VCO Tail Harmonic Filter Tuning for Low Phase Noise
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Solution Overview
Problem
Existing voltage-controlled oscillators (VCOs) require additional circuitry for fine-tuning the tail harmonic filter to maintain optimal phase noise performance across varying input voltage conditions.
Innovation Solution
The proposed VCO structure includes an inductor-capacitor resonant circuit with varactors connected to a voltage input node, cross-coupled field effect transistors with back gates connected to receive a variable back gate bias voltage dependent on the input voltage, and an inductor-capacitor filter connected to the shared source node. This configuration allows for automatic fine-tuning of the LC filter in response to changes in input voltage, eliminating the need for additional circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional circuitry is added to enable fine tuning of the LC filter, then phase noise performance is improved, but device complexity increases
Solution Approach 1:
The system uses the existing variable input voltage that controls the LC resonant circuit frequency to automatically tune the LC filter frequency. The back gate bias voltage, which is dependent on the variable input voltage, adjusts the transistors' capacitance to fine-tune the filter's resonant frequency, eliminating the need for separate control circuitry.
Solution Approach 2:
The variable input voltage serves dual purposes: it controls both the LC resonant circuit frequency through varactors and the LC filter frequency through the back gate bias voltage. This multi-functionality allows a single voltage signal to perform multiple tuning operations, reducing the need for additional dedicated circuitry.
2Reliability
If the LC filter is fine tuned to compensate for input voltage changes, then phase noise is reduced, but the number of circuit components increases
Solution Approach 1:
The existing back gate bias voltage, already required for transistor operation, is utilized to perform the additional function of fine-tuning the LC filter. This self-service approach allows the system to achieve phase noise reduction without adding dedicated filter tuning components.
Solution Approach 2:
The back gate bias voltage is varied as a function of the variable input voltage to dynamically adjust the transistors' capacitance parameters. This parameter change enables continuous fine-tuning of the LC filter resonant frequency to track the resonant circuit frequency, maintaining optimal phase noise performance across the tuning range.
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 solution effectively maintains the optimal relationship between the resonant frequency of the LC resonant circuit and the LC filter, thereby minimizing phase noise across the entire range of possible voltage input signals without requiring additional circuitry for fine-tuning.
Implementation Method 1
a VCO includes an inductor-capacitor (LC) resonant circuit (also referred to herein as an LC tank) to generate the oscillating output signal at a particular oscillating frequency
Implementation Method 2
inductor-capacitor resonant circuit with varactors connected to a voltage input node
Implementation Method 3
an inductor-capacitor filter connected to the shared source node
Data Source
AI summary
Disclosed is a voltage-controlled oscillator (VCO) including at least an inductor-capacitor (LC) resonant circuit (including varactors that receive a variable input voltage), cross-coupled transistors connected to the LC resonant circuit, and an LC filter connected to a shared source node of the cross-coupled transistors. The cross-coupled transistors can have back gates connected to receive a variable back gate bias voltage (Vbg), which is dependent on Vin to ensure that an optimal relationship between the oscillating frequency (ω0) of the LC resonant circuit and the resonant frequency (ω1) of the LC filter is continuously maintained to minimize phase noise. For example, if Vin is increased to increase varactor capacitance and, thereby decrease ω0, then Vbg is also increased, thereby increasing the voltage (Vs-s) and the capacitance (Cs-s) on the shared source node connected to the LC filter, decreasing ω1, and maintaining an optimal relationship of ω0=ω1/2.