NEETS 12 Modulation Principles Flashcards
What is modulation?
Modulation is the impressing of intelligence on a
transmission medium.
What is a transmission medium?
May be anything that transmits information, such as light, smoke, sound, wire lines, or
radio-frequency waves.
What is heterodyning?
Mixing two frequencies across a nonlinear impedance.
What is demodulation?
The process of recovering intelligence from a modulated
carrier.
What waveform is the basis of all complex waveforms?
The sine wave.
What is the purpose of using vectors?
To represent quantities that have both magnitude and
direction.
What is the trigonometric ratio for the sine of an
angle?
Sine Ө = opposite side ÷ hypotenuse.
What is the mathematical formula for computing the
output voltage from a moving coil in a
magnetic field?
E=EmaxsineӨ
What is the instantaneous amplitude of a sine wave?
The value at any given point on the sine wave.
What term describes how much of a cycle has been
completed?
Phase or phase angle.
What determines the frequency of a sine wave?
The rate at which the vector which is generating the sine
wave is rotating.
What is the period of a cycle?
The elapsed time from the beginning of cycle to its
completion.
How do you calculate the wavelength of a sine wave?
Wavelength = rate of travel × period.
Define the heterodyne principle.
Process of combining two signal frequencies in a
nonlinear device.
What is a nonlinear impedance?
An impedance in which the resulting current is not
proportional to the applied voltage.
What is spectrum analysis?
The display of electromagnetic energy that is arranged
according to wavelength or frequency.
What two conditions are necessary for heterodyning
to take place?
At least two different frequencies applied to a nonlinear impedance.
What is amplitude modulation?
Any method of modulating an electromagnetic carrier
frequency by varying its amplitude in accordance with
the intelligence.
What are the three requirements for cw
transmission?
A method of generating oscillations, a method of turning
the oscillations on and off (keying), and an antenna to
radiate the energy.
Name two methods of oscillator keying.
Plate keying and cathode keying.
State the method used to increase the speed of
keying in a cw transmitter.
Machine keying.
Name three advantages of cw transmission.
A high degree of clarity even under severe noise
conditions, long-range operation, and narrow
bandwidth.
Name a disadvantage of a single-stage cw
transmitter.
Antenna-to-ground capacitance can cause the oscillator
frequency to vary.
What is the purpose of the power-amplifier stage
in a master oscillator power amplifier cw
transmitter?
To isolate the oscillator from the antenna and increase
the amplitude of the rf oscillations to the
required output level.
What is the purpose of frequency-multiplier stages
in a VHF transmitter?
To raise the low frequency of a stable oscillator to the
vhf range.
What is a microphone?
An energy converter that changes sound energy into
electrical energy.
What special electromechanical effect is the basis
for carbon microphone operation?
The changing resistance of carbon granules as pressure
is applied to them.
What is a major disadvantage of a carbon microphone?
Background hiss resulting from random changes in the
resistance between individual carbon granules.
What property of a crystalline material is used in
a crystal microphone?
The piezoelectric effect.
What is the difference between a dynamic microphone
and a magnetic microphone?
A dynamic microphone has a moving coil and the magnetic
microphone has a moving armature.
What are the two major sections of a typical AM
transmitter?
Rf and af units
When 100 kilohertz and 5 kilohertz are heterodyned,
what frequencies are present?
100 kHz, 5 kHz, 95 kHz, and 105 kHz.
What is the upper sideband of an AM transmission?
All of the sum frequencies above the carrier.
Where is the intelligence in an AM transmission
located?
The intelligence is contained in the spacing between the
carrier and sideband frequencies.
What determines the bandwidth of an AM
transmission?
The highest modulating frequency.
What is percent of modulation?
The depth or degree of modulation.
With a single modulating tone, what is the amplitude
of the sideband frequencies at 100-percent
modulation?
One-half the amplitude of the carrier.
What is the formula for percent of modulation?
%M=Em/Ec x100%
What is high-level modulation?
Modulation produced in the plate circuit of the last
radio stage of the system.
For what class of operation is the final rf power amplifier of a plate-modulator circuit biased?
Class C.
The modulator is required to be what kind of a
circuit stage in a plate modulator?
Power amplifier.
How much must the fpa plate current vary to produce
100-percent modulation in a plate modulator?
Between 0 and nearly two times its unmodulated value.
The collector-injection modulator is similar to
what type of tube modulator?
Plate modulator.
When is a control-grid modulator used?
In cases when the use of a minimum of af modulator power
is desired.
What type of modulator is the cathode modulator
(low- or high-level)?
Low-level.
What causes the change in collector current in an
emitter-injection modulator?
Gain is varied by changing the voltage on the emitter.
What are the two types of angle modulation?
Frequency and phase.
Name the modulation system in which the frequency
alternates between two discrete values in response to
the opening and closing of a key?
Frequency-shift keying.
. What is the primary advantage of an fsk transmission
system?
Resistance to noise interference.
What characteristic of a carrier wave is varied in
frequency modulation?
Instantaneous frequency.
How is the degree of modulation expressed in an fm
system?
As the ratio of the frequency deviation to the maximum
frequency deviation allowable.
What two values may be used to determine the bandwidth
of an fm wave?
The number of significant sidebands and the modulating
frequency.
How does the reactance-tube modulator impress
intelligence onto an rf carrier?
By changing the reactance of an oscillator circuit in
consonance with the modulating voltage.
What characteristic of a transistor is varied in a
semiconductor-reactance modulator?
Collector-to-emitter capacitance.
What circuit section is required in the output of a
multivibrator modulator to eliminate unwanted output
frequencies?
An LCR filter.
What characteristic of a varactor is used in an fm
modulator?
Capacitance
What type of modulation depends on the carrier-wave
phase shift?
Phase.
What components may be used to build a basic phase
modulator?
A phase-shift network such as a variable resistor and
capacitor in series.
Phase-shift keying is similar to what other two
types of modulation?
Cw and frequency-shift keying
Overmodulating an rf carrier in amplitude
modulation produces a waveform which is similar to what
modulated waveform?
Pulse modulation.
What is prt?
Pulse-repetition time.
What is non-pulse time?
Rest Time
. What is average power in a pulsed system?
Peak power during a pulse averaged over pulse time plus
rest time.
What is the primary component for a spark-gap
modulator?
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.
What are the basic components of a thyratron
modulator?
Power source, a circuit for storing energy, a circuit
for discharging the storage circuit, and a pulse
transformer.
What action is necessary to impress intelligence on
the pulse train in pulse modulation?
Some characteristic of the pulses has to be varied.
To ensure the accuracy of a transmission, what is
the minimum number of times a modulating wave should be
sampled in pulse modulation?
2.5 times the highest modulating frequency.
What, if any, noise susceptibility advantage exists
for pulse-amplitude modulation over analogamplitude
modulation?
Both are susceptible to noise and interference.
What characteristics of a pulse can be changed in
pulse-time modulation?
The time duration of the pulses or the time of occurrence
of the pulses.
Which edges of the pulse can be modulated in
pulse-duration modulation?
Either, or both at the same time.
What is the main disadvantage of pulse-position
modulation?
It requires synchronization between the transmitter and
receiver.
What is pulse-frequency modulation?
A method of pulse modulation in which a modulating wave
is used to frequency modulate a
pulse-generating circuit.
Pulse-code modulation requires the use of
approximations of value that are obtained by what
process?
Quantization
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?
50.
What is the primary advantage of pulse-modulation
systems?
Low susceptibility to noise.
What is demodulation?
Re-creating original modulating frequencies
(intelligence) from radio frequencies.
What is a demodulator?
Circuit in which intelligence restoration is achieved.
What is the simplest form of cw detector?
A circuit that can detect the presence or absence of rf
energy.
What are the essential components of a cw receiver
system?
An antenna, tank circuit for tuning, rectifier for
detection, filter to give constant output, and an
indicator device.
What principle is used to help distinguish between
two cw signals that are close in frequency?
Heterodyning.
How does heterodyning distinguish between cw
signals?
By giving a different beat frequency for each signal.
What simple, one-transistor detector circuit uses
the heterodyne principle?
Regenerative detector.
What three functions does the transistor in a
regenerative detector serve?
Oscillator, mixer, and detector.
What are the three requirements for an AM
demodulator?
(1) Sensitive to the type of modulation applied, (2)
nonlinear, and (3) provide filtering.
What does the simplest diode detector use to
reproduce the modulating frequency?
The modulation envelope.
What is the function of the diode in a series-diode
detector?
Rectifies the rf pulses in the received signal
In figure 3-5, what is the function of C2?
To filter the rf pulses and develop the modulating wave
(intelligence) from the modulation
envelope.
How does the current-diode detector differ from the
voltage-diode detector?
The current-diode detector is in parallel with the input
and load.
Under what circuit conditions would the shunt
detector be used?
When the input voltage variations are too small to give
a usable output from a series detector.
Which junction of the transistor in the
common-emitter detector detects the modulation
envelope?
Emitter-base junction.
Which component in figure 3-7 develops the af signal
at the input?
R1.
How is the output signal developed in the
common-emitter detector?
By the collector current flow through R4.
Which junction acts as the detector in a common-base
detector?
Emitter-base junction.
To what circuit arrangement is a common-base
detector equivalent?
A diode detector followed by a stage of audio
amplification.
In figure 3-8, which components act as the filter
network in the diode detector?
C1 and R1
What is the simplest form of fm detector?
Slope detector.
What is the function of an fm detector?
Converting frequency variations of received fm signals
to amplitude variations.
What type of tank circuit is used in the
Foster-Seeley discriminator?
A double-tuned tank circuit.
What is the purpose of CR1 and CR2 in the
Foster-Seeley discriminator?
Rectify the rf voltage from the discriminator.
What type of impedance does the tank circuit have
above resonance?
Inductive.
What is the primary advantage of a ratio detector?
Suppresses amplitude noise without limiter stages.
What is the purpose of C5 in figure 3-12?
It helps to maintain a constant circuit voltage to
prevent noise fluctuations from interfering with
the output.
What circuit functions does the tube in a gated-beam
detector serve?
Limits, detects, and amplifies.
What condition must exist on both the limiter and
quadrature grids for current to flow in a gated-beam
detector?
Both grids must be positively biased.
Name two advantages of the gated-beam detector
Extreme simplicity, few components, and ease of
adjustment.
Where is the intelligence contained in a
phase-modulated signal?
In the amount and rate of phase shift of the carrier
wave.
Why can phase-modulated signals be detected by fm
detectors?
Because of the incidental frequency shift that is caused
while phase-shifting a carrier wave that is similar to
fm modulation.
How is a quadrature detector changed when used for
phase demodulation?
The quadrature grid signal is excited by a reference
from the transmitter.
In its simplest form, what functions must a radar
detector be capable of performing?
Detecting the presence of rf energy.
What characteristic of a pulse does a peak detector
Pulse amplitude or pulse duration.
. What is the time constant of the resistor and
capacitor
in a peak detector for pam?
At least 10 times the interpulse period.
How can a peak detector for pam be modified to detect
pdm?
By making the time constant for charging the capacitor
at least 10 times the maximum received pulse width.
How does a low-pass filter detect pdm?
By averaging the value of the pulses over the period of
the pulse-repetition rate.
How is conversion used in pulse demodulation?
Ppm, pfm, and pcm are converted to either pdm or pam for
demodulation.
What is the discharge rate for the capacitor in a
pcm converter?
It will discharge to one-half its value between pulses.
