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8/10/2019 PERDC-CNS-ST1.1

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CNSST 1.1: INTRO TO COMMS, NOISE AND dB MEASURMENTS

1. The theory of radio waves was originated by:Ans. Maxwell *

2. The first person who sent the first radio signal across the Atlantic ocean:Ans. Marconi *

3.

The transmission of radio waves was first done by:Ans. Hertz *

4. When two or more signals share a common channel, it is called:Ans. Multiplexing *

5. Man-made noise can come from:Ans. equipment that sparks *

6. Thermal noise is generated in:Ans. transistors and diodes, copper wire, and resistors *

7. Shot noise is generated in:Ans. transistors and diodes *

8. The power density of flicker noise is:Ans. greater at low frequencies *

9. So called 1/f noise is also called:Ans. pink noise *

10. Pink noise has:Ans. equal power per octave *

11. Noise figure is a measure of:

Ans. how much noise an amplifier adds to a signal *

12. Resistor that generates the lowest thermal noise:Ans. wire-wound *

13. Resistor with typical voltage range ofAns. metal film*

14. Reference for noise temperature in C:Ans. 17 *

15. Standard test tone connected on audio equipment?Ans. 1.0 kHz tone *

16. Reference standard test tone normally used is indicated in:Ans. dBm *

17. Power lost in device, due by the path of energy flow.Ans. insertion loss *

18. The noise generated with semiconductor devices.Ans. shot noise *

19. Bandwidth is approximately _____ the highest baseband frequency.Ans. 2 times *

20.

Flicker noise in radio communications is also known as.Ans. pink noise *

21. What determines the BW of a transmitted signal?Ans. the highest frequency component of modulating signal *

22. What formula is used to calculate the overall noise performance of thereceiver or of multiple stages if RF amplification?

Ans. Friis formula *


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23. If the bandwidth is doubled, considering all other parameters unchangedexcept the normal thermal noise only. The S/N will be___

Ans. decreased by 3 dB *

24. Noise at the receiver is in terms of:Ans. V *

25. Reference tone level for Ba:Ans. 85 dBm *

26. Reference tone level for dBrn:Ans. -90 dBm *

27. Reference tone level for F1A:Ans. -85 dBm *

28. Reliable measurement for comparing amplifier noise characteristics:Ans. noise factor *

NOISE PROBLEMS

29.

A receiver has noise power bandwidth of 10 kHz. A resistor that matches thereceiver input impedance is connected across its antenna terminals. What isthe noise power contributed by that resistor in the receiver bandwidth ifthe resistor has a temperature of 27 degrees Celsius?

Ans. 30. A 300 ohm resistor is connected across the 300 ohm antenna input of the

television receiver. The bandwidth of the receiver is 6 MHz, and theresistor is at room temperature. Find the noise power and noise voltageapplied to the receiver input.

Ans. 24.2 fW, 2.7 uV.

31. A diode noise generator is required to produce a 10 uV of noise in areceiver with an input impedance of 75 ohms, resistive, and a noise powerbandwidth of 200 kHz. What must be the current through the diode be?

Ans. 276 mA 32. Two noise-source resistors R1and R2connected in series at different

temperatures, 300K and 400K respectively. If R1= 100, R2= 200, find:A. the total noise voltage

B. the noise power at the load with RL= 300, over a BW = 100 kHz.Ans. A. 779 nV; B. 0.506 fW

33. A receiver produces a noise power of 200mW with no signal. The output level

increases to 5W when a signal is applied. Calculate (S+N)/N as a powerratio and in decibels.

Ans. 25, 14 dB 34. The signal power at the input to an amplifier is 100 uW and the noise power

is 1 uW. At the output, the signal power is 1W and the noise power is 30mW. What is the amplifier noise figure, as a ratio? In dB?

Ans. 3; 4.77dB

35. The signal at the input of an amplifier has an S/N of 42 dB. If the

amplifier has a noise figure of 6 dB, what is the S/N at the output indecibels?


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Ans. 36 dB 36. An amplifier has a noise figure of 2 dB. What is the equivalent noise

temperature?Ans. 170 K 37. A three-stage has stages with the following specifications: First stage with

power gain and noise figure of 10 and 2 respectively, 25 and 4 for thesecond stage and 30 and 5 for the third stage. Find the noise temperature.

Ans. 382 K

AMPLITUDE MODULATION

38. What is heterodyning?Ans. Mixing two frequencies across a nonlinear impedance.

39. What waveform is the basis of all complex waveforms?Ans. The sine wave

40. What is the instantaneous amplitude of a sine wave?Ans. The value at any given point on the sine wave

41. What term describes how much of a cycle has been completed?Ans. Phase or phase angle.

42. Define the heterodyne principle.Ans. Process of combining two signal frequencies in a nonlinear device.

43. What is a nonlinear impedance?Ans. An impedance in which the resulting current is not proportional to the

applied voltage.

44. What is spectrum analysis?Ans. The display of electromagnetic energy that is arranged according to

wavelength or frequency.

45. What two conditions are necessary for heterodyning to take place?Ans. At least two different frequencies applied to a nonlinear impedance.

46. Name two methods of oscillator keying.

Ans. Plate keying and Cathode keying

47. State the method used to increase the speed of keying in a CW transmitter.Ans. Machine keying

48. Name a disadvantage of a single-stage CW transmitter.Ans. Antenna to-ground capacitance can cause the oscillator frequency to vary

49. What is the purpose of frequency-multiplier stages in a VHF transmitter?Ans. To raise the low frequency of a stable oscillator to the vhf range.

50. What are the two major sections of a typical AM transmitter?

Ans. Rf and af units

51. When 100 kilohertz and 5 kilohertz are heterodyned, what frequencies arepresent?

Ans. 100 kilohertz, 5 kilohertz, 95 kilohertz, and 105 kilohertz

52. What determines the bandwidth of an AM transmission?Ans. The highest modulating frequency

53. What is percent of modulation?Ans. The depth or degree of modulation


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54. With a single modulating tone, what is the amplitude of frequencies at 100-

percent modulation?Ans. One-half the amplitude of the carrier

55. What is the formula for percent of modulation?Ans. %M = Em/Ec x 100%

56. What is high-level modulation?

Ans. Modulation produced in the plate circuit of the last radio stage of thesystem.

57. For what class of operation is the final rf power amplifier of a plate-modulator circuit based?

Ans. Class C

58. The modulator is required to be what kind of a circuit stage in a platemodulator?

Ans. Power amplifier

59. How much must the fpa plate current vary to produce 100-percent modulation in

a plate modulator?Ans. Between 0 and nearly two times its unmodulated value

60. The collector-injection modulator is similar to what type of tube modulator?Ans. Plate modulator

61. When is a control-grid modulator used?Ans. In cases when the use of a minimum of af modulator power is desired.

62. What type of modulator is the cathode modulator (low- or high-level)?Ans. Low-level

63. What causes the change in collector current in an emitter-injectionmodulator?Ans. Gain is varied by changing the voltage on the emitter

ANGLE AND PULSE MODULATION

64. What are the two types of angle modulation?Ans. Frequency and phase

65. Name the modulation system in which the frequency alternates between twodiscrete values in response to the opening and closing of a key?

Ans. Frequency-shift keying

66. What is the primary advantage of an fsk transmission system?Ans. Resistance to noise interference

67. What characteristic of a carrier wave is varied in frequency modulation?Ans. Instantaneous frequency

68. How is the degree of modulation expressed in an fm system?Ans. As the ratio of the frequency deviation to the maximum frequencydeviation allowable

69. What two values may be used to determine the bandwidth of an fm wave?Ans. The number of significant sidebands and the modulation frequency.

70. How does the reactance-tube modulator impress intelligence onto an rfcarrier?

Ans. By changing the reactance of an oscillator circuit in consonance with themodulating voltage.

71. What characteristic of a transistor is varied in a semiconductor-reactancemodulator?

Ans. Collector-to-emitter capacitance


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72. What circuit section is required in the output of a multivibrator modulatorto eliminate unwanted output frequencies?

Ans. An LCR filter

73. What characteristic of a varactor is used in an fm modulator?Ans. Capacitance

74. What type of modulation depends on the carrier-wave phase shift?Ans. Phase

75. What components may be used to build a basic phase modulator?Ans. A phase-shift network such a s a variable resistor and capacitor in series

76. Phase-shift keying is similar to what other two types of modulation?Ans. Cw and frequency-shift keying

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

Ans. Pulse modulation

78. What is prt?

Ans. Pulse-repetition time

79. What is nonpulse time?Ans. Rest time

80. What is average power in a pulsed system?Ans. Peak power during a pulse averaged over pulse time plus rest time

81. What action is necessary to impress intelligence on the pulse train in pulsemodulation?

Ans. Some characteristic of the pulses has to be varied

82. To insure the accuracy of a transmission, what is the minimum number of timesa modulation wave should be sampled in pulse modulation?Ans. 2.5 times the highest modulation frequency

83. What, if any, noise susceptibility advantage exists for pulse-amplitudemodulation over analog-amplitude modulation?

Ans. Both are susceptible to noise and interference

84. What characteristics of a pulse can be changed in pulse-time modulation?Ans. The time duration of the pulses or the time of occurrence of the pulses

85. Which edges of the pulse can be modulated in pulse-duration modulation?Ans. Either, or both at the same time

86. What is the main disadvantage of pulse-position modulation?Ans. It requires synchronization between the transmitter and receiver.

87. If a modulating wave is sampled 10 times per cycle with a 5-element binarycode, how many bits of information are required to transmit the signal?

Ans. 50

88. What is the primary advantage of pulse-modulation systems?Ans. Low susceptibility to noise

DEMODULATION

89. What is the simplest form of cw detector?Ans. A circuit that can detect the presence or absence of rf energy

90. What principle is used to help distinguish between two cw signals that areclose in frequency?

Ans. Heterodyning

91. How does heterodyning distinguish between cw signals?Ans. By giving a different beat frequency for each signal

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


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Ans. balance modulator *

114. Carrier is said to be overmodulated if the positive peak rises to a value_____ of the maximum unmodulated carrier.

Ans. more than twice *

115. Class of bias produce least harmonicsAns. class A *

116. Devices used to make modulated envelope visible.Ans. oscilloscope *

117. What will be the result in balanced modulation if not perfectly balanced.Ans. the carrier is transmitted *

118. Advantage of series modulationAns. generate high power *

119. Filter attenuates signals, passes below and above that band.Ans. band stop *

120. To provide 2 or more voice currents with same carrier.Ans. ISM emission *

121. To raise the power levels of AM signals, the class of amplifier used is_____.

Ans. class A *

122. Supposed a voice frequency of 400 Hz is transmitted on an AM radio stationoperating on 590 kHz, the voice frequency 400 Hz is NOT the ______frequency.

Ans. modulated *

123. What will normal AM receiver detect from an unmodulated RF AC wave?Ans. nothing *

124. Splatter is the result of ________.Ans. overmodulation *

125. What happens in standard AM transmission, no modulating signal is beingtransmitted?

Ans. there are no sidebands *

126. B8E, form of modulation also known as ______.Ans. Independent Sideband Transmission *

127. Colloquial term describes additional side frequencies produced byovermodulation or distortion in AM.

Ans. splatter *

128. Shape trapezoidal pattern at 100% modulation.Ans. triangle

129. What is the effect if the gain level being too high for signals entering themodulator?

Ans. distortion and splatter *

130. The RF signal produce; carrier frequency (fc) minus modulating frequency(fm).

Ans. LSB *

131. Mixer is also known as:Ans. converter *

132. In filter design, the maximum SB suppression is:Ans. 50 dB *

133. Transmitter power output in SSB operation is expressed in terms of


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Ans. PEP *

134. For SSB transmitter, the average power is typically _____ of the peakenvelope power, with the typical human speech.

Ans. 1/4to 1/3 *

135. Modulation system most noise resistantAns. FM *

136. Pre-emphasis provides extra noise immunity byAns. converting phase modulation to FM *

137. The three major types of demodulators:Ans. Foster-Seely, Quadrature, PLL *

138. In a frequency synthesizer, smallest amount which output frequency can bechanged?

Ans. resolution *

139. Internal capacitance, causes feedback produces same effect onAns. Miller effect *

140. Small length of wire found in some RF equipment, connected only at one endand use as a capacitance to ground.

Ans. gimmick *

141. Movement of signal from one frequency to another using mixer-oscillatorcombination.

Ans. frequency translation *

142. Feature of modulating tone, FM deviation is proportionalAns. amplitude *

143. Modulating 2 waves of the same frequency, but with _____ phase difference isequivalent to modulating both amplitude and phase of the same carrier.Ans. 90 degrees *

144. Frequency of unmodulated carrier of an FM.Ans. rest frequency *

145. What determines stations that will be selected by a tuner?Ans. resonant frequency of tuner *

146. Periodic waveforms consist of add harmonics.Ans. square wave *

147. Major problem with VHF oscillator.Ans. poor frequency stability *

148. What happens to a spectrum of repetitive pulse as the pulse width decrease?Ans. more harmonics of the same phase *

149. Keyed AGC is AGC thatAns. is used in TV receivers *

150. Main disadvantage of single-tube transmitter.Ans. frequency instability *

151. What is reduced by rounding off squarewave emission.Ans. bandwidth *

152. Gained by operating oscillator on some subharmonic of frequency.Ans. frequency stability *

153. Multiplexing, oldest and simplest.Ans. space division multiplexing *

154. Device display signal state of many lines simultaneously.Ans. logic analyzer *


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155. Most common IF carrier frequency.Ans. 70 MHz *

156. Citizen Band (CB) Radio Service is a two-way voice communication device, ituses frequency range from ______ MHz.

Ans. 26.965 to 27.405 *

157. What determines the rate of frequency swing for an FM broadcast transmitter?

Ans. modulation frequency *

158. In PLL demodulating an FM signal.Ans. VCOout = FMin *

159. In PLL frequency modulator, fmAns. error signal *

160. Find the modulation index if a 10V carrier is amplitude-modulated by threedifferent frequencies with amplitudes of 1V, 2V, and 3V, respectively.

Ans. 0.374

161. Calculate the modulation index for a waveform with a maximum voltage of 150V

and minimum voltage of 70V.Ans. 0.364 or 36.4%

162. An AM broadcast transmitter has a carrier power output of 50 kW. What is the

total power would be produced with 80% modulation?Ans. 66 kW

163. An AM broadcast transmitter has a carrier power output of 50 kW. What is the

power in one sideband with 80% modulation?Ans. 8 kW

164. An AM broadcast transmiiter has a carrier power output of 50 kW. What is the

total sideband power with 80% modulation?

Ans. 16 kW 165. An AM broadcast transmitter radiates 66 kW power when 100% modulated. If

the carrier and one sideband is suppressed, how much power is save?Ans. 55 kW

166. A transmitter with 10 kW carrier power transmit 12 kW when modulated with a

single sine wave. When modulated with another sinewave at 50% modulation,calculate the total transmitted power.Ans. 13.25 *

167. The rms antenna current of a transmitter is 10 A when unmodulated, it

increases by 2 A when modulated. Calculate the modulation index.Ans. 93.8% *


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168. In SSBSC system, if the peak envelope power (PEP) is 10 W, what will be the

value of the maximum instantaneous peak power?Ans. 20 W *

169. In SSBSC system, if the peak voltage is 25 volts and the load resistance is50 ohms, what will be the value of the peak envelope power (PEP)?

Ans. 6.25 W *

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170. A carrier wave with an RMS voltage of 20 V and a frequency = 1.5 MHz, is

modulated by a sine wave with a frequency of 500 Hz and amplitude of 10 VRMS. Determine the peak voltage of the carrier and of the lower sidefrequency.

Ans. 28.3 V; 7.1 V * 171. An FM modulator has a modulator deviation constant of kf = 30 kHz/V and

operates at a carrier frequency of 175 MHz. Find the output frequencyinstantaneous value of the modulating signal equal to -2V.

Ans. 174.94 MHz 172. An FM modulator has a modulator deviation constant of kf = 30 kHz/V and

operates at a carrier frequency of 175 MHz is modulated by a 3V sine wave.Calculate the deviation.

Ans. 127.2 kHz 173. An FM broadcast transmitter operates at its maximum deviation of 75 kHz.

Find the modulation index for a sinusoidal modulation signal with afrequency of 50 Hz.

Ans. 1500

174. A phase modulator has a phase modulator sensitivity of kp = 2 radians pervolt. What RMS voltage of a sine wave would cause a peak phase deviation of60 degrees?

Ans. 0.37 V

175. An FM communications transmitter has a maximum frequency deviation of 5 kHz

and a range of modulating frequencies from 300 Hz to 3 kHz. What is themaximum phase shift that it produces?

Ans. 16.7 radians

176. An FM communications transmitter has a maximum frequency deviation of 5 kHz

and a range of modulating frequencies from 300 Hz to 3 kHz. What is theminimum phase shift that it produces?

Ans. 1.67 radians

177. A phase modulation system, has a modulation index mp = 1.5, what is maximumphase shift?


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Ans. 86 degrees * 178. A phase modulator has a sensitivity of kp = 3 radians per volt. How much

frequency deviation does it produces with a sine-wave input of 2V peak at afrequency of 1 kHz?

Ans. 6 kHz

179. An FM signal has a deviation of 3 kHz and a modulating frequency of 1 kHz.Its total power is 5W, developed across a 50-ohm resistive load. Thecarrier frequency is 160 MHz. Using Carsons rule, calculate the bandwidth

of the signal.Ans. 16 kHz ( ) 180. An FM signal has a modulation index, mf = 3 and a modulating frequency of 2

kHz. Its total power is 5W, developed across a 50-ohm resistive load. Thecarrier frequency is 160 MHz. Using Carsons rule, calculate the bandwidthof the signal.

Ans. 16 kHz

( ) 181. An FM signal has a frequency deviation of 5 kHz and a modulating frequency

of 1 kHz. The signal to noise ratio at the input to the receiver detectoris 20 dB. Calculate the approximate signal-to-noise ratio at the detectoroutput.

Ans. 34 dB

182. A crytal oscillator is accurate within 0.0005%. How far off frequency could

be at 27 MHz?Ans. 135 Hz

183. A transmitter has a carrier power output of 10W at an efficiency of 70%.How much power must be supplied by the modulating amplifier for 100%modulation?

Ans. 7.14 W

184. A transmitter operates from a 12V supply, with a collector current of 2A.

The modulation transformer has a turns ratio of 4:1. What is the loadimpedance seen by the audio amplifier?

Ans. 96 ohms

185. A collector modulated class C amplifier has a carrier output power Pc of

100W and an efficiency of 70%. Calculate the supply power and the transistorpower dissipation with 100% modulation.Ans. Ps = 214W; Pd= 64W

186. An AM transmitter is required to produce 10W of carrier power when operating

from a 15V supply. What is the required load impedance as seen from thecollector?

Ans. 11.25 ohms


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187. A portable radio transmitter has to operate transmitter has to operate at

temperature from -5 to 35 degrees C. If its signal is derived from acrystal oscillator with a temperature coefficient of +1 ppm per degreeCelsius and it transmits at exactly 146 MHz at 20 degrees C, find thetransmitting frequency at the upper temperature limit.

Ans. 146.00219 MHz 188. A phase-locked loop has a VCO with a free-running frequency of 12 MHz. As

the frequency of the reference input is gradually raised from zero, the looplocks at 10 MHz and comes out again at 16 MHz. Find the capture range andlock range.

Ans. Capture range = 4 MHz, Lock range = 8 MHz

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