The basic function of sigma delta modulator is to feedback the noise and from the output to the input this will make the noise higher but it is pushed outside of the signal bandwidth. This will reduce the noise in the signal!!
"how Delta-Sigma mechanism can move the spur noise to high frequency and lower the spur noise power "
To iemotions
The Delta-Sigma modulator has the transfer function as below:
Y(z)=X(z)-N(z)*(1-z^-1)^n
for noise N(z), it is high pass
for signal X(z), it is all pass
The qutization noise from Divider of the fractional N PLL is high passed by delta sigma modulator.
so the low frequency spurs noise is supressed and moved to high frequency.