LC VCO Common-Mode Isolation for Lower Phase Noise
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Solution Overview
Problem
Voltage-controlled oscillators (VCOs) generate undesired common-mode (CM) noise due to non-linear capacitors, causing phase and frequency offsets in output signals, which affect signal quality and increase power consumption.
Innovation Solution
The VCO includes a first and second cell coupled via common-mode isolation circuitry, using a transmission line, transformer, or impedance circuit to reduce CM noise amplitude, improving signal-to-noise ratio (SNR) without increasing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If non-linear capacitors are used in the VCO circuitry, then the VCO can generate output signals with desired frequency and voltage amplitude, but common-mode noise is generated causing phase and frequency offsets
Solution Approach 1:
The VCO is divided into a first cell and a second cell, each containing separate circuitry including voltage-controlled current sources, inductor-capacitor tank circuits, and non-linear capacitors. This segmentation isolates the common-mode noise generated by non-linear capacitors in each cell, preventing it from affecting the entire circuit while maintaining the frequency and voltage amplitude generation function.
Solution Approach 2:
The common-mode noise is extracted and removed from the output signal through differential coupling between the first and second cells. The differential configuration cancels out the common-mode components while preserving the differential signal, thereby separating the harmful noise from the useful output.
2Device complexity
If traditional VCO circuitry is used, then the circuit can operate with simple structure, but signal-to-noise ratio is degraded due to common-mode noise
Solution Approach 1:
The VCO is segmented into two independent cells with separate voltage-controlled current sources and inductor-capacitor tank circuits. This segmentation enables differential operation that improves signal-to-noise ratio by canceling common-mode noise, while the modular structure maintains reasonable circuit complexity.
Solution Approach 2:
A differential coupling mechanism acts as an intermediary between the first and second cells, transferring the useful differential signal while blocking and canceling the common-mode noise. This intermediary structure improves SNR without requiring complete circuit redesign.
3Reliability
If common-mode isolation circuitry is added to reduce CM noise, then SNR is improved, but device complexity increases
Solution Approach 1:
The circuit is segmented into two symmetric cells that inherently provide common-mode rejection through differential operation. This segmentation approach improves SNR by canceling common-mode noise while maintaining a relatively simple and symmetric circuit structure that is easy to implement.
Solution Approach 2:
The first and second cells are designed with homogeneous structures, including matched voltage-controlled current sources, inductor-capacitor tank circuits, and non-linear capacitors. This homogeneity ensures that common-mode noise is generated equally in both cells and can be effectively canceled through differential coupling, improving SNR without adding complex asymmetric components.
4Adaptability or versatility
If non-linear capacitors are used to control oscillation frequency, then frequency tuning is achieved, but phase noise increases due to capacitance variations
Solution Approach 1:
The frequency tuning function is segmented across two independent cells, each with its own non-linear capacitors. The differential configuration allows frequency tuning while the common-mode rejection cancels the phase noise generated by capacitance variations, maintaining phase precision during frequency adjustment.
Solution Approach 2:
The oscillation frequency is controlled by changing the capacitance values of non-linear capacitors in response to control voltages. The differential configuration and common-mode rejection ensure that these parameter changes achieve frequency tuning while minimizing the impact on phase noise.
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 reduces phase noise and improves SNR by uncoupling and inverting CM noise, enhancing signal quality and reducing power consumption in VCOs.
Implementation Method 1
a second inductor-capacitor tank circuit coupled to the first inductor-capacitor tank circuit and may inductively couple to the first inductor-capacitor tank circuit
Implementation Method 2
generate an output signal having a desired oscillation frequency within a resonant frequency range
Implementation Method 3
inductively uncouple from the first inductor-capacitor tank circuit based on signals having an undesired oscillation frequency outside the resonant frequency range
Implementation Method 4
signals having an undesired oscillation frequency outside the resonant frequency range
Data Source
AI summary
This disclosure is directed to a Voltage-Controlled Oscillator (VCO) with improved phase noise compared to other VCOs. The VCO may include a first cell and a second cell separated by common-mode (CM) isolation circuitry to reduce an amplitude of CM noise at an output signal. The CM isolation circuitry may inductively couple the first cell to the second cell for generating the output signal based on a resonant frequency range including resonant frequencies of the CM isolation circuitry, the first cell, and the second cell. As such, the first cell and the second cell may each include a portion of an entirety of the CM noise. In some cases, the portions of the CM noise on the first cell and/or the second cell may improve a signal to noise ratio of the output signal.