NEETS 12 2,3 Flashcards
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 nonpulse 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 signalA-12. To filter the rf pulses and develop the modulating wave (intelligence) from the modulation envelope.
In figure 3-5, what is the function of C2?
The current-diode detector is in parallel with the input and load.
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 sample?
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.3-35
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.
