Angle Modulation (Answers)
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Frequency-shift keying resembles what type of AM modulation? a. CW modulation b. Analog AM modulation c. Plate modulation d. Collector-injection modulation Answer: A Frequency-shift keying is generated using which of the following methods? a. By shifting the frequency of an oscillator at an AF rate b. By shifting the frequency of an oscillator at an RF rate c. By keying an AF oscillator at an RF rate d. By keying an AF oscillator at an AF rate Answer: A In a frequency-shift keyed signal, where is the intelligence contained? a. In the duration of the RF energy b. In the frequency of the RF energy c. In the amplitude of the RF energy d. In the spacing between bursts of RF energy Answer: B If an FSK transmitter has a MARK frequency of 49.575 kHz and a SPACE frequency of 50.425 kHz, what is the assigned channel frequency? a. 49 kHz b. 49.575 kHz c. 50 kHz d. 50.425 kHz Answer: C FSK is NOT affected by noise interference for which of the following reasons? a. Noise is outside the bandwidth of an FSK signal b. FSK does not rely on the amplitude of the transmitted signal to carry intelligence c. The wide bandwidth of an FSK signal prevents noise interference d. Each of the above Answer: B In an FSK transmitter, what stage is keyed? a. The oscillator b. The power supply c. The power amplifier d. The buffer amplifier Answer: A When the amount of oscillator frequency shift in an FSK transmitter is determined,
which of the following factors must be considered? a. The number of buffer amplifiers b. The transmitter power output c. The frequency multiplication factor for the transmitter amplifiers d. The oscillator rest frequency Answer: C 8) In an FSK transmitter with a doubler and a tripler stage, the desired frequency shift is 1,200 Hz. To what maximum amount is the oscillator frequency shift limited? a. 60 Hz b. 100 Hz c. 120 Hz d. 200 Hz Answer: D 1,200 Hz / (3 x 2) = 200 Hz 9) FSK has which of the following advantages over CW? a. FSK has a more stable oscillator b. FSK is easier to generate c. FSK rejects unwanted weak signals d. FSK does not have noise in its output Answer: C 10) The "ratio of transmitted powers" provides what information? a. Transmitter power out in a CW system b. Transmitter power out in an FSK system c. Improvement shown using CW instead of FSK transmission d. Improvement shown using FSK instead of CW transmission methods Answer: D 11) In an FM signal, (a) the RATE of shift is proportional to what characteristic of the modulating signal, and (b) the AMOUNT of shift is proportional to what characteristic? a. (a) Amplitude (b) amplitude b. (a) Amplitude (b) frequency c. (a) Frequency (b) frequency d. (a) Frequency (b) amplitude Answer: D
Figure A. — Oscillator circuit. IN ANSWERING QUESTIONS 12 THROUGH 14, REFER TO FIGURE A. 12) When a sound wave strikes the condenser microphone (M), it has which, if any, of the following effects on the oscillator circuit? a. It changes output phase b. It changes output voltage c. It changes output frequency d. It has no effect Answer: C
When no sound waves strike M, the frequency is the RF carrier frequency. Any excitation of M will alter its capacitance and, therefore, the frequency of the oscillator circuit. 13) What is the purpose of capacitor C in the circuit? a. It helps set the carrier frequency of the oscillator b. It prevents amplitude variations in the oscillator output c. It sets the maximum frequency deviation of the oscillator d. It varies the output frequency in accordance with the modulating voltage Answer: A 14) A 1,000-Hz tone of a certain loudness causes the frequency-modulated carrier for the circuit to vary 1,000 Hz at a rate of 1,000 times per second. If the AMPLITUDE of the modulating tone is doubled, what will be the maximum carrier variation? a. 1,000 Hz at 1,000 times per second b. 1,000 Hz at 2,000 times per second c. 2,000 Hz at 1,000 times per second d. 2,000 Hz at 2,000 times per second Answer: C 15) The maximum deviation for a 1.5 MHz carrier is set at 50 kHz. If the carrier varies between 1.5125 MHz and 1.4875 MHz
(12.5 kHz), what is the percentage of modulation? a. 25 % b. 50 % c. 75 % d. 100 % Answer: A M = freq. deviation / modulating freq. 16) An FM transmitter has a 50-watt carrier with no modulation. What maximum amount of output power will it have when it is 50 percent modulated? a. 25 watts b. 50 watts c. 75 watts d. 100 watts Answer: B 17) Frequencies that are located between adjacent channels to prevent interference are referred to as a. sidebands b. bandwidths c. guard bands d. blank channels Answer: C 18) Modulation index may be figured by using which of the following formulas? a. 2f/fm b. fm/2f c. fm/f d. f/fm Answer: D 19) A 50-MHz FM carrier varies between 49.925 MHz and 50.075 MHz 10,000 times per second. What is its modulation index? a. 5 b. 10 c. 15 d. 20 Answer: C (150 kHz / 10 kHz)
following effects on the output of an oscillator? a. It will decrease amplitude b. It will increase amplitude c. It will change resonant frequency d. It will have no effect Answer: C 24) The reactance-tube frequency modulates the oscillator by which of the following actions? a. By shunting the tank circuit with a variable resistance b. By shunting the tank circuit with a variable reactance c. By shunting the tank circuit with a variable capacitance d. By causing a resultant current flow in the tank circuit which either leads or lags resonant current Answer: B Figure B. — Modulation index table. IN ANSWERING QUESTIONS 20 AND 21, REFER TO FIGURE B. 20) An FM-modulated carrier varies between 925 kHz and 1,075 kHz 15,000 times per second. What is the bandwidth, in kHz, of the transmitted signal? (HINT: You will need to figure MI to be able to find the sidebands.) a. 340 b. 420 c. 480 d. 560 Answer: B (M = 150,000/15,000 = 10) M = 10; sidebands = 28 28 x 15,000 = 420 kHz 21) The spectrum of a 500 kHz FM-modulated carrier has a 60-kHz bandwidth and contains 12 significant sidebands. How much, in kHz, is the carrier deviated? a. 5 b. 7.5 c. 10 d. 15 Answer: B 22) In a reactance-tube modulator, the reactance tube shunts what part of the oscillator circuitry? a. The amplifier b. The tank circuit c. The biasing network d. The feedback network Answer: B 23) With no modulating signal applied, a reactance tube has which, if any, of the
Figure C. — Semiconductor reactance modulator. IN ANSWERING QUESTIONS 25 AND 26, REFER TO FIGURE C. 25) The semiconductor reactance modulator in the circuit is in parallel with a portion of the oscillator tank circuit coil. Modulation results because of interaction with which of the following transistor characteristics? a. Collector-to-emitter resistance b. Collector-to-emitter capacitance c. Base-to-emitter resistance d. Base-to-emitter capacitance Answer: B 26) With a positive-going modulating signal applied to the base of Q2, (a) what will
circuit capacitance do and (b) what will the output frequency do? a. (a) Decrease (b) decrease b. (a) Decrease (b) increase c. (a) Increase (b) increase d. (a) Increase (a) decrease Answer: D 27) What type of circuit is used to remove the AM component in the output of a semiconductor reactance modulator? a. A mixer b. A filter c. A limiter d. A buffer amplifier Answer: C
Figure D. — Multivibrator modulator. IN ANSWERING QUESTIONS 28 AND 29, REFER TO FIGURE D. 28) The multivibrator modulator produces FM modulation by which of the following actions? a. By modulating the collector voltages b. By modulating the base-return voltages c. By modulating the value of the base value of the base capacitors d. By modulating the value of the base resistors Answer: B 29) What is the purpose of the filter on the output of the multivibrator modulator? a. To establish the fundamental operating frequency b. To eliminate unwanted frequency variations c. To eliminate unwanted odd harmonics d. To eliminate unwanted even harmonics Answer: C 30) A multivibrator frequency modulator is limited to frequencies below what maximum frequency? a. 1 MHz
b. 2 MHz c. 5 MHz d. 10 MHz Answer: A 31) To ensure the frequency stability of an FM transmitter, which, if any, of the following actions could be taken? a. Modulate a crystal-controlled oscillator at the desired frequency b. Modulate a low-frequency oscillator, and use frequency multipliers to achieve the operating frequency c. Modulate a low-frequency oscillator, and heterodyne it with a higher frequency oscillator to achieve the desired frequency d. None of the above Answer: C 32) A varactor is a variable device that acts as which of the following components? a. Resistor b. Inductor c. Capacitor d. Transistor Answer: C 33) As the positive potential is increased on the cathode of a varactor, (a) what happens to reverse bias and (b) how is dielectric width affected? a. (a) Increases (b) increases b. (a) Increases (b) decreases c. (a) Decreases (b) decreases d. (a) Decreases (b) increases Answer: A
Figure 2E. — Phase relationships. IN ANSWERING QUESTION 34, REFER TO FIGURE E. 34) In the figure, (a) waveform X has what phase relationship to waveform Y, and (b) waveform Y has what relationship to waveform Z? a. (a) Lags (b) leads b. (a) Lags (b) lags c. (a) Leads (b) lags d. (a) Leads (b) leads Answer: A 35) A 10 kHz, 10-volt square wave is applied as the phase-modulating signal to a transmitter with a carrier frequency of 60 MHz. What is
the output frequency during the constantamplitude portions of the modulating signal? a. 10 kHz b. 59,990 kHz c. 60,000 kHz d. 60,010 kHz Answer: C 36) In a phase modulator, the frequency during the constant-amplitude portion of the modulating wave is the a. peak frequency b. rest frequency c. deviation frequency d. modulating frequency Answer: B 37) In phase modulation, (a) the AMPLITUDE of the modulating signal determines what characteristic of the phase shift, and (b) the FREQUENCY of the modulating signal determines what characteristic of the phase shift? a. (a) Rate (b) rate b. (a) Rate (b) amount c. (a) Amount (b) amount d. (a) Amount (b) rate Answer: D 38) The frequency spectrums of a phasemodulated signal resemble the spectrum of which, if any, of the following types of modulation? a. Amplitude modulated b. Frequency modulated c. Continuous-wave modulated d. None of the above Answer: B 39) Compared to FM, increasing the modulating frequency in phase modulation has what effect, if any, on the bandwidth of the phasemodulated signal? a. It increases b. It decreases c. Remains constant d. None Answer: A 40) A simple phase modulator consists of a capacitor in series with a variable resistance. What total amount of carrier shift will occur when X C is 10 times the resistance? a. 0 degrees b. 45 degrees c. 60 degrees d. 90 degrees Answer: D Xc = R 45 degrees XC>10R 0 degrees XC=10R 90 degrees
41) The primary advantage of phase modulation over frequency modulation is that phase modulation has better carrier a. power stability b. amplitude stability c. frequency stability d. directional stability Answer: C 42) Phase-shift keying is most useful under which of the following code element conditions? a. When mark elements are longer than space elements b. When mark elements are shorter than space elements c. When mark and space elements are the same length d. When mark and space elements are longer than synchronizing elements Answer: C 43) When a carrier is phase-shift keying modulated, (a) a data bit ONE will normally cause the carrier to shift its phase what total number of degrees, and (b) a data bit ZERO will cause the carrier to shift its phase what total number of degrees? a. (a) 60 (b) 0 b. (a) 0 (b) 180 c. (a) 180 (b) 180 d. (a) 180 (b) 0 Answer: D 44) Which of the following circuits is used to generate a phase-shift keyed signal? a. Logic circuit b. Phasor circuit c. Phasitron circuit d. Longitudinal circuit Answer: A 45) When a carrier is modulated by a square wave, what maximum number of sideband pairs will be generated? a. 1 b. 9 c. 3 d. An infinite number Answer: D 46) As the square wave modulating voltage is increased to the same amplitude as that of the carrier, what will be the effect on (a) the carrier amplitude and (b) amplitude of the sidebands? a. (a) Remains constant (b) Increases b. (a) Decreases (b) Increases c. (a) Increases (b) Remains constant d. (a) Increases (b) Decreases
Answer: A 47) In a square-wave modulated signal, total sideband power is what percentage of the total power? a. 0 percent b. 25 percent c. 33 percent d. 50 percent Answer: D 33% when sine wave is used as modulating signal.
Figure 2. — Waveform. IN ANSWERING QUESTIONS 48 THROUGH 51, REFER TO FIGURE 2. SELECT THE FIGURE LETTER THAT CORRESPONDS WITH THE WAVEFORM LISTED IN THE QUESTIONS. LETTERS MAY BE USED ONCE, MORE THAN ONCE, OR NOT AT ALL. 48) Pulse width. a. A b. B (Answer) c. C d. D 49) Rest time. a. B b. C c. D d. E (Answer) 50) Pulse duration. a. A b. B (Answer) c. D d. E 51) Pulse-repetition time. a. B b. C (Answer) c. D d. E 52) Which of the following ratios is used to determine pulse-repetition frequency (PRF)? a. PRF = 1/PRT (Answer) b. PRF = 1/PW c. PRF = 1/PD d. PRF = 1 / RT 53) Average power in a pulse-modulation system is defined as the a. power during rest time b. power during each pulse c. power during each pulse averaged over rest time
d. power during each pulse averaged over one operating cycle Answer: D Peak power is the maximum value of the transmitted pulse; average power is the peak power value averaged over the pulserepetition time. 54) In pulse modulation, what term is used to indicate the ratio of time the system is actually producing RF? a. Rest cycle b. Duty cycle c. Average cycle d. Transmit cycle Answer: B 55) In a pulse-modulation system, which of the following formulas is used to figure the percentage of transmitting time? a. PW/PRT x 100 b. PRT/PW x 100 c. PW/RT x 100 d. RT/PW x 100 Answer: A 56) When pulse modulation is used for range finding in a radar application, which of the following types of pulse information is used? a. Reflected pulse return interval b. Reflected pulse duration c. Reflected pulse amplitude d. Reflected pulse frequency Answer: A 57) In a spark-gap modulator, what is the function of the pulse-forming network? a. To store energy b. To increase the level of stored energy c. To act as a power bleeder d. To rapidly discharge stored energy Answer: A
The SPARK-GAP MODULATOR consists of a circuit for storing energy, a circuit for rapidly discharging the storage circuit (spark gap), a pulse transformer, and an ac power source. The circuit for storing energy is essentially a short section of artificial transmission line which is known as the PULSE-FORMING NETWORK (pfn). The pulse-forming network is discharged by a spark gap. Two types of spark gaps are used: FIXED GAPS and ROTARY GAPS. The fixed gap, discussed in this section, uses a trigger pulse to ionize the air between the contacts of the spark gap and to initiate the discharge of the pulse-forming network. The
rotary gap is similar to a mechanically driven switch. 58) The damping diode in a thyratron modulator serves which of the following purposes? a. It discharges the pulse-forming network b. It limits the input signal c. It prevents the breakdown of the thyratron by reverse-voltage transients d. It rectifies the input signal Answer: C
The hydrogen THYRATRON MODULATOR is an electronic switch which requires a positive trigger of only 150 volts. 59) Compared to a spark-gap modulator, the thyratron modulator exhibits which of the following advantages? a. Improved timing b. Higher output pulses c. Higher trigger voltage d. Operates over a narrower range of anode voltages and pulse-repetition rates Answer: D 60) To transmit intelligence using pulse modulation, which of the following pulse characteristics may be varied? a. Pulse duration b. Pulse amplitude c. Pulse-repetition time d. Each of the above Answer: D 61) To accurately reproduce a modulating signal in a pulse-modulated system, what minimum number of samples must be taken per cycle? a. One b. Two c. Three d. Four Answer: B According to Nyquist criterion the minimum sampling rate is TWICE the highest modulating frequency. The standard sampling rate is 2.5 x maximum modulating frequency. 62) What is the simplest form of pulse modulation? a. Pulse-code modulation b. Pulse-duration modulation c. Pulse-frequency modulation d. Pulse-amplitude modulation Answer: D
63) The same pulse characteristic is varied in which of the following types of pulse modulations? a. PAM and PDM b. PDM and PWM c. PWM and PPM d. PPM and PAM Answer: B
Figure G. — Waveform. IN ANSWERING QUESTION 64, REFER TO FIGURE G. 64) Which of the points shown in the waveform may be varied in pulse-duration modulation? a. A only b. B only c. C only d. A and/or C Answer: D 65) Which, if any, of the following is the primary disadvantage of pulse-position modulation? a. It depends on transmitter-receiver synchronization b. It is susceptible to noise interference c. Transmitter power varies d. None of the above Answer: A 66) A PFM transmitter transmits 10,000 pulses per second without a modulating signal applied. How, if at all, will a modulating signal affect the transmitted pulse rate? a. It will decrease the transmitted pulse rate b. It will increase the transmitted pulse rate c. Both a and b above d. It will not affect the transmitted pulse rate Answer: C 67) The process of arbitrarily dividing a wave into a series of standard values is referred to as a. arbitration b. quantization c. interposition d. approximation Answer: B 68) A PCM system is capable of transmitting 32 standard levels that are sampled 2.5 times per cycle of a 3-kHz modulating signal.
What maximum number of bits per second are transmitted? a. 18,750 b. 37,500 c. 75,000 d. 240,000 Answer: B C = 5 levels x 2.5 samples/sec x 3000 cycles/level = 37,500
69) Which of the following is a characteristic of a PCM system that makes it advantageous for use in multiple-relay link systems? a. Average power is constant b. Average power decreases with each relay c. Noise is not cumulative at relay stations d. Quantization noise decreases with each relay Answer: C
Figure 1. Reactance-tube FM modulator
Multivibrator Modulator Inserting the modulating af voltage in series with the base-return of the multivibrator transistors causes the gate length, and thus the fundamental frequency of the multivibrator, to vary. The amount of variation will be in accordance with the amplitude of the modulating voltage. One requirement of this method is that the fundamental frequency of the multivibrator be high in relation to the highest modulating frequencies. A factor of at least 100 provides the best results.
