Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

252 FREQUENCY MODULATION, Volume Iseparated by the intermediate frequency, and spurious responses causedby oscillator and signal harmonics.In actual practice, stations will notbe so allocated in any given location, but the curves are valid in determiningthe best intermediate frequency to be used to reduce the numberof responses, regardless of how many stations are assigned in anyparticular location, provided of course the stations are uniformlylocated in the band.The relative severity of spurious responses is difficult to evaluateexactly without data as to the magnitude of signal-harmonic generationin the converter, which in turn depends upon the signal amplitude.However, an estimate may be made of the effect of oscillator harmonicsand instructive deductions made therefrom.In a typical converter of the pentagrid type, the amplitude of oscillatorharmonic has been found to be approximately proportional to*8\\\ ST\ \\ \i\\ T"\\2 4 « a 'o/MTfflU£D//>T£ Fff£Ol/eNCV /N M£G/ICYCtfSFig. 3—Interference caused by the image response and by two stationsseparated by the intermediate frequency versus intermediate frequency.Signals : from 40 channels between 42 and 50 megacycles.Tuning: from 42 to 50 megacycles.Oscillator either higher or lower than tune frequency.image.two stations separated by the intermediate frequency.1/N 2 ,where N is the order of the harmonic. The conversion gain isnot constant with respect to oscillator amplitude but reaches a maximum,usually a broad maximum (the conversion gain is substantiallyconstant over an appreciable oscillator range)at some value of oscillatorvoltage.For values of oscillator voltage below this maximum theconversion is, to a first approximation, proportional to the oscillatorvoltage. Since the oscillator amplitude is 1/JV 2 and the conversionproportional to amplitude, the conversion is approximately proportionalto 1/iV 2 if the converter is operated so that the fundamental does notexceed the optimum. If the fundamental somewhat exceeds the optimumconversion point, there will be no appreciable decrease in conversionfrom the optimum value. But under these conditions the secondharmonic may produce as much conversion gain as the fundamental.Consequently to minimize harmonic generation it is desirable to limitthe oscillator injection to a value only sufficient to develop maximum