Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

248 FREQUENCY MODULATION, Volume IIntermediate-Frequency Harmonic KesponseResponse at frequencies which are harmonics of the intermediatefrequency may be generated in the converter, but, unless an unusuallyhigh intermediate-frequency value is used, only high harmonic orderscan fall within the frequency-modulation band. As the amplitude ofthe harmonics decreases rapidly with harmonic order, littleis to be expected in practice from this type response.difficultyInfluence of Intermediate Frequency on Spurious ResponsesThe value of intermediate frequency chosen has a large influenceon the number of spurious responses produced.It is evident that, witha frequency range of 8 megacycles, an intermediate frequency higherthan 4 megacycles will eliminate image response due to frequencymodulationstations.Similarly an intermediate frequency higher than8 megacycles will eliminate responses from two stations separated bythe intermediate frequency and an intermediate frequency higher than16 megacycles will eliminate the half-intermediate-frequency image.The effect of intermediate frequency on combinations of signal andoscillator harmonics is not as obvious, and detailed analysis is requiredto determine them.Any combination of signal and oscillatorharmonics which resultsin the chosen intermediate frequency will produce a spurious response.It is assumed that the receiver can be tuned to any frequency between42 and 50 megacycles. Also only signals between 42 and 50 megacyclesare considered. The analysis of the number of spurious responses producedis made without regard to the radio-frequency selectivity. Thatis,for any signal between 42 and 50 megacycles any tune point in thesame range may be chosen. It is, therefore, necessary to consider everypossible tune frequency for every signal frequency in the range.In determining the number of spurious responses, consideration ofthe relative magnitudes is reserved for later discussion. The lowestfrequency channel is 42.1 megacycles and the highest frequency 49.9megacycles, a total of 40 in all. It is assumed that the total numberof spurious responses from any cause is additive. That is, if for anygiven tune frequency there are three different signal frequencies whichcan cause a spurious response, three spurious responses are countedfor that tune frequency. Thus, for some values of intermediate frequency,while there are 40 frequency-modulation channels, the numberof spurious responses is appreciably higher than 40. This form ofanalysis is more illuminating than indicating only the actual channelson which interference can occur without regard to the frequencies