1 RF Vector Impedance Analyser Josh McIntyre Supervisor: Nasser Asgari

2 Summary of content Project overview Background Methods used Key results Project achievements

3 Project Overview Task: To redesign the “insides” of an existing analyser – faster, better, cheaper…. Using digital technology

4 Background

5 Method

6 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

7 Inputs - Probe Model PSpice model Existing Probe Model outputs

8 Using a mixer as a VGA Frequency conversion & gain control

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

10 Filtering the DDS output PSpice Model of DDS filter Simulation results DDS output Mixer Input

11 Block Diagrams Bringing all design concepts together

12 Selection of parts Affordability Suitability Availability

13 Schematics DDS Schematic

14 Schematics RF Schematic

15 Schematics Microprocessor Schematic

16 Schematics Power supply, clock and IO Schematic

17 PCB Design Prototype 1 - Separate ground planes - Wrong footprints - Noisy DDS outputs Prototype 2 - Single ground plane - Extra filtering - ‘maximal copper’ to reduce noise on ground plane Prototype 3 - External ADC

18 Programming Interfacing  P with all the peripheral devices Coordinating gain control for V and I signals Achieving quadrature sampling at 5KHz

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

20 Sampling Technique Oscilloscope showing quadrature timing for sampling.

21 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

22 Outcome Reduction of PCB size, complexity, power consumption & manufacturing cost. =

23 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%

24 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);

25 The End…. ?