PLL Loop Filter Buffer With Level Shifting for VCO Tuning Range
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Solution Overview
Problem
Current phase-locked loop (PLL) circuits lack a loop filter that includes a standard integrator, pole zero, and RLC post-filter, and do not have a buffer to isolate the integrator from the impedance of the post-filter, limiting the tuning voltage range and efficiency of the charge pump's output to the voltage-controlled oscillator (VCO).
Innovation Solution
A phase-locked loop circuit with a loop filter in series with the phase detector and charge-pump, featuring an integrator, pole zero, and RLC post-filter, along with a buffer that isolates the integrator from the post-filter impedance, and a multiplexer for selecting between N-type and P-type source followers for level shifting, extending the tuning voltage range by approximately ±0.5 volts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a loop filter with integrator and post-filter is added to extend tuning voltage range, then the VCO tuning range is improved, but the circuit complexity increases
Solution Approach 1:
The loop filter is divided into distinct functional segments: an integrator section for DC component generation, a pole-zero section for frequency response control, and an RLC post-filter for signal conditioning. This segmentation allows each section to be optimized independently while contributing to the overall extended tuning range capability.
Solution Approach 2:
A buffer circuit is introduced as an intermediary between the integrator and the RLC post-filter. This buffer isolates the integrator from the impedance effects of the post-filter, preventing loading effects that would otherwise limit the tuning voltage range while maintaining circuit functionality.
2Stability of the object's composition
If a buffer is inserted to isolate the integrator from post-filter impedance, then the tuning voltage stability is improved, but the device complexity increases
Solution Approach 1:
The buffer acts as an electrical intermediary that provides high input impedance to the integrator output and low output impedance to the post-filter input. This isolation prevents the post-filter impedance from affecting the integrator's voltage stability while maintaining signal integrity throughout the loop filter chain.
3Adaptability or versatility
If level shifting is applied to extend charge pump output range, then the VCO tuning range is improved, but the circuit complexity increases
Solution Approach 1:
Level shifting is achieved by changing the DC bias parameters of the charge pump output through the loop filter configuration. The integrator and pole-zero sections work together to adjust the voltage levels, effectively extending the charge pump's output range without requiring additional active level-shifting components.
Data Source
AI summary
A system in accordance with the present invention may include a phase-locked loop circuit, comprising a first input signal oscillating at a reference frequency, a second input signal received from a voltage-controlled oscillator (VCO) after passing through an N-divider, a phase detector and charge-pump, the phase detector comparing a phase of the first input signal and a phase of the second input signal, a loop filter in series with the phase detector and charge-pump, the loop filter having an integrator, a pole zero, and a post-filter, and a buffer in parallel with the integrator and in series with the post-filter, the buffer receiving an output signal from the integrator and isolating the integrator from an input impedance of the post-filter, and the buffer having a multiplexer for selecting between a plus and minus level shift signal, wherein the VCO is in series with the loop filter and the N-divider, and the VCO is configured to receive a tuning voltage signal from the loop filter.