Digital Crystal Oscillator Compensation Using Temperature-Sensed Capacitance
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Solution Overview
Problem
Crystal oscillators often fail to maintain a stable clock frequency over varying temperatures, particularly in applications like GPS receivers, due to significant frequency variations, which existing technologies like VCTCXOs cannot affordably address.
Innovation Solution
A temperature compensation circuit comprising a temperature sensor, a digitally-controlled capacitor, and a processor that uses parametrized models to adjust the frequency of a crystal oscillator, directly measuring temperature and translating it into a control signal to maintain frequency accuracy within narrow specifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a VCTCXO is used to maintain stable clock frequency over varying temperatures, then frequency stability is improved, but cost increases significantly
Solution Approach 1:
The patent uses a temperature sensor to measure the actual temperature of the crystal oscillator and uses this information to digitally adjust capacitance values, creating a virtual compensation mechanism that replicates the function of expensive analog temperature compensation circuits without the high cost
Solution Approach 2:
The patent replaces the mechanical/analog temperature compensation mechanism of VCTCXOs with a digital system that uses temperature sensing, microprocessor-based calculations, and digitally controlled capacitors to achieve frequency stabilization, thereby reducing cost while maintaining performance
2Ease of manufacture
If a simple crystal oscillator is used, then cost is reduced, but frequency stability over temperature deteriorates
Solution Approach 1:
The patent implements a feedback loop where a temperature sensor continuously monitors the crystal oscillator temperature, the microprocessor calculates the frequency deviation based on temperature, and digitally controlled capacitors adjust the frequency in real-time to compensate for temperature-induced drift
Solution Approach 2:
The patent changes the capacitance parameters of the crystal oscillator circuit based on temperature measurements, using digitally controlled capacitors to adjust the total capacitance value and thereby compensate for frequency drift caused by temperature variations
3Measurement precision
If temperature compensation is implemented using traditional methods, then frequency accuracy is improved, but device complexity increases
Solution Approach 1:
The patent integrates the temperature compensation function into the existing microprocessor system, using the same processor to perform both signal processing and temperature compensation calculations, and using the same digital output channels to control both normal operation and compensation capacitance, thereby avoiding additional dedicated compensation circuitry
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
This solution allows for high-frequency accuracy in low-cost crystal oscillators, reducing costs while meeting stringent temperature compensation requirements, such as ±0.5 ppm deviation over −40° C to 80° C, without the need for expensive VCTCXOs.
Implementation Method 1
The temperature sensor includes a Negative Temperature Coefficient (NTC) resistor
Implementation Method 2
The digitally-controlled capacitor is connected to the crystal oscillator and configured to receive a control signal and, based on the control signal, to control a frequency of an output signal generated by the crystal oscillator
Data Source
AI summary
An apparatus includes a temperature sensor, a digitally-controlled capacitor and a processor. The temperature sensor is coupled to a crystal oscillator and configured to generate an input signal depending on a temperature of the crystal oscillator. The digitally-controlled capacitor is connected to the crystal oscillator and configured to receive a control signal and, based on the control signal, to control a frequency of an output signal generated by the crystal oscillator. The processor is configured to receive the input signal from the temperature sensor, to convert the input signal into the control signal based on parameters that characterize the crystal oscillator and the digitally-controlled capacitor, and to apply the control signal to the digitally-controlled capacitor.