RF circuits, Telecomunications, Wireless

AM and FM, understand the difference

Everyone have heard of radio AM and FM, but what it means and what the difference between one and other? This is this post`s subject.


The signal which carries information has a frequency very low to be transmitted by air. For that it needs to be modulated, in other words, needs to be transformed in another frequency to carry information by radio waves. The signal which carries the information is called base band or modulating. It is involved a wave signal with frequency to be transmitted by air, this signal is called carrier.

The modulated signal will be transmitted through air. The receiver has a demodulator to get the original information. These concepts are applied to both AM and FM.

AM radio

AM means amplitude modulation. The signal AM has frequencies from 540 kHz to 1600 kHz. The figure below shows how the low wave stays modulated in AM.

The AM transmitter`s flowchart. The local oscillator generate the carrier wave and it is necessary to implement amplifiers to transmitter and receiver.

The carrier`s equation.

c(t)=A_c cos(2\pi f_ct)

A_c is the amplitude and f_c is the frequency. Here we have the modulated AM wave equation.

s(t)=A_c[1+k_a m(t)] cos(2\pi f_ct)

k_a is the amplitude sensitivity constant, m(t) is the signal which has the information. When \left | k_am(t) \right |>1 there is phase inversion in the modulated wave. This is the generic AM receiver.

FM radio

FM means frequency modulation. Operate with frequencies from 87,5 MHz to 108 MHz. While the modulated AM wave change amplitude, the FM change the frequency.

This is modulated FM wave`s equation.

s(t)=A_c cos[2\pi f_ct+\beta sen(2\pi f_mt)]

f_m is the modulating signal`s frequency, \beta is the modulation index and is calculated this way:

\beta =\frac{\Delta f}{f_m}

\Delta f is the frequency deviation, how much the carrier frequency stay away from nominal frequency. The value of \beta determine if the FM modulation is narrow or wide band. Narrow band is when \beta is small comparing to a radian, the figure below shows a FM transmitter in narrow band.

In wide band modulation, \beta is greater than 1 and has a much bigger modulation frequency range. The wide band FM transmitter has a frequency multiplier between the adder (Σ) and RF amplifier. While the narrow band is used to FM mobile communication, the wide band is used to FM broadcast. The FM receiver stays to another post.


AM and FM have your pros and cons.

  • AM change amplitude, so it is more subject to interference than FM;
  • AM is simpler than FM;
  • FM needs more frequency width band than AM;
  • Due to bigger width band, FM has a better sound quality.






About Pedro Ney Stroski

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