Biased Cross-Coupled Oscillator for Lower 1/f Noise Upconversion
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Solution Overview
Problem
Existing oscillator designs, particularly those using CMOS transistors, face challenges in minimizing 1/f noise upconversion due to sensitivity to resonance frequency alignment and parasitic capacitance, which affects the accuracy and efficiency of radio frequency communications systems.
Innovation Solution
The introduction of a tail resistor in the oscillator circuit, coupled with a tail capacitor and current source, biases the cross-coupled transistors to operate in an active region, counteracting the Groszkowski effect and reducing 1/f noise upconversion by tuning the tail impedance to maintain the oscillation frequency close to the resonant frequency, thereby suppressing changes in oscillation frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a noise filter with inductor Lf and capacitor Cf is used to raise impedance at the common source node, then 1/f noise is reduced, but the circuit becomes very sensitive to the precise alignment of the resonance frequency of the noise filter with twice the oscillation frequency
Solution Approach 1:
The patent changes the operating parameters of the CMOS transistors by biasing them to operate in the active region rather than allowing them to enter the triode region. This is achieved through specific bias circuitry (Rb1, Rb2, Cb1, Cb2) that lowers the gate bias voltage, thereby changing the transistor operating point to suppress 1/f noise upconversion without requiring precise noise filter alignment
Solution Approach 2:
The patent introduces a tail capacitor CT as an intermediary element that absorbs parasitic capacitance at the common source node. This intermediary component forces the transistors to deliver narrow current pulses rather than overlapping rectangular pulses, effectively mediating between the transistor operation and the resonant circuit to reduce 1/f noise without sensitivity to filter alignment
2Object-affected harmful factors
If damping resistors (Rdmp) are placed in series with each CMOS source to minimize 1/f noise, then 1/f noise is reduced, but the gain of each transistor is negatively impacted
Solution Approach 1:
The patent uses a tail capacitor CT as an intermediary that absorbs parasitic capacitance and shapes the current waveform to narrow pulses. This intermediary approach achieves 1/f noise reduction without requiring series damping resistors that would degrade transistor gain, thereby maintaining both noise performance and transistor effectiveness
3Object-affected harmful factors
If resistances RD are placed in series with the drains of each transistor to minimize 1/f noise upconversion, then 1/f noise is reduced, but the circuit becomes sensitive to parasitic capacitance
Solution Approach 1:
The patent introduces a tail capacitor CT connected to the common source node as an intermediary element that absorbs parasitic capacitance. This approach suppresses 1/f noise upconversion by forcing narrow current pulses without requiring drain resistances that would make the circuit sensitive to parasitic capacitance effects
4Productivity
If Class-C operation with overlapping rectangular current pulses is used, then the oscillator operates efficiently, but 1/f noise upconversion occurs due to harmonic waveforms changing oscillation frequency
Solution Approach 1:
The patent changes the transistor operating region from Class-C (allowing triode region entry) to active region operation through bias circuitry. This parameter change transforms the current waveform from overlapping rectangular pulses to narrow current pulses, suppressing 1/f noise upconversion while maintaining oscillator efficiency
Solution Approach 2:
The patent utilizes periodic narrow current pulses delivered by the transistors operating in the active region. This periodic action with controlled pulse width and timing maintains oscillation efficiency while reducing the harmonic content that causes 1/f noise upconversion in traditional Class-C operation
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 approach effectively reduces 1/f noise upconversion, improving the accuracy and efficiency of radio frequency communications by minimizing phase noise and making the oscillator less sensitive to harmonic waveforms, while maintaining robust operation across varying conditions.
Implementation Method 1
A tail capacitor CT absorbs all the parasitic capacitance at the common source node
Implementation Method 2
the resonant frequency of the resonant circuit Lr, Cr. A problem with this technique however is that the circuit is very sensitive to the precise alignment of the resonance frequency of the noise filter with (twice) the oscillation frequency
Data Source
AI summary
Embodiments disclosed herein relate to oscillators including methods of operating the same, for example for use in radio frequency circuits. In an embodiment, an oscillator has cross-coupled transistors connected between a resonant circuit and a tail circuit. The resonant circuit and tail circuit have respective supply connections for powering the oscillator with an external power supply and the cross-coupled transistors have a bias circuit coupled to respective gates of the cross-coupled transistors and arranged to bias said transistors in an active region of operation. The tail circuit has a current source, a tail capacitor and a tail resistor coupled between a common node of the cross-coupled transistors and the supply connection of the tail circuit.