Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

INTERMEDIATE-FREQUENCY VALUES 253conversion. With the oscillator injection limited to such value, theprobability of a given signal causing a spurious response isinverselyproportional to the square of the oscillator-harmonic order involved.This factor may be applied to the calculations for signal- and oscillatorharmoniccombinations.Considering the image response; this isthe signal and the oscillator soasare the harmonic combinations.due to the fundamental ofshould not be decreased by any factorIn applying weighting to the case of two signals separated by theintermediate frequency, the oscillator isnot involved but the strengthof the spurious response is proportional to the strength of the signalsinvolved.This response will in general be less than that for the image.Accordingly a factor of 0.5 was applied to the two signals separatedby the intermediate frequency, giving ita weight between the imageand harmonic combinations due to the second harmonic of the oscillator.When weighting factors are applied we can no longer say that theordinates represent the number of spurious response signals. Theordinates in this case represent the probability of spurious responsesfor various intermediate-frequency values in a receiver having noselectivity ahead of the converter, with the probability for an intermediatefrequency of 1 megacycle considered to be 100. The variationof likelihood of interference is shown in Figure 4.This figure combinesthe weighted effect of image, two signals separated by the intermediatefrequency, and oscillator-signal harmonic combinations up to the fourthof both signal and oscillator.Itillustrates that the probability of interference decreases rapidlywith increasing intermediate frequency at firstand then more slowly,and that over the entire range of intermediate frequency there islittlechoice between oscillator lower and oscillator higher conditions. It doesnot indicate definitely any best intermediate frequency but does showthat a high intermediate frequency is better.While, as has been stated,direct intermediate-frequency interference is not believed to be a majorfactor, and as the image ratio for any intermediate frequency over 8megacycles should be adequately high for any receiver having morethan one tuned circuit ahead of the converter, nevertheless it is wellto minimize spurious response from these sources where no other disadvantageaccrues therefrom.In considering spurious responses, one characteristicof frequencymodulation, not heretofore mentioned, should be borne in mind, namely,that such responses when they occur on the frequency of a desiredsignal do not cause an audible whistle as they do on amplitude modulationbut appear essentially as cross talk.