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A Level     Modulation     AM     Digital     >FM<     PAM     PCM     PPM     PWM    

Modulation FM


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  A Level    Modulation  0 of 22    Question 1125    Bandwidth AM Transmissions 
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Deadline not set1 of 22    Question 1126    Bandwidth AM Transmissions 
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Deadline not set3 of 22    Question 1128    Bandwidth AM Transmissions 
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Deadline not set4 of 22    Question 1129    Bandwidth AM Transmissions 
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Deadline not set5 of 22    Question 1130    Bandwidth AM Transmissions 
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Deadline not set6 of 22    Question 1131    Bandwidth AM Transmissions 
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Deadline not set7 of 22    Question 114    FM Deviation Calculation 
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Deadline not set8 of 22    Question 53    Frequency Bands and Modulation Types 
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Deadline not set9 of 22    Question 109    Modulation - AM Bandwidth Calculation 
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Deadline not set10 of 22    Question 113    Modulation - AM bandwidth Calculation 
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Deadline not set11 of 22    Question 105    Modulation - Analogue Types 
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Deadline not set12 of 22    Question 108    Modulation - Analogue Types 
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Deadline not set13 of 22    Question 106    Modulation - Terminology 
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Deadline not set14 of 22    Question 107    Modulation - Terminology 
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Deadline not set15 of 22    Question 110    Modulation Depth 
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Deadline not set16 of 22    Question 112    Modulation Depth 
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Deadline not set17 of 22    Question 111    Modulation Depth Calculation 
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Deadline not set18 of 22    Question 1259    Radio Systems 
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Deadline not set19 of 22    Question 1260    Radio Systems 
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Deadline not set20 of 22    Question 307    Sub-systems 
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Deadline not set21 of 22    Question 50    Transmitter Basics 
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Deadline not set22 of 22    Question 51    Transmitter Terminology 
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Deadline not set  Questions 0 to 22   -->  View All  

Frequency Modulation / VHF Broadcasting Band from 88 to 108 MHz

Amplitude and Frequency Modulation Animation

FM on a Picoscope

Frequency Modulation

Increasing the Audio Amplitude

Frequency Modulation - Greater Deviation

Increasing the Audio Frequency

Frequency Modulation - Higher Information Signal 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

Frequency Modulation Spectrum Diagram

FM Slope Demodulator

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

Frequency Modulation Slope Demodulator

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 - SA602 Chip

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

Frequency Modulation FM Stereo

Stereo Receiver

Frequency Modulation FM 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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