How High Can You Make UGF of Your PLL? (Relative to f_REF)
“The key to science is not the answers, it’s the questions we ask.” - RPF Great question — this is a subtle but very important point in sampled-data feedback systems like digital PLLs. Background In a continuous-time loop, you can safely push the unity-gain bandwidth (UGBW) to a reasonable fraction of the loop's “available bandwidth” without worrying about aliasing or sampling. But in a sampled-data loop (like one with a PFD + divider running at reference frequency f REF ), the loop transfer function is effectively sampled at f REF . That means the Nyquist frequency of the sampled loop is only: f N = f REF / 2 The phase of the open-loop transfer function has to be evaluated not in continuous-time only, but also under the constraint that the digital updates occur every T REF = 1 / f REF . Quantitative Effect on Phase Margin The main penalty comes from extra delay equivalent to half a sampling period. This delay manifests as extra phase lag in the open-loop ...
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