Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

250 FREQUENCY MODULATION, Volume Ision for all tune frequencies between 42 and 50 megacycles. It isapparent that consideration should be given to the oscillatorhigherthan the tune frequency separately from the condition for oscillatorlower than the tune frequency, since the oscillator cannot change fromone side to the other during the tuning process.To illustrate, suppose a receiver has an intermediate frequency of4.0 megacycles and is tuned to 49 megacycles ; the oscillator is then at45 megacycles and its second harmonic is 90 megacycles. If the secondharmonic of the signal isat either 86 or 94 megacycles, the resultantintermediate frequency is 4.0 megacycles and a spurious responseoccurs. That is, the fundamental of the signal may be either 43 or 47megacycles.t 4 t a 10 it 14 » la to & -m/A/TeBIHE0l/IT£Ffl£Cl/£NCV M M£G/KYCL£$Fig. 1—Spurious responses caused by harmonic combinations of signal andoscillator versus intermediate frequency.Signals: from 40 channels between 42 and 50 megacycles.Tuning: from 42 to 50 megacycles.M = signal-harmonic order.N = oscillator-harmonic order.Oscillator lower than tune frequency.From an examination of the above expressions it may be seen thatvariation of spurious responses with intermediate frequency is a linearfunction, so that a plot of the variation of spurious responses withintermediate frequency will consist of a series of straight lines.A plot of the number of spurious responses as a function of intermediatefrequency for the case where the oscillator frequency is lowerthan the tune frequency is shown in Figure 1 and for the case wherethe oscillator frequency is higher than the tune frequency in Figure 2.An examination of the expressions for spurious responses as afunction of intermediate frequency, along with the plots of Figures 1and 2, reveals that for the same harmonic order of signal and oscillatorfrequency, the number of spurious responses as a function of inter-