CHAPTER 5: FREQUENCY MODULATION

1. In the stabilize reactance modulator AFC system,

a. the discriminator must have a fast time constant to prevent

demodulation

b. the higher the discriminator frequency, the better the oscillator

frequency stability

c. the discriminator frequency must not be too low, or the system will

fail

d. phase modulation is converted into FM by the equalizer circuit

C

2. In the spectrum of a frequency-modulated wave

a. the carrier frequency disappears when the modulation index is large

b. the amplitude of any sideband depends on the modulation index

c. the total number of sidebands depends on the modulation index

d. the carrier frequency cannot disappear

B

3. The difference between phase and frequency modulation

a. is purely theoretical because they are the same in practice

b. is too great to make the two system compatible

c. lies in the poorer audio response of phase modulation

d. lies in the different definitions of the modulation index

D

4. Indicate the false statement regarding the Armstrong modulation system

a. The system is basically phase, not frequency modulation,

b. AFC is not needed, as crystal oscillator is used.

c. Frequency multiplication must be used.

d. Equalization is unnecessary

D

5. An FM signal with a modulation index mf is passed through a frequency

tripler. The wave in the output of the tripler will have a modulation

index of

a. mf/3

b. mf

c. 3mf

d. 9mf

C

6. An FM signal with a deviation δ is passed through a mixer, and has its

frequency reduced fivefold. The deviation in the output of the mixer is

a. 5δ

b. Indeterminate

c. δ/5

d. δ

D

7. A pre-emphasis circuit provides extra noise immunity by

a. boosting the bass frequencies

b. amplifying the higher audio frequencies

c. preamplifying the whole audio band

d. converting the phase modulation to FM

B

8. Since noise phase-modulates the FM wave, as the noise sideband

frequency approaches the carrier frequency, the noise amplitude

a. remains constant

b. is decreased

c. is increased

d. is equalized

B

9. When the modulating frequency is doubled, the modulation index is

halved, and the modulating voltage remains constant. The modulation system

is

a. amplitude modulation

b. phase modulation

c. frequency modulation

d. any of the three

C

10. Indicate which one of the following is not an advantage of FM over AM:

a. Better noise immunity is provided

b. Lower bandwidth is required

c. The transmitted power is more useful

d. Less modulating power is required

B

11. One of the following is an indirect way of generating FM. This is the

a. reactance FET modulator

b. varactor diode modulator

c. Armstrong modulator

d. reactance bipolar transistor modulator

C

12. In an FM stereo multiplex transmission, the

a. sum signal modulates 19 kHz subcarrier

b. difference signal modulates the 19 kHz subcarrier

c. difference signal modulates the 38 kHz subcarrier

d. difference signal modulates the 67 kHz subcarrier

C