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Frequently Asked Question

Why do I want to run a fast PFD frequency?

The phase noise from the PFD, reference path, and feedback divider is reduced by running a faster PFD frequency. This can be seen from the PLL FOM (figure of merit) from the datasheet, which gives,

In band PN = FOM + 10log(Fpfd)+20logN-20log(DIV)

Where Fpfd is the PFD frequency and N is the N divider (feedback divider) setting (ratio of VCO frequency to PFD frequency), and DIV is the distribution divider setting. From this it can be seen that while increasing the PFD frequency 2X raises the in-band PN 3dB, the N will also reduce by a factor of 2, giving a net 3dB REDUCTION in the in-band phase noise. Therefore, for the same VCO frequency, every 2X increase in PFD frequency reduces the in-band phase noise 3dB.