Using LT SPICE to Simulate Phase Noise and Small Signal Performance of Phase Lock Loops

Simulating phase noise performance of PLL’s can be a challenge. Many of the manufactures of “single chip” PLLs provide proprietary software for phase noise simulation. However, for more complicated loops that use separate phase detectors, dividers and VCOs, another method needs to be used. Mathematical software such as MathCad or Matlab can be used, if all the relevant transfer functions are known. Using the LT SPICE resistor noise and diode noise models, the noise performance of VCOs, dividers and phase detectors can be modeled. LT SPICE automatically includes all the resistor noise sources in PLL and it is easy to include op-amp noise sources. These concepts will be illustrated with real world examples and the results compared to the mathematical model, the results using Keysight’s Advanced Design System and the measured results.