AdvancedA-005-005-011
What is the frequency deviation for a 12.21-MHz reactance-modulated oscillator in a +/- 5-kHz deviation, 146.52-MHz FM-phone transmitter?
C
Answer
Antennas and transmission lines
Type
A
+/- 41.67 Hz
B
+/- 5 kHz
C
+/- 416.7 Hz
D
+/- 12 kHz
Answer Notes
Frequency multiplier stages in an FM transmitter multiply both the base operating frequency and the frequency deviation. To find the deviation at the oscillator, you first need to determine the multiplication factor of the transmitter.
Divide the final output frequency (146.52 MHz) by the oscillator frequency (12.21 MHz) to find the multiplication factor, which in this case is exactly 12.
Next, divide the total target deviation (+/- 5 kHz, or 5000 Hz) by this multiplication factor of 12. This yields 416.666... Hz, matching the correct answer of +/- 416.7 Hz. Distractors are either the un-divided final deviation or result from incorrect arithmetic.
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What is the significant bandwidth of an FM-phone transmission having a +/- 5-kHz deviation and a 3-kHz modulating frequency?
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