nco configuration in iq demodulator

Hi,

i've implemented an IQ demodulator as in figure

http://www.alteraforum.com/forum/attachment.php?attachmentid=12075&stc=1

In order to test it i used an NCO to put a 25KHz pwm signal (Fcarrier=50Hz) through analog active filtering channel and through A/D the signal is sampled @ 4KHz and submitted to iq receiver.

the NCO used for signal generation has the following features:

http://www.alteraforum.com/forum/attachment.php?attachmentid=12076&stc=1

the NCO used for iq demodulator has the following features:

http://www.alteraforum.com/forum/attachment.php?attachmentid=12077&stc=1

the I & Q components vary slownly (170mHz).

I suppose that there is not a real 50Hz signal so the phase always moves. Is it true?

The problem is in the transmitter or in the receiver?

Is there any way of generating 50Hz signal in order to avoid phase drift in time?

What parameters of NCO could i modify to improve signal precision?

Thanks