RF Downconversion Mixer Using Common-Mode Sensing for IM2 Suppression
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Solution Overview
Problem
Existing mixers face challenges in reducing second-order distortion (IM2) due to self-mixing effects, which can interfere with signal processing, especially since common-mode feedback loops using coils occupy significant area and lower drain impedance, reducing conversion gain.
Innovation Solution
A mixer design that incorporates a common mode sensing component to detect deviations in the common mode output and uses a feedback path to subtract these deviations from the drain current of RF transistors, either by amplification or direct subtraction, thereby reducing IM2 without relying on coils.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If coils are used in the common mode feedback loop to short IM2 at the input transconductor, then second order distortion is reduced, but the area occupied increases and drain impedance decreases
Solution Approach 1:
The patent extracts and removes the coils from the common mode feedback loop architecture. Instead of using coils to short IM2 at the input transconductor, the invention implements a coil-free feedback mechanism that achieves the same distortion reduction function through alternative circuitry, thereby eliminating the area occupation problem while maintaining the distortion reduction benefit
Solution Approach 2:
The common mode feedback loop is designed to perform multiple functions without requiring coils: it detects common mode voltage deviations, generates appropriate feedback signals, and reduces second order distortion all within a coil-free architecture. This multi-functional approach replaces the single-function coil-based solution with a more versatile circuit implementation
2Manufacturing precision
If coils are used in the common mode feedback loop, then second order distortion is reduced, but conversion gain decreases due to lower drain impedance
Solution Approach 1:
The patent removes coils from the feedback loop to prevent the degradation of drain impedance. By eliminating the coils, the circuit maintains higher drain impedance values which are critical for achieving high conversion gain, while still achieving second order distortion reduction through the alternative feedback mechanism
Solution Approach 2:
The invention changes the architectural parameters of the feedback loop by replacing the coil-based implementation with a transistor-based implementation. This parameter change in the circuit topology allows the maintenance of optimal drain impedance levels while achieving the desired distortion reduction through different operational mechanisms
3Manufacturing precision
If duty-cycle errors occur due to mismatches in switching transistors, then IM2 leaks into differential mode at mixer output, but adding complex feedback increases device complexity
Solution Approach 1:
The patent implements a common mode feedback loop that detects deviations in common mode voltage at the mixer output and feeds back correction signals to the switching transistors. This feedback mechanism actively compensates for duty-cycle errors and prevents IM2 leakage into the differential mode, achieving high precision without excessive complexity through elegant feedback control
Solution Approach 2:
The common mode feedback loop acts as an intermediary mechanism between the switching transistors and the output. It mediates the effect of duty-cycle errors by detecting common mode deviations and introducing corrective feedback, thereby preventing IM2 leakage without requiring direct complex modifications to the switching transistor mismatches themselves
Data Source
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AI summary
The present invention provides a mixer for downconversion of RF signals, comprising: at least one RF transistor (305); at least one switching pair (303) connected to the drain current (301) of the at least one RF transistor (305); and a common mode sensing component (300) configured to detect deviations in the common mode output of the switching pair (303) from a specified output; wherein the deviations are subtracted from the current of the at least one switching pair or modify the voltage (307) of the RF transistors.