Discussion
At certain modulation indices the FM carrier component vanishes. This is used to
Answer: D
The Bessel function \( J_0 \) has zeros, the first near an index of 2.405, so a carrier null observed on a spectrum analyser fixes the deviation exactly for a known modulating frequency. It is the classic deviation measurement. The null is a spectral accident rather than an efficiency technique, and FM power is constant regardless. Bandwidth is unaffected. And a limiter is needed for noise rejection whatever the index. This is a good example of theory yielding a practical measurement.
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