Modeling and analysis of interference between phase-locked loops

We study the influence of the interference between the integrated closed-loop circuits such as PLL circuits based on circuit simulation. Two different types of oscillators which are the ring and the LC-tank oscillators are applied to this analysis. We investigate the influence of the shared analog supply of two PLLs and the capacitive connection between the loop filter of the PLL and the digital circuit of the other PLL. In the circuit simulations, we see that the output of the VCO becomes unstable without any external noise due to these interconnections. From these results, we present the guideline for the design of the low phase noise against the interference between PLLs.