Crystal Oscillator Bias Current Control for Reliable Start-Up
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Solution Overview
Problem
The shrinking dimensions of piezoelectric crystals in crystal oscillators lead to increased motional loss and a large spread in resistance values, making it challenging to design a crystal oscillator circuit that can effectively cope with the variability in resistance, and ensuring oscillator start-up without excessive bias current that may result in unwanted parasitic oscillations.
Innovation Solution
An oscillator circuit with a bias current generator that adjusts the bias current to match the resistance of the crystal, using a feedback mechanism to terminate the increasing bias current once an amplitude threshold is reached, allowing for stable and power-efficient oscillation, and enabling fast start-up by storing the final bias current level for subsequent operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the bias current is increased to ensure oscillator start-up for crystals with high resistance, then the oscillator can start reliably, but excessive bias current may result in unwanted parasitic oscillations
Solution Approach 1:
The bias current is made dynamic rather than fixed. The bias current generator increases the bias current from a first level to a second level during start-up, then terminates the increasing in response to a feedback signal when the oscillation amplitude reaches a threshold. This dynamic adjustment allows the circuit to adapt to varying crystal resistance values while preventing excessive current that could cause parasitic oscillations.
Solution Approach 2:
A feedback stage monitors the oscillation amplitude and generates a feedback signal when the amplitude reaches a predetermined threshold. This feedback signal controls the bias current generator to terminate the increasing of bias current. The feedback mechanism ensures that the bias current is adjusted precisely when needed, preventing both insufficient current (which would prevent start-up) and excessive current (which would cause parasitic oscillations).
2Device complexity
If a fixed bias current is used in the oscillator circuit, then the circuit design is simple, but it cannot adapt to the large spread in resistance values between different crystals
Solution Approach 1:
The bias current generator dynamically adjusts the bias current level based on the actual oscillation amplitude and crystal resistance characteristics. The circuit transitions from a fixed bias current approach to a dynamic adjustment approach, where the bias current increases from a first level to a second level during start-up and is then maintained or reduced based on feedback. This resolves the contradiction by making the circuit adaptable to resistance variations while maintaining reasonable design complexity through the use of standard circuit blocks.
Solution Approach 2:
The bias current parameter is changed dynamically during operation rather than being fixed. The bias current generator modifies the bias current level in response to the oscillation amplitude feedback, effectively changing the operating parameters of the oscillator to match the specific crystal being used. This allows the circuit to adapt to the large spread in resistance values without requiring completely different designs for different crystals.
3Reliability
If excessive bias current is supplied to ensure start-up for all possible crystals, then all crystals can start oscillation, but power consumption increases
Solution Approach 1:
The bias current is dynamically adjusted rather than maintained at a constantly high level. During the start-up phase, the bias current increases to ensure reliable oscillation initiation even for crystals with high resistance. Once the oscillation amplitude reaches the threshold, the feedback signal causes the bias current generator to terminate the increasing, and the bias current is maintained at a lower second level during subsequent operation. This dynamic approach ensures start-up reliability while minimizing power consumption during normal operation.
Solution Approach 2:
The bias current generator operates in distinct phases: an initial start-up phase where current increases to ensure oscillation begins, followed by a sustained operation phase where current is maintained at a lower level. This periodic action pattern allows the circuit to use higher current only when necessary for start-up, thereby ensuring all crystals can start while reducing overall power consumption during the majority of the operational time.
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
The solution allows for stable and power-efficient oscillation while preventing excessive bias current that could cause parasitic oscillations, ensuring reliable start-up and reducing power consumption by adapting the bias current to the crystal's resistance, thus addressing the variability in crystal resistance and improving start-up times.
Implementation Method 1
The resonator, which comprises a piezoelectric crystal
Implementation Method 2
a crystal oscillator arranged to generate an oscillation signal
Implementation Method 3
The spread of resistance between different crystals is typically large compared to the mean value of resistance
Data Source
AI summary
An oscillator circuit comprises a crystal oscillator arranged to generate an oscillation signal, a bias current generator arranged to supply a bias current to the crystal oscillator, and a feedback stage arranged to generate a feedback signal in response to an amplitude of the oscillation signal reaching an amplitude threshold. The bias current generator is arranged to: in response to a supply of power to the oscillator circuit being switched on, generate the bias current at an increasing level commencing from a first level; in response to the feedback signal, terminate the increasing; and during subsequent oscillation of the crystal oscillator, supply the bias current at a second level dependent on a final level of the bias current reached when the increasing is terminated.