01 Avionics Systems

7/23/2019 01 Avionics Systems

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Avionics Systems

AERO 5409


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The fundamental concepts relating to radio communication.

The basic operation of radio navigation equipment.

The basic operation of associated radio systems.

The basic operation of radar fundamentals and systems. The operation of non radio/radar/magnetic navigation

equipment.

The basic operation of air data computers.

The basic operation of flight director systems.

The basic operation of automatic flight control systems.

The system requirements for flight data recorders (FDR).


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Radio transmission

and modulation The purpose and functions of aircraft radio

transmitters

Construction operation and characteristics ofaudio transducers used in aircraft communicationssystems

Terms associated !ith modulation and encoding" carrier

"Intelligence

" modulation


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Radio transmission

and modulation#hy a modulated carrier must be used for

transmission of information

The relationship bet!een the carrier signal and theintelligence of the follo!ing transmission systems$

"amplitude modulation (AM)

"frequency modulation (F%)

"phase modulation (PM)"pulse code modulation (&C%)

"single sideband modulation (SSB)


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Principal functions of aircraft communication

transmitters

%any and varied. The pilot has to communicate !ith the to!er to transmit a transmitter is

needed. Transmitter only needs to have reasonably lo! po!er to contact !ith 'TC. ormally the pilot is reasonably close to the to!er.

&ilot selects the appropriate channel assigned to a particular to!erfrequency and then transmits.

Communications have been established !hen the 'TC operator ans!ers. Frequency used !ould probably beVHF (very high frequency). ome operators useUHF (ultra high frequency) for communication.

*ong range communication bet!een aircraft and ground or ground to

aircraft can only be established by using a different system called HF(high frequency).

To propagate this information !e need an antenna. +n aircraft transmitters !e need to convert the sound into electrical energy. #hen the audio is received the electrical energy needs to be converted

bac, into sound. To achieve this !e need transducers.


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Operation and construction of audio

transducers used in

aircraft

'udio transducers used in aircraft are$

"microphones"earphones

"spea,ers.


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Microphones

%icrophones convert sound energy into electricalenergy. This electrical energy is the audio signaland consists of a number of frequencies. 'ircraft

operation normally consists of a frequency rangefrom - to .0,12. The microphone output isnormally fed to an amplifier the microphoneoutput needs to be even in amplitude over thefrequency range. ' microphone that can do that issaid to have a flat frequency response. +t is alsoimportant that the microphone impedance ismatched to the amplifier for ma3imum po!ertransfer.


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%icrophones used in aircraft are$

the carbon microphone

4lectret microphonecondenser microphone

dynamic microphone.


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Carbon microphones


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Electret microphone

The electret is one of the most popular microphones used in aircraft.

'n electret microphone operates similarly to a variable capacitor. Theelectret is an insulator !ith a permanent charge on it. +t consists oft!o plates one attached to the diaphragm !hich is varied the otheris fi3ed. 'ir pressure caused by speech moves the diaphragm

thereby varying the capacitance.

5ou may recall the formula for 6 !hich is$

6 7 C8

#here$ 6 7 charge in coulombs

8 7 voltage

C 7 capacitance in farads.

+f !e transpose this formula !e can see that$

8 7 6

C


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Electret microphone


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Condenser microphone

The condenser microphone is not that

different to an electret microphone and its

operation is also very similar. The maindifference is that in an electret microphone

a charge is permanently put on the plates of

the microphone and it can operate !ithout

an e3ternal voltage being put on.


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Condenser microphone


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Dynamic microphone


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Dynamic microphone

The dynamic microphone is very robust and isvery popular in aircraft use as it provides a largeconstant output over the audio frequency range.

To summarise the dynamic microphone !e cansay that this microphone."is commonly used in aircraft

"has a large constant output

"has a relatively lo! output impedance !hich allo!sthe output to be fed directly into a 9:T

"is relatively simple in construction and therefore cheapto manufacture.


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%icrophones$

"convert sound energy into electrical energy

"the electret and dynamic microphones are the

most commonly used in aircraft"require a flat audio response

"depending on type have different output

impedances

"may require an e3ternal voltage for their

operation.


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pea!ers

pea,ers !or, opposite to microphones in

that they convert the electrical energy into

sound energy.pea,ers for aircraft fit into areas.$

; earphones

; loudspea,ers.


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Earphones


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"oudspea!ers


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#ircraft application microphones

and spea!ers%icrophones and spea,ers used in aircraft

are$

"hand


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Modulating and decoding

T!o !aves are involved$

"the carrier orRF (radio frequency) sometimes

called CW (continuous !ave)"the modulating signal !hich can be an audio

signal amplified from the microphone.


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To achieve modulation !e need

a carrier

intelligence

to combine the t!o.


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Carrier

#e said this !as a continuous !ave an RF

(radio frequency) signal. This !ave consists

of sine !ave oscillations repeated manytimes. +f the carrier has a frequency of

MHz (%egahert2) that means the sine

!ave is repeated million times per second.


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Carrier


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$ntelligence

+ntelligence can be$

"audio

"video"a code

"data

"or any other types of useful information thatcan modulate a carrier and can be e3tracted at

the receiver modulation.


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Modulation

There are several !ays to achieve this.ome of the types !e are going to considerare$"amplitude modulation ('%)

"frequency modulation (F%)

"phase modulation (&%)

"pulse code modulation (&C%)

"single sideband modulation(9).


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Modulation relationship carrier

and intelligence

There are three !ays the carrier can be

changed to enable it to carry

intelligence. #e can change its$"amplitude

"&hase

"frequency.


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6uestions

#hy do !e need a carrier to transmit

intelligence=

ame four types of modulation


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The operation of '% radio transmitters

; signal generation

; buffer amplifier; audio amplifier

; modulator

; po!er amplifier


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#mplitude modulation


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#mplitude modulated signal

To achieve amplitude modulation !e need

to combine the carrier and the lo!

frequency signal in a non


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%on&linear mi'ing

on


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'% signal


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raphic presentation #M signal

'n '% signal can be represented

graphically in t!o !ays that is in the$

"time domain"frequency domain.


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raphic presentation #M signal


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#M ransmitter signal

The '% transmitter signal contains the

follo!ing frequencies$

"the carrier frequency"the lo!er sideband frequency (carrier minus

modulating frequencies)

"the upper sideband frequency (carrier plus

modulating frequencies).


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ideband frequency calculations

+f the carrier frequency !as > kHz and themodulating signal !as a single frequency tone of >,12 !e can calculate the sideband frequencies as

follo!s$" the upper sideband frequency 7 fc ? fs

" 7 > ,12 ? > ,12

"7 >> ,12

" the lo*er sideband frequency 7 fc < fs" 7 > ,12 < > ,12

" 7 @@ ,12.


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Po*er relationship in an #M

signal

+n '% the po!er contained in the RF

carrier remains constant. This is

independent of the po!er contained in thesidebands.


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otal po*er

#hen !e tal, about total po!er of an '%

transmission !e are considering all the

po!er contained in the modulated signal.This is po!er contained in the$

"carrier

"sidebands.


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ransmitter band*idth (+,)

+f !e !ill loo, at a '% transmitted signal

of > ,12 !hich is modulated !ith an

audio signal. The highest modulatingfrequency being .0 ,12. ound in the

range of 0Hz to ,12 can be heard by a

person !ith good hearing. +n airborne

communication the range that is required is

only bet!een - 12 to 0 12.


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'% transmitter band!idth


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'% Transmitter


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'% uperheterodyne Receiver


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-requency and phase modulation

There are t!o other !ays of

modulating a carrier by changingthe$

"frequency

"phase.


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#ngle modulation

Frequency and phase modulation are

methods of modulation that are related. 5oucannot change the frequency of a carrier

!ithout changing its phase.


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-requency modulation

Frequency modulation is angle modulation !here theamplitude of the modulating signal causes theinstantaneous frequency of the carrier to vary by anamount proportional to it.

The rate of deviation is determined by the frequency of themodulating signal.

Frequency modulation is used !hen the intelligence(audio) is to be transferred !ith lo! noise. 'ustralian T8uses frequency modulation for audio most stereo

broadcasts use F%.


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+n F%$

"the po!er of the output signal does not change !ith

modulation

"the sidebands e3tract their po!er from the carrier.

The number of significant side frequencies

contained in F% depends on the modulated inde3

!hich is related to the$"ma3imum frequency deviation

"modulating signal frequency.


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Modulation percentage -M

%odulation percentage in F% is predetermined by

the type of transmission and the ma3imum

deviation required.

For e3ample in an F% broadcast transmitter A B0

,12 frequency deviation is defined as >

modulation.

These transmitters are so designed so that this A B0,12 frequency deviation occurs !hen the loudest

audio signal is modulating the carrier.


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F% Transmitter


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F% Receiver


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The operation of F% radio transmitters

; signal generation

; buffer amplifier ; automatic frequency control (AFC)

; audio amplifier

; modulator

; frequency multiplier

; po!er amplifier


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Modulation inde' The ma3imum deviationrelates to modulation inde3.

+n '% the modulation inde3 lets you ,no! the depthof modulation that has ta,en place.

+n F% the modulation inde3 also lets you ,no! the

degree of modulation that has ta,en place.no!ing this figure is necessary to find out the

follo!ing information$"the number of significant side frequencies contained in

the modulated signal"the amplitude of the significant side frequencies

" the band!idth.


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To calculate the modulation inde3 for F%the formula is as follo!s$

mf 7 E f ma3fm ma3

#here$"mf 7 modulation inde3 (F%)

"E f ma3 7 ma3imum frequency deviation

"fm ma3 7 highest modulating signal frequency.


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Therefore for an F% broadcast transmitter !hichuses >0 ,12 as its highest modulation frequencyand the ma3imum frequency deviation equals B0

,12 the modulation inde3 can be calculated asfollo!s$

mf 7 E f ma3 B0 ,12

fm ma3 7 >0 ,12

7 0


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' mathematical chart

For this reason a mathematical function chart

called the 9essel function chart !as produced.

This chart sho!s the number of significant side

frequencies and their relative amplitude for each

modulation inde3 number. +t allo!s you to read

the number of significant side frequencies and

their amplitude for a particular modulation inde3figure.


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' mathematical chart

9essel function chart

#here$

; %f 7 modulation

inde3

; :o 7 carrier

amplitude

; :> to :> sho!s

the significant side

frequencies

contained andtheir amplitude.


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+and*idth for -M

's !e have seen !hen !e considered '% theband!idth needs to be !ide enough to transmit all therequired side frequencies.

To calculate the band!idth !e use the band!idthrequirements of an F% broadcast transmitter as ane3ample.

#e have calculated the modulation inde3 of a broadcasttransmitter !hich !as 0. Reading the 9essel chart for amodulation inde3 of 0 you can see there are Gsignificant side frequencies sho!n. These occur on bothsides of centre frequency.


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+and*idth for -M

To calculate the total band!idth !e need to consider >Hside frequencies for transmission. 5ou ,no! that thema3imum audio frequency transmitted is >0 ,12 and thetotal band!idth required is therefore equal to >H times >0,12 !hich equals ,12.

+n a broadcast transmitter the band!idth is large comparedto some industry applications !here F% is used forcommunications only. The ma3imum modulatingfrequency required for communications is only about -,12 and the band!idth allocated in some of these systemscould be as lo! as >0 ,12.

The band!idth required for F% is still substantially !iderthan that required for '%.


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Phase modulation

&hase modulation is angle modulation

!here the amplitude of the modulating

signal causes the phase angle of the carrierto depart from its reference value by an

amount proportional to it.


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Phase modulation


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6uestions

ame the frequency components of an '%signal.

#hat determines ho! many significant sidefrequencies are contained in an F% signal=

1o! is band!idth determined in a F% signal=

The operation of 9 transmitter to


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The operation of 9 transmitter to

bloc, diagram

; signal generation

; buffer amplifier

; audio amplifier; modulator

; filtering

; frequency translation

; po!er amplifier


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Radio #ave &ropagation

Iround #aves

,y #aves

pace #aves


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Iround #aves

Jsed belo! %12

Travels along 4arthKs surface

8ertically polarisedTerrain affects ground !aves

ot affected by daily or seasonal changes

4g$ '%


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Iround !ave propagation


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,y #aves

Reflected/refracted from ionosphere

Jsed up to - %12

+onosphere effects " ight effectReception beyond line of sight


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'ircraft Communication ystems

1F $ -


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'ntennas

Device that transforms electrical signals

into 4% !aves

Reciprocity Theorem

&olarisation

Radiation &attern

'ntenna Types$ 1ert2 Dipole %arconi'ntenna 'rrays


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'/C antenna


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81F 'ntenna


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Transmission *ines

T!isted &air

Coa3ial

#aveguides


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Transmission *ines in 'ircraft


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#aveguides

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01 Avionics Systems