Some types of deviation meters work on the principle of:

A classic and highly accurate method for measuring Frequency Modulation (FM) deviation relies on the mathematical properties of Bessel functions. At specific modulation index values (such as 2.405, 5.52, and 8.65), all the transmitter's power is distributed into the sidebands, leaving zero power in the carrier itself. This phenomenon is known as a carrier null. Some deviation meters and testing procedures use this principle by monitoring the carrier signal while adjusting the modulating audio frequency. When the carrier disappears or 'nulls' on a spectrum analyzer or receiver, the operator knows a specific, exact modulation index has been reached. Once the carrier null is found, the peak deviation is easily calculated. Since the modulation index equals the frequency deviation divided by the modulating frequency, the deviation is found by simply multiplying the known modulating frequency by the specific modulation index at that null.