Crystal Oscillator Bias Current Control for Stable 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 parasitic oscillations.
Innovation Solution
An oscillator circuit with a bias current generator that adjusts the bias current level in response to 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 operation, and enabling fast start-up with a stored bias current level for subsequent power-ons.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the bias current is increased to ensure oscillator start-up with high resistance crystals, then the oscillator can start up reliably, but parasitic oscillations may occur and power consumption increases
Solution Approach 1:
The bias current is made dynamic rather than fixed. The circuit automatically adjusts the bias current level based on oscillation detection: a higher bias current is supplied during start-up to ensure reliable oscillation establishment even with high-resistance crystals, and once oscillation is detected via the feedback stage, the bias current is reduced to a lower steady-state level to prevent parasitic oscillations and reduce power consumption.
Solution Approach 2:
A feedback stage is introduced that monitors the oscillation signal and provides feedback to the bias current generator. When the oscillation amplitude reaches a threshold level, the feedback signal triggers the bias current generator to switch from high current mode to low current mode, thereby automatically adapting the bias current to the actual oscillation state and preventing excessive current conditions that cause parasitic oscillations.
2Reliability
If a fixed high bias current is used to ensure start-up with all crystals, then start-up reliability improves, but power consumption increases during normal operation
Solution Approach 1:
The bias current transitions from a static fixed value to a dynamic variable that adapts to operational conditions. During start-up, the bias current generator supplies a high bias current to ensure reliable oscillation establishment. Once oscillation is detected by the feedback stage, the bias current automatically transitions to a lower steady-state level, optimizing power consumption during normal operation while maintaining start-up reliability.
Solution Approach 2:
The bias current is applied in two distinct phases: an initial high-current phase during start-up to ensure reliable oscillation establishment, followed by a lower-current phase during normal operation. This periodic action pattern allows the circuit to achieve reliable start-up with high-resistance crystals while minimizing power consumption during sustained operation.
3Reliability
If the bias current is increased to compensate for crystal resistance spread, then oscillation start-up succeeds, but the negative resistance of the oscillator core becomes excessive causing instability
Solution Approach 1:
The bias current generator dynamically adjusts the bias current level based on oscillation detection. During start-up, a higher bias current is supplied to ensure the negative resistance of the oscillator core is sufficient to overcome crystal resistance and establish oscillation. Once oscillation is detected by the feedback stage, the bias current is reduced, thereby reducing the negative resistance to an appropriate level that maintains stability and prevents excessive oscillation conditions.
Solution Approach 2:
The feedback stage monitors the oscillation signal and provides feedback to the bias current generator. When oscillation amplitude reaches a threshold, the feedback signal triggers reduction of the bias current, which in turn reduces the negative resistance of the oscillator core to an appropriate level for stable operation, preventing the excessive negative resistance that would cause instability and parasitic oscillations.
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 adapts the bias current to the crystal's resistance, preventing excessive bias current and parasitic oscillations, while conserving power and ensuring stable oscillations, with the ability to quickly restart the oscillator circuit after power-off periods.
Implementation Method 1
The resonator, which comprises a piezoelectric crystal
Implementation Method 2
a resonator arranged to resonate at a frequency of the oscillation signal
Implementation Method 3
compensate for the resistance of the crystal during start-up of the crystal oscillator
Implementation Method 4
a feedback stage arranged to generate a feedback signal in response to an amplitude of the oscillation signal reaching an amplitude threshold
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.