I don't think you are grasping the idea of the low pass filter.  It is to r=
eject the carrier, not just the aliasing.  You want the modulated signal wi=
thout the carrier.  So the cutoff frequency of the filter would be above yo=
ur modulating signal frequency and below the carrier frequency.  Everything=
 else you decide will depend on the details of your problem. =20

BTW, you can do better if you first run the rectified signal through a peak=
 pick algorithm with a time window of about one cycle of your carrier.  The=
n you will have a data set to filter that has already been significantly sm=
oothed.  In fact, you can skip the rectification and just use the peak dete=
ctor instead.=20

Rick