Switchable PLL Loop Filter for Locking and Phase Tracking
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Solution Overview
Problem
Phase locked loop (PLL) circuits face a dilemma in balancing locking and tracking performance due to the conflicting characteristics of Type 1 and Type 2 PLL designs, where Type 1 PLLs excel in tracking but struggle with initial locking, and Type 2 PLLs excel in locking but poorly track phase changes.
Innovation Solution
A PLL circuit with a switchable loop filter that can operate in both locking and tracking modes by incorporating a switch and operational amplifier, allowing the circuit to initially lock onto a reference signal like a Type 2 PLL and then switch to Type 1 PLL characteristics for improved tracking performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Type 2 PLL design is used, then locking performance is improved, but tracking performance deteriorates
Solution Approach 1:
The loop filter is configured to dynamically switch between Type 1 and Type 2 PLL modes based on operational requirements. During initial acquisition, the filter operates in Type 2 mode for rapid locking. Once locked, it transitions to Type 1 mode for precise phase tracking, making the system adaptable rather than static.
Solution Approach 2:
The invention changes the electrical parameters of the loop filter by switching between different circuit configurations. The switch reconfigures the feedback network to alter the loop filter transfer function, effectively changing the PLL type and its associated performance characteristics without replacing the entire filter circuit.
2Measurement precision
If a Type 1 PLL design is used, then tracking performance is improved, but locking performance deteriorates
Solution Approach 1:
The system performs preliminary locking using Type 2 PLL characteristics before transitioning to Type 1 characteristics for tracking. The preliminary action of establishing lock with Type 2 ensures that when the system switches to Type 1 mode, the VCO is already close to the reference frequency, making tracking more effective.
Solution Approach 2:
The loop filter dynamically reconfigures its circuit topology using a switch element. During locking phase, the filter is configured for Type 2 operation with higher bandwidth for fast acquisition. During tracking phase, it reconfigures to Type 1 operation with lower bandwidth for reduced group delay variation and better phase accuracy.
Data Source
AI summary
An embodiment pertains to a phase locked loop (PLL) circuit. The PLL includes a voltage controlled oscillator which outputs a signal at a desired frequency. A phase detector is coupled to an output from the voltage controlled oscillator. The phase detector compares the phase of a signal output from the voltage controlled oscillator (VCO) with the phase of a reference signal. A loop filter is coupled to the VCO and the phase detector. The loop filter has a locking mode of operation for locking the phase of the VCO signal to the phase of the reference signal. The loop filter can subsequently be placed in a tracking mode of operation which adjusts the phase of the VCO signal to track the phase of the reference signal.