VCO Tail Inductor Layout Using Parallel Inductors
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Solution Overview
Problem
Designing a voltage controlled oscillator (VCO) with a tail inductor of smaller inductance becomes challenging as the operating frequency increases, leading to difficulties in selecting appropriate components and potential errors in inductor layout due to asymmetry, which affects noise performance.
Innovation Solution
The use of at least two inductors connected in parallel as a tail inductor connected to a cross-coupled transistor, allowing for a larger inductance and reducing the total inductance, thereby addressing the design difficulty and asymmetry issues while maintaining high impedance characteristics to suppress flicker noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a tail inductor with smaller inductance is used to achieve high operating frequency, then the operating frequency increases, but the design difficulty increases and asymmetry errors occur
Solution Approach 1:
The tail inductor is divided into multiple parallel inductors (e.g., two inductors L3 and L4) instead of using a single small inductance inductor. This segmentation allows each individual inductor to have larger inductance value while the parallel combination achieves the required small total inductance, making the design easier and reducing asymmetry errors.
2Speed
If a tail inductor with smaller inductance is used to achieve high operating frequency, then the operating frequency increases, but the impedance characteristics become harder to control
Solution Approach 1:
By segmenting the tail inductor into multiple parallel inductors, the total inductance is reduced while maintaining better control over impedance characteristics. The parallel configuration provides more stable and predictable impedance behavior compared to a single small inductor, especially at high frequencies.
3Reliability
If a single small inductance inductor is used as tail inductor, then the high impedance characteristic is achieved, but the inductor size becomes very small leading to layout asymmetry
Solution Approach 1:
The tail inductor is segmented into multiple parallel inductors that can be symmetrically arranged in the layout. For example, two inductors can be placed symmetrically on either side of the circuit center, maintaining layout symmetry while achieving the required small total inductance and high impedance characteristic.
Solution Approach 2:
By using multiple inductors that can be symmetrically arranged, the design actually applies the reverse of asymmetry - it uses symmetry to eliminate layout asymmetry errors. The parallel inductor configuration allows for symmetric placement patterns that reduce parasitic effects and improve matching.
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 enables the use of a tail inductor with a relatively large size to achieve small impedance, simplifying the design and reducing noise impact, while maintaining effective suppression of flicker noise and improving the oscillator's performance across a wide range of power voltages.
Implementation Method 1
a first tail inductor 111, a resonant circuit 130... The first tail inductor 111 includes at least two first inductors connected in parallel
Implementation Method 2
The LC tank may also be referred to as a resonant circuit... when a resonance frequency between the inductor and the capacitor connected in parallel is twice an operating frequency of the VCO, the group of inductances including the capacitor and the inductor has a high impedance characteristic
Implementation Method 3
Due to an inherent disadvantage of the MOS transistors, the cross-coupled transistor generates flicker noise. The flicker noise enters the LC tank and affects noise performance of the oscillator
Data Source
AI summary
An oscillator and a clock circuit are disclosed. In an oscillator (100), a tail inductor connected to a cross-coupled transistor includes at least two inductors connected in parallel. Therefore, an inductance of the tail inductor is less than an inductance of any one of the inductors. This can address a design difficulty that a tail inductor with a smaller inductance needs to be used as an operating frequency of a VCO increases. The oscillator (100) includes a first cross-coupled transistor (121) and a first tail inductor (111). The first tail inductor (111) includes at least two inductors connected in parallel. The first tail inductor (111) is coupled to a source of the first cross-coupled transistor (121). The source of the first cross-coupled transistor (121) is coupled to a power supply or a ground through the first tail inductor (111).