I'm interested in fractional-N synthesizers, I've built a couple for hobby projects, and I'm playing with software simulations to evaluate the Fourier series of (a) the MASH output, and (b) the waveform at the PFD output. Noise at the MASH output rises at 80 dB per decade for a 4-stage MASH. My simulation of the PFD output for a synthesizer using the same MASH shows a
20 dB per decade slope. I don't know if this is right. My question is: what should the noise at the PFD output look like, and why? Since the output of the MASH controls frequency via the PLL divide-by-N, and the PFD output is measuring phase, I was expecting them to be seperated by an integration i.e. 20 dB per decade, but it seems that either my simulation is wrong, or it's more complicated than that. Any pointers would be appreciated.TIA Andrew.
MASH:
- this is a 16-bit MASH, but I get the same slope for 24-bits.