RF Mixer Switching Circuit With Parallel Capacitors for 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 small capacitors in parallel with switching diodes or transistors to compensate for discontinuity during local oscillator signal transitions, thereby reducing temporal discontinuity and improving linearity characteristics such as IIP3 and OIP3, while using a low noise amplifier to compensate for potential conversion gain reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If small capacitors are added in parallel with switching diodes to compensate for temporal discontinuity, then linearity (IIP3, OIP3) is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
The patent introduces capacitors as intermediary elements coupled in parallel with the switching diodes. These capacitors act as mediators that smooth out the temporal discontinuities in the switching action, thereby improving linearity without fundamentally changing the mixer's core switching mechanism. The capacitors fill in the gaps during transition periods when diodes switch states, maintaining continuous signal flow.
Solution Approach 2:
The patent modifies the electrical parameters of the switching circuit by adding parallel capacitors with specific capacitance values (e.g., 0.5pF to 5pF). This parameter change alters the frequency response and impedance characteristics of the switching path, compensating for nonlinear effects and improving IIP3 and OIP3 measurements by up to 6 dBm.
2Manufacturing precision
If capacitors are coupled across differential ports of input balun to reduce temporal discontinuity, then linearity is improved, but manufacturing cost and device complexity increase
Solution Approach 1:
Capacitors are introduced as intermediary elements across the differential ports of the input balun to mediate the signal transition during local oscillator switching. These capacitors smooth out abrupt voltage changes and reduce temporal discontinuities in the differential signal path, improving linearity characteristics.
Solution Approach 2:
The patent addresses linearity issues by adding components in a different dimensional configuration - coupling capacitors across differential ports rather than in series with individual diodes. This dimensional change in circuit topology provides an alternative approach to compensating for nonlinear effects while maintaining compatibility with existing mixer architectures.
3Power
If a low noise amplifier is used to compensate for conversion gain reduction, then signal amplitude is maintained, but noise figures may be affected and device complexity increases
Solution Approach 1:
The patent employs a low noise amplifier with gain configured to provide feedback compensation for the conversion gain reduction caused by the parallel capacitors. The amplifier monitors the output signal level and adjusts its gain to maintain the desired signal amplitude, compensating for the attenuation introduced by the capacitance additions while attempting to preserve noise figures.
Solution Approach 2:
The patent changes the gain parameter of the amplifier stage to compensate for the conversion loss introduced by the parallel capacitors. By adjusting the amplifier gain (e.g., +6 dB to +10 dB), the system restores the original signal amplitude levels while the low noise design of the amplifier minimizes the impact on overall noise figures.
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3A
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.