Sampling PLL Injection Oscillator for Fast Startup and Low Phase Noise
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Solution Overview
Problem
Existing crystal oscillators face challenges in achieving low phase noise while maintaining a large swing, which increases standby power consumption and degrades mobile device performance, and designing an injection oscillator with frequency error less than 20 ppm is difficult due to PVT changes.
Innovation Solution
A sampling PLL-based injection oscillator that uses a crystal oscillation circuit, injection circuit, dithering circuit, and PLL circuit to inject first and second injection signals in specific phases, reducing oscillation time and compensating for frequency deviations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large swing is maintained to achieve low phase noise, then phase noise performance is improved, but initial oscillation time increases and standby power consumption increases
Solution Approach 1:
The injection circuit pre-injects a signal at a specific phase ahead of time to kick-start the oscillation process. This preliminary action provides an initial voltage swing that accelerates the startup phase, allowing the oscillator to reach its steady-state large swing faster without compromising phase noise performance.
Solution Approach 2:
The injection circuit operates periodically during the startup phase, applying injection signals at specific phases to sustain and amplify the oscillation. This periodic injection maintains the build-up of voltage swing efficiently, reducing the time to reach stable operation while preserving low phase noise characteristics.
2Reliability
If a large swing of 1.2V or more is required for low phase noise, then phase noise performance is improved, but frequency tolerance error exceeds 20 ppm
Solution Approach 1:
The PLL circuit continuously monitors the frequency of the injection signal and adjusts the control voltage applied to the VCO to maintain the frequency within 20 ppm tolerance. This feedback mechanism ensures that even with large swing requirements, the frequency accuracy is preserved by dynamically compensating for deviations caused by PVT variations.
Solution Approach 2:
The system dynamically adjusts the control voltage parameter of the VCO based on PLL feedback to maintain frequency accuracy. By changing the control voltage in response to frequency deviations, the system achieves both large swing amplitude (1.2V or more) and precise frequency tolerance (within 20 ppm) simultaneously.
3Measurement precision
If compensation is applied to the injection oscillator, then frequency accuracy is improved, but the system becomes impractical when XO is not operating
Solution Approach 1:
The PLL circuit uses the output of the crystal oscillator itself as the reference signal for frequency comparison. This self-service approach eliminates the need for external reference clocks or separate compensation systems, allowing the oscillator to maintain frequency accuracy autonomously whether the XO is operating or not.
Solution Approach 2:
The PLL circuit serves multiple functions: it compensates for frequency deviations during normal operation and continues to provide frequency control even when the XO is not operating. This multi-functionality ensures frequency accuracy is maintained across different operational states without requiring separate compensation mechanisms.
4Adaptability or versatility
If PVT changes are considered, then design robustness is improved, but achieving frequency error less than 20 ppm becomes difficult
Solution Approach 1:
The PLL continuously monitors frequency deviations caused by PVT changes and adjusts the VCO control voltage in real-time to compensate for these variations. This feedback mechanism maintains frequency accuracy within 20 ppm despite process, voltage, and temperature changes, achieving both PVT robustness and frequency precision simultaneously.
Solution Approach 2:
The system dynamically adjusts the VCO frequency in response to PVT variations through PLL feedback. Rather than relying on static design margins, the system adaptively changes operating parameters to maintain frequency accuracy across varying conditions, achieving both robustness and precision.
Data Source
AI summary
An oscillator includes a crystal oscillation circuit configured to generate an oscillation signal having a natural frequency, an injection circuit configured to inject a first injection signal and a second injection signal into the crystal oscillation circuit, a dithering circuit configured to transmit a first control signal for generating the first injection signal to the injection circuit, and a phased-lock loop (PLL) circuit configured to lock a phase of the first injection signal to the natural frequency, to transmit a second control signal for generating the second injection signal to the injection circuit.