Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FREQUENCY MODULATION 61frequency increases the deviation by an equal factor. Thus a tloublerstage not only doubles the carrier frequency but also doubles thedeviation width.VI.Modulation of Phase to Produce Frequency ModulationOne means of producing frequency modulation is first to "integrate"the program material and then phase modulate some low-frequencysource of energy; subsequent frequency multiplication can increasethe resulting frequency deviation to any desired amount. After thedesired band width has been obtained, the carrier with its modulatingcomponents can be heterodyned either directly to the transmitterfrequency or to some sub-multiple thereof.deviation and band width are obtained by thesubsequent multiplicationto the transmitter frequency.In the latter case, additionalThe integrating process usually consists of obtaining the potentialsacross a pure capacitive reactance in the plate circuit of a highimpedance vacuum tube whose grid issupplied with the normal audiocomponents. This causes a phase delay of 90° at all modulating frequenciesand gives the program material an amplitude characteristicwhich is inversely proportional to frequency. If high-frequency preemphasisis to be used, the plate circuit may have a resistance in serieswith the capacitance with the output potentials taken across both.This allows more of the high-frequency components to enter themodulating circuit than would otherwise be the case. Present-daypractice in pre-emphasis of both amplitude and frequency-modulatedwave transmitters is to use a pre-emphasizing circuit equivalent to aseries L/R network having a time constant of about 100 microseconds.Therefore, if the program integrating circuits for a phase-modulatingfrequency-modulated wave transmitter are to perform the pre-emphasisfunction also, the CR series combination must have a time constant ofthat value. Of course, the integration and pre-emphasis functions maybe performed in separate stages, but it seems desirable to combine thetwo and thus prevent the extremely wide discrepancies between thelevels of the low-frequency and high-frequency components of theprogram material at any point in the preparatory networks.If the original frequency which is to be phase modulated by theprepared (integrated) program is produced by a crystal oscillator orother stable source of oscillations, the usual method of procedure is toamplitude modulate some of the energy at that frequency in a balancedmodulator and then to re-insert carrier energy with an exactly 90°phaserotation. The result is the same vectorially as is shown inFigure 6. The output contains both phase and amplitude modulation.