Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

402 FREQUENCY MODULATION, Volume Ii'erentiating 3 the wave to change the frequency variations into anamplitude-modulated wave. Amplitude detection follows this differentiation(or discrimination) to obtain the amplitude envelope. If thewaves represented by Figures 4 and 6 are differentiated, the originalmodulation frequencies are obtained intact. However, if one or moreof the terms in these side frequencies are eliminated before differentiating,the differentiated (or discriminated wave) tone will containterms proportional to the derivative of these missing terms. Any twoof these waves can then beat together inthe amplitude detector thatfollows this discriminator to give cross talk.Bandwidth limitation could occur in the transmitter itself, but themore likely place will be in the receiver intermediate-frequency amplifieror discriminator circuits. This is especially true in receivers hav-2075 KC DEV-3T2S-~>oI 40 SO 60 70 IKILOCYTLESFig. 5—Sideband amplitude.lij-L.90 100ing good adjacent channel selectivity and consequent narrow band-passaction in the i-ftransformers.After detecting, the requirements of linearity in any tube that bothaural and facsimile signals pass through together are the same as foramplitude multiplexing.In the case of two modulating frequencies which differ in characteras greatly as do those of sound and facsimile programs the linearityrequirements imposed on the system are much more severe than in thecase of single program modulation. With a single program, any nonlinearityof the modulator, the detector, or the audio-frequency portionof the receiver causes harmonic distortion. Distortion may also becaused by too narrow a receiver passband. This latter distortion willbe decreased appreciably, but not entirely eliminated by a limiter.of these causea of distortion may be classed as non-linearity.The distortion which results from non-linearity in the case of asingle program, if not too great, is not readily distinguished by theear. For example, there are many radio receivers having 3 per centor even 5 per cent distortion which are not objectionable to the major-3Carson and Fry, "Variable Frequency Electric Circuit Theory WithApplication to the Theory of Frequency Modulation," Bell System Tech.Jour., Oct. 1937.All