Wideband FM is the form of frequency modulation used for commercial radio broadcasting. This page covers what WBFM means, how it differs from narrowband FM, its bandwidth using Carson’s rule, and the points usually asked in exams.
What is WBFM?
WBFM stands for Wideband Frequency Modulation. It is frequency modulation in which the modulation index is large — greater than one, and typically much greater — so the transmitted signal occupies a wide band of frequencies around the carrier.
The large modulation index is what gives FM broadcasting its characteristic high audio quality and strong resistance to noise.
Modulation index
The modulation index of an FM signal is the ratio of the frequency deviation to the highest modulating frequency:
β = Δf / fm
- Δf is the peak frequency deviation — how far the carrier shifts from its resting frequency.
- fm is the maximum frequency present in the modulating signal.
When β is much less than 1 the signal is narrowband FM. When β is greater than 1 it is wideband FM. Commercial FM broadcasting uses a deviation of 75 kHz and an audio bandwidth of 15 kHz, giving β = 5.
Bandwidth and Carson’s rule
An FM signal theoretically contains an infinite number of sidebands, but only those carrying significant power matter in practice. Carson’s rule gives the bandwidth that contains about 98 per cent of the total power:
BW = 2 (Δf + fm)
For commercial FM: BW = 2 (75 + 15) = 180 kHz. Broadcast channels are allocated 200 kHz to allow a guard band between adjacent stations.
Carson’s rule can also be written as BW = 2 fm (β + 1), which is the form usually more convenient when the modulation index is given.
WBFM compared with NBFM
- Modulation index — WBFM: greater than 1, often 5 or more. NBFM: much less than 1.
- Bandwidth — WBFM: wide, around 200 kHz for broadcasting. NBFM: narrow, roughly twice the modulating frequency.
- Sidebands — WBFM: many significant pairs. NBFM: essentially one significant pair, so it resembles AM in spectrum.
- Noise immunity — WBFM: excellent. NBFM: moderate.
- Typical use — WBFM: FM radio broadcasting, television audio. NBFM: two-way radio, police and taxi communication, walkie-talkies.
Advantages of WBFM
- High fidelity audio, which is why music broadcasting uses it.
- Strong noise immunity — noise affects amplitude, and FM carries no information in amplitude.
- Constant transmitted power regardless of modulation depth.
- The capture effect: a receiver locks onto the stronger of two signals on the same frequency instead of hearing both.
Limitations
- Requires much more bandwidth than AM or NBFM.
- Receiver and transmitter circuits are more complex.
- Range is limited to line of sight at VHF frequencies.
Frequently asked questions
What is the full form of WBFM?
Wideband Frequency Modulation.
What modulation index counts as wideband?
Any value greater than 1. Commercial FM broadcasting uses about 5.
What is the bandwidth of commercial FM?
By Carson’s rule, 2 (75 + 15) = 180 kHz. Channels are spaced 200 kHz apart to provide a guard band.
Why does FM sound better than AM?
Noise disturbs amplitude, and FM carries its information in frequency. A limiter in the receiver removes amplitude variation, and with it most of the noise, before demodulation.