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Frequency Modulation

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Frequency Modulation / VHF Broadcasting Band from 88 to 108 MHz



FM on a Picoscope



Increasing the Audio Amplitude



Increasing the Audio Frequency



Amplitude Noise Rejection

AM suffers badly from noise. FM suffers much less. Noise varies the amplitude of the received signal in a random way. It sounds like a hissing noise (waterfall) with additional cracks and pops caused by lightning strikes or electric circuit switching. AM radios can not reject these unwanted signals.

FM signals can be amplified so much that the amplifier limits. Any amplitude noise will be removed as long as the amplifier continues to limit. The frequency information is undamaged by this limiting. The FM demodulator comes after the limiting amplifier. This works extremely well as long as the FM signal is significantly stronger than the background noise in the receiver.


FM Bandwidth and Deviation


FM Bandwidth Calculation

    Bandwidth = 2 x (deviation + fmax)    


FM Spectrum



FM Slope Demodulator

Demodulating FM properly is quite complex but the slope detector (demodulator) is simple enough to explain here.


This solution works but not well. The FM signal has to be de-tuned from the resonance point of the tuned circuit. If a stronger station is on the resonant frequency, the FM signal will be drowned out.

FM Demodulator - SA602 Chip

An untested design. The tuned circuit should be centered on the FM frequency to be received.

FM Demodulator


FM Stereo

Stereo audio has two channels (left and right) and two loudspeakers are needed. This arrangement gives the illusion of space and sounds appear to be coming from the left, middle, right or anywhere in between. To record stereo, at least two microphones are needed.

FM stereo uses a simple frequency division multiplex system.

The left and right stereo channels are transmitted as a monophonic signal L+R using ordinary FM. A low cost monophonic receiver picks this up and demodulates the L+R transmission in the normal way.

The stereo information is modulated onto a 38 kHz sub-carrier. This is outside the range of human hearing so the simple radio (above) will not not produce a noticeable output. L-R is encoded onto the sub-carrier. A stereo receiver is designed to demodulate this signal as well as the conventional L+R FM signal.

To produce stereo ...


Stereo Transmitter



Stereo Receiver



Digital Frequency Shift Keying (FSK)

Binary zeros and ones are encoded as two slightly different carrier frequencies. This modulation scheme is very common on short wave radio bands where news agencies and business transmit text using FSK. It sounds like a very long and boring bird song.




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