Electronically Variable Inductor Circuit for Wider VCXO Tuning
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Solution Overview
Problem
The tuning range of voltage-controlled crystal oscillators (VCXO) is limited due to the large fixed case capacitance, making it difficult to effectively decrease the resonant frequency using variable capacitance, as the motional capacitance is several orders of magnitude smaller, resulting in diminishing returns from adding variable capacitance.
Innovation Solution
A simulated variable inductor circuit is introduced, where a variable inductor is connected in parallel with the case capacitance, and an electronically controllable inductor is used to extend the tuning range by altering the equivalent capacitance, allowing for increased frequency control through electronic means rather than mechanical.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If variable capacitance is added in parallel with case capacitance to decrease resonant frequency, then frequency tuning capability is improved, but the effectiveness diminishes due to the large fixed case capacitance
Solution Approach 1:
The patent changes the electrical parameters of the resonant circuit by introducing a simulated inductor with variable inductance. By controlling the inductance parameter electronically (through voltage or current control of the operational amplifier and associated components), the resonant frequency can be tuned effectively despite the large fixed case capacitance. This parameter change approach allows the system to overcome the limitation of fixed capacitance values.
2Adaptability or versatility
If a variable inductor is used to extend tuning range, then frequency control range is improved, but device complexity increases
Solution Approach 1:
The patent replaces mechanical variable inductors (which would have moving parts) with an electronically controlled simulated inductor implemented using operational amplifiers, resistors, and capacitors. This substitution eliminates mechanical components while achieving the same variable inductance function through electronic control, thereby extending tuning range without proportionally increasing mechanical complexity.
Solution Approach 2:
The patent introduces an operational amplifier-based simulated inductor as an intermediary element between the control voltage and the resonant circuit. This intermediary converts voltage control signals into the appropriate current-voltage relationship that simulates inductive behavior, enabling electronic tuning without direct mechanical intervention in the resonant circuit.
3Adaptability or versatility
If moving parts are used in variable inductors to achieve frequency tuning, then frequency control is improved, but reliability decreases due to mechanical wear and limited applications
Solution Approach 1:
The patent completely replaces mechanical variable inductor structures with an electronic simulated inductor using operational amplifiers and passive components. This substitution eliminates all moving parts, mechanical wear, and associated reliability issues while maintaining the ability to vary inductance for frequency control. The electronic implementation is more suitable for integrated circuit applications and miniaturized systems.
Data Source
AI summary
A variable simulated inductor comprises an integrator connected to receive the voltage across the input to the circuit. The output of the inductor is connected to a control terminal of a transconductor connected across the input of the circuit. The gain of the transconductor is electronically controllable in order to control the inductance of the circuit. An oscillator using a variable simulated inductor and a piezoelectric resonator connected in parallel is also provided.