Reactance modulation versus phase modulation
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The most direct way to get FM is to apply the audio signal to a varactor diode in a VFO circuit. An example of this scheme, known as reactance modulation, is shown in Fig. 26-10. The varying voltage across the varactor causes its capacitance to change in accordance with the audio waveform. The changing capacitance results in an up-and-down swing in the frequency generated by the VFO. In the illustration, only the tuned circuit of a Hartley oscillator is shown.
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26-10 Reactance modulation to obtain FM.
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Another way to get FM is to modulate the phase of the oscillator signal. This causes small fluctuations in the frequency as well, because any instantaneous phase change shows up as an instantaneous frequency change (and vice versa). This scheme is called phase modulation. The circuit is more complicated than the reactance modulator. When phase modulation is used, the audio signal must be processed, adjusting the amplitude versus-frequency response of the audio amplifiers. Otherwise the signal will sound muffled in an FM receiver.
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484 Data transmission
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The amount by which the carrier frequency varies will depend on the relative audio signal level, and also on the degree to which the audio is amplified before it s applied to the modulator. The deviation is the maximum extent to which the instantaneous carrier frequency differs from the unmodulated-carrier frequency. For most FM voice transmitters, the deviation is standardized at plus or minus 5.0 kHz (Fig. 26-11).
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26-11 Frequency-versus-time rendition of an FM signal.
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The deviation obtainable by means of direct FM is greater, for a given oscillator frequency, than the deviation that can be gotten via phase modulation. But the deviation of a signal can be increased by a frequency multiplier. This is an RF amplifier circuit whose output is tuned to some integral multiple of the input. A multiply-by-two circuit is called a frequency doubler; a multiply-by-three circuit is a frequency tripler. When an FM signal is passed through a frequency multiplier, the deviation gets multiplied along with the carrier frequency. If a modulator provides plus or minus 1.6 kHz deviation, the frequency can be doubled and the result will be a deviation of plus or minus 3.2 kHz. If the frequency is tripled, the deviation increases to plus or minus 4.8 kHz, which is just about the standard amount for FM voice communications.
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In FM hi-fi broadcasting, and in some other applications, the deviation is much greater than plus or minus 5.0 kHz. This is called wideband FM, as opposed to narrowband FM just discussed. For ordinary voice communications, there s nothing to be gained by using wideband FM. The only result will be that the signal will take up an unnecessary amount of radio spectrum space. But for music, the fidelity improves as the bandwidth increases. The deviation for an FM signal should be equal to the highest modulating audio frequency, if optimum fidelity is to be obtained. Thus, plus or minus 5.0 kHz is more than enough for voice (3.0 kHz would probably suffice). For music, a deviation of about plus or minus 15 kHz or 20 kHz is needed for excellent hi-fi reception.
Pulse modulation 485 The ratio of the frequency deviation to the highest modulating audio frequency is called the modulation index. For good fidelity, it should be at least 1: 1. But it should not be much more; that would waste spectrum space.
Still another method of modulation works by varying some aspect of a constant stream of signal pulses. Several types of pulse modulation (PM) are briefly described below. They are diagrammed in Fig. 26-12 as amplitude-versus-time graphs. The modulating waveforms are shown as curvy lines, and the pulses as vertical lines.
26-12 Pulse modulation. At A, pulse amplitude modulation; at B, pulse width modulation; at C, pulse interval modulation; and at D, pulse code modulation.