Cascaded pll for reducing low-frequency drift in holdover mode

Abstract

A cascaded phase-locked loop (PLL) clock generation technique reduces frequency drift of a low-jitter clock signal in a holdover mode. An apparatus includes a first PLL circuit configured to generate a control signal based on a first clock signal and a first divider value. The apparatus includes a second PLL circuit configured to generate the first clock signal based on a low-jitter clock signal and a second divider value. The apparatus includes a third PLL circuit configured to generate the second divider value based on the first clock signal, a third divider value, and a second clock signal. The low-jitter clock signal may have a greater temperature dependence than the second clock signal and the second clock signal may have a higher jitter than the low-jitter clock signal.

Description

BACKGROUND[0001]1. Field of the Invention[0002]This invention relates to integrated circuits and more particularly to clock generator integrated circuits.[0003]2. Description of the Related Art[0004]High speed communication systems require high speed clock signals for transmission and reception of information. In a typical clock generation circuit, a phase-locked loop (PLL) receives a synchronization input reference clock signal and generates one or more high speed clock signals suitable for use in transmitting or receiving data. According to one aspect of such a communication system, when the synchronization input reference clock signal is lost, the system enters a mode known as “holdover” mode and continues to output a clock signal.[0005]The accuracy with which the clock generation circuit provides the clock signal in holdover mode is typically specified in a system standard. The clock generation circuit tries to maintain the output clock signal during holdover mode at a frequency...

AI Technical Summary

Benefits of technology

This patent describes a technique for generating a stable clock signal with low jitter during a holdover mode. The technique involves using multiple phase-locked loops (PLLs) and divider values to reduce frequency drift. The accuracy of the clock signal is improved by using a low-jitter clock signal with higher temperature dependence and a higher jitter second clock signal. This allows for a smoother transition into the holdover mode while maintaining a stable clock signal.

Problems solved by technology

However, the frequency of the clock signal generated in holdover mode may still drift to such an extent as to fail to meet the holdover requirements.

For example, certain PLLs may fail to meet holdover requirements because the voltage controlled oscillators utilized in such systems have too much frequency variation over temperature (e.g., 100 ppm / ° C.).

Images