Part 3 - Radio/Radar Fundamentals Flashcards
1
Q
Name three types of modulation used in radio signals?
A
- Pulse width modulation
- Frequency modulation
- Amplitude modulation
2
Q
In a normal AM voice transmission, how many sidebands are there?
A
Two, upper and lower
3
Q
An oscillator will be more stable when it is operating at what frequencies?
A
Low frequency
4
Q
A frequency multiplier will normally use what class of amplifier?
A
Class C as it gives more harmonics
5
Q
Which type of radio communications uses ‘On-Off Keying’?
A
Continuous wave or radio telegraphy
6
Q
Which type of modulation works by varying the strength of the carrier wave by the information being sent?
A
AM
7
Q
Which type of modulation has the greatest bandwidth - AM or FM?
A
FM
8
Q
A ‘current’ or ‘centre-fed’ aerial has an input impedance of:
A
73 Ω
9
Q
How can the ‘earth connection of a ‘Marconi’ aerial be enhanced?
A
By the use of copper matting or by locating onto a metallic plate
10
Q
What type of aerial uses ‘directors’ and ‘reflectors’?
A
Directional array aerials
11
Q
Which type of transmission cable is most suitable to carry frequencies up to 1000 MHz?
A
Co-axial
12
Q
What are the four types of aerial coupling in radio receivers?
A
- Transformer
- Auto transformer
- Capacitive
- Stagger tuned
13
Q
In radio receivers, what will transformer coupling give?
A
The best selectivity
14
Q
In radio receivers, which type of aerial coupling will give the best sensitivity?
A
Auto transformer or capacitive coupling
15
Q
In radio receivers, which type of aerial coupling will give the widest bandwidth?
A
A stagger tuned coupler
16
Q
If a receiver is tuned to 80 MHz, and the local oscillator produces 100 MHz, what will the intermediate frequency (IF) be?
A
20 MHz
17
Q
What are the three types of external noise, a sub-division of noise interference?
A
- Man made
- Atmospheric
- Cosmic
18
Q
What is the signal/noise ratio at the output of a receiver?
A
Ratio of signal power to noise power
19
Q
What is the noise factor of an ideal receiver?
A
Unity
20
Q
A receiver’s ability to select the desired signal frequency and reject, or attenuate to a low level, other unwanted signal frequencies, is known as what?
A
Selectivity
21
Q
What is the typical power output for a HF radio system?
A
400 W
22
Q
What is fidelity, as it pertains to receivers?
A
Ability of a receiver to reproduce an output which is an exact copy of the modulation of the input signal
23
Q
If a receiver’s bandwidth is too small, what does this mean?
A
Loss of the modulating frequencies on AM
Clipping of the modulation signal on FM
Transmitted signal in either case won’t be faithfully reproduced
24
Q
What is a Tuned Radio Frequency (TRF) receiver?
A
Basic receiver operation on one frequency only
25
Where might Tuned Radio Frequency (TRF) receiver be used?
ILS and single channel domestic radios
26
What is the name of the radio receiver which uses a fixed frequency called an intermediate frequency (IF)?
Super Heterodyne Receiver (SuperHet)
27
In an AM transmitted wave, what is the total power equal to?
The carrier power and the two sideband powers
28
What is the total power, if a carrier wave has a power of 50W, and a modulation index of 0.5?
50.625
PT=PC+PC(m2/2)
29
What are the significant problems which exist with transmission in the UHF and VHF bands?
* Skin effect
* Stray capacitance
* Lead inductance
* Stray resonances
* Parasitic oscillation
* Component size
30
Why might parasitic oscillations occur at lower frequencies?
Poor decoupling of supply lines
31
Where is the maximum skin effect in a conductor?
In the centre
32
What are the two types of frequency mixing processes in radio receivers?
Additive or multiplication
33
What does the choice of mixer type depend on?
* Noise
* Frequency
* Gain
* Type of active device
* Value of the IF
34
Why is an additive mixer used for frequencies above 50 MHz?
Additive mixing introduces less noise, which is important at frequencies above 50 MHz
35
When RF and LO frequencies are added, what frequencies are produced?
* RF
* LO
* RF + LO
* RF - LO
36
What is the importance of the difference frequency in an additive mixer?
It is known as the intermediate frequency which is then selected and amplified further before demodulation
37
What is the bias of a mixer used for additive mixing of RF and LO signals?
Class B
38
What is the bias of a dual gate FET mixer and why?
Class A
Avoids distortion of the modulated signal
39
In frequency mixing, which mixers will normally have 'coupling' between the inputs (RF and LO)?
Diodes
40
What does a low IF give?
* Easier tracking (LO closer to SF)
* Good adjacent channel rejection
* Low LO frequency, hence stable operation
* Narrow bandwidth (good selectivity)
41
What are the typical compromise IFs for AM and FM?
AM - 465 kHz
FM - 10.7 MHz
42
How many IFs will a double superhet use?
Two
43
What is the impact on bandwidth if IF amplifiers are used in cascade?
Reduced
44
What does the so called 'linear' method of demodulating the AM signal involve?
Feeding the signal, appropriately amplified, to a simple rectifier and then filtering
45
What are the relative properties of a series as compared to a shunt demodulator detector?
* In basic circuit, good RF filter action
* Relatively high input impedance
* Relatively high output impedance
* Audio output level slightly less than on modulated RF input waveform
* Reservoir and load values quite critical if distortion is to be avoided
46
A 'Beat Frequency Oscillator' is used during the demodulation of which type of signals?
Continuous wave
47
What is delayed AGC?
No AGC is produced until the receiver reaches a pre-set minimum level
The receiver gain is then at a maximum on weak input signals
48
Noise can be removed from the signal by using what type of circuit?
Noise limiter
49
What type of circuit is used under 'no signal' conditions, to prevent 'white noise'?
Squelch, quiescent AGC or muting
50
Inside an AF amplifier, distortion can be caused by what 4 things?
* Harmonics
* Intermodulation
* Incorrect loading
* Waveform shape
51
What is a transducer?
A device that converts energy from one form into another
52
Which type of microphones are used in aircraft?
* Carbon
* Moving coil
* Dynamic
* Condenser
53
Explain a carbon microphone?
Consists of a cup shaped metal container having a carbon 'back' electrode fixed inside
A carbon plunger is positioned at the entrance to the cup and is attached to a light aluminium or paper diaphragm
The space between the carbon plunger and the carbon 'back' electrode is filled with carbon granules
The diaphragm vibrates when stimulated by sound, which pushes the plunger in and out of the cup, compressing and relaxing the carbon granules
As the contact area between adjacent carbon granules increases, so the resistance increases
A power source between the carbon elements means voltage is induced through the granules of varying DC which is a facsimile to the sound waves
54
Why is there more noise in the carbon microphone than in the moving coil microphone?
The movement of the carbon granules causes the noise
55
Which type of microphone requires an energising voltage, is reasonably robust and as a single ended output?
Carbon
56
What is the construction of a moving coil microphone?
Consists of a specially shaped permanent magnet in the form of a ring, with a central pole or limb
Around the central pole in a loose fit is a light paper tube, which is attached to a light metal or paper diaphragm
On the outside of the tube is a winding of fine insulated copper wire known as the speech coil
57
What are the characteristics of a moving coil microphone?
* No energising voltage
* Good signal/noise ratio
* Good frequency response
* Low voltage output
* Low impedance
* Balanced output, normally connected to the primary of an audio transformer with its centre tap at earth
58
What is the name of the microphone that is a carbon and moving coil hybrid?
Dynamic microphone
59
On what principle does noise cancelling work on?
Equal pressure on either side of the microphone diaphragm to cancel the effect of this ambient noise
Speech only comes from one side of the microphone
60
What type of microphone is often used as a cockpit area microphone and what is its principle of operation?
Condenser microphone
Sounds waves move a diaphragm which results in a changing capacitance between the diaphragm and a back plate
61
Why is a condenser microphone usually used with an integral amplifier?
It has a low voltage output and high impedance
62
What is the purpose of the permanent magnet in a moving iron telephone?
Polarises the device, ensuring that the device is polarised otherwise the generated sound waves would be twice the frequency of the audio currents in the coils
63
What is used to prevent the sound wave from the front of a loudspeaker cone cancelling the one from the back?
Baffle board
64
What is the size of a baffle board dictated by?
Lowest frequency which must be reproduced without a loss of power
At least one quarter wavelength at this frequency around the speaker
65
How does a SSB differ from an AM transmission?
* Only one sideband is present
* Carrier power can be very reduced or virtually non-existent
66
What are the advantages of SSB over DSB AM?
* Power saving
* Reduced bandwidth
* Noise reduction
* Reduction in multipath or selective fading
67
What are the three types of SSB transmissions?
* Suppressed carrier
* Pilot carrier
* Controller carrier
68
What does the efficiency of a controller carrier system depend on?
The carrier is only on when no information is being transmitted, so the more information that is sent the more efficient the system
69
How is the problem of detection being complicated in the suppressed carrier system overcome in the pilot carrier system?
The carrier is sent at a reduced signal together with one sideband, making detection easier
70
What is the output of balanced modulator?
Upper and lower sidebands without the carrier
71
The diode quad is an example of which type of radio circuit?
Balanced modulator
72
Name some of the common filters?
* Simple LC filters
* Electro-mechanical filters
* Crystal lattice filters
* Ceramic filters
73
What principle does an electro-mechanical filter work on?
Magnetostriction effect
74
What does the frequency response of a electro-mechanical filter depend on?
Dimensions of the elements
75
Which types of filters have a well defined pass band?
Magneto-striction and crystal types
76
How does the pilot carrier SSB circuit differ from the suppressed carrier SSB circuit?
A combining circuit is included after the lower sideband is filtered out, to add in the carrier at a reduced amplitude
77
What is the purpose of the LO in an SSB receiver?
Maintain a stable output, thus maintaining a constant IF
78
What is the purpose of the detector in an SSB receiver?
Extracts the required information from the signal as faithfully as possible
79
Why is the triple superhet receiver the most suitable receiver for the reception of narrow band SSB?
It has high gain and selectivity
80
What does a suppressed carrier SSB (SCSSB) rely on for stable frequency operation?
* Transmitted signal remaining precisely on frequency
* Ver stable, locally generated carrier signal being incorporated in the circuitry
81
What formula can be used to calculate the modulation index of an FM signal?
M = m (fd/fm)
Where - m is modulation factor, fd is the frequency deviation and fm is the modulating signal frequency
82
How is the bandwidth of an FM signal calculated?
2 * the number of sidebands * modulating frequency
83
How does the amplitude of an AF signal affect the FM signal?
Amount by which the carrier deviates from the nominal (centre) frequency
84
How does the frequency of the AF signal affect the FM signal?
Rate at which the carrier deviates
85
Why is the frequency content of an FM wave very much more complicated than that of an AM waveform?
Contains many more sidebands, spaced at intervals determined by the modulating signal frequency
86
How does the modulation index vary with a) frequency deviation and b) modulating frequency?
a) directly
b) inversely
87
Why is FM normally confined to frequency ranges of VHF and above?
Due to the large bandwidth required
88
Why is an FM signal affected less by external noise than an AM signal?
The signal is of constant amplitude so less influenced by amplitude modulated noise
89
Why is frequency multiplication necessary in FM transmitters?
* Achieves a high operating frequency normal in FM from a low frequency
* Allows the MO to operate at low frequency which helps its stability
* Enables the large frequency deviation in FM to be obtained
90
How can a crystal oscillator be used to control an FM transmitter?
The frequency of a crystal oscillator is compared to the frequency of the master oscillator
A discriminator produces a dc output, which, when applied to a reactance device, changes the L or C of the master oscillator, thus changing its frequency
91
What is the purpose of the limiter and frequency discriminator in a FM receiver without automatic frequency control?
Frequency discriminator is in effect the demodulator
Limiter is needed if the frequency discriminator is frequency sensitive
92
What components make up automatic frequency control and where would it connect in an FM receiver?
Output from a detector stage is passed through an AFC filter and then to a variable reactance circuit which feeds into the LO
An AFC defeat switch open circuits the AFC loop, allowing manual tuning to a weaker station
93
What is a pre-emphasis and de-emphasis circuit in an FM receiver?
Pre-emphasis - output of the discriminator has the same signal/noise ratio for both high and low audio frequencies
De-emphasis - output of the discriminator is flattened
94
What are the differences between FM receiver stages and AM?
RF amp - not always required in AM receivers
IF amp - may be more stages of IF amplifications, last stage may also act as a limiter and bandwidth is much greater
95
What are the advantages of FM over AM?
* Reduced adjacent channel problems
* Less affected by noise
* High quality due to large bandwidth
* Efficient
* On the same frequency, only the strongest signal is processed
96
What are the disadvantages of FM over AM?
* Large bandwidth
* Can only use VHF and above due to the large bandwidth
* Line of sight transmission only
97
Where is AGC in an FM receiver derived and where is it applied?
Derived from the amplitude of the input signal to the limiter stage, and applied to the earlier IF amplifier stages and the RF amplifier
98
What are the advantages of the ratio discriminator?
* Changes in amplitude do not affect it
* Output can be used for AGC
99
What is the disadvantage of the ratio discriminator?
Not very linear, therefore the distortion levels are greater
100
How does an FM discriminator ensure the accuracy of the re-inserted carrier in automatic frequency control?
The discriminator compares the filtered carrier and give an output proportional to the deviation of the carrier from the frequency to which the discriminator is tuned
Output is fed to a variable reactance device, which varies the frequency of the second LO
101
What are the chief disadvantages of SSB?
* More complex
* Frequency stability is more critical in SSB systems
* Can be overpowered by stronger signals
102
Describe a Transverse Electromagnetic Wave?
* Made up of an electric (E) field and a magnetic (H) field
* Both fields vary sinusoidally and are of the same frequency and phase
* Fields are at 90° to the direction of propagation
* Polarisation refers to the E field direction
103
What is an isotropic radiator?
Truly omnidirectional source of RF
May be represented by an expanding sphere
104
How much power is required to double the range of a transmission?
Four times
105
What are the two general modes of propagation utilised by radio waves?
Those which make use of layers of ionised gases and those that do not
106
Describe groundwaves?
Primary mode for frequencies below 2 MHz
Great benefit is that it is little affected by seasonal or diurnal variations, unlike skywaves
Disadvantage is that is requires very powerful transmitters and large aerials
107
Why are higher frequencies attenuated more at surface level?
More penetrations of the underlying terrain, where each penetration contributes to overall attenuation
108
What are the advantages of open twin feeder?
* High transmitter power outputs can be handled without danger of 'flash over' between conductors
* Standing waves can be easily measured and maintenance of line is easy
* The line is balanced
109
What are the disadvantages of open twin feeders?
* Bulky and rigid and can only be used in static installations
* Practical upper frequency limit is about 100 MHz
* Line must be kept clear of ground and walls
110
What does an open (twin wire) feeder consist of?
2 parallel wires maintained at a fixed distance apart by insulating spacers
