topic 2 electronic display

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Malaysian Institute of Aviation Technology

TOPIC 2

ELECTRONIC DISPLAY

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INTRODUCTION

The data contained in a digital device has to bepresented to the user in a way that can be easilyunderstood.

This could be done by numbered lights, punched as

holes on paper, printed by a typewriter. There are 3 types of display used in modern aircraft :

- Cathode Ray Tubes

- Light Emitting Diodes

- Liquid Crystal Display The differences between them in terms of weight,

reliability, power consumption and others.

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CRT

The cathode ray tube (CRT), is an evacuated glassenvelope containing an electron gun (a source ofelectrons) and a fluorescent screen, usually with internalor external means to accelerate and deflect theelectrons.

When electrons strike the fluorescent screen, light isemitted.

The electron beam is deflected and modulated in a waywhich causes it to display an image on the screen.

The image may represent electrical waveforms(oscilloscope), pictures (Television, computer monitor)and echoes of aircraft detected by radar.

The generation of an image on a CRT by deflecting anelectron beam requires the use of an evacuated glassenvelope which is large, deep, heavy, and relatively

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General Description

Consist of :

Evacuated Glass Envelope (Screen)

Electron Gun

Beam Focusing

Beam Deflecting

Electron Gun

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CRT CONTSRUCTION

Cathode ray tube employing electromagnetic focus and deflection

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Screen Inside surface is coated with crystalline solid material

known as phosphor.

Electron Gun An indirectly heated cathode biased negatively wrtscreen to produce electron.

A cylindrical grid which maintained at a negativepotential, to control the current thus modulate the

beam passing through. 2 or 3 anode. Positive potential with respect to thecathode. To accelerate the electrons. Also providemeans of focusing.

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Malaysian Institute of Aviation TechnologyDEFLECTION SYSTEM

To deflect the electrons in order to providea trace out of display or to scan the

display. Two types of deflection system :

Electrostatic deflection Consists of 1 set of horizontal deflection plates and

1 set of vertical deflection plates (Oscilloscope).

Electromagnetic deflection Using magnetic field to produce deflection instead

of electrical field (television and other large CRTs).

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Malaysian Institute of Aviation TechnologyBASIC OPERATION OF

MONOCROMATIC CRT

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Malaysian Institute of Aviation TechnologyBASIC OPERATION OF

MONOCROMATIC CRT

Electron gun produce electron and create

electron beam.

Accelerate by means of anodes and highpositive voltage applied to the conductive

coating.

The beams deflected by deflecting system top tobottom and side to side..

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In absence of input signal, deflection system

produces a set of white line on screen.

When input available, the signal applied to thecontrol grid via amplifier.

Control grid modulates the intensity of the

electron beam base on input signal to produce

area on the screen.

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Malaysian Institute of Aviation TechnologyBASIC OPERATION-SCANNING

There are 3 main scanning methods used

in CRTs display :

Raster Scanning

Interlace

Stroke

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Malaysian Institute of Aviation TechnologyRASTER SCANNING

A high frequency sawtooth wave is applied to thehorizontal deflection system.

At the same time, a relatively low frequency

sawtooth wave is applied to the vertical deflectionsystem.

So the electron beam will deflected rapidly acrossthe CRT from left to the right and top to bottom.

At the end of horizontal sweep, the beam movedrapidly back to the left hand side (flyback) of CRTand start the process again.

During flyback, the grid shut the beam off.

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Malaysian Institute of Aviation TechnologyRASTER SCANNING

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Malaysian Institute of Aviation TechnologyINTERLACE SCANNING

MODIFIED SYSTEM OF RASTER SCANNING USED TO

IMPROVE DEFINITION WITHOUT INCREASING THE

SCANNING RATE.

THE LINES OF ODD AND EVEN NUMBERED FIELDS FALLBETWEEN ONE ANOTHER.

THIS IS ACHIEVED BY DOUBLING THE FREQUENCY OF

THE VERTICAL SAWTOOTH WAVE WHILE KEEPING THE

HORIZONTAL WAVE THE SAME.

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Malaysian Institute of Aviation TechnologyINTERLACE SCANNING

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STROKE SCANNING

HIGH FREQUENCY TRIANGULAR WAVE APPLIED TO THEHORIZONTAL DEFLECTION SYSTEM.

RELATIVELY LOW FREQUENCY STEP PULSE WAVE IS

APPLIED TO THE VERTICAL DEFLECTION SYSTEM. THE BEAM WILL BE DEFLECTED RAPIDLY ON THE

HORIZONTAL MAKING LINE (STROKE).

AT THE END OF THE HORIZONTAL LINE, THE VERTICAL WAVEWILL CAUSE THE BEAM TO STEP DOWN THE SCREEN(PULSE).

THE PROCESS WILL REPEATED UNTIL THE BEAM REACHESTHE BOTTOM LEFT HAND CORNER OF THE SCREEN ANDTHEN FLYBACK TO THE TOP.

THE PROCESS WILL BEGIN AGAIN.

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STROKE SCANNING

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COLOUR CRT

COLOUR CRT HAS 3 ELECTRON GUNS.

SCREEN IS COATED WITH 3 DIFFERENT KIND OFPHOSPHOR, WHICH WILL LUMINESCE IN EACH OF 3

PRIMARY COLOURS RED, GREEN AND BLUE. THE COLOUR LUMINESCE IS CORRESPOND TO

WHICH THE ELECTRON EMITTED FORM.

OTHER COLOUR CAN BE DISPLAY BY MIXING AND

CONTROLLING THE BEAM CURRENT OF EACHELECTRON GUN.

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Malaysian Institute of Aviation TechnologyCOLOUR CRT

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COLOUR CRT HAS 3 ELECTRON GUNS

COLOUR CRT


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SCREEN FORMAT

THE VIDEO DATA RECEIVED FROM RADAR ANNTENA IS

CONVENTIONALLY IN WHAT TERMED RHO-THETA (figure a)

CORRESPONDING TO THE SWEEP OF THE ANTENNA.

IN EARLIER TYPE OF MONOCHROME INDOCATORS THE

INDICATOR ITSELF CORRESPOND TO THIS RHO-THETA

SWEEP.

IN CRT DISPLAYS THAT ADOPT RASTER, INTERLACE OR

STROKE PULSE, THE DISPLAY IS IN X-Y (RECTANGULAR)FORM (figure b).

IN CRT, THE RECEIVED STILL IN RHO-THETA (POLAR CO-

ORDINATE) FORM, THUS CONVERTED INTO X-Y FORM.

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Malaysian Institute of Aviation TechnologySCREEN FORMAT

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Malaysian Institute of Aviation TechnologyLCD

For the recent cockpits of aircraft, the conventional CRTdisplay have been being placed by Liquid Crystal Display(LCD) panels.

The LCD panels are superior to CRT display units with

regards to space, viewability both in sunlight and atnight, weight, reliability, power consumption and others.

The construction of LCD panels used for aircraft isbasically the same as those for LCDs used in personalcomputers and others, but the LCDs for aircraft are

subjected to severer environments as to temperature,barometrics pressure, vibration and impact.

Moreover, they are also required to demonstrate displayperformance specific to the cockpits of aircraft.

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Example of LCD display

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Malaysian Institute of Aviation TechnologyCHARACTERISTICS

Operate from low voltage (3 to 15 Vrms), low

frequency ( 2560Hz) AC signal.

Often used is 7-segment display format fornumerical readouts.

Slower than LED, but draw much less current.

Often used in battery operated device.

Passive devicedoes not emit light so required

an external light.

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Malaysian Institute of Aviation TechnologyLCD

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Malaysian Institute of Aviation TechnologyLCD-OPERATION

WHEN NO CURRENT PASSING THROUGH A LCD, THE

MOLECULES IN THE SUBSTANCE ARE STABLE, SO THE

DEVICE IS TRANSPARENT.

IF A CURRENT PASS THROUGH A LCD, BETWEEN THE

SEGMENT AND BACKPLANE, THE MOLECULES IN THEMATERIAL VIBRATE CAUSING IT BECOME OPAQUE.

THE LCD IS NORMALLY SANDWICHED BETWEEN TWO PIECES

OF GLASS. THE BOTTOM PIECE OF GLASS HAS A MIRRORED

UPPER SURFACE, SO ANY LIGHT SHONE WILL REFLECTED

BACK, EXCEPT WHERE THE LCD IS OPAQUE, THUSDISPLAYING NUMBER.

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Malaysian Institute of Aviation TechnologyLCD OPERATION

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Malaysian Institute of Aviation TechnologyLED

One of most common displays used in digital systems. The LED is based on the semiconductor diode.

As an electron moves across forward bias junction it willgive off its energy as it falls into a hole.

This energy is given off in form of heat for germaniumand silicon but in form of light for gallium arsenidephosphide.

LEDs present many advantages including lower energyconsumption, longer lifetime, improved robustness,smaller size, faster switching, and greater durability andreliability. However, they are relatively expensive andrequire more precise current and heat management thantraditional light sources.

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Red, green and blue LEDs

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Malaysian Institute of Aviation TechnologyADVANTAGES

lower energy consumption

longer lifetime

improved robustness

smaller size

faster switching

greater durability

Greater reliability

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Malaysian Institute of Aviation TechnologyLED

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H d it k?


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Malaysian Institute of Aviation TechnologyHow does it work?

P-n junction ElectricalContacts

A typical LED needs a p-n junction

Junction is biased to produce even more

e-h and to inject electrons from n to p forrecombination to happen

There are a lot of electrons and holes at

the junction due to excitations

Electrons from n need to be injected to p

to promote recombination

Recombination

produces light!! 33


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APPLICATION OF LED

CAN BE USED TO INDICATE IF A

CIRCUIT IS OPERATING BY USING AN

OPEN CATHODE NAND GATE TOSUPPLY THE EARTH FOR A LED.

CAN BE USED TO DISPLAY

NUMERICAL IN WHICH INDIVIDUALLEDs ARE USED IN COLUMNS TO

DISPLAY MULTI DIGIT NUMBER.

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Malaysian Institute of Aviation TechnologyAPPLICATION OF LED

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A li ti f LED


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Malaysian Institute of Aviation TechnologyApplications of LEDs

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topic 2 electronic display

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