Crystal Oscillator Waveform Shaping for Low Noise and Emissions
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Solution Overview
Problem
Conventional crystal oscillators face issues with noise contribution from self-biasing feedback resistors and high-frequency emissions due to square waveforms with rich harmonics, which existing solutions fail to adequately address.
Innovation Solution
A crystal oscillator design incorporating a feedback network with diodes stacked in forward and reverse directions, a waveform shaper with NMOS and PMOS transistors, and shunt capacitors to reduce noise and emissions, featuring a feedback network with diodes and transistors to manage voltage transitions and suppress high-frequency harmonics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a self-biasing feedback resistor is used in the crystal oscillator, then the oscillator can maintain stable oscillation, but it introduces significant noise that degrades performance
Solution Approach 1:
The patent removes the self-biasing feedback resistor from the oscillator circuit entirely. Instead, it uses a different biasing approach through the inverter's internal structure and external capacitors to achieve stable oscillation without the noisy resistor component.
Solution Approach 2:
The inverter is designed to perform multiple functions: providing amplification, establishing bias conditions, and enabling oscillation stability without requiring a separate feedback resistor. This multi-functionality eliminates the noise source while maintaining reliability.
2Power
If a conventional crystal oscillator produces a square wave output, then it provides strong fundamental frequency, but it generates rich harmonics that cause undesired high-frequency emissions
Solution Approach 1:
The patent introduces an output buffer stage with series capacitors and parallel resistors as an intermediary between the oscillation core and the output. This buffer shapes the waveform to reduce high-frequency harmonics while maintaining the fundamental frequency strength.
Solution Approach 2:
The output waveform parameters are modified by changing the rise and fall times through the buffer circuitry. By controlling the transition edges, the circuit reduces high-frequency content while preserving the fundamental oscillation power.
Data Source
AI summary
A crystal oscillator includes an inverter configured to receive a first voltage at a first node and output a second voltage at a second node, a stacked-diode feedback network inserted between the first node and the second node, a waveform shaper configured to couple the second node to a third node in accordance with the first voltage, a crystal inserted between a fourth node and a fifth node, wherein the fourth node is coupled to the third node, and the fifth node is coupled to the first node, a first shunt capacitor inserted between the fourth node and a ground node, and a second shunt capacitor inserted between the fifth node to and the ground node.