XM Radio: Understanding How Radio Waves are Generated

Published: 01st July 2007
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There are two types of devices in which it may be necessary to generate amplitude modulation. The first of these is, the amplitude modulation transmitter generates such high power that its prime requirement is efficiency, so quite complex means of amplitude modulation generation may be used. The other device is the amplitude modulation generator. Here the amplitude modulation is produced at such a low power level that the simplicity is a more important requirement than efficiency. Although the methods of generating amplitude modulation can be different but generating high power are always taken care of.



In order to generate amplitude modulation wave it is necessary merely to apply the series of current pulses to a tuned circuit. Each pulse, if it were the only one, would initiate a damped oscillation in the tuned circuit. The oscillation would have initial amplitude proportional to the size of the current pulse and a decay rate dependent on the time constant of the circuit. Since a train of pulses is fed to the tank circuit here, each pulse will cause a complete sine wave proportional in amplitude to the size of this pulse. This will be followed by the next sine wave proportional to the size of the next applied pulse and so on. Bearing in mind that at least ten times as many pulses per audio cycle are fed to a practical circuit we see that an extremely good approximation of an amplitude modulated wave will result if the original current pulses are made proportional to the modulating voltage. The process is known as the flywheel effect of the tuned circuit, and it works best with a tuned circuit whose operating point is not too low.



It is possible to make the output current of a class C amplifier proportional to the modulating voltage by applying this voltage in series with any of the direct current supply voltages for this amplifier. Accordingly cathode or emitter, grid or base and anode modulation of class c amplifiers are all possible, as is any combination of these methods. Each has its own applications, advantages and drawbacks.



In an amplitude modulation transmitter, amplitude modulation can be generated at any point after the radio frequency source. As a matter of fact even a crystal oscillator could be amplitude modulated, except that this would be an unnecessary interference with its frequency stability. If the output stage in a transmitter is plate modulated the system is called high level modulation. If the modulation is applied at any other point, including some electrode of the output amplifier, then so called low level modulation is produced. Naturally the end product of both systems is the same but the transmitter circuit arrangements are different.



It is not feasible to use plate modulation of the output stage in a television transmitter, because of the difficulty of generating high video powers at a large bandwidth required. Grid modulation of the output stage is the highest level of the modulation employed in television transmitters. It is called high level modulation in television broadcasting and any thing else is then called low level modulation.



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Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.

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