Low-Gain VCO with Current Mirror Filter for PLL Noise Reduction
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Solution Overview
Problem
High-gain voltage-controlled oscillators in integrated circuits suffer from high phase noise when implemented in phase locked loops, leading to increased jitter and significant chip area consumption due to large loop filter components.
Innovation Solution
A current mirror circuit is used to supply drive current to the voltage-controlled oscillator, coupled with a filter and tuning capacitors to reduce noise and gain, allowing for coarse and fine frequency control while minimizing the size and noise of the loop filter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-gain voltage-controlled oscillator is used, then oscillation frequency control is achieved, but phase noise increases and chip area increases
Solution Approach 1:
The VCO is divided into two independent control paths: a coarse control path using a current mirror circuit to adjust the main drive current, and a fine control path using tuning capacitors for precise frequency adjustment. This segmentation allows the majority of frequency control to be achieved through current adjustment while fine-tuning is done through capacitance, reducing the need for high gain and thereby reducing phase noise.
Solution Approach 2:
The invention changes the control parameter from voltage (which requires high gain) to current through the current mirror circuit. By controlling the drive current directly rather than using voltage gain, the system achieves frequency control with lower noise. The tuning capacitors provide additional parameter adjustment through capacitance variation.
2Measurement precision
If high-gain voltage-controlled oscillator is used, then oscillation frequency control is achieved, but chip area increases
Solution Approach 1:
The frequency control function is segmented between the current mirror circuit (coarse control) and tuning capacitors (fine control). This allows the use of smaller capacitors compared to what would be needed in a high-gain voltage-controlled system, reducing the overall chip area required for the loop filter and associated components.
Solution Approach 2:
The invention substitutes the voltage-gain-based control mechanism with a current-based control mechanism through the current mirror circuit. This substitution eliminates the need for high-gain voltage amplification stages and large loop filter components, significantly reducing chip area while maintaining frequency control precision.
3Speed
If drive current is increased to achieve higher transition speed, then oscillation frequency increases, but noise contribution increases
Solution Approach 1:
The current control is segmented between the main drive current from the current mirror (providing the necessary drive strength for fast transitions) and the fine adjustment current through the tuning capacitors. This allows the main drive current to be optimized for speed while the fine control current makes minimal adjustments, reducing the noise contribution from current variations.
Data Source
AI summary
A phase locked loop includes a voltage-controlled oscillator and a current mirror circuit that supplies a drive current to the voltage-controlled oscillator. The current mirror circuit includes a filter between a bias current generator and current mirror transistor. The filter includes a first and a second switch driven in unison with a small duty cycle.