LC VCO Resistor Isolation for Phase Noise and Frequency Drift
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Solution Overview
Problem
LC-based voltage-controlled oscillators (VCOs) face issues with increased phase noise and frequency drift due to clamping to supply voltages, which degrades performance and fails to meet target criteria for noise, power, and area in integrated circuit (IC) designs.
Innovation Solution
Incorporating a series connection of resistors and an LC circuit with CMOS transistors in the VCO topology, which includes first and second differential output terminals, and resistors coupled between these terminals and the drain terminals of CMOS transistors, to provide surge protection and improve phase noise and frequency stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If LC-based VCO topology is used, then output frequency control is achieved, but phase noise increases due to clamping to supply voltages
Solution Approach 1:
Resistors are introduced as intermediary elements between the LC circuit nodes and the supply voltages. These resistors prevent direct clamping of the LC circuit to the supply rails, thereby reducing phase noise while still allowing voltage control functionality. The resistors act as mediators that isolate the sensitive LC tank from the noisy supply voltage connections.
2Measurement precision
If LC-based VCO topology is used, then frequency control is achieved, but frequency drift increases with temperature
Solution Approach 1:
The resistors serve as thermal buffers between the LC circuit and the supply voltages, reducing the direct coupling of temperature-induced supply variations to the oscillation frequency. This intermediary structure helps stabilize frequency against temperature changes by isolating the frequency-determining LC tank from temperature-sensitive supply connections.
3Measurement precision
If series resistors are added to reduce phase noise, then phase noise decreases, but device complexity increases
Solution Approach 1:
The VCO structure is segmented by introducing discrete resistor elements at specific locations within the circuit. Rather than using a monolithic complex structure, the solution divides the voltage control path into separate segments (supply voltage, resistors, LC circuit nodes), allowing independent optimization of each segment for noise performance while maintaining overall simplicity.
Data Source
AI summary
A system includes a data path and a phase-locked loop (PLL) coupled to the data path. The system also includes a voltage-controlled oscillator (VCO) coupled to the PLL. The VCO includes an LC circuit with first and second differential output terminals. The VCO also includes a first resistor coupled between the first differential output terminal and drain terminals of a first pair of complementary metal-oxide semiconductor (CMOS) transistors. The VCO also includes a second resistor coupled between the second differential output terminal and drain terminals of a second pair of CMOS transistors.