency noise that would otherwise alias.
It's not feasible to achieve this using traditional analog AAF
devices, since a very high order analog filter would be required, and this would likely bring problems with phase matching and inband distortion. However, a combination of moderate analog AAF
, oversampling, and high-order decimating digital FIR
filter can be deployed to meet the requirement.
The unfiltered square wave input shows the 3rd harmonic (1800kHz, reflected at 200kHz) as expected; in the FIR filtered case, the 3rd harmonic is in the stop band and is completely eliminated.