nav topic 3 radio components

AV2220 - Aircraft Communication Systems Chapter 1 1

AVIONICS AVIONICS TECHNOLOGYTECHNOLOGY

Basic Radio System Basic Radio System ComponentsComponents

An amplifier is a device that increases the strength of a signal. They are found in both

transmitters and receivers.

A transmitter must increase the strength of the signal sent to the antenna so that the EM waves will travel a useful distance outward from the antenna.

A receiver needs amplifiers because the strength of the signal from the antenna is very low and must be increased to enable the signal to be heard.

Amplifiers can be categorized as Class A, Class B and Class C. The difference between these is

the shape of the output waveform.

The output of a Class A amplifier is complete sine waves just like the input. The Class B amplifier has an output which shows only half of each sine wave.The Class C amplifier has an output waveform which is less than half of the sine wave

Amplifiers



The Class C amplifier is often used as a power amplifier because of its higher efficiency.

The output of the Class C amplifier can be sent through an LC circuit or other device to restore the complete sine wave shape.

Amplifiers (cont’d)

An example of the output waveform for Class A, B and C amplifiers



An oscillator is a device that produces the frequencies needed by both receivers and transmitters.

A simple oscillator is an LC circuit or tank circuit made up of a capacitor and inductor in parallel.

The LC circuit will have a resonant frequency which matches the desired frequency.

An LC circuit by itself will not continue to oscillate because of resistance in the components and wires.

Oscillators

A parallel LC tank circuit.



An LC tank circuit which connected to a battery can produce oscillations. If the switch in Figure on the right-side is moved to position A, the capacitor will be charged by the battery. If the switch is then moved to position C, the tank circuit will start to oscillate as energy is transferred rapidly back and forth between the capacitor and inductor. The oscillations will become weaker and die out because of the resistance in the circuit.

Oscillators (cont’d)

A parallel LC tank circuit



In order to maintain oscillations, some energy must be fed back into the tank circuit.

The resonant frequency or oscillation frequency is determined by the values of capacitance and inductance in the tank circuit.

The LC circuit will not be stable over a period of time and may drift off the correct frequency.


An oscillator circuit with feedback supplied by a transistor



A common technique to stabilize the oscillator and produce a more accurate frequency is to use a crystal.

The piezoelectric effect of the crystal will produce a more accurate and consistent output frequency from the oscillator.


A crystal controlled oscillator



In the radio transmitter, a device is needed which will combine the AF signal with the RF carrier wave signal before it is sent to the antenna.

This is the function of a modulator, it combines the AF and RF signals so that information can be transmitted.

The output of the modulator is called modulated RF.

The signal produced by the antenna in a receiver will be modulated RF.

In order to hear the voice as an output of the receiver, the AF component must be separated out.

The demodulator removes the RF component of the modulated RF signal and produces an AF output.

When the AF and RF signals are combined in the modulator, they must have the proper relative strengths for maximum efficiency.

The amount of modulation is called the modulation rate.

Modulators and Demodulators



If the AF signal is too weak compared to the RF signal, the modulation rate will be low and the efficiency will also be low.If the modulation rate is over 100%, there will be distortion in the signal due to the gaps created. Figure (on the right-side) gives examples of 50%, 100% and more than 100% modulation rates with AM modulation.Most radio transmitters are adjusted to about 90 95% modulation to ‑provide a little margin to prevent distortion.

Modulators and Demodulators

Examples of different modulation rates (A) 50%; (B) 100%;(C) Over 100%



A filter is used in a radio circuit to remove or filter out unwanted frequencies.

The signals that are processed by the circuits in a radio often have additional frequencies present that are not needed.

If the proper filter is installed, it will filter out the frequency or frequencies that are not wanted.

A filter is usually made up of an arrangement of inductors and capacitors.

Filters

Radio frequency filters are combinations of inductors and capacitors



A low pass filter will remove all frequencies above a certain value and pass the low ones.

A high pass filter does the opposite. If a range of frequencies must be blocked, a band reject filter will be used.

A bandpass filter will allow a certain band of frequencies to go through and block frequencies either above or below that range.

Filters (cont’d)

Radio frequency filters are combinations of inductors and capacitors



An antenna is a device that transforms electrical signals into EM waves in the case of a transmit antenna, or transforms EM waves into electrical signals in the case of a receive antenna. An antenna may be used for transmit only, receive only, or both, depending on the particular radio system involved.The maintenance, inspection and installation of antennas are usually the responsibility of the airframe technician since they are attached to the structure or skin of the aircraft.

Three characteristics of an antenna are critical: length, polarization directivity.

For an antenna to be most efficient, its length must be one-half the wavelength of the signal being transmitted or received. This length allows the antenna

current to be maximum.

Antennas



To induce the maximum amount of voltage into the receiving antenna, the antenna must be installed in such away that it is: perpendicular to the magnetic,

H, field in the radio waves. parallel to the electric, E, field

in the radio waves.

When the transmitting antenna is vertical, the E field is vertical, and the radiation is said to be vertically polarized. The maximum reception is picked

up with a vertical antenna.

When the transmitting antenna is horizontal, the radiation is horizontally polarized, and is best received on a horizontal antenna. When the transmitting antenna is vertical, its electric field is vertical and the magnetic field is horizontal. It is picked up best by a vertical

antenna. Most LF, MF, and HF communication use horizontally polarized antennas, and Higher frequency systems use vertically polarized antennas.

Antennas (cont’d)



The dipole antenna in A transmits its signal strongest in a direction perpendicular to its length.

The vertical whip antenna in B has a uniform field strength in all directions and is called an onmidirectional antenna.

The loop antenna in C is highly directional. Its strength is sharply reduced in

the direction perpendicular to its plane.

Antennas (cont’d)

Directional characteristics of typical antennas



Antennas often have general names that describe some of their basic characteristics.

Two of the more common types are the Hertz dipole antenna and the Marconi monopole antenna.

The Hertz dipole antenna has two metal conductors in a straight line with the connection in the middle. It is called a half wave ‑

antenna because the overall length is equal to one half the wavelength of the EM wave it is designed to be used with.

Antennas (cont’d)

The Hertz dipole antenna is a half wave antenna‑



The Marconi antenna is a single metal conductor with a length of ¼ wavelength.

In order to work properly, the Marconi antenna must have metal surrounding the mounting base. The metal at the base is needed

for efficient operation of the antenna.

The necessary metal at the base is called the groundplane or counterpoise.

Antennas (cont’d)

The Marconi antenna is a ¼ wave monopole antenna ‑that requires a groundplane



The groundplane is the four metal rods at the base of the antenna; the metal skin of an aircraft is

used as a groundplane for most aircraft antennas.

Most antennas must be installed with the correct polarization.

Polarization refers to the orientation of the electric field relative to the earth. If the electric field is vertical, it

has vertical polarization.

Antennas (cont’d)

The Marconi antenna is a ¼ wave monopole antenna ‑that requires a groundplane



Antennas (cont’d)

A horizontally polarized Hertz antenna will produce this type of radiation pattern

A vertically polarized Marconi antenna will produce this type of radiation pattern



Antennas (cont’d)

The connection between an antenna and a radio normally requires a coupler in order to give the best transfer of energy between the two of them.

Two common types of antenna couplers are the LC circuit and the transformer types.

An isolation transformer can be used as an antenna coupling device.

A transformer type of coupler antenna



The use of an LC circuit as a coupler between the coax and antenna is shown in Figure below.

In the past, long-wire trailing antennas were used for HF communication. But advances in communication technology have developed tuned antennas that are actually part of the aircraft structure. Other aircraft use a copper-clad steel wire enclosed in a polyethylene covering run from outside the fuselage above the cockpit to the top of the vertical fin.

Antennas (cont’d)

An LC circuit used as an antenna coupler



VHF communication uses the frequencies between 118 and 136 megahertz, which are just above the VOR frequencies, and the antenna used is normally a quarter-wavelength, vertically polarized whip.

The metal in the aircraft structure provides the other quarter-wavelength to make the antenna electrically a half-wavelength long.

Many whip antennas are bent so they can also pick up horizontally polarized signals.

Broad-band blade antennas provide more efficient transmission and reception than simple whips.

Antennas (cont’d)



An antenna will intercept many different EM waves of different frequencies so some method must be used to separate out the desired frequency. The tuning circuit performs this

function. As the tuning knob is rotated on the radio, it moves the variable capacitor until the resonant frequency of the circuit matches the frequency of the desired station. This signal is passed into the

radio and the other frequencies are blocked out.

Tuning Circuits

A simple tuning circuit which consists of a variable capacitor and an inductor in parallel



A better type of tuner which is found on most modern radios uses a frequency synthesizer which contains a number of crystals that can be combined to match the desired frequency. Each crystal has a particular frequency and by using switches the crystals can be combined to produce many additional frequencies. When two frequencies are combined, two new frequencies are created that are equal to the sum and the difference of the two frequencies.

Tuning Circuits (cont’d)

The basic operation of a frequency synthesizer



Transmitters

Simplified block diagram of a voice radio transmitter



A simple voice radio transmitter consists of: Frequency oscillator which operates at one half the carrier frequency Buffer amplifier and frequency doubler which assigned to amplify and doubling

the signal so that it has enough power to radiate into space when it goes to the antenna.

The modulator combines the AF and RF signals The power amp which operates to amplify signal before being sent down the

coax to the antenna.

The transmitter uses a crystal oscillator to produce an accurately controlled carrier frequency, and only this one frequency radiates from the transmitter antenna.

In order for a receiver to be useful, it must filter out every frequency except the one that is wanted. To do this, it employs a special superheterodyne circuit.

Transmitters (cont’d)



In the 1920s, a new type of radio receiver was invented that produced better sound quality. It was called the superheterodyne or superhet radio.

The only major difference between the superhet and earlier radios was that it reduced the modulated RF signal from the antenna to an AF signal in more than one jump or stage.

Receivers



Receivers

Simplified block diagram of a superheterodyne VHF voice radio receiver



In a VHF superhet aircraft receiver block diagram above: The RF signal from the antenna is combined with a local oscillator frequency to

produce a lower IF frequency. The intermediate frequencies found in a superhet radio are abbreviated as IF. The output of the mixer is the difference between the RF frequency and the

local oscillator frequency. The basic principle of the mixer is: two different frequencies are combined, two new frequencies are created; the sum and the difference of the two

combined frequencies. The IF signals is amplified and then sent to the detector and demodulator. The detector chops off half of each sine wave to produce a varying DC signal

from an AC signal. The AF signal is amplified and used to drive the speaker.

Receivers (cont’d)




A tunable local oscillator is included in this circuit. The frequency of this oscillator is

varied so it is always a specific frequency higher than the frequency to which the preamplifier is tuned.

For most broadcast band receivers, the frequency of the local oscillator is always 455 kilohertz higher than the frequency tuned on the preamplifier. In this case, the local oscillator

produces a signal with a frequency of 1,655 kilohertz (1,200 + 455).

The antenna picks up all the radio signals in the area and they are taken into a tunable preamplifier. This preamplifier uses an

electronic filter circuit that passes only the frequency to which the receiver is tuned and sends all of the other frequencies to ground.

The preamplifier amplifies any signal with a frequency of e.g. 1,200 kHz and passes all other frequencies to ground.




The four signals will have frequencies of: 1,200 kHz 1,655 kHz 2,855 kHz (1,200 + 1,655) 455 kHz (1,655 - 1,200)

The four signals from the mixer are sent into the intermediate frequency (IF) amplifier. This is a very narrow-band amplifier that is tuned to 455 kHz. It amplifies the 455 kHz signal and attenuates, or diminishes, all other frequencies.

The signals from the preamplifier and the local oscillator are sent to the mixer.

When signals with two frequencies are mixed, they produce two other signals: one with a frequency that is the

sum of the original two frequencies and

the other with a frequency that is the difference between the two.



The amplified 455 kHz signal is sent to the detector/demodulator It removes the 455 kHz IF carrier and leaves the AF envelope that has both

halves of the audio signal.

The detector rectifies the AF signal and removes one half of the envelope.

The AF signal is amplified by a power amplifier stage and drives the speaker.

The output of the speaker is the same as the input to the microphone at the transmitter.




Communication receivers such as those used in aircraft are more sensitive than the normal household broadcast receiver, and they have more stages.


A simplified block diagram of a VHF superhet communication receiver



The signal is picked up on the antenna and amplified by the tuned preamplifier.

The local oscillator produces a frequency that is 10.8-megahertz different from the frequency to which the preamplifier is tuned.

These two frequencies are fed into the mixer where they produce a 10.8-MHz intermediate frequency.

This IF is amplified by two stages of IF amplification and sent into the detector/demodulator, where it emerges as an audio frequency signal that duplicates the AF produced by the microphone at the transmitter. Some of the detector output is sent into a squelch circuit that controls the audio frequency amplifier. When no signal is being received, the AF amplifier output is attenuated, or decreased, so the background noise that makes a hissing sound in the speaker is not loud enough to be annoying.




Aircraft radios often supply an audio output for the pilot and voice transmitters require an audio input from a microphone.

A speaker is a device that transforms electrical signals into sound waves.

When the audio frequency signal is applied to the windings in the speaker, it sets up a magnetic field that expands and contracts at an audio rate.

Speakers and Microphones

A dynamic speaker



The magnetic field causes the metal diaphragm to vibrate at a corresponding rate to produce the movement of air that generates sound waves.

Dynamic microphones are available which operate in the opposite way.

Many newer and more efficient types of microphones are now being manufactured, but they all work by transforming the vibrations of sound waves into varying electrical signals.

Speakers and Microphones (cont’d)

A dynamic speaker



When an aircraft has more than one radio, an efficient means of switching the microphone and speaker connections from one radio to another is needed. The audio control panel performs

this function.

An audio control panel is not a radio because it only uses audio frequencies, but it is associated with the radios in the aircraft.

Audio Control Panel

A typical audio control panel



This audio control panel has a row of toggle switches that can be used to connect the audio output of the various radios to the speaker or headphones.

It also has a rotary selector switch to connect the microphone audio output to the different radio transmitters and intercom systems available for the aircraft.

The audio control panel illustrated also has three lights that are the indicators for the marker beacon system on the aircraft.

Audio Control Panel (cont’d)

A typical audio control panel



Regulations and Regulations and Standards for Radios Standards for Radios

Aircraft avionics equipment might have to comply with a number of different regulations and standards depending on the type of equipment and the type of aircraft in which it is installed.

Regulations from the FAA and the FCC apply to the manufacture and use of most types of equipment and carry the force of law.

FAA standards for equipment are usually in the form of TSO (Technical Standard Order) approvals.

FCC rules generally apply to equipment which produces radio waves.

An FCC Station License is required for aircraft that have radio transmitters other than ELT.

Each different type of transmitter must be listed on the license that is displayed in the cockpit.

The role of ARINC in established standards apply to the equipment in air carrier jets and bizjets primarily.

There are some FAA Regulations concerning the use of radio equipment in flight.



No person may operate an aircraft in an Airport Radar Service Area (ARSA) unless two way radio communication is established and maintained with ATC. ‑

A transponder with Mode C automatic reporting of aircraft pressure altitude is also required in all ARSAs.

FAR 91.130 (c)

No person may operate an aircraft in a Terminal Control Area (TCA) unless it has:1. A two way radio with appropriate frequencies available.‑

2. An operable transponder with Mode C altitude reporting.

FAR 91.131 (c) and (d)



Two way radio communications and navigation equipment appropriate to the ‑ground facilities that will be used.

At and above 24,000 ft. MSL; approved DME (distance measuring equipment).

FAR 91.205 (d) and (e) Minimum Equipment Requirements for IFR



FAR 91.511

Large and turbine powered, multi engine airplanes, if operating over water ‑ ‑more than 30 minutes or 100 nautical miles from the nearest shoreline, must have:

1. Radio communication equipment appropriate to the ground facilities.

2. Two transmitters.

3. Two microphones.

4. Two headsets (or headset and speaker).

5. Two independent receivers.

6. If needed, one HF transceiver.

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