Current-Mode RF Downconverter With Passive Mixer for Low-Noise Linearity

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Solution Overview

Problem

Achieving highly linear and low noise RF downconverters in CMOS technology is challenging due to flicker noise and high power consumption, especially in low power wireless receivers, where conventional solutions result in noise figure degradation and high current consumption.

Innovation Solution

The implementation of a transconductance amplifier with a passive mixer for current domain mixing and a second-order complex load impedance, comprising a frequency-dependent negative resistance component, capacitor, and resistor, which reduces noise and improves linearity by using a general impedance converter and operational amplifiers to achieve a third-order elliptic filter response.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If CMOS passive mixers are used to reduce flicker noise, then noise figure is improved, but linearity deteriorates due to switching RF signal across low frequency impedance

Engineering Contradiction:

Improveflicker noiseVSAvoidlinearity

Core Design Contradiction:

Object-affected harmful factorsVSManufacturing precision

Solution Approach 1:

The patent replaces the conventional voltage-mode switching mechanism with a current-mode mixing mechanism. Instead of switching voltage signals across resistive loads, the mixer switches current signals across virtual ground nodes created by feedback amplifiers. This substitution of the fundamental mixing mechanism eliminates the linearity degradation caused by voltage switching across low frequency impedance while maintaining the low flicker noise advantage of passive mixers.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent introduces feedback amplifiers as intermediary elements between the passive mixer switches and the output nodes. These amplifiers create virtual ground nodes that provide low impedance paths for RF signals during switching transitions, thereby maintaining signal integrity and linearity. The feedback amplifiers act as mediators that decouple the switching action from the voltage nodes, allowing the passive mixer to operate in current mode without suffering from voltage switching limitations.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Manufacturing precision

If voltage mode buffers are added to provide low impedance voltage reference, then linearity is improved, but power consumption increases significantly

Engineering Contradiction:

ImprovelinearityVSAvoidpower consumption

Core Design Contradiction:

Manufacturing precisionVSUse of energy by moving object

Solution Approach 1:

The patent merges the functions of voltage buffering and impedance transformation into a single current-mode mixing stage. The feedback amplifiers integrated into the mixer circuit perform both the impedance transformation and provide the virtual ground references needed for linear operation, eliminating the need for separate voltage mode buffers. This consolidation achieves the same linearity improvement while consuming significantly less power by operating entirely in current mode throughout the signal path.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent changes the fundamental operating parameter domain from voltage mode to current mode throughout the mixer circuitry. By operating the passive mixer switches and feedback amplifiers in current mode rather than voltage mode, the circuit achieves low impedance references and high linearity without requiring high current consumption voltage buffers. The parameter change enables the same functional goals to be achieved with much lower power dissipation.

Inventive Principle:

Principle #35Parameter changes

3Speed

If wideband resistive feedback amplifiers are used, then bandwidth is improved, but noise figure deteriorates

Engineering Contradiction:

ImprovebandwidthVSAvoidnoise figure

Core Design Contradiction:

SpeedVSObject-affected harmful factors

Solution Approach 1:

The patent inverts the conventional approach by using capacitive feedback rather than resistive feedback in the amplifiers. Instead of using resistors for feedback which provide broadband operation but add thermal noise, the patent uses capacitors for feedback which provide frequency-dependent feedback that reduces noise while maintaining wide bandwidth through the reactive nature of the feedback path. This inversion of the feedback element type resolves the trade-off between bandwidth and noise figure.

Inventive Principle:

Principle #13The other way round (Inversion)

Data Source

PatentUS7546110B2Low power highly linear RF downconverter

Publication Date: 2009.06.09 ATMEL CORP

AI summary

A technique for downconverting a RF signal comprises an antenna adapted to receive an RF signal; a transconductance amplifier connected to the antenna and adapted to amplify the RF signal; a passive mixer connected to the transconductance amplifier and adapted for current domain mixing of electrical current transferred from the transconductance amplifier; and a load impedance connected to the passive mixer. The load impedance may comprise a parallel combination of a frequency dependent negative resistance component, a capacitor, and a resistor. The load impedance may comprise a pair of complex poles, a pair of imaginary zeros, and a real pole. Voltages at an input and an output of the passive mixer are related such that the input voltage of the passive mixer is an upconverted version of the output voltage of the passive mixer, wherein the input voltage of the passive mixer is at an output of the transconductance amplifier.