Crystal Oscillator Bias Feedback for Leakage-Resilient Oscillation
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Solution Overview
Problem
Crystal oscillators face issues with area occupation by feedback resistors, sensitivity to process, power supply, and temperature, as well as introduction of unbalanced DC voltage and increased costs due to the use of analog switches, and are limited by leakage current and circuit power in low-frequency designs.
Innovation Solution
A crystal oscillator control circuit that eliminates the need for resistors and analog switches by using a current source, peak detection and bias voltage adjustment circuit, low-pass filter, and feedback control circuit, coupled with capacitors to achieve optimal performance and leakage isolation without introducing side effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a resistor is used as feedback device in crystal oscillator, then the oscillation can be maintained, but the resistor occupies large area and increases cost
Solution Approach 1:
The patent replaces the physical resistor component with an electronic control mechanism using an analog switch and control circuit. The analog switch uses transistors to simulate resistance without requiring a physical resistor, thereby eliminating the area occupation while maintaining the feedback function necessary for oscillation stability.
Solution Approach 2:
The patent dynamically adjusts the resistance parameter through voltage control of the analog switch. By changing the gate voltage of the control transistor, the effective resistance can be varied without changing the physical component, allowing the same circuit element to serve multiple resistance values as needed for different oscillation conditions.
2Area of stationary object
If an analog switch is used instead of resistor, then area is reduced, but the resistance is sensitive to process, power supply and temperature
Solution Approach 1:
The patent implements a feedback control circuit that continuously monitors the oscillation signal and adjusts the control voltage of the analog switch accordingly. This feedback mechanism compensates for variations in resistance caused by process, temperature, and power supply changes, maintaining stable oscillation performance despite environmental variations.
Solution Approach 2:
The control circuit automatically adjusts the analog switch parameters based on the actual oscillation conditions without external intervention. The circuit self-regulates by detecting changes in oscillation amplitude and frequency, and correspondingly adjusting the switch control voltage to maintain optimal performance.
3Extent of automation
If analog switch is used with large signal amplitude, then feedback control is achieved, but unbalanced DC voltage is introduced between input and output
Solution Approach 1:
The patent separates the AC feedback function from the DC path by using the analog switch only for AC signal feedback while maintaining separate DC biasing paths. The control transistor is configured to affect only the AC impedance without disturbing the DC operating point, thus achieving feedback control while preventing DC voltage imbalance.
Solution Approach 2:
The patent designs the circuit to maintain equipotential conditions for DC levels at the input and output of the analog switch. By ensuring that the DC paths are symmetrically designed and properly biased, the circuit prevents potential differences from developing across the switch terminals during normal operation.
4Reliability
If on-chip capacitor series with resistor is inserted to reduce PCB leakage current sensitivity, then leakage sensitivity is reduced, but resistor area increases tremendously
Solution Approach 1:
The patent replaces the series resistor used for leakage compensation with an active electronic circuit using the analog switch and control transistor. This electronic solution provides the same leakage current compensation function without requiring a large physical resistor, as the active circuit can achieve the same effect with minimal area occupation.
5Use of energy by moving object
If crystal oscillation amplitude is limited by loop to reduce power, then power consumption is reduced, but AGC loop and low-pass filter are required
Solution Approach 1:
The patent combines the amplitude control function with the existing feedback mechanism. The same analog switch and control circuit that provide frequency stabilization also perform amplitude limiting by adjusting the feedback impedance based on the oscillation amplitude, thereby achieving power reduction without requiring separate AGC and filtering circuits.
Data Source
AI summary
A crystal oscillator control circuit includes a first terminal and a second terminal, a current source, and a peak detection and bias voltage adjustment circuit. The first terminal and the second terminal are arranged to couple the crystal oscillator control circuit to a crystal. The current source is coupled to a power supply voltage and generates a bias current. The peak detection and bias voltage adjustment circuit is coupled between the bias current and a ground voltage and coupled to the first terminal, and performs peak detection and bias voltage adjustment to correspondingly generate a first signal at a node. The low-pass filter low-pass filters the first signal to generate a filtered signal. The feedback control circuit is arranged to perform feedback control according to the filtered signal to generate an oscillation signal at one or both of the first terminal and the second terminal.