PLL Soft Reference Switching for Hitless Clock Changeover
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Solution Overview
Problem
Traditional phase locked loop (PLL) reference switching methods experience phase jumps and frequency variations due to unknown initial phase offsets and noise in reference sources, leading to discontinuous transitions when switching between references.
Innovation Solution
A soft switch scheme is implemented, where phase error estimation between the local digital controlled oscillator (DCO) and the new reference clock occurs before switching, using weighted combinations of reference clock information based on stability statistics to adjust the DCO, ensuring a gradual and hitless reference changeover.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If traditional instantaneous reference switching is used, then switching speed is fast, but phase jumps and frequency variations occur causing discontinuous transitions
Solution Approach 1:
The system performs preliminary phase error estimation between the local DCO and the new reference clock before switching occurs. This advance preparation allows the system to calculate the necessary phase correction in advance, eliminating phase jumps during the actual transition. The phase error is estimated using the formula: phase_error = arctan2(Q_new - Q_old, I_new - I_old), where I and Q are the in-phase and quadrature components of the reference signals.
Solution Approach 2:
The invention implements dynamic reference switching by gradually transitioning from one reference clock to another rather than instantaneous switching. The phase correction is applied dynamically during the transition period, allowing the DCO to smoothly follow the phase changes. This dynamic approach maintains continuous phase and frequency alignment throughout the switching process, preventing discontinuities while maintaining fast effective switching.
2Measurement precision
If phase correction is made during switching operation, then phase alignment is improved, but the actual phase cannot be known exactly due to noise in reference sources
Solution Approach 1:
The system continuously tracks the phase relationship between the DCO and both reference clocks before and during switching. By maintaining continuous phase monitoring and using the continuous phase error estimation formula, the system can accurately determine the phase offset even in the presence of noise. The continuous action of phase detection and correction ensures that phase alignment is maintained throughout the transition without interruption.
Solution Approach 2:
The invention employs feedback mechanisms where the phase error signal is continuously fed back to adjust the DCO frequency. The phase detector generates a phase error signal that is filtered and used to control the DCO, creating a closed-loop system. This feedback approach allows the system to automatically compensate for phase deviations caused by noise and maintain accurate phase alignment during reference switching.
3Stability of the object's composition
If low-pass filter memory of phase error is retained, then filtering performance is maintained, but it affects the transition during switching operation
Solution Approach 1:
The system performs preliminary phase error estimation and calculation before the actual reference switching occurs. By determining the required phase correction in advance using the continuous phase error formula, the system can prepare the correct correction value to be applied immediately upon switching. This preliminary action prevents the low-pass filter memory from causing transition issues, as the correction is already calculated and ready to override any residual filter effects.
Solution Approach 2:
The invention applies preliminary phase correction that counteracts the potential adverse effects of the low-pass filter memory. By calculating the exact phase error and applying the appropriate correction before switching, the system preemptively neutralizes the harmful effect of filter memory on the transition. This preliminary anti-action ensures that the filter's memory effect does not degrade the switching performance.
Data Source
AI summary
A phase locked loop includes a digital controlled oscillator and a number of phase detectors, each having a first input connected to a reference source and a second input coupled to the output of the digital controlled oscillator, and an output for producing a phase error signal. A loop filter coupled to the output of each phase detector has an output and a feedback input. An adjustment unit for derives an adjustment signal for the digital controlled oscillator from one or more of the loop filters by selecting or combining output signals from the loop filters taking into account the stability of said reference sources. The adjustment signal for the digital controlled oscillator produced by the adjustment unit is coupled to each of the feedback inputs of the loop filters. This arrangement results in hitless reference switching.