Parameter fm is the frequency of the modulating signal, δmax is the maximum deviation, and m is the corresponding modulation index. If a range of frequencies is used to modulate the carrier, the maximum modulating frequency and its corresponding modulation index are used.
Frequency deviation () is used in FM radio to describe the maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency. The term is sometimes mistakenly used as synonymous with frequency drift, which is an unintended offset of an oscillator from its nominal frequency. FM applications use peak deviations of 75 kHz (200 kHz spacing), 5 kHz (25 kHz spacing), 2.5 kHz (12.5 kHz spacing), and 2 kHz (8.33 kHz spacing). The ratio between the deviation of a frequency modulated signal and the modulating frequency. The processor index. The FM signal-to-noise ratio. The ratio of the maximum carrier frequency deviation to the highest audio modulating frequency. 3-57H6: In an FM-phone signal, what is the term for the maximum deviation from the carrier frequency divided by the maximum audio modulating frequency? Deviation ratio.  For example [2], in North America, the maximum frequency deviation, ∆ f, is 75 kHz for commercial FM broadcasting. If the maximum message frequency is equal to 15 kHz for audio, then β= kHz kHz =75 15 5, and the FM bandwidth is BW FM = + 2(5 1)15 kHz =180 kHz. This is close to the allotted 200 kHz channel bandwidth. The FCC has set modulation limits of 100% modulation (an instantaneous frequency deviation of 75 kHz corresponds to a 100 percent modulation) for stereophonic transmission and up to 110. percent modulation for SCA multiplex subcarriers under certain conditions [5]. Parameter fm is the frequency of the modulating signal, δmax is the maximum deviation, and m is the corresponding modulation index. If a range of frequencies is used to modulate the carrier, the maximum modulating frequency and its corresponding modulation index are used. Commercial FM broadcasting uses a maximum deviation of 75 KHz and a maximum modulating frequency of 15 KHz.  Whenever the maximum allowable frequency deviation and the maximum modulating frequency are used in computing the modulation index, mf is known as the deviation ratio. The audio broadcast in T.V. is transmitted by FM. In most communications systems using FM, maximum limits are put on both the frequency deviation and modulating frequency. For example, in standard FM broadcasting, the maximum permitted frequency deviation is 75 KHz, while the maximum permitted modulating frequency is 15 KHz. Example: suppose in FM radio that the audio signal to be transmitted ranges from 20 to 15,000 Hz (it does). If the FM system used a maximum modulating index, b, of 5.0, then the frequency would swing by a maximum of 5 x 15 kHz = 75 kHz above and below the carrier frequency.  Frequency modulation uses the instantaneous frequency of a modulating signal (voice, music, data, etc.) to directly vary the frequency of a carrier signal. Modulation index, β, is used to describe the ratio of maximum frequency deviation of the carrier to the maximum frequency deviation of the modulating signal. The concept was pioneered by Edwin H. Armstrong in the late 1920s and patented in the early 1930s. Here, the maximum frequency (f max) causes a maximum deviation of 5*f max in the carrier. From the modulation index formula: Modulation Index (β) = 25. Here, the maximum frequency (f max) causes a maximum deviation of 2 The modulation index is a measure of radian phase shift of the modulated FM signal compared to the phase of the un-modulated carrier alone. In most communications systems using FM, maximum limits are put on both the frequency deviation and modulating frequency. For example, in standard FM broadcasting, the maximum permitted frequency deviation is 75 KHz, while the maximum permitted modulating frequency is 15 KHz. This produces a modulation index of Where. D = Deviation ratio. To give an example of how the deviation ratio may be calculated and used, take the example of an FM broadcast transmitter. For these the maximum deviation is ±75 kHz and the maximum modulation frequency is 15 kHz. This means that the deviation ratio is 75 / 15 = 5 The FM broadcasting range (87.5–108 MHz, NOTE: In some countries the 87.5–88.0 MHz part of the band is not used) uses a channel spacing of 200 kHz, with a maximum frequency deviation of 75 kHz, leaving a 25 kHz buffer above the highest and below the lowest frequency to reduce interaction with other channels. FM applications use peak deviations of 75 kHz (200 kHz spacing), 5 kHz (25 kHz spacing), 2.5 kHz (12.5 kHz spacing), and 2 kHz (8.33 kHz spacing).
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frequency modulation maximum deviation