Questions for Review
1. Amplitude modulation refers to varying the size, or amplitude, of a
carrier wave in a well-defined way, i.e. at a set of audio frequencies. Frequency modulation refers to periodic changes in
the frequency of the carrier wave.
2. Audio frequencies from a source that fall into the range of 200 Hz - 5 kHz are used to amplitude modulate a carrier wave with frequency in the range 535-1605 kHz. The amplitude modulated carrier wave is amplified and fed to an antenna for broadcast.
3. A light wave differs from a radio wave simply by the range of wavelengths into which it falls. Visible light has a wavelength of between approximately 0.4 µm and 0.7 µm, while radio wavelengths are much longer, in the range of meters and larger.
4. A superheterodyne receiver combines incoming radio wave signals with the oscillations of a single-frequency local oscillator, producing beats, or amplitude fluctuations, at a range of frequencies which are simply the differences between the radio frequencies and the local oscillator frequency. The tuner knob adjusts the local oscillator frequency to select the radio wave frequency that produces beats at 455 kHz, for AM, and 10.7 Mhz, for FM. This now produces an intermediate frequency carrier with the audio modulation signal, which is at a fixed frequency and can be amplified and processed by subsequent electronics.
5. AM broadcasts from 200 Hz to 5 kHz, a ratio of 5,000/200 = 25 which is a little less than 25, or about 4.5 octaves. FM broadcasts from 50 Hz to 15,000 Hz, a ratio of 15,000/50 = 300, somewhat more than 256 = 28, or somewhat more than 8 octaves.
6. An electromagnetic wave is a combination of an oscillating electric field and oscillating magnetic field that propagates through space, away from its source. It can be produced by accelerating charges, such as occurs when a frequency generator is connected to a pair of wires set up in a linear fashion. Charges accelerate up and down the wires, creating a plane polarized EM wave.
7. Each radio station has its own carrier frequency. An additional band of frequencies, centered around the carrier, is required to accommodate the audio frequency information that is being broadcast. For AM, this amount is 5 kHz on either side of the carrier, for a total of 10 kHz. An additional space of 5 kHz is designated as a buffer between stations, to avoid overlap, which would cause interference between adjacent stations.
8. Figures 8.17 and 8.18 of the text illustrate how a frequency modulated signal contains both the amplitude and frequency of the modulator, which could be, for example, an audio signal. The frequency of an audio signal determines how often the carrier is modulated, while the amplitude of an audio signal determines by how much the frequency is modulated.
9. For AM broadcasting, the rate at which the amplitude of the carrier changes is proportional to the frequency of the modulating (audio) signal, while the degree to which the carrier wave amplitude changes is related to the amplitude of the modulating signal. See Fig. 8.13, e.g.
10. An antenna allows an oscillating potential to be spread out in space, allowing charge carriers to accelerate along the antenna's path. This acceleration of charges gives rise to oscillating electric and magnetic fields that detach from the antenna and propagate through space. If, for example, one has a linear antenna, a plane-polarized EM wave will be generated, with the oscillation of the EM wave's electric vector along the same direction as the length of antenna.
11. Figure 8.42 sketches out how a pair of mutually perpendicular antennas can generate a circularly polarized radio wave. If the current oscillates between the horizontal and vertical antennas in a regular fashion at the carrier frequency, the electric field vector of the EM wave will oscillate around the direction of propagation as the wave moves away from the antenna. This constitutes a circularly-polarized wave.
Exercises
1. A. sensitivity is related to the ability to pick up weak radio
signals.
2. C. 1 µvolt to 1,000 µvolts, i.e. between 0.001 and 1 millivolt.
3. B. FM broadcasts with the shortest wavelength and highest
frequency.
4. C. a small capture ratio means the tuner could distinguish easily
between two radio signals
whose amplitudes were very similar.
5. A. Loudness doubles for each 10 dB, so 70 dB is 27 =
128.
6. B. Good sensitivity is required if you are far from radio
stations.
7. A. capture ratio
8. B. AM transmits only 200 Hz - 5 kHz.
9. A. FM carrier frequencies fall in the range from 88 Mhz - 108
Mhz.
10. A. AM wavelengths are 10 times longer than typical FM waves.
11. B. FM broadcasts pre-emphasize high audio frequencies.
12. B. FM transmission
13. C. if the amplitude of the audio increases, the carrier will have
a larger frequency shift
14. C. 112.7 Mhz
15. A. 15 kHz
16. B. AM
17. A. For FM, the higher the audio frequency, the more times per
second the carrier frequency
must change.
18. C. 100 dB - 65 dB = 35 dB.
19. A. S/N ratio
20. E. 8.745 Mhz
21. B. 1.5 meters
22. D. 10 times longer for AM than for FM
23. A. 1 Mhz/500 Hz = 2,000
24. A. broadcasted AM waves have vertical polarization
25. C. 3 meters