Problem Solving (Communications)
1. In the the equat equation ion of of the the instan instantan taneous eous voltage as a function of time e (t ) = 5 sin(5000t + 30) , what is the amplitude of the wave in rms? a. 5 volts b. 30 volts c. 3.54 volts d. None of the above 2. What What is the frequen frequency cy of of the the signa signall in Problem 1? a. 5000 Hz b. 5 Hz c. 30 Hz d. 796 Hz 3. Calcul Calculate ate the wavele wavelengt ngth h in free free space corresponding to a frequency of CB radio band. a. 300 m b. 11.1 m c. 7.5 cm d. 3 cm 4. A rece receiv iver er has has a noise noise pow power er 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 27ºC? a. 4.14 x 10-17 W b. 4.14 x 10-18 W c. 4.14 x 10-16 W d. 4.14 x 10-15 W 5. A 300 Ω resis resistor tor is conne connecte cted d across across the 300 Ω antenna input of a television receiver. The bandwidth of the receiver is 6 MHz, and the resistor is at room temperature. Find the noise power applied to the receiver input? a. 24.2 nW b. 24.2 pW c. 24.2 fW d. 24.2 μW
6. Comp Comput utee for the the noise noise vol volta tage ge applied to the receiver input in Problem 5. a. 5.4 mV b. 5.4 μV c. 5.4 nW d. 5.4 pV 7. A diode diode nois noisee genera generator tor is requ require ired d to produce 10 μV of noise in a receiver with an input impedance of 75 Ω, resistive, and a noise power bandwidth of 200 kHz. (These values are typical of FM broadcast receivers.) What must be the current through the diode be? a. 0.133 μA b. 276 mA c. 7.5 μA d. 13.3 nA 8. Two Two res resis isto tors rs (R 1 and R 2) are in series at two different temperatures (R 1 = 300 K and R 2 = 400K). Find the total noise voltage produced at the load (R L=300 Ω), over a bandwidth of 100 kHz. a. 779 mV b. 779 μV c. 779 nV d. 779 pV 9. Find Find the the nois noisee powe powerr prod produce uced d by the circuit in Problem 8. a. 0.605 μW b. 5.06 nW c. 0.6 pW d. 0.506 fW 10. A receiver receiver produces produces a noise power of 200 mW with no signal. The output level increases to 5 W when a signal is applied. Calculate (S+N)/N as a power ratio. a. 25 b. 0.10 c. 0.04 d. 5.2
11. The signal power at the input to an amplifier is 100 μW and the noise power is 1 μW. At the output, the signal power is 1 W and the noise power is 30 mW. What is the amplifier noise figure, as a ratio? a. 100 b. 33.3 c. 3 d. None of the above 12. 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 dB)? a. 48 dB b. 7 dB c. 36 dB d. None of the above 13. An amplifier has a noise figure of 2 dB. What is the equivalent noise temperature? a. 169.6 K b. 156.6 K c. 126.7 K d. 196.7 L 14. An RF amplifier has a tank consisting of L=25 μH and C = 50 pF. Find the operating frequency. a. 300 kHz b. 4.50 MHz c. 1.50 MHz d. 600 kHz 15. What is the bandwidth needed for Problem 14, assume the Q is 15? a. 300 kHz b. 4.50 MHz c. 1.50 MHz d. 600 kHz 16. A portable radio transmitter has to operate at temperatures from -5ºC to 35ºC. If its signal is derived from a crystal oscillator with a temperature coefficient of +1 ppm/degree C and it transmits at exactly 146 MHz at 20ºC, find the maximum transmitting frequency?
a. 145.99635 MHz b. 146.00219 MHz c. 146.01542 MHz d. 145.98654 MHz 17. Find the minimum transmit frequency for Problem 16. a. 145.99635 MHz b. 146.00219 MHz c. 146.01542 MHz d. 145.98654 MHz 18. Sine-wave signals with frequencies of 10 MHz and 11 MHz are applied to a square-law mixer. What frequencies appears at the output? a. 22 MHz and 20 MHz b. 21 MHz and 1 MHz c. 10 MHz and 11 MHz d. All of the above 19. A PLL has a VCO with a freerunning 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 of lock again at 16 MHz. Find the capture range of the PLL. a. 8 MHz b. 4 MHz c. 14 MHz d. 16 MHz 20. Find the lock range of the PLL in Problem 19. a. 8 MHz b. 4 MHz c. 14 MHz d. 16 MHz 21. Find the modulation index if a 10 V carrier is amplitude-modulated by thee different frequencies with amplitudes of 1 V, 2V and 3V respectively. a. 0.1 b. 0.2 c. 0.374 d. 0.3
22. Calculate the modulation index if the maximum voltage is 150 mV and the minimum voltage is 70 mV. a. 0.364 b. 2.75 c. 0.47 d. 2.14 23. CB radio channels are 10 kHz apart. What is the maximum modulation frequency that can be used if a signal is to remain entirely within its assigned channel? a. 10 kHz b. 5 kHz c. 270 kHz d. None of the above 24. An AM broadcast transmitter has a carrier power output of 50 kW. What total power would be produced with 80% modulation? a. 66 kW b. 75 kW c. 80 kW d. 16 kW 25. What is the modulation index for an AM having Vmax and Vmin of 2.6 amd 0.29, respectively? a. 0.1115 b. 0.7793 c. 8.9655 d. 1.2511 26. Determine the bandwidth of an AM signal modulated by a 5-kHz tone. a. 5 kHz b. 10 kHz c. 15 kHz d. 20 kHz 27. An AM transmitter radiates 100 W at 100% modulation. How much power is required for the carrier? a. 66.67 W b. 150 W c. 33.33 E d. None of the above
28. An AM broadcast station operates at its maximum allowed total output of 50,000 W and at 85% modulation. How much of the transmitted power contains the intelligence? a. 36.73 kW b. 13.3 kW c. 6.65 kW d. None of the above 29. For an AM DSB-FC transmitter with an unmodulated carrier power of 500 W that is modulated simultaneously by four tones, m1=0.1,m2=0.2,m3=0.4, and m4=0.5, determine the total modulation index? a. 0.91 b. 1.2 c. 0.68 d. 0.86 30. Determine the total sideband power of Problem 29. a. 615.6 W b. 115.6 W c. 57.8 W d. None of the above 31. Determine the total transmitted power of Problem 29. a. 615.6 W b. 115.6 W c. 57.8 W d. None of the above 32. An antenna current of an AM transmitter is 11 A when unmodulated but increases to 14 A when modulated. Find the percentage of modulation. a. 111.34% b. 89.56% c. 65% d. 50% 33. An AM transmitter has a carrier voltage of 50 Vrms at a carrier frequency of 10 MHz. it is modulated at 75% by a 1-kHz sine wave. How much is the total rms voltage transmitted?
a. 66.67 Vrms b. 150 Vrms c. 56.6 Vrms d. 96 Vrms 34. An Am transmitter must transmit 39.9-kW PEP. It maintains an average current of 1.5 A and an 80% plate efficiency. Determine the average output power at 100% modulation. a. 15 kW b. 10 kW c. 39.9 kW d. 50 kW 35. Determine the unmodulated carrier power of Problem 34. a. 15 kW b. 10 kW c. 5 kW d. 50 kW 36. Determine the audio power present in the transmitted power of Problem 34. a. 10 kW b. 15 kW c. 5 kW d. 50 kW 37. Calculate the percentage power saving when the carrier and one of the sidebands is suppressed in an AM signal if the modulation index is 25%. a. 83.3% b. 98.5% c. 75% d. 25% 38. A SSB transmission drives 110-V peak into a 75-Ω antenna. Calculate the PEP. a. 161 W b. 125 W c. 80.67 W d. 65 W Answer: C 39. What is the modulation index of an FM transmitter whose frequency
deviation is 50 kHz while its audio frequency is 10 kHz? a. 5 b. 0.2 c. 0.66 d. 1.5 40. Determine the approximate bandwidth of an FM signal modulated by a 15-kHz tone whose frequency deviation is 45 kHz. a. 120 kHz b. 180 kHz c. 240 kHz d. 60 kHz 41. For a shorted coaxial transmission line with the following characteristics: Zo = 75 Ω, Єr = 0.7, f = 1.5 GHz, and Zin = -j70 Ω, determine the length of the transmission line. a. 38 cm b. 11.9 cm c. 23.9 cm d. 9.1 cm 42. A 75-Ω transmission line is terminated with a load equal to 25 j50Ω, determine the reflection coefficient of the line. a. 0.362∠108.43° b. 0.132∠ 54° c. 0.632∠ − 1 08.43° d. None of the above 43. Determine the distance the quarterwave transformer must be placed from the load to provide matching of Problem 42. a. 0.0994λ b. 0.3494λ c. 0.1234λ d. 0.2314λ
