Current-Mode Image Reject Mixer for Low-Noise RF Reception

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Solution Overview

Problem

Existing RF receiver mixer architectures face challenges in achieving high image rejection with low noise, particularly in systems like satellite radio, where the attenuation of RF bandpass filters is insufficient, leading to noise addition from resistors in polyphase filters and increased noise with higher order filters.

Innovation Solution

The implementation of a transconductance mixer, a polyphase filter operating in current mode, and a transimpedance amplifier in an image reject mixer architecture, which reduces noise by converting RF voltage signals to current signals and then to output voltage signals, effectively rejecting image frequencies while minimizing thermal noise from resistors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a polyphase filter with resistors is used to reject image signals, then image rejection is improved, but thermal noise increases

Engineering Contradiction:

Improveimage rejectionVSAvoidthermal noise

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent replaces the conventional voltage-mode polyphase filter using resistors with a current-mode polyphase filter. This substitution eliminates the need for resistors that generate thermal noise, thereby maintaining image rejection performance while significantly reducing thermal noise contribution from the filter components.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the operating domain from voltage mode to current mode in the polyphase filter. By operating the filter in the current domain rather than voltage domain, the design achieves the same image rejection function without requiring resistive elements, thus reducing thermal noise while maintaining the filtering performance.

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If higher order polyphase filters are used to increase image rejection, then image rejection is improved, but noise and device complexity increase

Engineering Contradiction:

Improveimage rejectionVSAvoidnoise

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent replaces higher order voltage-mode polyphase filters that require additional resistors with a current-mode implementation. This substitution allows achieving the same or better image rejection without proportionally increasing noise, as current-mode operation eliminates the thermal noise contribution from resistors that would otherwise be required in higher order voltage-mode filters.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Device complexity

If voltage mode mixing is used, then simplicity is maintained, but noise performance deteriorates

Engineering Contradiction:

Improvemixer architectureVSAvoidnoise

Core Design Contradiction:

Device complexityVSObject-generated harmful factors

Solution Approach 1:

The patent substitutes voltage-mode mixing with current-mode mixing in the polyphase filter stage. This substitution improves noise performance by eliminating resistor thermal noise while maintaining relatively simple circuit implementation through direct current-mode operation, avoiding the need for additional voltage-to-current conversion stages.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS7457605B2Low noise image reject mixer and method therefor

Publication Date: 2008.11.25 XENOGENIC DEVELOPMENT LIMITED LIABILITY COMPANY

AI summary

A receiver (200) includes a transconductance mixer (310), a polyphase filter (320), and a transimpedance amplifier (330). The transconductance mixer (310) mixes a radio frequency (RF) voltage signal into a current signal having a plurality of phases using a local oscillator signal. The polyphase filter (320) filters the current signal to provide a filtered current signal. The transimpedance amplifier (330) converts the filtered current signal into an output voltage signal.