Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

46 FREQUENCY MODULATION, Volume III.Deviation and Band WidthThe simple conception of a frequency-modulated wave, in whichthe frequency of a carrier wave of unvarying amplitude is alteredcyclically about its mean value, is sufficiently adequate if the maximumfrequency excursions of the carrier-wave frequency are large comparedto the modulating frequency, for example, in the case of a wave beingvaried between limits separated by, say, 150 kc with the variationoccurring sinusoidally at a 100-cyele per second rate.If, however, the maximum frequency excursion is of the sameorder of magnitude, or equal to,or less than the frequency of change,the case is not so simple. It has often been proposed in all good faiththat a communication channel could be narrowed by the use of frequencymodulation in which the deviation in cycles per second isthan the modulating frequency.However, it is axiomatic that any sidebandenergyless(and consequently the necessary intelligence component)in any type of modulation, whether frequency, phase, or amplitude, willnot approach the carrier closer than a frequency interval equal to themodulating frequency. Therefore, if a narrow-band receiver is usedto intercept a small deviation frequency-modulated wave whose modulatingfrequency is greater than one-half of the band width of thereceiver, none of the modulation will be received. If we assume thecase of a frequency-modulated wave with a modulating frequency of10,000 cycles per second, which causes a deviation of + and — 10,000cycles in the frequency of the carrier, a receiver with theoreticallyperfect band pass in its r-f and i-f circuits 19.9 kc wide with perfectlyperpendicular sides,tuned with the carrier at the center of the bandto receive that frequency-modulated wave, would not give any evidenceof what modulating frequency was being used. A pure unvaryingcarrier with neither frequency, phase, nor amplitude modulation wouldbe applied to its demodulating detector. It is true that in the examplecited the received carrier would be somewhat decreased in value(actuallyto about 76 per cent) from its unmodulated amplitude, but thisneed not concern us for the moment. The example is cited simply toemphasize the fact that in frequency and phase modulation, as wellas in amplitude modulation, the intelligence components are carriedin side-band energy, and that that side-band energy must be receivedin order to interpret or reproduce the originalmodulation.In the example cited above, side bands will exist not only at frequenciesof + and — 10 kc with respect to the carrier, but also at+ and — 20 kc, as well as at 30 kc, etc. 1 If the theoretically perfect1See publications listed in the bibliography for more complete explanationof this characteristic.