CMOS VCO Phase Noise Reduction via Current Mirror Biasing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional voltage controlled oscillators (VCOs) suffer from increased phase noise due to the noisy power supply voltage, which degrades their performance and requires additional passive elements to control 1/f noise.
Innovation Solution
A CMOS VCO design that uses a current mirror to apply a gate bias voltage independent of the power supply voltage VDD and supplies stable power to the inverter buffer, reducing phase noise by removing the DC component and applying AC voltage through resistors, thereby minimizing the impact of power supply noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional tail current source is used in the VCO, then the circuit structure is simple, but phase noise increases due to noisy power supply voltage
Solution Approach 1:
A current mirror circuit is introduced as an intermediary between the power supply voltage VDD and the tail current source. The current mirror isolates the tail current source from power supply noise, providing a stable bias current while blocking noise transmission. This resolves the contradiction by adding a noise-isolating intermediary component.
Solution Approach 2:
The DC component of the power supply voltage is extracted and removed using a capacitor, leaving only the AC signal component to control the varactor diodes. This separates the noise-containing DC bias from the signal path, reducing phase noise while maintaining circuit functionality.
2Reliability
If additional passive elements are added to control 1/f noise, then phase noise is reduced, but device complexity increases
Solution Approach 1:
The current mirror circuit serves multiple functions simultaneously: it provides the tail current for oscillation, isolates power supply noise, and stabilizes the bias voltage. This multi-functionality reduces phase noise without requiring separate dedicated noise-filtering components, thus avoiding increased complexity.
Solution Approach 2:
The noise filtering function is merged into the existing current mirror structure rather than being implemented as a separate filter circuit. The current mirror inherently provides noise isolation while maintaining its current source function, combining multiple benefits in a single circuit element.
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 reduces phase noise in the VCO, as demonstrated by simulation results showing a significant improvement in noise frequency characteristics, from −108.3 dBc to −120.0 dBc at 1 MHz, enhancing the oscillator's stability and output power.
Implementation Method 1
uses a current mirror to apply a gate bias voltage independent of a power supply voltage VDD
Implementation Method 2
removing the DC component and applying AC voltage through resistors
Implementation Method 3
a noise filter 140 preventing noise generated from the tail current source 130 from flowing in the VCO, and includes an inductor L13 and a bypass capacitor C13
Data Source
AI summary
A complementary metal oxide semiconductor voltage controlled oscillator is provided. The voltage controlled oscillator includes an LC tank which is supplied with a power supply voltage, the LC tank oscillating at a certain frequency; a negative resistor including first and second N-channel metal oxide semiconductor field effect transistors (NMOS FETs) to sustain the oscillation of the LC tank; a direct current block to remove a direct current component from the power supply voltage; an alternating current block to apply an alternating current voltage to the gates of the first and second NMOS FETs; a first current mirror including third and fourth NMOS FETs and allowing a current to symmetrically flow in the voltage controlled oscillator, a drain and the gate of the third NMOS FET being connected to a reference voltage supply; and the reference voltage supply applying a direct current voltage to the first current mirror.