Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

4 FREQUENCY MODULATION, Volume IThe purpose of the limiter is to remove unwanted amplitude modulationso that only the frequency modulation component of the signalwill be received. It may take the form of an overloaded amplifier tubewhose output cannot rise above a certain level regardless of the input.Care must also be exercised to insure that the output of the overloadedamplifier does not fall off as the input is increased since this wouldintroduce amplitude modulation of reverse phase, but of equally undesirablecharacter.The main requirement of the conversion circuit for converting thefrequency modulation into amplitude modulation is that it slope linearlyfrom a low value of output at one side of the intermediate-frequencychannel to a high value at the other side of the channel. To dothis, an off-tuned resonant circuit or a portion of the characteristic ofFig.FREQUENCy2— -»Ideal sloping filter characteristics.one of the many forms of wave filters may be utilized. The ideal slopefilter would be one which gave zero output at one side of the channel,an output of one voltage unit at carrier frequency, and an output oftwo units at the other side of the channel. Such a characteristic isgiven by the curve JOC of Fig. 2. From this curve it is easily seen thatif the frequency is swung between the limits F and F h about the meanfrequency F c ,the output of the filter will have an amplitude modulationfactor of unity. The modulation factor for a frequency deviation,F d , will be given byFi F "2FM*{= — = =(1)(F t- F c )(F c- F ) Fiwhere F (= intermediate-frequency channel width.When the converting filter departs from the ideal characteristic inthe manner of the filter of curve HGOE of Fig. 2, the modulation factorproduced by a given frequency deviation is reduced by a factor equal