Study Guide Electronics About Principles of Communication

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CHAPTER 7
1. The ability of a receiver to choose a desired signal frequency while rejecting closely adjacent signal frequencies is known as selectivity. 2. Decreasing the Q of a resonant circuit causes its bandwidth to increase. 3. Good selectivity usually means narrow bandwidth.

4. True. A tuned circuit can provide voltage gain.
5. Cascading tuned circuits cause the selectivity to increase. 6. If the selectivity of a tuned circuit is too sharp, the sidebands of the received signal may be attenuated. 7. A tuned circuit has a Q of 100 at its resonant frequency of 500 kHz. Its bandwidth is 5 kHz. 8. A parallel LC tuned circuit has a coil of 3µHz and a capacitance of 75 pF. The coil resistance is 10 Ω. The circuit bandwidth is 0.53 MHz or 530 kHz. 9. A tuned circuit has a resonant frequency of 10 MHz and a bandwidth of 100 kHz. The upper and lower cutoff frequencies are f1 (upper) = 9.95 MHz, f2 (lower) = 10.05 MHz. 10. To achieve a bandwidth of 3 kHz at 4 MHz, a Q of 1333.3 is required. 11. To narrow the bandwidth of a tuned circuit, the coil resistance must be decreased. 12. A filter has a 6-dB bandwidth of 500 Hz and a 60-dB bandwidth of 1200 Hz. The shape factor is 2.4. 13. The greater the gain of the receiver, the better its sensitivity. 14. A receiver that uses only amplifiers and a detector is known as a(n) tuned radio frequency receiver. 15. A receiver that uses a mixer to convert the received signal to a lower frequency is called a(n) superheterodyne. 16. The acronym IF means intermediate frequency.

17. Tuning a superheterodyne is done by varying the frequency of its local oscillator. 18. Most of the gain and selectivity in a superhet is obtained in the IF amplifier. 19. The automatic gain control circuit in a receiver compensates for a wide range of input signal levels. 20. The mixer output is usually the difference between the signal or input frequency and the local oscillator frequency. 21. The AGC voltage controls the gain of the IF amplifier. 22. For best selectivity and stability, the IF should be c. Low 23. An interfering signal that is spaced from the desired signal by twice the IF is called a(n) image. 24. A superhet has an input signal of 15MHz. The LO is tuned to 18.5 MHz. The IF is 3.5 MHz. 25. Images are caused by the lack of selectivity at the mixer input. 26. A desired signal at 27 MHz is mixed with an LO frequency of 27.5 MHz. The image frequency is 28 MHz. 27. True. The LO may be above or below the signal frequency. 28. The main feature of a dual-conversion superhet is that it has two mixer circuits. 29. The image problem can be solved by proper choice of the intermediate frequency. 30. A dual-conversion superhet has an input frequency of 50 MHz and LO frequencies of 59 MHz and 9.6 MHz. The two Ifs are 9 MHz and 600 kHz. 31. List three sources of external noise.

industrial, atmospheric, space
32. List three main types of internal noise.
a. thermal agitation
b. shot noise
c. transit-time noise
33. Noise from the sun and stars is called extraterrestrial or space noise. 34. Atmospheric noise comes primarily from lightning.
35. List four sources of industrial noise.
automobiles, motors, generators, fluorescent lights
36. The main source of internal noise is thermal agitation.
37. The S/N ratio is usually expressed in decibels.
38. For best reception, the S/N ratio should be high.
39. Increasing the temperature of a component causes its noise power to increase. 40. Thermal noise is sometimes called white or Johnson noise. 41. Narrowing the bandwidth of a circuit causes the noise level to decrease. 42. The noise voltage produced across a 75-Ω input resistance at a temperature of 25˚C with a bandwidth of 1.5 MHz is 1.36 µV. 43. Two types of noise caused by tubes or transistors are shot noise and transit-time noise. 44. False. The noise at the output of a receiver will be less than the noise at the input. 45. True. The...
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