Question

15. What is the frequency swing of an FM broadcast transmitter when modulated 80%?

16. What is the modulation index of an FM transmitter whose frequency deviation is 50Khz, while its audio frequency is 10 kHz?

17. An FM signal has a center frequency of 100MHz but is swinging between 100.01 MHz and 99.999MHz at a rate of 100 times per second. What is the modulation index of the signal?

18. What is the modulation index of an FM transmitter whose frequency deviation is 50kHz, while its audio frequency is 10kHz?

19. The carrier swing necessary to provied 80% modulation in the FM broadcasting band is ?

20. An FM broadcast station transmits a 3kHz test tone, and a frequency deviation of 15kHz. Determine the modulation index.

Answer 1

Question (1)

the relation between modulation index and frequency swing or peak frequency deviation is given by

beta = swing / f_m   where f_m is modulating frequency. and beta is modulation index

Therefore, swing = beta * f_m

Question (2)

from the above relation, modulation index, beta = swing/f_m = 50 k / 10 k = 5

beta = 5

Question (3)

modulation index = swing / f_m = swing/ rate of change in carrier frequency = 0.01M / 100 = 10000 / 100 = 100

Question (4)

from the above relation, modulation index, beta = swing/f_m = 50 k / 10 k = 5

Question (5)

modulation index = 0.8.

swing = index * f_m

Question (6)

index = 15 k / 3 k = 5