CNS–ST 1.1: INTRO TO COMMS, NOISE AND dB MEASURMENTS 1.
The theory of radio waves was originated by: Ans. Maxwell Maxwell * 2.
The first person who sent the first radio signal across the Atlantic ocean: Ans. Marconi Marconi * 3.
The transmission of radio waves was first done by: Ans. Hertz Hertz * 4.
When two or more signals share a common channel, it is called: Ans. Multiplexi Multiplexing ng * 5. “Man-made” noise can come from: Ans. equipment equipment that sparks sparks * 6. Thermal noise is generated in: Ans. transistor transistors s and diodes, diodes, copper copper wire, wire, and resistors * 7. Shot noise is generated in: Ans. transistor transistors s and diodes diodes * 8.
Flicker noise in radio communications is also known as. Ans. pink pink noise * 21.
What determines the BW of a transmitted signal? Ans. the highest frequency frequency component component of modulating modulating signal signal * 22.
What formula is used to calculate the overall noise performance of the receiver or of multiple stages if RF amplification?
Ans. Frii’s Frii’s formula formula *
23.
If the bandwidth is doubled, considering all other parameters unchanged except the normal thermal noise only. The S/N will be___ Ans. decreased decreased by 3 dB * 24.
Noise at the receiver is in terms of: Ans. µV * 25. Ans.
Reference tone level for µBa: – 85 dBm *
26. Ans.
Reference tone level for dBrn: -90 dBm *
The power density of “flicker” noise
is: Ans. greater greater at low frequencies frequencies * 9.
20.
So called “1/f” noise is also
called: Ans. pink pink noise * 10. “Pink” noise has: Ans. equal equal power power per octave octave * 11. Noise figure is a measure of: Ans. how how much noise noise an amplifier amplifier adds to a signal * 12.
Resistor that generates the lowest thermal noise: Ans. wire-wound wire-wound * 13.
Resistor with typical voltage range of Ans. metal metal film* film* 14. Reference for noise temperature in ˚C: Ans. 17 *
15.
Standard test tone connected on audio equipment? Ans. 1.0 1.0 kHz tone tone * 16.
Reference standard test tone normally used is indicated in: Ans. dBm dBm * Power lost in device, due by the path of energy flow. Ans. insertion insertion loss *
27. Reference tone level for F1A: Ans. -85 dBm * 28.
Reliable measurement for comparing amplifier noise characteristics: Ans. noise noise factor factor * NOISE PROBLEMS PROBLEMS 29. A receiver has noise power bandwidth of 10 kHz. A resistor that matches the receiver input impedance is connected across its antenna terminals. What is the noise power contributed by that resistor in the receiver bandwidth if the 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 the resistor is at room temperature. Find the noise power and noise voltage applied to the receiver input.
Ans. 24.2 24.2 fW, 2.7 2.7 uV.
17.
18.
The noise generated with semiconductor devices. Ans. shot shot noise * 19.
Bandwidth is approximately _____ the highest baseband frequency. Ans. 2 times *
31. A diode noise generator is required to produce a 10 uV of noise in a receiver with an input impedance of 75 ohms, resistive, and a noise power bandwidth of 200 kHz. What must be the current through the diode be? Ans. 276 mA
32.
Two noise-source resistors R 1 and R 2 connected in series at different temperatures, 300˚K and 400˚K respectively. If R 1 = 100Ω, R2 =
200, find: A. the total noise voltage B. the noise power at the load with R L = 300Ω, 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 power ratio 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 30 mW. 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 in decibels? 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 the second stage and 30 and 5 for the third stage. Find the noise temperature. Ans. 382 K
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 frequencymultiplier 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 are present? 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
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?
54. With a single modulating tone, what is the amplitude of frequencies at 100percent 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 the system.
Ans. By changing the reactance of an oscillator circuit in consonance with the modulating voltage.
57. For what class of operation is the final rf power amplifier of a platemodulator circuit based? Ans. Class C
71. What characteristic of a transistor is varied in a semiconductor-reactance modulator? Ans. Collector-to-emitter capacitance
58. The modulator is required to be what kind of a circuit stage in a plate modulator? Ans. Power amplifier
72. What circuit section is required in the output of a multivibrator modulator to eliminate unwanted output frequencies? Ans. An LCR filter
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
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
61. When is a control-grid modulator used? Ans. In cases when the use of a minimum of af modulator power is desired.
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
62. What type of modulator is the cathode modulator (low- or high-level)? Ans. Low-level
76. Phase-shift keying is similar to what other two types of modulation? Ans. Cw and frequency-shift keying
63. What causes the change in collector current in an emitter-injection modulator? Ans. Gain is varied by changing the voltage on the emitter
77. Overmodulating an rf carrier in amplitude modulation produces a waveform which is similar to what modulated waveform? Ans. Pulse modulation
ANGLE AND PULSE MODULATION 64. What are the two types of angle modulation? Ans. Frequency and phase
78. What is prt? Ans. Pulse-repetition time
65. Name the modulation system in which the frequency alternates between two discrete 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 frequency deviation 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 rf carrier?
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 pulse modulation? Ans. Some characteristic of the pulses has to be varied 82. To insure the accuracy of a transmission, what is the minimum number of times a 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-amplitude modulation 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 binary code, 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 are close 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? Ans. Regenerative detector 93. What three functions does the transistor in a regenerative detector serve? Ans. Oscillator, mixer, and detector 94. What does the simplest diode detector use to reproduce the modulating frequency? Ans. The modulation envelope 95. What is the function of the diode in a series-diode detector? Ans. Rectifies the rf pulses in the received signal. 96. Which junction of the transistor in the common-emitter detector detects the modulation envelope? Ans. Emitter-base junction 97. How is the output signal developed in the common-emitter detector? Ans. By the collector current flow through R4
100. What is the simplest form of fm detector? Ans. Slope detector 101. What type of tank circuit is used in the Foster-Seeley discriminator? Ans. A double-tuned tank circuit 102. What is the primary advantage of a ratio detector? Ans. Suppresses amplitude noise without limiter stages. 103. What circuit functions does the tube in a gated-beam detector serve? Ans. Limits, detects, and amplifies. 104. What condition must exist on both the limiter and quadrature grids for current to flow in a gated-beam detector? Ans. Both grids must be positively biased 105. Name two advantages of the gated-beam detector. Ans. Extreme simplicity, few components, and ease of adjustment. 106. Where is the intelligence contained in a phase-modulated signal? Ans. In the amount and rate of phase shift of the carrier wave. 107. How is a quadrature detector changed when used for phase demodulation? Ans. The quadrature grid signal is excited by a reference from the transmitter. 108. In its simplest form, what functions must a radar detector be capable of performing? Ans. Detecting the presence of rf energy. 109. What characteristic of pulse does a peak detector sample? Ans. Pulse amplitude or pulse duration 110. What is the time constant of the resistor and capacitor in a peak detector for PAM? Ans. At least 10 times the interpulse period MODULATION/RECEIVERS/TRANSMITTERS 111. The power output of a single-sideband transmitter is normally expressed as the _____ power. Ans. peak envelope *
112. SSB modulation is classified as ______ Ans. AM * 113. Used to suppress carrier in single sideband transmitters. Ans. balance modulator *
98. Which junction acts as the detector in a common-base detector? Ans. Emitter-base junction
114. Carrier is said to be overmodulated if the positive peak rises to a value _____ of the maximum unmodulated carrier. Ans. more than twice *
99. To what circuit arrangement is a common-base detector equivalent? Ans. A diode detector followed by a stage of audio amplification.
115. Class of bias produce least harmonics Ans. class A *
116. Devices used to make modulated envelope visible. Ans. oscilloscope *
133. Transmitter power output in SSB operation is expressed in terms of Ans. PEP *
117. What will be the result in balanced modulation if not perfectly balanced. Ans. the carrier is transmitted *
134. For SSB transmitter, the average power is typically _____ of the peak envelope power, with the typical human speech. Ans. 1/4to 1/3 *
118. Advantage of series modulation Ans. 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 station operating 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 being transmitted? 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 by overmodulation 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 the modulator? Ans. distortion and splatter *
135. Modulation system most noise resistant Ans. FM * 136. Pre-emphasis provides extra noise immunity by Ans. 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 be changed? Ans. resolution * 139. Internal capacitance, causes feedback produces same effect on… Ans. Miller effect *
140. Small length of wire found in some RF equipment, connected only at one end and use as a capacitance to ground. Ans. gimmick * 141. Movement of signal from one frequency to another using mixer-oscillator combination. Ans. frequency translation * 142. Feature of modulating tone, FM deviation is proportional Ans. amplitude * 143. Modulating 2 waves of the same frequency, but with _____ phase difference is equivalent 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 *
130. The RF signal produce; carrier frequency (fc) minus modulating frequency (fm). Ans. LSB *
147. Major problem with VHF oscillator. Ans. poor frequency stability *
131. Mixer is also known as: Ans. converter *
148. What happens to a spectrum of repetitive pulse as the pulse width decrease? Ans. more harmonics of the same phase *
132. In filter design, the maximum SB suppression is: Ans. 50 dB *
149. Keyed AGC is AGC that Ans. 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 *
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
154. Device display signal state of many lines simultaneously. Ans. logic analyzer * 155. Most common IF carrier frequency. Ans. 70 MHz * 156. Citizen Band (CB) Radio Service is a two-way voice communication device, it uses 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 *
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% *
159. In PLL frequency modulator, f m Ans. error signal * 160. Find the modulation index if a 10V carrier is amplitude-modulated by three different frequencies with amplitudes of 1V, 2V, and 3V, respectively. Ans. 0.374
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 *
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%
169. In SSBSC system, if the peak voltage is 25 volts and the load resistance is 50 ohms, what will be the value of the peak envelope power (PEP)? Ans. 6.25 W *
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
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 V RMS. Determine the peak voltage of the carrier and of the lower side frequency. 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 frequency instantaneous value of the modulating signal equal to -2V. Ans. 174.94 MHz
Carson’s rule, calculate the
bandwidth of the signal. Ans. 16 kHz 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
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. The carrier frequency is 160 MHz. Using Carson’s rule, calculate
173. An FM broadcast transmitter operates at its maximum deviation of 75 kHz. Find the modulation index for a sinusoidal modulation signal with a frequency of 50 Hz. Ans. 1500
174. A phase modulator has a phase modulator sensitivity of kp = 2 radians per volt. What RMS voltage of a sine wave would cause a peak phase deviation of 60 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 the maximum 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 the minimum phase shift that it produces? Ans. 1.67 radians 177. A phase modulation system, has a modulation index mp = 1.5, what is maximum phase shift? 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 a frequency 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. The carrier frequency is 160 MHz. Using
the bandwidth of 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 detector is 20 dB. Calculate the approximate signal-tonoise ratio at the detector output. 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 load impedance 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 transistor power 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 the collector? Ans. 11.25 ohms
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 a crystal oscillator with a temperature coefficient of +1 ppm per degree Celsius and it transmits at exactly 146 MHz at 20 degrees C, find the transmitting 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 loop locks at 10 MHz and comes out again at 16 MHz. Find the capture range and lock range. Ans. Capture range = 4 MHz, Lock range = 8 MHz
