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the output power level, but leave R2 set at midpoint so that the signal is still of constant frequency.

Fig. 4, If the speed control is left atone but the field power control is varied regularly from minimum to maximum and back again, the output of the alternator will be almost zero when field power is minimum and will increase as field power increases until it reaches a maximum, then fall back as field power is decreased.. The resulting waveform looks like this, and corresponds to amplitude modulation of a constant-frequency carrier signal.

Fig. 4, If the speed control is left atone but the field power control is varied regularly from minimum to maximum and back again, the output of the alternator will be almost zero when field power is minimum and will increase as field power increases until it reaches a maximum, then fall back as field power is decreased.. The resulting waveform looks like this, and corresponds to amplitude modulation of a constant-frequency carrier signal.

The result looks like Fig. 4. The period (which determines frequency) and the phase remain the same as in Fig. 3, but the amplitude varies from near zero to maximum and back down again as the field-coil power varies. This is amplitude modulation.

For the next illustration, we'll leave R1 set at midpoint so that output power level is constant, but vary R2 at the same rate as we did R! before. Assuming that the dc driving motor can change speed instantly as we change the amount of power supplied to it,

Fig. 5, When field power level is left constant, but the driving speed is varied from high to low and back, the alternator's output will be of constant amplitude but will vary in frequency. This is shown in this waveform; the tick marks on the time axis are the times at which the waveform of Fig. 3 crosses the axis, assuming that the leftmost zero-crossing occurs at the same time for boht waveforms. This corresponds to FM. All of these waveforms have been traced from an X-Y plot produced by an electronic computer, using a carrier frequency three times that of the modulating signal in order to clearly show the actions. Normal practice is to use a carrier of several hundred to several million times the frequency of the modulating signal.

and that the rotating system has so little inertia that it also can change speed instantly, our output signal in this case will resemble Fig, 5.

The amplitude is constant, but the period and the phase both vary as R2 is varied. The tick marks on ^the baseline in Fig. 5 show where the waveform of Fig. 3 crosses the zero axis. You can see that as the dc motor turns faster and frequency rises, the period of the output signal shortens, and as it turns

Fig. 6. If the circuit of Fig. 2 is connected to a constant-frequency generator as shown here, frequency of the output signal wilt be locked to that of the second generator. Varying speed of the alternator now cannot produce permanent change in frequency, but will produce change in phase of output signal while speed is changing, Result corresponds to PM, and has same waveform as FM shown in Fig, 5; differences between FM and PM are largely a matter of definitions.

Fig. 6. If the circuit of Fig. 2 is connected to a constant-frequency generator as shown here, frequency of the output signal wilt be locked to that of the second generator. Varying speed of the alternator now cannot produce permanent change in frequency, but will produce change in phase of output signal while speed is changing, Result corresponds to PM, and has same waveform as FM shown in Fig, 5; differences between FM and PM are largely a matter of definitions.

slower, the period lengthens again. This is frequency modulation.

We could perform the same actions again as we did to produce Fig. 5, but connect the output to a fixed-frequency generator through large series inductances as shown in Fig. 6. When we do this, the frequency is locked to that of the fixed-frequency generator - but the phase will vary while the dc motor's speed is changing. This is phase modulation. We don't show a separate illustration of it, because for any single-frequency modulating signal, there is essentially no way to tell the difference between FM and PM at the output signal. The outputs of each are identical.

The functional difference between FM and PM lies in the fact that the amount of change in period or phase which occurs in the modulated signal is determined by different factors. When FM is employed, the amount of change is determined by both the strength of the modulating signal, and by its frequency- Lower-frequency modulating signals give greater change for the same level than do higher-frequency signals. When PM is employed, the amount of change depends only upon the strength of the modulating signal.

There is no practical difference between the two types of modulation, since any filtering of the modulating signal ahead of the point at whici modulation occurs can compensate for the functional difference between 7M and PM, and permit an FM-type output signal to be produced by a PM modulator, or a PM output signal to be produced by an FM modulator. Most commercial FM broadcasting uses an output signal which is about halfway between true FM and true PM characteristics. Almost all commercial two-way FM equipment actually uses phase modulation, in order to permit crystal control of center frequency.

How Can We Modulate A Signal? Now that we have an idea of the basic characteristics and differences, if any, between the three major types of modulation — AM, FM, and PM - we need to know how we can modulate a signal with any of these types. The circuit of Fig. 2 is obviously not very practical for use at radio frequencies; we need something with a lot less inertia than a physical generator, and we must be able to control it with an audio-frequency speech signal.

Let's see just what we have to work with, and go from there. If we're going to modulate a radio signal, we have some type of rf carrier generated by an oscillator and brought up to the output power level we

desire by a series of rf power amplifiers, and we have some sort of modulating signal which (except for TV) is an af signal.

To modulate that carrier, we must change its amplitude if we want AM, its frequency if we want FM, or its phase if we want PM. Whichever of these characteristics we change, the change must be controlled by the af modulating signal.

We can vary the amplitude of the signal in either of two basic ways. We can vary the amount of power supplied to one of the rf power amplifier stages, or we can vary the operating efficiency of one of those amplifiers. This can be done at any amplifier stage, but if modulation is applied to any

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