Subelement H: Modulation— Topic 57: Frequency Modulation
Question 3-57H3
Element 3 (GROL)What is the deviation ratio of an FM-phone signal having a maximum frequency swing of plus or minus 7.5 kHz and accepting a maximum modulation rate of 3.5 kHz?
Explanation
The deviation ratio for an FM signal is a key parameter that describes the relationship between its maximum frequency swing and the highest modulating frequency. It is calculated by dividing the maximum frequency deviation (or peak frequency swing) by the highest modulating frequency.
Using the provided values:
Maximum frequency swing ($\Delta$f) = 7.5 kHz
Maximum modulation rate (f$_{m(max)}$) = 3.5 kHz
Deviation Ratio = $\Delta$f / f$_{m(max)}$
Deviation Ratio = 7.5 kHz / 3.5 kHz = 2.1428...
Rounding to two decimal places, the deviation ratio is 2.14.
Option A is correct. Option B results from incorrect decimal placement. Option C results from inverting the ratio (3.5/7.5). Option D is incorrect by a large magnitude.
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