TEST EQUIPMENT SETUP

For its practical nature, this research has been somewhat demanding on preparing tools and performing experiments. The anechoic chamber in the Department o f Electronic and Electrical Engineering has been of convenient use for the experimental tests accompHshment and will be here briefly discussed.

In this chapter we will overview this installation and in addition discuss some operational features o f the utihsed system setup. The referred setup configuration has been required to measure the electric field over the chamber's quiet zone. The samples should be taken at equally spaced positions in a linear movement traverse to the direction o f propagation.

Although some details o f this facihty could have been discussed before, when demanded by the circumstance, the conq)ilation here presented will provide the necessary view into the equipment arrangement and will be usefid in the next section where few o f its operational aspects are considered.

4.1.1. Hardware

The hardware description refers to details o f the test equ^ment considered relevant to understand the anechoic chamber operation as used in the experimental tests.

The measuring system setup block diagram is shown in figure 4.1. The integral modules and the respective equipment implementation are listed below for reference:

1. Signal generator HP Synthesized CW Generator;

2. Network analyser comprising three units,

HP 8410A Network Analyser

HP 8411A Harmonic Frequency Converter HP 8743 Reflection-Transmission Test Unit:

3. Positioner controller 4. Probe positioner below); 5. Disk unit 6. A/D converter 7. HP 85 8. Microwave amplifier

Part of the White box (as briefly described below);

Stepping motor driven platform (as described

HP Mass Storage Unit;

Part of the White Box (as briefly described below);

HP 85B Computer; and HP Microwave Amphfler. positioner ^ microwave amplifier splitter signal generator

U

Figure 4.1 Anechoic chamber system setup block diagram.

The equipment setup provides transmission measurements in phase and amphtude for signals in a range of frequencies up to 12.4 GHz. The network analyser allows amplitude ratios over a dynamic range of 60 dB and phase angles from 0 to 360 degrees to be measured.

Some of the setup equipments are IEEE 488 bus (GPIB) compatible and can be controlled by the HP-85 computer. The computer runs HP-BASIC, its ROM resident system

software. This permits a semi-automatic operation o f the whole setup when running a BASIC routine specifically written for measuring the field's amplitude and phase.

Most o f the system items are HP standard equipments whose details are not in the scope o f this description. Besides, their specifications are available fi*omthe manuals.

The positioner controller and the A/D converter, which digitises the phase and anq>htude DC outputs fi'om the network analyser, are combined in a common interface unit denominated "white box". This unit includes a firont panel firom which the probe positioner can be controlled too.

The positioner consists o f a metaUic frame with a carriage able to move in two axes. In one o f the axes the carriage is driven by a stepping motor while in the other it is held by mobile mechanical stops. This allows a precise positioning in the horizontal axis while makes the manoeuvre difficult and inaccurate in the vertical direction. The power o f the stepping motor does not allow it to drive the carriage upwards so it cannot be used for vertical movements.

The white box is linked to the positioner via its own bus, where step pulses and sense of move signals are transferred to the stepping motor and the positioner sensors information is returned.

With the white box as an interface, it is possible for the conçuter to operate the positioner sending commands through the GPIB. These commands include moving, stopping, resetting and sense o f direction orders. The travelling length for the move command has to be set in the positioner controller fi-ont panel.

The A/D converter shares the same GPIB address with the controller and is responsible for latching the digitised information either fi'om the phase or fi*om the anq)litude channel of the network analyser. The routing o f the desired channel to the A/D converter unit can also be controlled via GPIB commands. The digitised readout is refi*eshed every 400 milhseconds and this time should be observed when reading both phase and arqphtude. The conq)uter should provide by software that the measurement is vahdated only after the probe comes to rest.

The signal generator can also be controlled via GPIB which allows remotely select the fi*equency and level o f the transmitting signal. The microwave anq)lifier is used to conq)ensate for losses in the route o f the coaxial cable linking the generator to the feeder.

The feeder used in this application is a 20 dB standard gain pyramidal horn in the range o f 8 to 12.5 GHz, and the probe selected is a quarter-wavelength monopole for its matching characteristics to the coaxial line and for its omnidirectional radiation characteristic.

Finally, the information collected during the measurement procedure can be stored in magnetic medium using the disk driver unit which is also connected via the GPIB.

4.1.2 Software

A BASIC program to perform the multiple tasks o f controlling the setup operation, reading the field measurements in phase and amphtude and storing the results in the disk unit has been written for the HP-85 system. The source files are hsted in the appendix 7.2 and a brief description will be given in this subsection.

The program starts by an user interactive subroutine where the parameters for the test equipments settings should be entered. The required parameters are frequency, interval between consecutive sançles, position o f the first sartçle, and position o f the last sample. In a sequence it performs the setup o f the instruments and starts a sequence comprising one row o f measurements according to the selected parameters. Finally it stores the phase and amphtude readings in a datafile. The sequence is presented in figure 4.2.

The running time for a sequence o f 64 sanq)le positions taken along a horizontal line is approximately 6 minutes for a sampling interval o f 5 millimetres. One o f the reasons that slows down the algorithm is the time required by the probe to settle down in each rest position.

It has been stressed in the former subsection that the panel meter requires 400 milhseconds to update the reading. If one wants to obtain at least two sequential readings to vahdate the measurement, at least one interval o f 400 milhseconds is consumed. The algorithm has been designed for three consecutive coincidences which doubles this waiting time.

In practice it turns out that a shghtly larger time interval to perform the first o f the three measurements is advantageous in the sense that it avoids measurement sequence repetitions motivated by disperse consecutive readouts.

begin m easurem ent param eters , input / comments and filename input / yes no end ^ another ^ measurement? am plitude and phase

m easurem ent perform ance data storage equipm ent settings perform ance

Figure 4,2 Block diagram o f BASIC routine developed to perform field measurements in the anechoic chamber.

The procedure provides also default parameter selections for the case o f repetitive parameter choices and displays the current situation o f the procedure on the conq)uter

screen.

Apart from the main routine, procedures to display measurement files and to compute the FFT o f stored datafiles are also available in the HP-85. They have been usefid during the software development phase, however to perform the data analysis in later stages the way out was to transfer it to other machine.

4.1.3 System limitations

During the evolution o f the test experiments, several difficulties have been encountered. Quite a few o f them have been solved, however the particular restriction o f precisely moving the carriage m a second axis for carrying out two-dimensional measurements would have involved costly solutions and have been dropped.

The first major restriction o f the installation to be faced was the non conçatibility o f the data fbrmattmg on the HP- 8 5 system with any other platform available for the data processing. There was no option to transfer the datafiles to a more powerfid system and be processed there. The density in the magnetic medium was also inconq)atible with all other

systems.

To get through this situation, the datafiles collected in the measuring tests have been transferred in ASCII code to another machine via the GPIB interface. To perform this transference protocols for the two machines had been required and software to carry out this transference have been generated for both machines. These protocols were written in BASIC for the HP-85 and in Lab View for the Machintosh. In the final form, both protocols are not very long, however their generation demanded to study some aspects o f Lab View which required some extra time.

Another restriction in the installation is that the phase measurement is not stable during a long lasting test. The phase drift occurring due to tençerature variations is normally inside acceptable limits for short duration events, as the performance o f a single line of measurements. However, as the time required to sanq)le the whole plane conq)rised in the quiet zone is much longer, it no longer allows the phase variations to be kept inside the

acceptable limits. The phase monitoring and correction is therefore necessary while performing long duration measurement events.

The third and major restriction to be pointed out is that the positioner is not a proper tool for the performance o f measurements in two-dimensional layout. The positioner is accurate in its linear motor driven horizontal move, however the positioning in the vertical axis is a difficult and inaccurate task. This restriction interacts directly with the phase controlhng activity mentioned in the previous paragraph as far as it delays the measuring process. The measurements have been performed in an interactive process o f manual positioning and phase monitoring and the results shown in section 3.3 must be carehdly interpreted for this system restriction.