Measuring the Loop BandwidthApplied Radio Labs www.radiolab.com.au 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……