PLL Charge Pump Attenuation Circuit for Lower In-Band Noise
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Solution Overview
Problem
Conventional charge pump circuits in phase locked loops (PLLs) introduce undesirable in-band noise, limiting their effectiveness in generating stable frequencies over a wide band of frequencies.
Innovation Solution
The proposed solution involves a charge pump circuit with a phase frequency detector, an attenuation circuit, and a loop filter, which includes current sources and capacitors configured to adjust the voltage at the VCO control node based on phase differences between input and feedback signals, reducing noise through capacitive division and increased current sources, thereby reducing thermal noise and amplifier noise feedthrough.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a conventional charge pump circuit is employed to generate control signals for wide band frequency operation, then the phase locked loop can track frequencies over a wide band, but the charge pump circuit introduces undesirable in-band noise
Solution Approach 1:
The charge pump circuit is segmented into multiple parallel pump circuits, each handling a portion of the frequency range. This segmentation allows each pump to operate at lower noise levels while collectively covering the wide frequency band, resolving the contradiction between wide frequency tracking and noise generation
Solution Approach 2:
The system dynamically switches between different charge pump circuits based on the desired output frequency. By selecting the appropriate pump circuit for the current frequency range, the system maintains wide band adaptability while minimizing noise through optimal pump selection rather than continuous operation of a single noisy pump
2Power
If the charge pump circuit current is increased to improve loop gain, then the loop gain is enhanced, but the thermal noise from the current sources increases
Solution Approach 1:
The total loop gain requirement is distributed across multiple parallel charge pump circuits. Each pump operates at a lower current level than a single pump would need to provide the same total gain, thereby achieving the required loop gain while reducing thermal noise since noise scales with the square root of current rather than linearly
Solution Approach 2:
Multiple charge pump circuits are merged in parallel to achieve the cumulative loop gain effect. The combined gain of multiple low-current pumps equals or exceeds that of a single high-current pump, while the noise from multiple lower-current sources is lower than that of one high-current source
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 output noise by a factor of 1/A, enhances loop gain, and suppresses in-band phase noise, improving the stability and frequency tracking capabilities of the PLL.
Implementation Method 1
reducing noise through capacitive division and increased current sources
Implementation Method 2
reducing thermal noise and amplifier noise feedthrough
Data Source
AI summary
A phase-locked-loop includes a phase-frequency-detector (PFD) comparing phases of an input signal and feedback signal, and generating therefrom control signals. An attenuation circuit in series with the PFD includes a filter between a voltage-controlled-oscillator (VCO) control node and ground. A buffer is coupled to the VCO control node. An impedance network is coupled to the VCO control node and has an impedance element coupled to a first current source so voltage at the VCO control node increases when control signals indicate the phase of the input signal leads the feedback signal, and coupled to a second current source so voltage at the VCO control node decreases when control signals indicate a lagging phase. A VCO is coupled to the VCO control node to generate an output signal, with the phase of the output signal matching the input signal. The feedback signal is based upon the output signal.