Fast and accurate time-domain simulations of integer-N PLLs

We present a methodology to simulate industrial integer-N phase-locked loops (PLLs) at a verification level, as accurate as and faster than transistor-level simulation. The accuracy is measured on the PLL factors of interest, i.e., locking time, power consumption, phase noise and jitter (period and long-term). The speedup factor tends to the division ratio N for device-noise simulations. We develop a unifying technique which is able to deal with both noise-free and device-noise analyses, taking into account nonlinear and second-order effects visible at transistor-level simulation only, whereas previous works focused on one of the two analyses, separately. The procedure is based on oscillator's sensitivity analysis and on the creation of a phase macromodel for the voltage-controlled oscillator (VCO) together with the loop divider (the phase model is called VCODIV), whilst the other PLL's blocks remain at transistor level. The macromodel's phase law is characterized by a piecewise linear curve, representing the sensitivity of the VCODIV output's phase deviation with respect to the voltage variation of the VCO's control pin, and by the effects of all the VCO's and divider's noise sources on the model's output. We show two experiments on industrial PLLs, and provide guidelines for designers which highlight the steps needed to implement the methodology by using well-known analyses in circuit simulation and Verilog-A for the creation of the macromodel.