Stability Consideration of the Voltage Switched Charge-pump Phase Locked Loop using Linear Approach

Ehsan Ali ； Abdul Sattar Sand ； Farida Memon ； Ghulam Mustafa Bhuto ； Nasreen Nizamani

《Research Journal of Applied Sciences, Engineering and Technology》 13卷3期 (2016/08) Pp. 257-264

https://doi.org/10.19026/rjaset.13.2939

摘要

The aim of this study to provide possible criteria to consider the stability of Voltage Switched Charge- Pump Pump Phase Locked Loop (VSCP-PLL). Stability of the conventional CP-PLL architecture is often determined by Gardner's stability plane, when PLL system is locked and near the fixed point. In the latest work it has been shown that, this stability criterion is not conservative enough when considering initial condition far from the fixed point. However, Gardner's criterion is based on the PLL operating with a Current Switched Charge Pump (CSCP), which provides an ideal pump current during switching states. Nevertheless, a Voltage Switched Charge Pump (VSCP) is often used in many commercially available CP-PLL circuits (like 4046 family). The VSCP offers design simplicity but generates a peculiar effect related to the non-constant current flowing through the electrical load of Loop Filter (LF) w. r. t. operating point (average input voltage) of the Voltage Controlled Oscillator (VCO). This non-linearity may complicate the PLL characteristics making it difficult to analyze the stability of the system, since there is no any particular criterion developed to study the stability of a VSCP-PLL. In this study, a procedure is proposed for PLL designers to apply similar Gardner's stability criterion developed for the CSCPPLL to a VSCPPLL. The stability simulations are performed using an efficient discrete time Event-Driven (ED) model. The stability condition is observed for the initial operating conditions far from the fixed point by simulating the transient domain of the VSCP-PLL in the Gardner’s plane considering symmetrical and asymmetrical pump current.

關鍵字

Event driven technique ； linear modeling ； stability ； voltage switched charge pump PLL