Engaging the multiplier in the reference path allows more flexibility in setting the PFD frequency. One example use case of this is if Fvco % Fpd is the dominant spur. This method can move the PFD frequency and thus the Fvco % Fpd.

Example: Fvco = 3720.12 MHz, Fosc = 300 MHz, Pre-R divider = 5, Fpd = 60 MHz, Fvco%Fosc = 120.12 MHz (Far out), Fvco%Fpd = 120 kHz (dominant). There is a Fvco%Fpd spur at 120 kHz (refer to Figure 8-5).

Figure 8-5 Fvco % Fpd Spur

Then second case, using divider and multiplier, is Fpd = 53.57 MHz away from 120-kHz spur. Fvco = 3720.12MHz, Fosc = 300MHz, Pre-R divider = 7, Multiplier = 5, Post-R divider = 4, Fpd = 53.57 MHz, Fvco%Fosc = 120.12 MHz (Far out). Fvco % Fpd = 23.79 MHz (far out). There is a 20–dB reduction for the Fvco % Fpd spur at 120 kHz (refer to Figure 8-6).

Figure 8-6 Moving Away From Fvco % Fpd Spur