Resistive Ladder Mixer for LO Harmonic Rejection
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Solution Overview
Problem
Conventional mixers, such as the four-switch mixer, introduce harmonics into the output signal, leading to unwanted down conversion of signals at multiples of the local oscillator frequency, which is problematic for harmonic rejection and results in parasitic mixing, requiring costly filtering or complex harmonic reject mixer designs.
Innovation Solution
A mixer structure comprising a ladder with series-connected resistances and switch-controlled branches, which generates a sinusoidal mixer signal by varying resistance between input and output, effectively rejecting harmonics related to the local oscillator frequency, allowing only fundamental frequency mixing and reducing even-order distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a four-switch mixer is used to mix input signal with local oscillator, then mixing function is achieved, but harmonics are introduced causing unwanted down conversion of signals at multiples of LO frequency
Solution Approach 1:
The mixer is segmented into multiple parallel switch pairs (first switch pair, second switch pair, third switch pair, fourth switch pair) instead of using a single four-switch configuration. Each switch pair processes the signal independently with different timing, and their combined output achieves the desired mixing function while canceling out harmonic components through differential operation.
Solution Approach 2:
The switches are controlled by periodic clock signals (first clock signal, second clock signal, third clock signal, fourth clock signal) with specific phase relationships. The periodic switching action creates a time-varying resistance profile that mixes the input signal with the local oscillator while the differential timing between switch pairs rejects harmonics.
2Productivity
If conventional mixer structures are used, then signal mixing is achieved, but parasitic mixing occurs due to even-order distortion
Solution Approach 1:
The mixer employs asymmetric switching sequences where the first and second switch pairs operate with different timing relative to the third and fourth switch pairs. This asymmetry in the switching pattern, combined with differential signaling, cancels even-order distortion components that would otherwise cause parasitic mixing.
Solution Approach 2:
The differential switch pairs are arranged to provide counterbalancing effects. The first switch pair and second switch pair generate distortion components that are equal in magnitude but opposite in phase to those generated by the third and fourth switch pairs, effectively canceling out the even-order distortion through destructive interference.
3Device complexity
If four-switch mixer is used, then compact structure is achieved, but costly filtering or complex harmonic reject designs are required
Solution Approach 1:
The invention converts the inherent distortion-generating switching action into a beneficial harmonic rejection mechanism. By carefully designing the switching sequences of the multiple switch pairs, the distortion components that would normally be harmful are instead used to cancel each other out, eliminating the need for external filtering while maintaining a relatively simple structure.
Solution Approach 2:
The mixer structure performs multiple functions simultaneously: it mixes the input signal with the local oscillator, rejects harmonics, and cancels even-order distortion all within a single integrated circuit block. This multi-functionality eliminates the need for separate filtering stages or complex harmonic rejection networks that would otherwise be required.
Data Source
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Figure 4a~4b
AI summary
A mixer comprising a ladder having at least two resistances arranged in series and an input configured to receive an input signal and apply it across the ladder, said ladder including an output arrangement comprising at least three branches, a first branch branching from a first end of the ladder, a second branch branching from between the at least two resistances and a third branch branching from a second end of the ladder, opposite the first end, each branch including a switch for controlling a connection between its branch and an output.