Measuring the Loop Bandwidth

Measuring the Loop BandwidthApplied Radio Labs
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Measuring the Loop Bandwidth of a PLL
Design File: DN003 15 October 1999

Introduction
Many modern communication systems utilise PLL frequency synthesizers. The PLL is a feedback loop, and the frequency domain characteristics of the loop transfer function (gain and phase response) have significant effect on the PLL operation, for example: § § § Locking times (e.g. overshoot and ringing for poor phase margin) Phase noise ‘ears’ if phase margin is poor Frequency response of modulation / demodulation
Link 1 VCO

F out Main Divider 1/N Kv/s F(s) Phase Detector Kd 1/M Reference Divider F ref

Loop Filter

Figure 1 - PLL Synthesizer

Measuring the Loop Bandwidth
For the PLL in Figure 1, we insert a low-pass filter in place of Link 1. A suitable filter is t