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.