Signal aliasing

hello all,
i used A2D module with timer2 interrupt, i set T2CON to 1B -1:2 postscale-// - 1:16 prescale - it must give 612 Hz (if my calculation is correct)
but the signal i get in my computer have aliasing from 60Hz and higher (mean that frequency sampling is less than 2*fs) ,, so what the problem guys
help me please .
thanks

I would check your actual sampling rate - toggle an I/O pin after each reading and check the frequency with a scope.

What is your oscillator frequency?

And, I second Nigel's suggestion. Performing this kind of "reality check" is ALWAYS a good idea with PICs if you aren't running simulations and/or in-circuit debugging to verify the exact timings of those types of things. It's all too common to spend hours of changes and failed attempts, only to find it was something simple like your interrupt rate was wrong.