The methods of reducing frequency modulation may be put under two broad categories mainly direct methods and indirect methods. Direct methods of producing frequency modulation utilizes an electron device such as an electron tube, field effect transistor, bipolar junction transistor which offers voltage variable reactance, either capacitive or inductive such a device is placed directly in shunt with the inductor capacitor tuned circuit of an oscillator. The oscillator frequency varies in accordance with the modulating voltage applied to the device resulting in frequency modulation. By proper adjustment of circuit parameters and operating conditions, an almost linear relation may be obtained between the frequency deviation and the instantaneous modulating voltage. The direct method of reducing frequency modulation is of four types, namely Frequency modulation using reactance tube, reactance field effect transistor; reactance bipolar junction transistor and Varactor diode. The indirect method of frequency modulation consists in pre-distorting the modulation voltage in an integrator to make its amplitude vary inversely with its frequency and making the modified modulating voltage to phase modulate the carrier.
This modulate of modulation was suggested by Armstrong.
Reactance tube modulation is classical method of producing frequency modulation. Before the advent of semiconductor devices this method was most popularly used however today it has been largely replaced by the Varactor diode method. Third method makes use of a reactance tube that is an electron tube which offers between its plate and cathode terminals a reactance varying in accordance with the modulating voltage applied between its control grid and cathode. This reactance tube is placed in shunt with the tuned circuit of an oscillator. This oscillator frequency varies in accordance with the modulating voltage resulting in frequency modulation. By proper adjustment of circuit parameters and operating conditions and almost linear relation may be obtained between the frequency deviation and the instantaneous modulating voltage.
The reactance tube may be either a capacitive reactance or an inductive inductance tube depending upon whether it offers a capacitance reactance or an inductive reactance across its plate to cathode circuit. Further this reactive tube operation may be achieved by use of either a resistance capacitor network or resistance inductor networks. However resistance capacitor is preferred. Hence we consider only the reactance tubes using resistance capacitor network
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