When using some deviation meters, it is important to know:

When using a deviation meter that relies on the carrier null (Bessel null) method, the peak deviation is not measured directly. Instead, it is calculated using a specific mathematical relationship: Deviation equals the modulation index multiplied by the modulating frequency. Because of this formula, it is essential to know both variables to find the deviation. The operator must know the exact audio modulating frequency being fed into the transmitter, as well as the specific modulation index that corresponds to the observed carrier null (for example, the first null always occurs at an index of 2.405). If either the modulating frequency or the modulation index is unknown, the calculation cannot be performed. Therefore, knowing just the modulation index or just the modulating frequency is insufficient to determine the transmitter's deviation.