Variable Simulated Inductor for Wider VCXO Tuning Range
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Solution Overview
Problem
The tuning range of voltage-controlled crystal oscillators (VCXO) is limited due to the small motional capacitance compared to case capacitance, making it difficult to effectively vary the resonant frequency, especially in miniaturized systems like RF integrated circuits and mobile devices.
Innovation Solution
An electronically variable simulated inductor is introduced in parallel with the variable capacitor, allowing for electronic control of the inductance, which extends the tuning range by creating a smaller equivalent parallel capacitance, thereby increasing the frequency range of the oscillator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a variable capacitor is used to control the resonant frequency in a crystal oscillator, then the frequency can be adjusted, but the tuning range is limited due to the small motional capacitance compared to case capacitance
Solution Approach 1:
The patent changes the electrical parameters of the oscillator circuit by introducing a simulated inductor with variable inductance. By controlling the inductance parameter electronically, the resonant frequency can be adjusted over a wider range than possible with variable capacitors alone, while maintaining frequency control accuracy through electronic regulation
Solution Approach 2:
The patent replaces the mechanical/physical limitation of crystal motional capacitance with an electronically simulated inductor. This substitution allows the system to overcome the inherent physical constraints of the crystal resonator and achieve extended tuning range through electronic control of the simulated inductance
2Adaptability or versatility
If the case capacitance is increased to extend the tuning range, then the resonant frequency can be lowered, but the effectiveness diminishes due to the already large fixed case capacitance
Solution Approach 1:
Instead of trying to increase capacitance to extend the tuning range (which has diminishing returns), the patent inverts the approach by introducing a simulated inductor. This allows frequency control through inductance variation rather than capacitance variation, effectively reversing the traditional method and achieving extended tuning range
Solution Approach 2:
The simulated inductor acts as an intermediary element between the crystal resonator and the frequency control mechanism. It provides an additional degree of freedom for frequency adjustment without directly modifying the crystal's inherent capacitance values, thereby extending the tuning range while maintaining circuit simplicity
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
The use of an electronically variable simulated inductor in a voltage-controlled crystal oscillator increases the tuning range beyond what is achievable with variable capacitors alone, providing a more effective method for frequency control in miniaturized systems.
Implementation Method 1
The circuit of Figure 4 comprises a voltage buffer 111, an integrator 112 and a variable transconductor 113
Implementation Method 2
The transconductance of 113 is g m2. The overall signal current I 1 is equal to the transconductor output current I L plus the input current of buffer 1
Implementation Method 3
In electro-mechanical resonators based on a piezoelectric ceramic resonator or quartz crystal
Implementation Method 4
The combination of an inductor and a capacitor in a loop, typically known as a resonant circuit, is an important building block in electronic circuits
Data Source
Figure 1a~3
Figure 4
Figure 5a~5b
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.