AM Transmission

This is a sketch of some of the components involved in AM radio transmission. You may click on the illustration for more detail.

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AM Radio Receiver

This is a sketch of some of the components involved in an AM radio receiver. A typical AM radio can be tuned to receive the output of any AM radio transmitter which is close enough to provide adequate signal strength.

The information signal as a low frequency 'audio" signal is superimposed upon a much higher radio frequency 'carrier' which can be transmitted and then received by the radio antenna. However, the antenna will receive the signal from all AM radio transmitters in the area simultaneously, and you would usually want to listen to just one. Therefore in the development of practical radio, it was necessary to develop a way to tune precisely to one carrier and reject all the others. Each radio station is assigned a precise carrier frequency and it must keep its transmitted information within +/- 5 kHz of that carrier frequency.

The scheme developed to make AM radio a practical means for mass communication was called "heterodyning". It involves using a local oscillator in the receiver that tunes along with the input radio frequency amplifier so that their difference or beat frequency remains the same. This beat frequency is called the "intermediate frequency" or IF, and for the United States that beat frequency is 455 kHz. After initial amplification, the radio frequency (RF) signal is mixed with the local oscillator frequency to produce the IF, and all the subsequent stages are tuned to 455 kHz. The IF carries any AM signal that was on the original RF. This process has the practical advantages of allowing a high quality tuned RF amplifier on the front end to tune to the different stations, and then a high quality single amplifier and detector chain tuned to the single IF of 455 kHz.

After the Mixer, the frequency fLocal Oscillator - fCarrier = 455kHz is amplified and then enters the detector stage. There it is rectified and fed into an AM detector circuit. This process amounts to a kind of filter that does not respond to the high frequency variations of the IF, but tracks the low audio frequency "envelope" of the IF signal. This gives an audio frequency output signal which is then amplified and supplied to a loudspeaker to convert it back into sound waves. That sound output is hopefully a faithful reproduction of the original input sound.

As a historical note, the reason for choosing 1480 kHz as an example AM frequency out of the range of 535kHz to 1605 kHz was that on one occasion when we were discussing AM radio, we were having a hard time eliminating a 1480kHz signal from some of our electronic equipment in the Department of Physics and Astronomy. For some reason, our whole building seemed to be an effective antenna for that particular radio station. With the help of the engineers from the station and some detective work on our own, we were able to solve the problem.

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AM Modulation Stage for AM Radio

In order to generate the radio frequency signal for AM radio transmission, the information contained in the original sound signal must be put in a form that can be sent to a great distance. When information from a voice is broadcast on an AM radio station, an electrical image of the sound is made by a microphone. A dynamic microphone is an example. That electrical signal or a signal from another recorded source is used to modulate the amplitude of the much higher frequency carrier wave. The electrical image of the sound information is used to vary the amplitude of the carrier wave by an amount porportional to the strength of the original sound signal. The modulated carrier wave is then amplified by the transmitter, which amounts to a power amplifier which can apply the modulated electrical wave to the conducting element in the transmitting broadcast antenna of the radio station.

The AM band of the Electromagnetic spectrum is between 535 KHz and 1605 kHz and the carrier waves are separated by 10 kHz.

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AM Transmission Stage

The electrical signal from the AM modulation stage contains the information to be transmitted superimposed on the precisely controlled carrier frequency wave. Neither sound waves themselves nor electrical signals at audio frequencies can be transmitted over large distances. The AM band of frequencies is between 535 KHz and 1605 kHz, corresponding to wavelengths between approximately 200 and 600 meters. These long wavelengths dictate long conducting antennas, so it is customary to see metal towers or antennas on top of mountains or tall buildings.

The radio transmitter is a power amplifier which provides enough power to generate radio frequency electric currents in the conducting broadcast antenna. Such antennae for AM radio are vertical, which leads to transmitted electromagnetic waves which are linearly polarized in the vertical direction. The electromagnetic waves, transmitted at the speed of light, are received by metallic antennae on the radio receivers. These antennae must also be oriented vertically for optimum reception. FM radio signals, by contrast, are transmitted with both horizontally and vertically polarized signals, so vertical antennae are not required.

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