Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

148 FREQUENCY MODULATION, Volume Ipart of the cycle when the piate potential of the 807 swings toward itslowest value, C discharges through diode d^ in series with its load R c ;thus, one pulse of current flows through the load R of diode d 2 foreach cycleof operation.The total charge acquired by the condenser C once each cycle isCE B (neglecting the contact potential of diode d x ) . The portion of thistotal charge which passes through diode d 2and R once each cycle isequal to that total charge (CE B ) minus the residual charge Ce pmin(again, neglecting the contact potential of diode d 2 ) . Then, as long asE B and e pmin remain constant and the time constants of the charge anddischarge circuits are sufficiently small compared to the period of theinput wave, the total quantity of electricity which flows through R ineach cycle is constant. Thus, an increase in the repetition rate willincrease the number of current pulses per unit of time and, therefore,*/ FIITCHFig. 1increase the average value of current flowing through R .Conversely,a decrease in repetition rate decreases the average current through R,and the average potential across R is thus a perfectly linear functionof the repetition rate or frequency of the input signal.Obviously, then, dynamic operation of the detector will result inessentially distortionless detection over that part of the characteristicfor which the previously discussed charge and discharge time-constantrequirements hold.Practical Detector CircuitThe circuit of the laboratory model of the monitor detector, shownin Figure 2, is essentially the same as the basic circuit of Figure 1,except that a 6AG7 amplifier stage has been added.Certain pertinent factors relating to the development and use ofthe monitor detector are listed below.(1) The sensitivity (audio volts developed per kc frequency deviationin the input wave) is approximately a linear function of the Bvoltage applied to the 807. Hence, variations in this B voltage will