Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FM EXCITER 173to be entirely satisfactory for automatic frequency control purposes.The circuit can be tuned over the required range by means of anadjustable iron slug located within the field of the coil. The lock-inrange may be as high as plus or minus five per cent. Division ratiosas high as12 have been successfully obtained.Having selected a circuit for frequency division the next considerationwas to fix the frequency at which the comparison of frequencieswas to be made. A crystal and holder with excellent performance designedto operate in the frequency range of 90 to 125 kilocycles is usedas the frequency standard for the exciter units.A preliminary test of the performance of the frequency control withfrequency comparison, made at 100 kilocycles, gave excellent resultsexcept for a loss of control in the presence of tone-frequency modulation6AC7 AS A TRIODEB +Fig. 10—Locked-In Oscillator Frequency Dividerwhen the modulating frequency was below 100 cycles. A fundamentalcharacteristic of frequency modulation is that the energy in the modulatedsignal does not change with the degree of modulation. However,as the amount of modulation is increased from zero, there is a transferof power from the carrier to the sidebands lying above and below thecarrier frequency. This effect continues with increasing modulationuntil a point is reached at which all the energy is in the sidebands andthere is no power in the carrier. With further increase in modulation,or frequency swing, the carrier again reappears. With the ± 75-kilocycleswing of frequency, used for broadcasting, there are several cyclesof this effect at low modulating frequencies. In the first experiments,where the frequency comparison was made at five megacycles, thisphenomenon was detected only in a loss of torque in the tuning motor.