ELECTRONICA DE COMUNICACIONES
QUIZ 2 PARCIAL 2
1.What is the purpose of modulation?
a) To improve the tonal quality of the signal at the receiver.
b)To limit the frequency deviation for improved signal transmission.
c) To move the information signal to a new position in the frequency spectrum so it
can be transmitted easily and accurately.
d)To permit engineers to view a signal on an oscilloscope.
2.Which type of modulation requires more bandwidth, SSB or AM?
c) They require the same amount of bandwidth.
d) Bandwidth does not apply to these types of modulation.
3.Why is AM used for entertainment radio broadcasting in the 550-kHz to 1600-kHz
a) Because AM stations broadcast to customers that are within line-of-sight range.
b) Early radio engineers didn't know how to generate signals above 1600 kHz.
c) This frequency band was mistakenly assigned to amateur radio.
d)AM is very easy to detect and the required detection circuitry is simple.
4.Generally does FM require more or less bandwidth than AM modulation?
a) FM and AM require equal amounts of bandwidth.
b) FM requires greater bandwidth.
c) AM requires greater bandwidth.
d) Bandwidth does not apply to these types of modulation.
5.Why is SSB modulation good for voice transmission in the high-frequency region?
a) It requires more bandwidth so it can accommodate more channels.
b) It is less susceptible to "selective fading."
c) SSB doesn't work at all at high frequencies.
d)Thunderstorms and man-made noise do not affect SSB.
6.How many sidebands are present in SSB, AM, and FM?
a) SSBhas one; AM has two; and FM has two or more.
b)They all have the same number of sidebands.
c) SSBand AM each have two; FM has four.
d) SSBand FM have multiple sidebands; AM has one.
7.What does 100% modulation mean for AM?
a) The signallevel is greatest and reception is best at 100% modulation.
b)The power in the sidebands is equal to the power in the carrier.
c) It is the maximum modulation possible without distortion.
d) It is the minimum modulation necessary to successfully transmit a signal.
8.What is meant by deviation with regard to an FM signal?
a) It is the "drift" in frequency that requires one to retune their radio from time to
b) It is the difference between the carrier frequency and the baseband frequency.
c) It is the method used for producing FM stereo.
d) It is the maximum carrier frequency change caused by the modulating frequency.
9.Why is FM used in entertainment radio broadcasting in the 88MHz to 108MHz
a) FM uses greater bandwidth, which is better accommodated at higher frequencies.
b) Because FM stations can broadcast for very long distances.
c) Radio manufacturers in the early 1950's agreed to this standard.
d)The amount of power to the FM antenna requires higher frequencies.
10. What are the beneficial characteristics of FM modulation?
a) There are none. AM signals are better than FM signals.
b) Improved S/N; processing gain; quieting; and capture.
c) FM has better tonal qua lity that SSB, and requires less bandwidth.
d) FM has more sidebands and is easier to produce than AM.
11. in AM modulation, all of the______________ is in the sidebands.
12. a 4-kHz baseband signal is used to AM modulate a earrier at 1-MHz, what are the
13. a 1000 watt carrier is 50% AM modulated. How much power is in both sidebands?
In each sideband?
14. A single 1-kHz signal is used to AM modulate a 1-MHz carrier. What is the total
bandwidth of the output signal?
15. In a single sideband signal, the_____________ is suppressed and one sideband is
16.A_________ modulator is used to generate SSB.
17. In___________ modulation, the phase of a signal is changed. In _________
modulation the frequency of the signal is changed. Both forms of modulation are
18. An FM carrier of 1000 watts is deviated by 3-kHz. How much power is in the
19.Calculate the deviation ratios for the following FM signals:
Maximum carrier frequencies (fcmax): Maximum modulation frequencies (fm max)
120 MHz 8 kHz
200 MHz 20 kHz
70 MHz 10 kHz
60 MHz 12 kHz
20. FM systems have a quality called ______________where the stronger station
signal will completely dominate the channel and eliminate interference from stations
broadcasting on the same frequency with a weaker signal.
21. How are mixing and modulation similar?
a) They both translate a signal in frequency
b) Mixing is easier to do
c) They both require a linear process
d)AII of the above
22.A converter circuit has what basic components?
a) A modulator and amplifier
b)A mixer and amplifier
c) A mixer and local oscillator
d) A mixer and a modulator
23. Heterodyning is a special type of mixing that ...
a) Converts radio station signals of different frequencies to the same fixed
b) Uses a tunable local oscillator
c) 15 a form of mixing used in receivers
d) AII of the above
24. Diodes are useful as mixers because they ...
a) Handle large amounts of power
b) Have non-linear characteristics
c) Have linear characteristics
d) Require low signal levels to drive
25.Active devices such as FET transistors are good mixers because ...
a) They have very low gain and linear characteristics
b) They have high gain and linear characteristics
c) They have high gain and non-linear characteristics
d) None of the above
26.A modulated signal can be easily translated by a mixer because ...
a) It is larger than the oscillator signal
b) The mixing process preserves all the frequencies in a translated signal
c) Signal envelopes will not pass through the mixing process
d) Mixers only work with sinewaves at a single frequency
27.Image frequencies arise because ...
a) Mixers have single frequency outputs
b) Mixer circuits may have to much gain
c) Mixers provide insufficient signal isolation
d) Mixers produce sum and difference frequency outputs
28. Mixing two signals, each with single frequencies, produces
a) Signals at twice the sum of the frequencies.
b) Signals at twice the difference of the frequencies.
c) Signals with frequencies that are the sum and difference of the original
d)AII of the above.
29.The intermediate frequency of heterodyning is used in receiver design
a) To provide high signal-to-noise ratios.
b)To provide for sensitivity and selectivity in the receiver.
c) To lower the receiver gain for lower signal-to-noise ratios.
d) So that two local oscillators can be used in the mixer circuit.
30. Mixing can have two input signal of the following type:
a) One a single-frequency input signal and one a saw-tooth wave local oscillator signal.
b) One a square-wave signal and the other a local oscillator signal.
c) One a single-frequency oscillator signal and one a modulated carrier signal.
d)a and c above.
31. Dual-gate MOSFET mixers are an advantage because they:
a) Provide two outputs that can be used.
b) Have very-Iow input impedance.
c) Have very-high input impedance.
d) Have no isolation between the inputs.
32. Diode mixers have the following characteristics:
a) Large gain, no conversion loss, narrowband frequency range.
b) Small gain, no conversion loss, narrowband frequency range.
c) No gain, conversion loss, broad-band frequency range.
d) No gain, no conversion loss, broad-band frequency range.
33. If the input signals to a mixer are 5 MHz and 17 MHz, what will the output
34. If you wish to receive a station transmitting at 1.8 MHz and the receiver IF
frequency is 455kHz, what frequency should the local oscillator be tuned to?
35. If the intended received frequency is 11 MHz, what is the image frequency if the IF
frequency is 9 MHz and the Local Oscillator is 20 MHz?
36. Is a diode mixer an active device mixer?
37. Why is an FET a candidate to be able to design a good mixer?
38. In order to design a receiver with very good selectivity and sensitivity, a_______
, and____________ ________ oscillator are required.
38. Semiconductor technology has produced diodes used in diode mixers up to a __
__Hz in frequency.
39. A spectrum analyzer is used to detect the frequency output of two mixed signals.
One has a frequency of 100 MHz and the other has two frequencies, one at 2 MHz and
another at 4 MHz. Where will the signals be detected?
40.calculate the percentage power saving the carrier and one of the sidebands are
suppressed in AM wave modulated to a depth of a) 100 percent and b) 50 percent.
41. A 1000-kHz carrier is simultaneously modulated with 300-Hz, 800-Hz and 2-kHz
audio sine waves. What will be the frequencies present in the output?
42. A broadcast AM transmitter radiates 50 kW of carrier power. What will be the
radiated power at 85 percent modulation?
43. When the modulation percentage is 75, an AM transmitter produces 10 kW. How
much of this is carriei power? What would be the percentage power saving if the
carrier and one of the sidebands were suppressed before transmission took place?
44. A 360-W carrier is simultaneously modulated by two audio waves with modulation
percentages of 55 and 65, respectively. What is the total sideband power radiated?
45. A transistor class e amplifier has maximum permissible collector dissipation of 20
W and a collector efficiency of 75 percent. lt is to be collector-modulated to a depth of
90 percent. (a)Calculate (i) the maximum unmodulated carrier power and (ii) the
sideband power generated. (b)If the maximum depth of mod!llation is now restricted
to 70 percent, calculate the new maximum sideband power generated. "
46. When a broadcast AM transmitter is 50 percent modulated, its antenna current is
12 A. What will the current be when the modulation depth is increased to 0.9?
47. The output current of a 60 percent modulated AM generator is 1.5 A. To what
value will this current rise if the generator is modulated additionally by another audio
wave, whose modulation index is 0.7? What will be the percentage power saving if the
carrier and one of the sidebands are now suppressed?
48. When the modulating frequency in an FM system is 400 Hz the modulating
voltage is 2.4 V, the modulating index is 60. Calculate the maximum deviation .what is
the modulation index when the modulating frequency is reduced to 250 hz and the
modulating voltage is simultaneously raised to 3.2 V.?
49. Describa las características de la señal de TV NTSC, monocromática y de color.
a)Calcular la resolución horizontal y vertical para señales de video, es decir las
frecuencias a las que operan tales resoluciones.
b) Mediante diagramas a bloques describa las operaciones de los receptores de TV.
c)Diga cuales son los valores de una señal RGB normalizados.
d) Explique cuales son los niveles IRE en televisión.
e) Diga cual es la utilidad de un vectorscopio en la señal de televisión.
f) Explique la diferencia entre un cuadro y un campo en TV.
g)Explique como es posible transmitir una señal a color por medio del mismo ancho
de banda utilizado para una señal monocromática.
h)Diga que frecuencia de FI de sonido se utiliza en Tv Y POR QUE esa frecuencia.
i)Diga por que se utiliza la transmisión de imagen negativa en TV.
50. Se tiene la señal modulada en amplitud v(t) encuentre la suma y diferencia d e
frecuencias , las amplitudes correspondientes y el índice de modulación.