fractional spur

My sigma delta PLL has a inband fractional spur. After research around I found it may come from the noise folding from out of band sigma delta quantization noise. I read Muer's paper on that and still donot understand why non-linearity of the PFD and CP will cause this kind of leakage.

Can you help on clearing the concept and recommend paper on it?

Thanks.

The effect is very similar to intermodulation. So if you have a second order nonlinearity and two tones out of the PLL bandwidth then the intermodulation tone found at the difference frequency shows up within the PLL bandwidth. There is no specific difference to normal intermodulation. No magic number effect or something else mention.

Thanks RFsystem,

I am kind of agree your explaination if I treat the charge pump as an black box with nonlinearity in transfering Phase error --> I. So multi tones from input can end up with multi tones in output I.

But after I double check the PLL spectrum I find there is even spur leakage out of the bandwidth of the PLL(not reference spur but a fraction of the ref frequency). I cannot understand this point since the loop filter will filter out those intermodulation current tones beyond the loop filter bandwidth.

Is that mean loop filter only has limited rejection of high frequency inputs?

Thanks.

Hi RFsystem I have to bother you again. I donot get any reply from the post listed below.

I have a setup in ADS to simulate the folding effect as we discussed.

For the fractional PLL:
Fref=1.728 MHz
divider N=1023+501/(2^10)=1023.48925
loop bandwidth=20kHz
3rd order loop filter and sigma delta modulator

In PLL output phase noise spectrum,a fixed fractional spur is just outstanding there at 37.14KHz offset. I tried around that how to relate this 37.14KHz to Fref and fraction part of the divider N, but cannot see any relationship among them. It is obviously located out of band. But why?

This spur is bigger but locate in the same frequency if I increase the deadzone of the PFD. Unfortranately if I change the fraction of the divider my SDM output screw up.