cdj-200 loops

Chances are your loops aren't perfect, thus the loop comes out of synchronization with the other track.  That and the CDJ200's resolution is 74fps, meaning that if your loop doesn't hit perfectly on one of the frame markers, you'll be out of sync regardless.

Let's say you're playing a track at 128BPM, that's one beat every 0.46875 seconds.  A perfect four beat loop would thus take 1.875 seconds.  Well, if you chop that up into frames (of which, each is 0.0135135 seconds long), you get a loop that's 138.75 frames ... which you can't have, so either you're going to make it 138 frames (1.864865 seconds) or more likely, 139 frames (1.87837838 seconds).

As perfect as you can try and be, you'll still be out by at least 0.003s per loop cycle.

Thanks for your reply so fast.

Does that mean I can make a loop "perfect"?

Because as I say, seems I have to be with the calculator, calculating frames and seconds, but I guess you will not need ..

Regards and thanks ...

Please answer me if you can

In many cases, you can get it "close enough," but it may not be perfect.  In any situation you can simply use the jog to either pitch bend it a bit or edit the loop's out-point to make it shorter or longer depending on the need.