PLL Loop Filter Capacitor Switching to Minimize Supply Noise
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Solution Overview
Problem
Parasitic capacitance on the control input of a current source device in phase-locked loops introduces noise into the power supply, affecting the loop filter's signal and current to the current controlled oscillator, leading to instability in frequency and phase locking.
Innovation Solution
A circuit architecture incorporating a binary-weighted capacitor array and switch configurations that minimize parasitic capacitance by connecting switches to either the charge pump or the filter, but not both, reducing supply noise injection into the control input of the current source device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If switches are connected to both the charge pump and the filter in conventional loop filters, then the circuit can operate, but parasitic capacitance is introduced on the control input which injects supply noise and causes instability
Solution Approach 1:
The switch network is segmented into two separate groups: first switches connecting capacitors to the charge pump output, and second switches connecting capacitors to the filter output and control input. This segmentation prevents any single switch from being connected to both charge pump and filter simultaneously, thereby eliminating the parasitic capacitance path that causes supply noise injection.
Solution Approach 2:
The harmful parasitic capacitance effect is extracted and eliminated by removing the direct connection path between charge pump switches and filter output. The patent separates the charge pump connection path from the filter output connection path, taking out the source of the harmful effect while preserving the necessary filtering function.
2Measurement precision
If a binary-weighted capacitor array is used with multiple switches per capacitor, then filtering precision is improved, but the complexity of switch connections increases
Solution Approach 1:
The switch array is segmented into two functional groups with distinct connection roles. First switches handle charge pump connections, while second switches handle filter and control input connections. This segmentation simplifies the overall connection architecture by assigning specific functions to each switch group, reducing the complexity of managing all possible connections.
Solution Approach 2:
Each capacitor in the binary-weighted array is equipped with both first and second switches, allowing the same capacitor structure to serve dual purposes: receiving charge from the charge pump and delivering filtered output to the control input. This multi-functionality reduces the need for separate circuit paths and simplifies the overall architecture.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design reduces parasitic capacitance, minimizing supply noise impact on the oscillator frequency and maintaining stable frequency and phase lock between reference and feedback clocks, allowing the phase-locked loop to operate effectively over a wide range of frequencies.
Implementation Method 1
Parasitic capacitance on the control input of a current source device in phase-locked loops introduces noise into the power supply
Data Source
AI summary
A circuit includes a first filter, a plurality of binary-weighted capacitors, and a current source device. The circuit also includes a first plurality of switches. Each of the first plurality of switches is connected to a separate capacitor of the plurality of binary-weighted capacitors. The first plurality of switches are connected together, and the first plurality of switches are not connected to the first filter. A second plurality of switches is also included, and each of the second plurality of switches is connected to a separate capacitor of the plurality of binary-weighted capacitors and to the first filter and to a control input of the current source device. The first plurality of switches are not connected to the control input.