RF Mixer Parallel Capacitor Circuit for Better Linearity
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Solution Overview
Problem
Mixers, particularly those used in RF receivers and transmitters, face challenges in achieving improved linearity due to inherent non-linear characteristics of diodes and transistors, which affect cross modulation, desensitization, harmonic generation, and signal-to-noise distortion ratio, making it difficult to enhance linearity without compromising noise figures or practicality in applications like cell phone wireless transmissions.
Innovation Solution
Incorporating capacitors in parallel with switching elements, such as diodes or field effect transistors, to allow leakage current during zero-crossing transitions of the local oscillator signal, which compensates for discontinuity and improves linearity by enhancing parameters like input and output 3rd order intercept points and 1-dB compression point.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional mixer designs using diodes or transistors are used, then the mixer can perform basic frequency conversion functions, but the linearity is degraded due to inherent non-linear characteristics of the switching elements
Solution Approach 1:
A capacitor is introduced as an intermediary element connected in parallel with the switching element. This capacitor provides a leakage current path during the transition period when the switching element is changing state, thereby maintaining signal continuity and improving linearity without requiring fundamental changes to the mixer architecture
Solution Approach 2:
The invention changes the electrical parameters of the switching circuit by adding a capacitor in parallel with the switching element. This modification alters the current-flow characteristics during switching transitions, specifically enabling leakage current to flow during the zero-crossing transition time of the local oscillator signal, which improves linearity parameters such as the 3rd order intercept point and 1-dB compression point
2Manufacturing precision
If switching elements are used for frequency conversion, then the mixer achieves the desired frequency shifting function, but discontinuity during zero-crossing transitions degrades linearity
Solution Approach 1:
The capacitor serves as a mediator that bridges the signal path during the discontinuous transition period of the switching element. By providing an alternative current path through the capacitor during zero-crossing transitions, the signal continuity is maintained and linearity is improved
Solution Approach 2:
The capacitor is positioned to provide cushioning effect beforehand during the transition period. It stores and releases energy during the zero-crossing transition, compensating for the discontinuity that would otherwise occur when the switching element changes state, thus maintaining smooth signal progression
Data Source
AI summary
Systems and methods are disclosed for improved linearity performance of a mixer. An example mixer includes switching circuit elements configured to be switched on and switched off based at least partly on a local oscillator signal and capacitors including a respective capacitor in parallel with each of the switching elements. The mixer is configured to mix the input signal with the local oscillator signal to thereby frequency shift the input signal.