Third-Overtone Crystal Oscillator With Tunable All-Pass RC Feedback
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Solution Overview
Problem
Existing integrated circuit oscillators face limitations in operating at higher frequencies and require external components for frequency tuning and temperature compensation, especially when using quartz crystals above 30 MHz, as they are typically limited by parallel resonance and require specific capacitance or resistance settings.
Innovation Solution
A cascaded arrangement of all-pass networks with a crystal resonator, providing negative feedback at low frequencies and positive feedback at the resonant frequency, allowing for tuning of the oscillation frequency without external components and enabling accurate temperature compensation and frequency modulation, utilizing tunable RC filters and differential amplifiers to adjust capacitance and resistance values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a quartz crystal is operated above 30 MHz at parallel resonance, then frequency control is achieved, but the crystal appears as an inductive reactance and additional circuit capacitance pulls the frequency down
Solution Approach 1:
The patent inverts the conventional approach by operating the crystal at series resonance (where impedance is minimum) rather than parallel resonance. This inversion allows the crystal to operate at higher frequencies (above 30 MHz) without the frequency pulling problem caused by circuit capacitance, as the crystal appears resistive rather than inductive at series resonance.
Solution Approach 2:
The patent changes the operating parameter from parallel resonance to series resonance, and further to third overtone series resonance. This parameter change enables operation at frequencies above 30 MHz while maintaining frequency stability, as the third overtone mode provides higher frequency operation with reduced sensitivity to circuit capacitance effects.
2Measurement precision
If external components are used for frequency tuning and temperature compensation, then accurate frequency control is achieved, but device complexity increases
Solution Approach 1:
The patent merges the frequency tuning and temperature compensation functions into the oscillator circuit itself by using tunable all-pass RC filters. The capacitors within these filters can be adjusted to compensate for crystal temperature coefficients and achieve accurate frequency tuning, eliminating the need for separate external tuning and compensation components.
Solution Approach 2:
The oscillator circuit performs its own temperature compensation and frequency tuning through the tunable all-pass RC filters. The circuit uses its internal capacitors to adjust for temperature drift and achieve precise frequency control, making the system self-sufficient without requiring external adjustment components.
3Speed
If the resonator operates at third overtone frequency, then higher frequency operation is enabled, but phase stability becomes more challenging
Solution Approach 1:
The patent employs cascaded all-pass networks that provide negative feedback at low frequencies and positive feedback at the resonant frequency. This feedback mechanism stabilizes the phase at the third overtone frequency by providing 180 degrees of positive feedback at resonance, ensuring stable oscillation at the higher frequency.
Solution Approach 2:
The patent uses dynamically tunable all-pass RC filters with adjustable center frequencies that can be set to match the third overtone frequency of the crystal. The tunable nature of these filters allows the circuit to adapt to the specific resonant characteristics of the crystal, maintaining phase stability at the higher third overtone frequency.
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 configuration stabilizes the oscillator at the fundamental frequency, allows for accurate frequency tuning, and reduces the need for external components, enhancing the oscillator's performance and flexibility in operating at higher frequencies by leveraging the third overtone frequency of the crystal resonator.
Implementation Method 1
A crystal oscillator is a circuit that uses the mechanical resonance of a vibrating crystal to create an electrical signal
Implementation Method 2
collectively provide 180 degrees of positive feedback within the oscillator at a resonant frequency of the resonator
Implementation Method 3
a second all-pass network configured to provide negative feedback to the first all-pass network at low frequencies far below the frequency of operation
Implementation Method 4
setting a center frequency of the all-pass networks to that of a third overtone crystal may yield an open loop phase of about 106 degrees at the fundamental frequency
Data Source
AI summary
Integrated circuit oscillators include a cascaded arrangement of first and second all-pass networks containing a resonator therein, such as a crystal-based resonator. The second all-pass network is configured to provide negative feedback to the first all-pass network at DC (i.e., very low frequencies) and is further configured to provide positive feedback with 90 degrees of phase to the first all-pass network at the resonant frequency of the resonator, which may be the third overtone frequency of a crystal resonator. The first all-pass network includes the resonator and the second all-pass network includes a resistor having resistance matched to a motional resistance of the resonator at the resonant frequency.