Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

'United258 FREQUENCY MODULATION, Volume 1insure maximum performance. This requirement makes it difficult toprovide the desired over-all stability without excessive shielding andother circuit complications. This problem has already been the subjectof a great deal of engineering investigation and one of the solutionswhich has been proposed is the use of the double heterodyne type ofsuperheterodyne receiver. A new approach to a solution of these problemsis provided by a frequency-dividing locked-in oscillator frequencymodulationreceiving system which has been developed. 1 It is thepurpose of this paper to describe the new receiving system and toindicate some of the factors which affect its operation.Description of SystemBasically the operation of the system depends on producing, in thereceiver, a local signal which is frequency-modulated by the receivedsignal. The local signal is provided by a continuously operating oscillator.The received signal, after it has been amplified by conventionalradio-frequency and intermediate-frequency amplifiers, is applied tothe oscillator in such a way as to cause its frequency to change inaccordance with the frequency variations of the received signal.In theparticular applications of the system to be described .in this paper, theoscillator is locked in with the received signal at one fifth the intermediatefrequency. With this 5:1 relationship between the intermediatefrequency and the oscillator frequency an equivalent reduction in thefrequency variations of the local oscillator is obtained.Eeceived-signalfrequency variations of ± 75 kilocycles are reproduced as ± 15-kilocyclevariations in the oscillator frequency. It should be noted that thelocked-in oscillator operating at one fifth the intermediate frequencyreduces the frequency deviation corresponding to any modulationfrequency but does not change the modulation frequency. The frequency-modulatedsignal derived from the oscillator is applied to adiscriminator which is designed for this reduced range of frequencies.The output voltage of the oscillator is independent of the strengthof a received signal, in fact, the same voltage is applied to the discriminatorwhen no signal is being received as when the receiver is tunedto a near-by transmitter.This feature makes it unnecessary to employthe conventional arrangements for minimizing amplitude variationsin the received signal.The adjacent-channel selectivity of a conventional frequency-modulationreceiver is determined by the selectivity characteristics of theradio-frequency and intermediate-frequency circuits.If these circuitsStates Patent No. 2,356,201, filed February 12, 1942.