Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

INTERMEDIATE-FREQUENCY VALUES 247choice of a frequency to minimize direct intermediate-frequency responseis desirable.Eesponse from Two Stations Separated by theIntermediate FrequencyIn this type of response one of the signals acts at the first detectoras the local oscillator for the other signal, the detector output being theintermediate frequency. This type of response is likely to be seriouson frequency modulation when a low intermediate-frequency value isused, because stations may be allocated every 400 kilocycles in a giventerritory, and the per cent frequency separation is small with respectto the signal frequency. This type of response is particularly troublesome,because in localities where two strong signals exist separatedby the intermediate frequency, they will be heard throughout thetuning range of the receiver.Combination of Signal and Oscillator HarmonicsMost oscillators have appreciable harmonic content, so that ifconditionsexist under which signal harmonics occur in the first detector,spurious responses will result.The most common condition for signalharmonicproduction is when the signal exceeds the bias. Many frequency-modulationreceivers have not used automatic volume control,depending upon the limiter to maintain a uniform signal at the seconddetector. Under such conditions, particularly with a radio-frequencyamplifier present, relatively low-intensity signals will generate harmonics.Even with automatic volume control, harmonics may be generatedby an undesired signal of high intensity when the receiver istuned to a weaker desired signal. Whenever the difference frequencybetween the harmonics of signal and oscillator is equal to the intermediatefrequency, a spurious response will occur.Half Intermediate-Frequency ImageThis type of response in which the interfering signal differs fromthe oscillator by half the intermediate-frequency value, depends uponproduction of the second harmonic of this difference frequency in theconverter plate circuit. The second-harmonic generation in the converteris usually sufficiently small so that this type of response is notserious despite the fact that the radio-frequency separation of interferingand desired signals necessary for its production is small.