Excel Review Center 1.
EST Exam - Problems
An AM transmitter is modulated by two sine waves at 1 kHz and 2.5 kHz, with modulations of 25% and 50%, respectively. What is the effective modulation index?
A. B. C. D. 9.
A. B. C. D. 2.
An FM transmitter has a carrier frequency of 220 MHz. Its modulation index is 3 with a m odulating frequency of 5 kHz. The total power output is 100 W into a 50 load. What is the deviation? A. B. C. D.
3.
164 dB 41 dB 20.5 dB 82 dB *
An AM transmitter supplies 10 kW of of carrier power to a 50 load. It operates at a carrier frequency of 1.2 MHz and is 80% modulated by a 3 kHz s ine wave. Calculate the total average power in the signal, in watts and dBW. A. B. C. D.
8.
620 kHz 1530 kHz * 1075 kHz 1350 kHz
An AM broadcast receiver has two identical tuned circuits prior to the IF stage. The Q of these circuits is 60 and the IF frequency is 455 kHz and the receiver is tuned to a station at 680 kHz. Calculate the amount of image frequency rejection. A. B. C. D.
7.
12 dB * 6 dB 24 dB 3 dB
Determine the image frequency for a standard broadcast band receiver using a 455-kHz IF and tuned to a station at 620 kHz. A. B. C. D.
6.
1.5 kW 500 W * 250 W 1000 W
A frequency-modulation system requires a signal-to-noise ratio of 40 dB at the detector output. The modulating frequency is 2 kHz, and the deviation is 50 kHz. Calculate the required m inimum S/N at the detector output. A. B. C. D.
5.
20 kHz 15 kHz * 30 kHz 40 kHz
Determine the maximum sideband power if the carrier output is 1 kW. A. B. C. D.
4.
0.56 * 0.65 0.43 0.83
12.3 kW (40.9 dBW) 13.2 kW (41.2 dBW) * 10.2 kW (40.09 dBW) 23.1 kW (43.64 dBW)
An FM broadcast-band receiver tunes from 88 to 108 MHz. The IF is 10.7 MHz, and the receiver us es high-side injection. Calculate the range of local oscillator frequencies.
97.8 MHz to 117.8 MHz 89.7 MHz to 118.7 MHz 98.7 MHz to 118.7 MHz * 98.7 MHz to 117.8 MHz
A 1.4-MHz carrier is modulated by a music signal that has frequency components from 20 Hz to 15 kHz. Determine the range of frequencies generated for the upper sideband? A. B. C. D.
1,400,020 Hz – 1,415,000 Hz * 1,385,000 Hz – 1,399,980 Hz 1,400,200 Hz – 1,415,020 Hz 1,385,000 Hz – 1,415,000 Hz
10. A 500-W 500-W carrier is to be modulated to a 90% level. Determine the total transmitted power. A. B. C. D.
202.5 W 405 W 702.5 W * 1.5 kW
11. CB radio was developed developed using fullcarrier DSB AM. It operates using 40 channels, each 10 kHz wide. hen SSBSC is used, the same carrier frequencies are used as with conventional AM, AM, but the transceivers are switchable to either lower or upper sideband. How many channels are available when SSB is used? A. B. C. D.
90 70 80 * 100
12. The antenna current of an AM transmitter is 12A when unmodulated but increases to 13 A w hen modulated. Calculate %m. A. B. C. D.
59 % * 34 % 95 % 43 %
13. An intelligence intelligence signal is is amplified by a 70% efficient amplifier before being combined with a 10-kW carrier carrier to generate the AM signal. If it is desired to operate at 100% modulation, what is t he dc input power to the f inal intelligence amplifier? A. B. C. D.
14.28 kW 5 kW 3.57 kW 7.14 kW *
14. A transmitter transmitter with a 10-kW carrier carrier transmits 11.2 kW when modulated with a single sine wave. Calculate the modulation index. A. B. C. D.
0.49 * 0.94 0.7 0.25
15. A transistor transistor has a power dissipation dissipation rating of 25W. Assume that the transistor is the only element that dissipates power in the circuit. Calculate the power an amplifier using this transistor could deliver to the load if it operates Class A with an efficiency of 30%.
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A. B. C. D.
100 W 70.1 W 10.7 W * 15.1 W
16. An AM signal has a carrier frequency of of 3 MHz and an amplitude of 5 V peak. It is modulated by a sine wave with a frequency of 500 Hz and a peak voltage of 2 V. Writ e the equation of this signal and calculate the modulation index. A. B. C. D.
v(t) = [3+3sin(3.14t)]sin (188t); m = 0.4 3 v(t) = [5+2sin(3.14x10 t)]sin 6 (18.8x10 t); m = 0.4 * v(t) = [10+3sin(3.14t)]sin (18t); m = 0.6 v(t) = [2+5sin(3.14t)]sin (1.5t); m = 0.3
17. The audio frequency response of of an AM transmitter, measured from microphone input to the secondary of t he modulation transformer, is 3 dB down at 3 kHz from its level at 1 kHz. If a 1 kHz signal modulates the transmitter to 90%, what will be the modulation percentage due to a 3 kHz signal with the same level at the input? A. B. C. D.
63.7% * 67.3% 73.6% 75.1%
18. An AM AM transmitter operates into into a 50- resistive load. The RMS volt age measured at the output is 250V without modulation and 300 V with modulation, using a true-RMS reading meter. Find the power with modulation. A. B. C. D.
2 kW 1.8 kW * 2.5 kW 0.5 kW
19. An AM AM transmitter uses high-level high-level modulation. The RF power amplifier runs from a 12-volt source, putting out a carrier power of 75 W at an efficiency of 75%. How much power must be provided by the modulating amplifier for 100% modulation? A. B. C. D.
100 W 20 W 50 W * 75 W
20. An AM AM transmitter uses high-level high-level modulation. The RF power amplifier draws 10 amperes from a 20-V supply, putting out a carrier power of 125 W. What is the efficiency of this stage? A. B. C. D.
65.2 % 100 % 62.5 % * 75.8 %
21. A receiver receiver has has a sensitivity sensitivity of 0.3 0.3 V. The same receiver can handle a sig nal level of 75 mV without overloading. What is its AGC range in decibel? A. B. C. D.
100 dB 108 dB * 98 dB 118 dB
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Excel Review Center 22. A receiver uses low-side injection for the local oscillator, with an IF of 1750 kHz. The local oscillator is operating at 15.750 MHz. What is the image frequency? A. B. C. D.
1000 Hz, 1700 Hz * 1000 Hz, 2000 Hz 2000 Hz, 2700 Hz 2000 Hz, 1700 Hz
25. A 10 MHz crystal oscillator in a PLL frequency synthesizer has been calibrated to a reference standard known to be accurate to 5 ppm. The oscillator is guaranteed to drift no m ore than 10 ppm per month. A year later, what are the maximum and minimum frequencies that the oscillator c ould be producing? A. B. C. D.
10.00125 MHz, 9.99875 MHz * 10.125 MHz, 9.8875 MHz 15.00145 MHz, 10.1257 MHz 20.0153 MHz, 75.265 MHz
26. In an FM system, when the audio frequency (AF) is 500 Hz and the AF voltage is 2.4 V, the deviation is 4.8 kHz. If the AF voltage is now increased to 7.2 V, what is the new deviation? A. B. C. D.
9.16 kHz 7.2 kHz 14.4 kHz * 20 kHz
27. Determine the permissible range in maximum modulation index for commercial FM that has 30 Hz to 15 kHz modulation frequencies. A. B. C. D.
5 to 75 5 to 15 5 to 2500 * 5 to 1500
28. Determine the relative total power of the carrier and side frequencies when m f = 0.25 for a 10-kW FM transmitter. A. B. C. D.
A. B. C. D.
5 kW 10 kW * 4.5 kW 9 kW
34.5 kW 69 kW 17.25 kW 15.5 kW *
30. A transmitter power amplifier has a gain of 30 dB and an efficiency of 75%. Its output power is 100 W. What will be the output power with 100% modulation, assuming the efficiency is the same with and without modulation?
30 kHz 25 kHz 15 kHz * 20 kHz
24. An SSBSC signal has a suppressed carrier frequency, in the IF a receiver, of 2 MHz. It is modulated with two audio tones having frequencies of 1500 and 2200 Hz. The receiver is m istuned so that the BFO is at 1.9995 MHz. What will be the output fr equencies of the demodulator if the signal is LSB? A. B. C. D.
29. An AM broadcast station operates at its maximum allowed total output of 50 kW and at 95% modulation. How much of its transmitted power is intelligence?
15 MHz 17.5 MHz 14 MHz * 28 MHz
23. A receiver’s IF filter has a shape factor of 2.5 and a bandwidth of 6 kHz at the 6 dB down point. W hat is its bandwidth at 60 dB down? A. B. C. D.
EST Exam - Problems
A. B. C. D. 31.
133 W 100 W 150 W * 250 W
An SSB transmitter transmits on the USB with a (suppressed) c arrier frequency of 7.2 MHz. It is modulated by two tones, with frequencies of 1 kHz and 2.5 kHz and equal amplitude. The transmitter PEP is 75 W into a 50 load. The carrier and unwanted sideband are each suppressed by 60 dB. Calculate the average power into the load? A. B. C. D.
75.3 W 37.5 W * 375 W 53.7 W
32. A superheterodyne receiver is tuned to a frequency of 5 MHz when the local oscillator frequency is 6.65 MHz. What is the IF? A. B. C. D.
1.65 MHz * 2.75 MHz 455 kHz 10.7 MHz
33. If the carrier of the above problem is simultaneously modulated with another sine wave at 50% modulation, c alculate the total transmitted power. A. B. C. D.
12.45 kW * 9.33 kW 6.22 kW 8.21 kW
34. A TRF receiver is to be designed with a single tuned circuit using a 10-H inductor. Calculate the capacitance range of the variable capacit or required to tune from 550 to 1550 kHz. A. B. C. D.
1.06 nF – 8.37 nF * 1.06 mF – 8.37 F 1.06 F – 8.37 nF 1.06 nF – 8.37 pF
35. The tuned circuit of the oscillator in a simple AM transmitter employs a 50-H coil and a 1-nF capacitor. If the oscillator output is modulated by audio frequencies up to 10 kHz, what is the frequency range occupied by the sidebands? A. B. C. D.
722 to 732 kHz 702 to 712 kHz 702 to 722 kHz * 712 to 722 kHz
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36. The power of an amplifier of an AM transmitter has an output carrier power of 25 W and an efficiency of 70% and is collector-modulated. How audio power will have to be supplied to this stage for 100% modulation? A. B. C. D.
19.7 W 17.9 W * 17.78 W 19.77 W
37. A transistor RF power amplifier operating class C is designed to produce 30 W output with a supply voltage of 50 V. If the efficiency of the stage is 70%, what is the average collector current? A. B. C. D.
785 mA 587 mA 857 mA * 857 A
38. A transmitter uses twelve modules in its solid-state output stage. Calculate th e power reduction (in decibels) that would occur if one module fails. A. B. C. D.
0.48 dB 0.84 dB 0.38 dB * 0.83 dB
39. A 400-W carrier is modulated to a depth of 75 percent. Calculate the t otal power in the modulated wave. A. B. C. D.
512.5 W * 712.5 W 256.25 W 128.125 W
40. A broadcast radio transmitter radiates 10 kW when the modulation percentage is 60. How much of this is carrier power? A. B. C. D.
8.47 kW * 8.74 kW 7.47 kW 4.78 kW
41. The antenna current of an AM transmitter is 8 A wh en only the carrier is sent, but it increases to 8.93 A when the carrier is modulated by a s ingle sine wave. Find the percentage modulation. A. B. C. D.
64 % 80 % 70.1 % * 51.2 %
42. A certain transmitter radiates 9 kW with the carrier unmodulated, and 10.125 kW when the carrier is sinusoidally modulated. Calculate the modulation index. A. B. C. D.
0.95 0.5 * 0.64 0.35
43. If another sine wave, corresponding to 40 percent modulation, is tr ansmitted simultaneously, determine the total radiated power of the above problem.
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Excel Review Center A. B. C. D.
108.4 kW 108.4 W 1084 W 10.84 kW *
44. The antenna current of an AM broadcast transmitter, modulated to a depth of 40% by an audio sine wave, is 11 A. It increases to 12 A as a result of simultaneous modulation by another audio sine wave. What is th e modulation index due to this second sine wave? A. B. C. D.
EST Exam - Problems A. B. C. D.
64 dB 30 dB 45 dB 54 dB * < < < Nothing Follows > > >
0.643 * 0.4 0.413 0.757
45. Calculate the percentage power saving when the carrier and one of the sidebands are suppressed in an AM wave modulated to a depth of 100 percent. A. B. C. D.
33.3 % 66.7 % 100 % 83.3 % *
46. Determine the worst-case output S/N for a broadcast FM program that has a maximum intelligence frequency of 5 kHz. The input S/N is 2. A. B. C. D.
30 * 60 15 45
47. An AM transmitter has a carrier power of 50 W at a carrier frequency of 12 MHz. It is modulated at 80% by a 1 kHz sine wave. How much power is contained in the sidebands? A. B. C. D.
16 W * 10 W 8W 25 W
48. A receiver has two uncoupled tuned circuits before the mixer, each with a Q of 75. The signal fr equency is 100.1 MHz, and the IF is 10.7 MHz. The local oscillator uses high-side injection. Calculate the image rejection ratio in decibels. A. B. C. D.
56.8 dB 58.6 dB * 65.8 dB 68.5 dB
49. A double-tuned IF transformer has k = 1.5kc. The primary Q is 50 and the secondary Q is 40. Calculate th e bandwidth of the transformer at a frequency of 10.7 MHz. A. B. C. D.
953 kHz 593 kHz 359 kHz * 935 kHz
50. When measuring the selectivity of a receiver, you discover that a si gnal level of 500 microvolts on an adjacent channel is required to give the same output as a 1 micr ovolt signal on the channel to which the receiver is t uned. What is the adjacent channel selectivity in dB? Baguio: PiRC Building, Gapuz Centrum II, 80 T. Alonso St., New Lucban CP: 0928 934 1637
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