Frequency Modulation (FM) Tutorial


Edwin H. Armstrong, identified as one particular of the founding fathers of radio engineering, invented the super heterodyne radio receiver in 1918 and frequency modulation (FM) in 1933. These two ideas, along with his regenerative circuit strategy created in 1912, formed the basis of radio frequency electronics as we know it currently. In the United States, FM radio stations broadcast involving radio frequencies of 88 MHz to 108 MHz with a channel bandwidth of 200 kHz. FM radio was initially deployed in monaural in 1940; and in 1960, FM stereo was introduced. This post presents a fundamental tutorial on FM with descriptions of multiplex (MPX) signaling and noise improvement methods such as stereo-mono blending and soft mute.

FM Fundamentals Frequency modulation is a form of analog angle modulation in which the baseband information carrying signal, usually known as the message or information signal m (t), varies the frequency of a carrier wave. Audio signals transmitted by FM radio communications are the most frequent.

Nonetheless, FM radio can also transmit digital information with the low bandwidth digital information recognized as Radio Information Technique (RDS) in Europe and Radio Broadcast Facts Program (RBDS) in the U.S. The easiest strategy to producing FM signals is to apply the message signal straight to a voltage-controlled oscillator (VCO).

A voltage message signal, m (t), is utilized to the control voltage of the VCO, and the output signal, xFM (t), is a continuous amplitude sinusoidal carrier wave whose frequency is ideally a linear function of its control voltage. When there is no message or the message signal is zero, the carrier wave is at its center frequency, fc. When a message signal exists, the instantaneous frequency of the output signal varies above and beneath the center frequency and is expressed by

f (t) f K m (t) i c VCO = +

Exactly where KVCO is the voltage-to-frequency get of the VCO expressed in units of Hz/V, and the quantity, KVCO*m (t), is the instantaneous frequency deviation.

Stereo FM Multiplex Signal

Proceeding to 1961, monaural broadcasting of audio signals was the basic for AM, FM and Television. FM broadcasts at that time also integrated Subsidiary Communications Authorization (SCA) solutions that had been multiplexed with the major monophonic channel to give background music and other solutions to offices and retailers. In 1961, the FCC authorized the transmission of stereophonic sound, which extends the idea of multiplexing signals to produce stereo audio.

One particular of the crucial needs of the stereo multiplex signal was to be backwards compatible with the significant current base of FM monophonic receivers. To achieve this target, the 0 to 15 kHz baseband part of the multiplex (MPX) signal had to include the left (L) and ideal (R) channel information (L+R) for monophonic reception. Stereophonic sound is attained by amplitude modulating the (L-R) information onto a suppressed 38 kHz subcarrier in the 23 to 53 kHz area of the baseband spectrum. A 19 kHz pilot tone is added to the multiplex signal to allow FM stereo receivers to detect and decode the stereo left and correct channels. The composite baseband signal format meets the backwards compatibility necessary for FM mono receivers though simultaneously giving adequate information for FM stereo receivers to decode the left and ideal stereo channel outputs. Today’s MPX signal involves a 57 kHz subcarrier that carries RDS and RBDS signals.


The mathematical evaluation presented in the preceding section assumes that the message signal, m (t), is one particular-tone sinusoidal signal. In reality, the message signal applied in today’s FM broadcasts is the MPX signal with a baseband spectrum equivalent to the single shown in Figure 5. The FCC has set modulation limits of one hundred% modulation (an instantaneous frequency deviation of 75 kHz corresponds to a one hundred percent modulation) for stereophonic transmission and up to 110 percent modulation for SCA multiplex subcarriers below particular circumstances.

The total modulation level for the MPX signal, assuming no correlation, is the arithmetic sum of every of the sub channel levels offering 102.67 percent modulation or a peak frequency deviation of 77.0025 kHz. From the final section, the frequency deviation is equivalent to the amplitude of the message signal by the continual, KVCO, because VCO m f = K A. Therefore, for a fixed KVCO, the amplitude of all the sub channel signals inside the MPX message signal should be scaled to offer the ideal total frequency deviation.

Noise Improvement Approaches

Current implementations of FM tuners, such as Silicon Laboratories Si4700 FM tuner and Si4701 FM tuner with RDS/RBDS, have incorporated noise improvement strategies such as stereo-mono blending and soft mute to improve the audio high quality of FM radios.

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