Crystal Oscillator Start-Up Circuit Using Zero-Crossing Energy Injection
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Solution Overview
Problem
Existing crystal oscillator start-up methods are limited by long start-up times and high energy consumption due to reliance on noise amplification and inefficient energy injection, particularly in low-power wireless systems like IoT and BLE devices.
Innovation Solution
A crystal oscillator start-up circuit that alternates between switch control phases to calibrate and compare inputs, using a comparator with a differential amplifier stage and switch control generator to optimize energy injection and reduce start-up time by dynamically adjusting resistance and sense resistance values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If noise amplification is used to start-up the crystal oscillator, then the oscillator can be started, but the start-up time is relatively long and energy consumption is high
Solution Approach 1:
The patent applies preliminary action by providing an initial kick or pulse to the crystal oscillator at start-up to establish a well-defined initial motional current. This preliminary energy injection prepares the oscillator for faster start-up by avoiding the slow noise amplification process, directly reducing start-up time while maintaining reliable oscillator start-up capability
Solution Approach 2:
The patent implements periodic action by periodically injecting energy pulses to the crystal oscillator in a well-defined manner. This periodic energy injection constantly increases the motional current up to the final value, optimizing start-up time to the minimum possible value while reducing energy consumption compared to continuous noise amplification
2Measurement precision
If a single kick pulse is applied to the crystal oscillator, then the initial motional current is well defined, but the maximum initial motional current is limited by the supply voltage
Solution Approach 1:
The patent resolves this contradiction by using periodic energy injection instead of a single kick pulse. Multiple periodic pulses accumulate energy in the crystal oscillator, allowing the motional current to build up constantly to higher values that exceed what a single voltage-limited kick pulse could achieve, while maintaining precise control over the initial motional current through well-timed pulse injection
3Loss of time
If periodic pulses are injected to increase motional current, then start-up time is improved, but the frequency and phase must be exactly matched to optimize start-up time
Solution Approach 1:
The patent applies self-service by using the crystal oscillator's own motional current to generate the timing signal for energy injection. The system detects zero-crossing events of the motional current and uses these events to trigger subsequent energy injection pulses, automatically achieving frequency and phase matching without external clocks or complex synchronization circuits, thus optimizing start-up time while avoiding increased device complexity
Solution Approach 2:
The patent implements feedback by using the crystal oscillator's motional current to control the timing of energy injection. The system continuously monitors the motional current and adjusts the injection timing based on detected zero-crossing events, creating a self-regulating mechanism that automatically optimizes frequency and phase matching for minimum start-up time without requiring complex external control
Data Source
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AI summary
A circuit and method for starting-up a crystal oscillator is described. A crystal resonator is configured to be coupled to a start-up circuit including an H-bridge circuit having a number of switches. A plurality of switch control signals are generated in response to detecting a zero-crossing event of the motional current in the crystal resonator. The switches of the H-bridge circuit are controlled by the switch control signals to apply a voltage to the terminals of the crystal resonator in a first polarity during a first switch control phase and a second opposite polarity during a second switch control phase. During a respective first subphase of the respective switch control phase, the plurality of switches are configured in a first configuration to couple the supply node to a respective crystal resonator terminal. During a respective second subphase of the respective switch control phase the plurality of switches are configured in a second configuration to couple the supply node to the respective crystal resonator terminal. The resistance between the supply node and the respective crystal resonator terminal is larger in the second configuration than the first configuration. A zero-crossing is detected during each respective second sub-phase.