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LOOKS FAM > NEETS 12 Modulation Principles > Flashcards

NEETS 12 Modulation Principles Flashcards

1
Q

What is modulation?

A

Modulation is the impressing of intelligence on a

transmission medium.

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2
Q

What is a transmission medium?

A

May be anything that transmits information, such as light, smoke, sound, wire lines, or
radio-frequency waves.

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3
Q

What is heterodyning?

A

Mixing two frequencies across a nonlinear impedance.

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4
Q

What is demodulation?

A

The process of recovering intelligence from a modulated

carrier.

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5
Q

What waveform is the basis of all complex waveforms?

A

The sine wave.

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6
Q

What is the purpose of using vectors?

A

To represent quantities that have both magnitude and

direction.

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7
Q

What is the trigonometric ratio for the sine of an

angle?

A

Sine Ө = opposite side ÷ hypotenuse.

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8
Q

What is the mathematical formula for computing the
output voltage from a moving coil in a
magnetic field?

A

E=EmaxsineӨ

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9
Q

What is the instantaneous amplitude of a sine wave?

A

The value at any given point on the sine wave.

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10
Q

What term describes how much of a cycle has been

completed?

A

Phase or phase angle.

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11
Q

What determines the frequency of a sine wave?

A

The rate at which the vector which is generating the sine

wave is rotating.

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12
Q

What is the period of a cycle?

A

The elapsed time from the beginning of cycle to its

completion.

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13
Q

How do you calculate the wavelength of a sine wave?

A

Wavelength = rate of travel × period.

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14
Q

Define the heterodyne principle.

A

Process of combining two signal frequencies in a

nonlinear device.

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15
Q

What is a nonlinear impedance?

A

An impedance in which the resulting current is not

proportional to the applied voltage.

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16
Q

What is spectrum analysis?

A

The display of electromagnetic energy that is arranged

according to wavelength or frequency.

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17
Q

What two conditions are necessary for heterodyning

to take place?

A

At least two different frequencies applied to a nonlinear impedance.

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18
Q

What is amplitude modulation?

A

Any method of modulating an electromagnetic carrier
frequency by varying its amplitude in accordance with
the intelligence.

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19
Q

What are the three requirements for cw

transmission?

A

A method of generating oscillations, a method of turning
the oscillations on and off (keying), and an antenna to
radiate the energy.

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20
Q

Name two methods of oscillator keying.

A

Plate keying and cathode keying.

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21
Q

State the method used to increase the speed of

keying in a cw transmitter.

A

Machine keying.

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22
Q

Name three advantages of cw transmission.

A

A high degree of clarity even under severe noise
conditions, long-range operation, and narrow
bandwidth.

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23
Q

Name a disadvantage of a single-stage cw

transmitter.

A

Antenna-to-ground capacitance can cause the oscillator

frequency to vary.

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24
Q

What is the purpose of the power-amplifier stage
in a master oscillator power amplifier cw
transmitter?

A

To isolate the oscillator from the antenna and increase
the amplitude of the rf oscillations to the
required output level.

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25
Q

What is the purpose of frequency-multiplier stages

in a VHF transmitter?

A

To raise the low frequency of a stable oscillator to the

vhf range.

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26
Q

What is a microphone?

A

An energy converter that changes sound energy into

electrical energy.

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27
Q

What special electromechanical effect is the basis

for carbon microphone operation?

A

The changing resistance of carbon granules as pressure

is applied to them.

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28
Q

What is a major disadvantage of a carbon microphone?

A

Background hiss resulting from random changes in the

resistance between individual carbon granules.

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29
Q

What property of a crystalline material is used in

a crystal microphone?

A

The piezoelectric effect.

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30
Q

What is the difference between a dynamic microphone

and a magnetic microphone?

A

A dynamic microphone has a moving coil and the magnetic

microphone has a moving armature.

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31
Q

What are the two major sections of a typical AM

transmitter?

A

Rf and af units

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32
Q

When 100 kilohertz and 5 kilohertz are heterodyned,

what frequencies are present?

A

100 kHz, 5 kHz, 95 kHz, and 105 kHz.

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33
Q

What is the upper sideband of an AM transmission?

A

All of the sum frequencies above the carrier.

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34
Q

Where is the intelligence in an AM transmission

located?

A

The intelligence is contained in the spacing between the

carrier and sideband frequencies.

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35
Q

What determines the bandwidth of an AM

transmission?

A

The highest modulating frequency.

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36
Q

What is percent of modulation?

A

The depth or degree of modulation.

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37
Q

With a single modulating tone, what is the amplitude
of the sideband frequencies at 100-percent
modulation?

A

One-half the amplitude of the carrier.

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38
Q

What is the formula for percent of modulation?

A

%M=Em/Ec x100%

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39
Q

What is high-level modulation?

A

Modulation produced in the plate circuit of the last

radio stage of the system.

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40
Q 
For what class of operation is the final rf power
amplifier of a plate-modulator circuit biased?
A

Class C.

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41
Q

The modulator is required to be what kind of a

circuit stage in a plate modulator?

A

Power amplifier.

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42
Q

How much must the fpa plate current vary to produce

100-percent modulation in a plate modulator?

A

Between 0 and nearly two times its unmodulated value.

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43
Q

The collector-injection modulator is similar to

what type of tube modulator?

A

Plate modulator.

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44
Q

When is a control-grid modulator used?

A

In cases when the use of a minimum of af modulator power

is desired.

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45
Q

What type of modulator is the cathode modulator

(low- or high-level)?

A

Low-level.

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46
Q

What causes the change in collector current in an

emitter-injection modulator?

A

Gain is varied by changing the voltage on the emitter.

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47
Q

What are the two types of angle modulation?

A

Frequency and phase.

48
Q

Name the modulation system in which the frequency
alternates between two discrete values in response to
the opening and closing of a key?

A

Frequency-shift keying.

49
Q

. What is the primary advantage of an fsk transmission

system?

A

Resistance to noise interference.

50
Q

What characteristic of a carrier wave is varied in

frequency modulation?

A

Instantaneous frequency.

51
Q

How is the degree of modulation expressed in an fm

system?

A

As the ratio of the frequency deviation to the maximum

frequency deviation allowable.

52
Q

What two values may be used to determine the bandwidth

of an fm wave?

A

The number of significant sidebands and the modulating

frequency.

53
Q

How does the reactance-tube modulator impress

intelligence onto an rf carrier?

A

By changing the reactance of an oscillator circuit in

consonance with the modulating voltage.

54
Q

What characteristic of a transistor is varied in a

semiconductor-reactance modulator?

A

Collector-to-emitter capacitance.

55
Q

What circuit section is required in the output of a
multivibrator modulator to eliminate unwanted output
frequencies?

A

An LCR filter.

56
Q

What characteristic of a varactor is used in an fm

modulator?

A

Capacitance

57
Q

What type of modulation depends on the carrier-wave

phase shift?

A

Phase.

58
Q

What components may be used to build a basic phase

modulator?

A

A phase-shift network such as a variable resistor and

capacitor in series.

59
Q

Phase-shift keying is similar to what other two

types of modulation?

A

Cw and frequency-shift keying

60
Q

Overmodulating an rf carrier in amplitude
modulation produces a waveform which is similar to what
modulated waveform?

A

Pulse modulation.

61
Q

What is prt?

A

Pulse-repetition time.

62
Q

What is non-pulse time?

A

Rest Time

63
Q

. What is average power in a pulsed system?

A

Peak power during a pulse averaged over pulse time plus

rest time.

64
Q

What is the primary component for a spark-gap

modulator?

A

Either a fixed spark gap that uses a trigger pulse to
ionize the air between the contacts, or a rotary gap that
is similar to a mechanical switch.

65
Q

What are the basic components of a thyratron

modulator?

A

Power source, a circuit for storing energy, a circuit
for discharging the storage circuit, and a pulse
transformer.

66
Q

What action is necessary to impress intelligence on

the pulse train in pulse modulation?

A

Some characteristic of the pulses has to be varied.

67
Q

To ensure the accuracy of a transmission, what is
the minimum number of times a modulating wave should be
sampled in pulse modulation?

A

2.5 times the highest modulating frequency.

68
Q

What, if any, noise susceptibility advantage exists
for pulse-amplitude modulation over analogamplitude
modulation?

A

Both are susceptible to noise and interference.

69
Q

What characteristics of a pulse can be changed in

pulse-time modulation?

A

The time duration of the pulses or the time of occurrence

of the pulses.

70
Q

Which edges of the pulse can be modulated in

pulse-duration modulation?

A

Either, or both at the same time.

71
Q

What is the main disadvantage of pulse-position

modulation?

A

It requires synchronization between the transmitter and

receiver.

72
Q

What is pulse-frequency modulation?

A

A method of pulse modulation in which a modulating wave
is used to frequency modulate a
pulse-generating circuit.

73
Q

Pulse-code modulation requires the use of
approximations of value that are obtained by what
process?

A

Quantization

74
Q

If a modulating wave is sampled 10 times per cycle
with a 5-element binary code, how many bits of
information are required to transmit the signal?

A

50.

75
Q

What is the primary advantage of pulse-modulation

systems?

A

Low susceptibility to noise.

76
Q

What is demodulation?

A

Re-creating original modulating frequencies

(intelligence) from radio frequencies.

77
Q

What is a demodulator?

A

Circuit in which intelligence restoration is achieved.

78
Q

What is the simplest form of cw detector?

A

A circuit that can detect the presence or absence of rf

energy.

79
Q

What are the essential components of a cw receiver

system?

A

An antenna, tank circuit for tuning, rectifier for
detection, filter to give constant output, and an
indicator device.

80
Q

What principle is used to help distinguish between

two cw signals that are close in frequency?

A

Heterodyning.

81
Q

How does heterodyning distinguish between cw

signals?

A

By giving a different beat frequency for each signal.

82
Q

What simple, one-transistor detector circuit uses

the heterodyne principle?

A

Regenerative detector.

83
Q

What three functions does the transistor in a

regenerative detector serve?

A

Oscillator, mixer, and detector.

84
Q

What are the three requirements for an AM

demodulator?

A

(1) Sensitive to the type of modulation applied, (2)

nonlinear, and (3) provide filtering.

85
Q

What does the simplest diode detector use to

reproduce the modulating frequency?

A

The modulation envelope.

86
Q

What is the function of the diode in a series-diode

detector?

A

Rectifies the rf pulses in the received signal

87
Q

In figure 3-5, what is the function of C2?

A

To filter the rf pulses and develop the modulating wave
(intelligence) from the modulation
envelope.

88
Q

How does the current-diode detector differ from the

voltage-diode detector?

A

The current-diode detector is in parallel with the input

and load.

89
Q

Under what circuit conditions would the shunt

detector be used?

A

When the input voltage variations are too small to give

a usable output from a series detector.

90
Q

Which junction of the transistor in the
common-emitter detector detects the modulation
envelope?

A

Emitter-base junction.

91
Q

Which component in figure 3-7 develops the af signal

at the input?

A

R1.

92
Q

How is the output signal developed in the

common-emitter detector?

A

By the collector current flow through R4.

93
Q

Which junction acts as the detector in a common-base

detector?

A

Emitter-base junction.

94
Q

To what circuit arrangement is a common-base

detector equivalent?

A

A diode detector followed by a stage of audio

amplification.

95
Q

In figure 3-8, which components act as the filter

network in the diode detector?

A

C1 and R1

96
Q

What is the simplest form of fm detector?

A

Slope detector.

97
Q

What is the function of an fm detector?

A

Converting frequency variations of received fm signals

to amplitude variations.

98
Q

What type of tank circuit is used in the

Foster-Seeley discriminator?

A

A double-tuned tank circuit.

99
Q

What is the purpose of CR1 and CR2 in the

Foster-Seeley discriminator?

A

Rectify the rf voltage from the discriminator.

100
Q

What type of impedance does the tank circuit have

above resonance?

A

Inductive.

101
Q

What is the primary advantage of a ratio detector?

A

Suppresses amplitude noise without limiter stages.

102
Q

What is the purpose of C5 in figure 3-12?

A

It helps to maintain a constant circuit voltage to
prevent noise fluctuations from interfering with
the output.

103
Q

What circuit functions does the tube in a gated-beam

detector serve?

A

Limits, detects, and amplifies.

104
Q

What condition must exist on both the limiter and
quadrature grids for current to flow in a gated-beam
detector?

A

Both grids must be positively biased.

105
Q

Name two advantages of the gated-beam detector

A

Extreme simplicity, few components, and ease of

adjustment.

106
Q

Where is the intelligence contained in a

phase-modulated signal?

A

In the amount and rate of phase shift of the carrier

wave.

107
Q

Why can phase-modulated signals be detected by fm

detectors?

A

Because of the incidental frequency shift that is caused
while phase-shifting a carrier wave that is similar to
fm modulation.

108
Q

How is a quadrature detector changed when used for

phase demodulation?

A

The quadrature grid signal is excited by a reference

from the transmitter.

109
Q

In its simplest form, what functions must a radar

detector be capable of performing?

A

Detecting the presence of rf energy.

110
Q

What characteristic of a pulse does a peak detector

A

Pulse amplitude or pulse duration.

111
Q

. What is the time constant of the resistor and
capacitor
in a peak detector for pam?

A

At least 10 times the interpulse period.

112
Q

How can a peak detector for pam be modified to detect

pdm?

A

By making the time constant for charging the capacitor

at least 10 times the maximum received pulse width.

113
Q

How does a low-pass filter detect pdm?

A

By averaging the value of the pulses over the period of

the pulse-repetition rate.

114
Q

How is conversion used in pulse demodulation?

A

Ppm, pfm, and pcm are converted to either pdm or pam for

demodulation.

115
Q

What is the discharge rate for the capacitor in a

pcm converter?

A

It will discharge to one-half its value between pulses.

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