FM (frequency modulated) radio
certainly was a hot topic beginning in the middle to late 1940s. With the war out
of the way, energies and resources were being redirected back to peacetime production.
Major Edwin Armstrong announced his FM scheme in 1935, and as with many new
inventions, it was met with skepticism by many who doubted his claim of static interference
immunity. For many, it was a lack of understanding that caused the negative reaction,
caused primarily by the increased level of sophistication of the transmitter and
receiver circuitry. Amplitude modulation (AM) was so
easy even a caveman could
understand it, but adding phase relationships into the equation (literally)
left many in the dust. This FM Radio Quiz from a 1950 issue of Radio & Television
News magazine tests your grasp of frequency modulation principles.
By Ed Bukstein
Northwestern Vocational Institute
1. On the commercial FM channels (88 to 108 mc.) , one hundred per-cent modulation
is defined as a frequency deviation of plus and minus (a) 88 kc. (b) 21.75 kc. (c)
75 kc. (d) 25 kc.
2. If a commercial FM station produces a signal whose deviation is plus and minus
60 kc., its percentage of modulation is (a) 20% (b) 60% (c) 80% (d) 100%.
3. The rate at which the FM signal swings back and forth is deter-mined by the
(a) amplitude of the audio signal (b) frequency of the audio signal (c) height of
the antenna (d) effective radiated power.
4. The deviation of an FM signal increases when the amplitude of the audio signal
(a) increases (b) decreases.
5. If a 30 kc. deviation is produced by a 5000 cycle audio signal, the modulation
index is (a) 150 (b) 25,000 (c) 0.16 (d) 6.
6. The ratio of the maximum frequency deviation to the highest audio modulating
frequency is called the (a) aspect ratio (b) deviation ratio (c) ratio detector.
7. If an FM signal having a deviation of plus and minus 2 kc. is fed successively
through a frequency doubler and a frequency quadrupler, the output signal will have
a frequency deviation of plus and minus (a) 6 kc. (b) 8 kc. (c) 16 kc. (d) 32 kc.
8. The circuit designed to remove any undesirable amplitude modulation from a
frequency modulated signal is known as a (a) limiter (b) reactance modulator (c)
9. In the circuit shown in Fig. 1, tube V, is a (a) reactance modulator (b) d.c.
restorer (c) ratio detector (d) Wien bridge.
10. Tube V1 of Fig. 1 behaves like (a) an inductance (b) a condenser.
11. The second detector of an FM receiver is more properly known as a (a) frequency
modulator (b) frequency divider (c) frequency multiplier (d) discriminator.
12. The circuit shown in Fig. 2 is a (a) pre-emphasis filter (b) high-pass filter
(c) de-emphasis filter.
13. As specified by the FCC, the pre-emphasis filter should have a time constant
of (a) 25 (b) 75 (c) 500 (d) 10,000 microseconds.
14. If C of Fig. 2 has a value of 0.0015 microfarads, R should have a value of
(a) 20,000 (b) 50,000 (c) 2,000,000 (d) 66 ohms.
15. An FM signal generator employing a motor-driven variable condenser is known
as a (a) selsyn (b) PPI (c) rotating vector (d) wobbulator.
Answers To "FM Quiz"
15 correct ... excellent
13 to 14 correct ...very good
10 to 12 correct ... good
8 to 9 correct ... fair
7 or less correct ... poor
Posted March 18, 2020