PLL
There Third-order is still one residual problem that we have overlooked. The phase detector produces pulses of variable width that activate the switches to either charge or discharge the capacitor CP in the case of the charge pump PFD-CP combination. Now that we have added the resistor RP, which is absolutely necessary for stability, we find that the control voltage coming out of the charge pump will jump up or down before settling to its steady state value. This occurs because you cannot change the voltage across a capacitor instantaneously, so the initial voltage drop occurs across RP, which then charges CP exponentially. This jumpy control voltage frequency modulates the VCO at the reference frequency, creating reference spurs. This is not such a big problem if N = 1 because the jump will be at the same frequency as the VCO. But, at larger N values, it creates low frequency jitter producing FM sidebands.
in(t) out(t) VCO
Phase detector
Loop Filter F(s)
1/N
out Frequency spectrum of output. Reference spur sidebands are spaced at intervals of in. So, we need to fix this by adding a second capacitor, C2, whose function is to filter out the jumpy response of the series RC network. The magnitude of the reference spur sidebands is reduced. Unfortunately, however, C2 adds a third pole of finite frequency that will reduce the stability of the PLL. Now, the handy tools we have been using for predicting performance of the second-order PLL no longer are accur...
TOPLL