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RF Vector Impedance Analyser

Josh McIntyreSupervisor: Nasser Asgari

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Summary of content

Project overview Background Methods used Key results Project achievements

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Project Overview

Task: To redesign the “insides” of an existing analyser – faster, better, cheaper….

Using digital technology

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Background

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Method

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Sequence of events

Goals, objectives Defining inputs, outputs to the system Block diagram of system concept Selection of parts Reading of datasheets Creation of schematic Creation of PCB Assembly and programming of

prototype Analysis of successes/failures

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Inputs - Probe ModelPSpice model

Existing Probe

Model outputs

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Using a mixer as a VGAFrequency conversion & gain control

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Digital frequency generation

Using Direct Digital Synthesizers (DDS) requires knowledge of output spectrum, and filter design.

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Filtering the DDS outputPSpice Model of DDS filter

Simulation results

DDS outputMixer Input

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Block DiagramsBringing all design concepts together

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Selection of parts

Affordability Suitability Availability

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Schematics DDS Schematic

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Schematics RF Schematic

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Schematics Microprocessor Schematic

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Schematics Power supply, clock and IO Schematic

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PCB Design

Prototype 1 - Separate ground planes

- Wrong footprints - Noisy DDS outputsPrototype 2 - Single ground plane - Extra filtering - ‘maximal copper’ to reduce noise on

ground plane

Prototype 3 - External ADC

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Programming

Interfacing P with all the peripheral devicesCoordinating gain control for V and I signalsAchieving quadrature sampling at 5KHz

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Sampling Technique

)sin(1 AV

)cos()90sin(2 AAV

)(tan2

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V

V

)sin(1

V

A

The low frequency signal contains the impedance information.Quadrature sampling provides a fast, effective way to extract this with a minimum number of samples.

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Sampling Technique

Oscilloscope showing quadrature timing for sampling.

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Key Results

3 prototypes built Successful replacement of analogue frequency

generators with digital Elimination of 4x high precision custom made crystals Reduction of frequency generation PCB area

Synchronisation of RF signals with system clock Elimination of Phase Locked Loops to synchronise signals

Using the mixer to compress dynamic range Elimination of variable broadband amplifiers

Successful implementation of the quadrature sampling technique

Fast results

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OutcomeReduction of PCB size, complexity, power consumption & manufacturing cost.

=

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Achievements

• Working prototype• Measures 10, 50, 1000 ohm loads to within 5%• Operating power reduced from 4.8 watts to 2.4 watts• Data output increased from 2 sps to 40 sps• Manufacturing cost reduced by 70%

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Lessons Learned

Never trust a datasheet!Very accurate reference!!!!

Buy the part and evaluate it yourself before using it in a design!!!

This

Plus this

==rand(4096);

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The End….

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