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Winter 2007 Issue |
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The Phase Modulator in NBFM |
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The phase modulator has been a point of discussion as to why it, and not a frequency modulator, is used in what are called FM communication systems. This article’s goal is to show why. There are a number of points regarding why phase modulators are used and they are probably valid, but this article will show that to meet technical requirements and create efficient operation with a voice response, a phase modulator is best. Looking at the characteristics of frequency and phase modulators and coming to a conclusion that one is better than the other without considering the system technical requirements can lead to incorrect conclusions. Frequency modulators and phase modulators have different characteristics as a function of the input modulation voltage frequency response, and they need to be understood in order to come to the correct conclusion. The explanation of why a phase modulator is the modulator of choice will be undertaken by breaking down the elements that make up the system performance. Once the elements are gone over, then the characteristics of these elements will be used to explain why the phase modulator is used in FM communication systems. The breakdown is as follows: The FM discriminator The voice response The modulation index
After the above discussions, the technical
requirements will be applied to sort out the reasons for using the phase
modulator. Also for this discussion, the phase modulator can be either a
phase modulator, or a frequency modulator preceded by a pre-emphasis or
differentiator circuit, making it a phase modulator. The merits, or lack
thereof, of phase modulation from a phase modulator or a frequency
modulator preceded by a differentiator circuit are not within the scope of
this article. FM receivers have discriminators that convert the incoming RF signals to an audio output. Since FM discriminators demodulate only the delta frequency modulation of the modulated signal (not the phase modulation), the receiver sees only the frequency modulation of the modulated signal. Because the receiver detects only the frequency modulation of the transmitter, the system is properly called an FM system. Both frequency and phase modulators generate modulation that has both a delta-f (deviation) component and a delta-p (modulation index—radians) component. (Note: delta-f and delta-p occur simultaneously and each represents a different aspect of the same thing—angle modulation). Figure 1 illustrates the classic plot, delta-f as a function of a flat modulation frequency response for both a frequency and phase modulator. |
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