Projects:2017s1-135 A Low Cost Impedance Analyser

Abstract

This project has commenced in order to develop a low cost impedance analyser that can be used to perform measurements in a laboratory environment. The project will be able to significantly cut costs by utilising instruments and software already available from the University. The impedance analyser will make use of the voltage divider principle, where the impedance of a device under test (DUT) can be found by using the voltage divider equation. A signal generator will feed a signal into the main circuit and a digital oscilloscope will measure the resultant voltage output of the DUT. A resistor is placed in series with the DUT, where impedance matching will allow for higher accuracy results. Thus, a control mechanism circuit will also be developed to complement the main circuit.

The control mechanism circuit is comprised of a network of parallel resistors, where there are switches in each branch. The values of these resistors are per decade, from a minimum of 0.1 up to a maximum of 1 M. An FPGA will be able to determine which of the resistors has the closest value to the magnitude of the DUT’s impedance. The FPGA will then cause the switch to be closed, whilst leaving all other switches open. As a result, that resistor will then be in series with the DUT, completing the voltage divider circuit. This control mechanism is essentially behaving as an impedance matching circuit. A LabView program running on a PC will be developed that will communicate and control the developed impedance analyser. The signal generator, oscilloscope and impedance analyser will be connected to the PC via the USB protocol.

Preliminary results show that the analyser is able to acquire measurement data and hence it should be able to find the impedance of the DUT. The analyser should also able to find the transfer function characteristics of a filter. The obtained values will be compared against known impedances that have been calibrated with the analyser. The impedance analyser should be able to accurately determine the value of the known DUT, with a 0.1% accuracy tolerance.

Background

The fundamental working of the impedance analyser is that it will incorporate the voltage divider principle. A signal generator (controlled by a PC via USB port) will send a signal through the circuit. There will be a voltage drop across a known impedance that will be in series to the DUT (the value of the known impedance is determined from one of the known resistors in the small impedance network). The figure below shows the circuit configuration for measuring impedances.

The most suitable value (the resistor with the impedance closest to the load/DUT value) is selected by one of the relay switches (controlled by the FPGA by toggling the switch on or off). An oscilloscope will measure the voltage drop across the known impedance that has been selected. Thus, the impedance of the DUT can be found using the equation:

Vout = (Vin - Vout) * Zknown / (Zknown + Zdut)

Where:

Vout, is the voltage across the DUT

Vin, is the voltage from the signal generator

Zknown, is the impedance across the known impedance selected by the relay

Zdut, is the impedance across the device under test (DUT)

LabVIEW

LabVIEW software is utilised to automate the hardware (AFG, oscilloscope and FPGA) functions of the low cost impedance analyser with a simple graphical user interface. The software stores results of the DUT measured by the low cost impedance analyser for further analysis. This section describes in-detail the developed LabVIEW software. To interface the hardware of the low cost impedance analyser with LabVIEW, instrument drivers are utilised. LabVIEW instrument drivers are a set of program-subroutines that create a communication link between LabVIEW and the connected instrument. In LabVIEW, predefined instrument drivers exist for both the arbitrary function generator and oscilloscope utilised in the design of the project. The Tektronix TDS 200 1000 2000 Series oscilloscope instrument driver is used to create a communication link between LabVIEW and the Tektronix TBS 1064 oscilloscope and the Tektronix AFG 1000 Series signal generator instrument driver is used to create a communication link between LabVIEW and the Tektronix AFG1062 arbitrary function generator [16] [17].

AFG Control Module The arbitrary function generator control module allows for the robust setting and injection of a signal into the low cost impedance analyser circuit.

Oscilloscope Control Module The oscilloscope control module allows for the systematic sampling and recording of voltage readings V1 and V2.

System-wide Control Automation

The below algorithm illustrates the skeleton of the algorithm of the system control under the user selecting the impedance analyser: <poem> Calculate the frequency sweep array according to user input. Send i^th frequency to the main loop, and define loop length (n) according to frequency sweep array size. Set arbitrary function generator control module frequency and voltage according to user input; send error query chain to oscilloscope control module. Commence oscilloscope control module once error query chain received. 4a. If first iteration: Perform oscilloscope setup prior to setting time base. 4b. Set time base according to frequency. 4c. send error query chain to signal processing module. Commence signal processing module once error query chain received. 5a. Obtain impedance and signal if impedance needs to be recalculated with differently closed resistor. 5b. If appropriate resistor closed: Update all relevant GUI outputs; impedance and voltage data. Else loop through 3 – 5b. until correct resistor switched and impedance obtained; update GUI outputs on loop termination. Repeat 2-6, n times. Plot results against frequency. </poem>

Project Team

Student Members

• Muhamad Shaabuddin:

Bachelor of Engineering (Honours)(Electrical and Electronic) with Computer Science

• Filip Karisik:

Bachelor of Engineering (Honours)(Electrical and Electronic) with Finance

Academic Supervisors

• Dr Keith Kikkert

• Dr Dr Cheng-Chew Lim