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NEETS > NEETS 12 2,3 > Flashcards

NEETS 12 2,3 Flashcards

1
Q

What are the two types of angle modulation?

A

Frequency and phase

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2
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.

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3
Q

What is the primary advantage of an fsk transmission system?

A

Resistance to noise interference.

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4
Q

What characteristic of a carrier wave is varied in frequency modulation?

A

Instantaneous frequency.

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5
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.

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6
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.

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7
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

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8
Q

What characteristic of a transistor is varied in a semiconductor-reactance modulator?

A

Collector-to-emitter capacitance.

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9
Q

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

A

An LCR filter.

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10
Q

What characteristic of a varactor is used in an fm modulator?

A

Capacitance.

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11
Q

What type of modulation depends on the carrier-wave phase shift?

A

Phase.

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12
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.

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13
Q

Phase-shift keying is similar to what other two types of modulation?

A

Cw and frequency-shift keying.

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14
Q

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

A

Pulse modulation.

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15
Q

What is prt?

A

Pulse-repetition time.

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16
Q

What is nonpulse time?

A

Rest time.

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17
Q

What is average power in a pulsed system?

A

Peak power during a pulse averaged over pulse time plus rest time.

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18
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.

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19
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.

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20
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.

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21
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.

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22
Q

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

A

Both are susceptible to noise and interference.

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23
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.

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24
Q

Which edges of the pulse can be modulated in pulse-duration modulation?

A

Either, or both at the same time.

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25
Q

What is the main disadvantage of pulse-position modulation?

A

It requires synchronization between the transmitter and receiver.

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26
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

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27
Q

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

A

Quantization.

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28
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.

29
Q

What is the primary advantage of pulse-modulation systems?

A

Low susceptibility to noise.

30
Q

What is demodulation?

A

Re-creating original modulating frequencies (intelligence) from radio frequencies.

31
Q

What is a demodulator?

A

Circuit in which intelligence restoration is achieved.

32
Q

What is the simplest form of cw detector?

A

A circuit that can detect the presence or absence of rf energy.

33
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.

34
Q

What principle is used to help distinguish between two cw signals that are close in frequency?

A

Heterodyning.

35
Q

How does heterodyning distinguish between cw signals?

A

By giving a different beat frequency for each signal.

36
Q

What simple, one-transistor detector circuit uses the heterodyne principle?

A

Regenerative detector.

37
Q

What three functions does the transistor in a regenerative detector serve?

A

Oscillator, mixer, and detector.

38
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.

39
Q

What does the simplest diode detector use to reproduce the modulating frequency?

A

The modulation envelope.

40
Q

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

A

Rectifies the rf pulses in the received signalA-12. To filter the rf pulses and develop the modulating wave (intelligence) from the modulation envelope.

41
Q

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

A

The current-diode detector is in parallel with the input and load.

42
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.

43
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.

44
Q

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

A

Emitter-base junction

45
Q

Which component in figure 3-7 develops the af signal at the input?

A

R1

46
Q

How is the output signal developed in the common-emitter detector?

A

By the collector current flow through R4.

47
Q

Which junction acts as the detector in a common-base detector?

A

Emitter-base junction.

48
Q

To what circuit arrangement is a common-base detector equivalent?

A

A diode detector followed by a stage of audio amplification.

49
Q

In figure 3-8, which components act as the filter network in the diode detector?

A

C1 and R1.

50
Q

What is the simplest form of fm detector?

A

Slope detector.

51
Q

What is the function of an fm detector?

A

Converting frequency variations of received fm signals to amplitude variations.

52
Q

What type of tank circuit is used in the Foster-Seeley discriminator?

A

A double-tuned tank circuit.

53
Q

What is the purpose of CR1 and CR2 in the Foster-Seeley discriminator?

A

Rectify the rf voltage from the discriminator.

54
Q

What type of impedance does the tank circuit have above resonance?

A

Inductive.

55
Q

What is the primary advantage of a ratio detector?

A

Suppresses amplitude noise without limiter stages.

56
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.

57
Q

What circuit functions does the tube in a gated-beam detector serve?

A

Limits, detects, and amplifies.

58
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.

59
Q

Name two advantages of the gated-beam detector.

A

Extreme simplicity, few components, and ease of adjustment.

60
Q

Where is the intelligence contained in a phase-modulated signal?

A

In the amount and rate of phase shift of the carrier wave.

61
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.

62
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.

63
Q

In its simplest form, what functions must a radar detector be capable of performing?

A

Detecting the presence of rf energy.

64
Q

What characteristic of a pulse does a peak detector sample?

A

Pulse amplitude or pulse duration.

65
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.

66
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.3-35

67
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.

68
Q

How is conversion used in pulse demodulation?

A

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

69
Q

What is the discharge rate for the capacitor in a pcm converter?

A

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

NEETS (11 decks)

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