Cross-Coupled RF Oscillator Bias Control for Low Phase Noise
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Solution Overview
Problem
Conventional oscillators face challenges in achieving low phase noise performance, particularly in advanced CMOS technology, due to limitations in oscillation amplitude swing and quality factor of the tank, which affects wireless communication systems' performance and channel utilization.
Innovation Solution
The proposed oscillator design incorporates a current source to control physical parameters such as clipping duration, oscillation amplitude, and current consumption, along with optimized capacitance values to enhance the equivalent quality factor of the tank, thereby improving phase noise performance and reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional cross coupled negative gm oscillators are used, then the oscillator can operate in standard CMOS technology, but the phase noise performance is limited due to circuit noise conversion
Solution Approach 1:
The patent changes the operating parameters of the oscillator by introducing a current source to control the gm-device operation point. By adjusting the current level and clipping duration, the oscillator minimizes the time spent in the triode region where noise sensitivity is highest, thereby improving phase noise performance while operating in standard CMOS technology
Solution Approach 2:
The patent introduces dynamic control of the gm-device operation through a current source that adjusts the bias current. This dynamic operation allows the device to transition between saturation and triode regions in a controlled manner, optimizing the clipping duration to reduce noise sensitivity during critical phases of the oscillation cycle
2Reliability
If oscillation amplitude swing is increased to improve phase noise, then phase noise performance improves, but the quality factor of the tank decreases due to transistor operation in triode region
Solution Approach 1:
The patent employs periodic clipping action controlled by the current source to shape the oscillation waveform. By periodically switching the gm-device between saturation and triode regions at optimal points in the oscillation cycle, the system achieves large amplitude swing when beneficial for phase noise while minimizing the duration in the noise-sensitive triode region, thus preserving tank quality factor
Solution Approach 2:
The patent dynamically adjusts the operating parameters including current level and clipping duration to optimize the trade-off between oscillation amplitude and quality factor. By controlling the clipping duration parameter, the system achieves sufficient amplitude for low phase noise while limiting the time spent in triode region to maintain high quality factor
3Reliability
If clipping duration is extended to reduce noise sensitivity, then phase noise improves, but oscillation amplitude and current consumption increase
Solution Approach 1:
The patent introduces a current source to independently control the clipping duration parameter. By optimizing this parameter, the system achieves the minimum necessary clipping duration to reduce noise sensitivity and improve phase noise, while avoiding excessive current consumption that would result from extended clipping periods. The current source allows precise adjustment of the bias current to achieve optimal power efficiency
Data Source
AI summary
An oscillator used in RF system. The oscillator includes a pair of transistors to which source terminals are interconnected and to which drain and gate terminals are coupled by a positive feedback loop comprising an oscillator tank, wherein the source terminals of the transistors are connected to a current source configured to control physical parameters of the oscillator.