Okay, here's the deal:
1) Learn about coherent sampling.
Yes i do know that, i just didn't know the exact term of "coherent sampling" , please keep in mind that i mentioned that the system is behaving perfectly as expected when i simulate using 4096 points. I mean that i am doing it right from the DFT point of view & input frequency. However when i use 8192 points (and of course change the input frequency accordingly) I get these strange results.
Bottom Line is (keeping in mind the system design "ideally" should give 75 db):
* the system (ALL IDEAL ) using 4096 --> SNR = 72.4 db
* the system (ALL IDEAL ) using 8192--> SNR = 70.9 db
* the system (ALL IDEAL except for the com ) using 4096 --> SNR = 72.4 db
(it improved, maybe i have a minor error in the ideal comparator)
* the system (ALL IDEAL except for the com ) using 8192 --> SNR is variable !!!!!!!!! yes i mean it, running the simulation several times with ALL the conditions are the same changes the results (the last 2 in the original post is an example) & right now i ran it again , and i got this DIFFERENT result !! SNR = 40.4 (and i got a warning that the system is running out of memory while simulating, but it continued anyway !)
So, my question is ,, is that normal?! Could that be from that memory warning ? Should i trust the 4096 reading and go on with my design ??
Thanks for you help ..