The use of automatic frequency control in the generation of FM signals results in a practical scheme but one which depends on feedback. For proper operation of the system all of the circuits must be made to track each other. Furthermore, the voltage-controlled capacitor used in the automatic frequency control circuit has a non-linearity as well as a temperature characteristic which must be compensated for. Evidently, a different scheme with less complexity is needed. The Armstrong system, named after its inventor, is one such scheme. The theoretical basis of the Armstrong system is given by Equation (4.2.7). It is clear that, in commercial FM systems, the frequency deviation must be small compared to the carrier frequency, that is:
—c(t) ^ k — | sin —st| = mf | sin —st| (4.4.1)
The following approximations can be applied to Equation (4.2.11):
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