Crystal Oscillator Digital Bias Control for Fast Low-Power Startup
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Solution Overview
Problem
Designing a low-power crystal oscillator that can achieve a startup time of less than 1 ms is challenging due to differing conditions for sustaining and initiating oscillations, and existing solutions like AGC loops are inefficient and sensitive to noise and environmental variations.
Innovation Solution
A system with a variable current source and control mechanism that adjusts the bias current from a large startup value to a smaller steady-state value after a predetermined number of oscillations, using a digital feedback loop to control the oscillator circuit, reducing power consumption and minimizing sensitivity to process, voltage, and temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a large bias current is used to initiate oscillations quickly, then the startup time is reduced, but the power consumption increases excessively
Solution Approach 1:
The bias current is made dynamic rather than static. The system automatically adjusts the bias current based on oscillation amplitude: using a large current during startup to achieve fast oscillation initiation, then automatically reducing it to a smaller steady-state current once oscillations are established. This dynamic adjustment resolves the contradiction by allowing high power only when necessary for startup.
Solution Approach 2:
An automatic gain control (AGC) loop with peak detector monitors the oscillation amplitude and provides feedback to control the bias current. When oscillations are small during startup, the feedback signals to maintain high current; when oscillations reach steady state, the feedback triggers current reduction. This closed-loop feedback system automatically manages the power consumption vs. startup speed tradeoff.
2Speed
If an AGC loop is used to control oscillation magnitude, then the startup time requirement is met, but the circuit becomes sensitive to noise and environmental variations
Solution Approach 1:
The patent replaces the traditional analog AGC loop with a digital control mechanism. Instead of using analog components (operational amplifiers, analog peak detectors) that are sensitive to noise and environmental variations, the system uses digital signal processing to detect oscillations and control the bias current. This substitution of digital for analog mechanisms significantly reduces sensitivity to noise and environmental factors while maintaining fast startup performance.
Solution Approach 2:
The patent introduces a digital signal processor as an intermediary between the oscillation signal and the bias current control. Rather than directly using analog feedback, the system samples the oscillation signal digitally, processes it through algorithms, and then controls the current based on digital decisions. This intermediary digital layer isolates the control system from analog noise and environmental variations.
3Speed
If the bias current is always maintained at a high level to ensure fast startup, then the startup time is reduced, but the power consumption increases unnecessarily during steady state
Solution Approach 1:
The system implements periodic monitoring of oscillation amplitude through the AGC loop, allowing the bias current to alternate between high and low states. During startup, the current remains high periodically until oscillations are detected; once steady state is achieved, the current switches to a low periodic state. This periodic adjustment ensures fast startup when needed while minimizing steady-state power consumption.
Solution Approach 2:
The system discards the high bias current once it has served its purpose of initiating oscillations. After detecting that steady-state oscillations are established, the system reduces the current to minimal levels needed to maintain oscillations. The high current is effectively discarded after use, and the system recovers power by operating at low current during steady state, thus resolving the power consumption issue.
Data Source
AI summary
Embodiments of the present invention provide a system for controlling a startup time of an oscillator circuit. The system includes a variable current source coupled to the oscillator circuit, wherein a startup time of the oscillator circuit is proportional to a bias current input into the oscillator circuit from the variable current source. The system also includes a control mechanism coupled to the variable current source and to the output of the oscillator circuit. Upon startup, the control mechanism is configured to adjust the variable current source to input a large startup bias current into the oscillator circuit. After the oscillator circuit outputs a predetermined number of oscillations during startup, the control mechanism is configured to adjust the variable current source to decrease its current to a smaller steady-state bias current into the oscillator circuit.