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RF Engineering: Modern Modulation Techniques
Modern society is definitely more mobile than it was several decades ago. Accompanying the increased mobility is a corresponding craving for greater quantities of information. The increased demand has called for a number of changes in modulation techniques. This month we look at several of them.
Initial wireless transmissions were an offshoot of telegraphy. The first spark gap transmitters worked well, but were of course quite “dirty” in their operation. In 1906 the venerable method of amplitude modulation radio was born.
While it would take several more years to reach commercial viability, this simple method of transmission would reign supreme for several decades. The development of AM came out of the realization that the waves produced by spark gaps were insufficient for the transmission of audio material. Instead of the on-off scheme under which spark gap transmitters operated, continuous waves would be required. This carrier wave is amplitude modulated by audio before transmission. (See Figure 1.)
Since free lunch rarely exists, the benefit of the simplicity of AM was offset by audio quality and inefficiency. The simplistic modulation scheme of AM makes it very susceptible to natural and man-made noise sources. In addition, the majority of the power is concentrated in the carrier, which lacks the original transmitted information.
In the more modern technique of FM, the transmitted information varies the instantaneous frequency of the carrier, while maintaining the amplitude. The frequency of the carrier, for the most part, will tend to reside within a certain range on either side of the carrier known as the frequency deviation. In a Fourier analysis, which breaks up the signal into combinations of simpler signals, we find that a much broader range of frequencies is required to fully characterize an FM signal. Indeed this range extends to infinity, but at that point, and well in advance of it, the amplitudes are so low, that they practically drop out of the equations.
Of course, a major advantage with FM is its ability to reduce noise. In reality, below the noise threshold, FM actually has a poorer signal-to-noise ratio than AM does; however, above this level the SNR is vastly improved. Wider deviations can increase this ratio, as can pre-emphasis. Limiter circuits provide additional benefit by removing AM noise, as the FM signal is of constant amplitude. (See Figure 2.)
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