Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

GENERATION AND DETECTION 149introduce undesired components into the detector output. Therefore,the B supply must be well regulated.(2) Approximately 20 peak-peak volts, at a center frequency of150 kc, are required as input to the 6AG7, when the circuit of Figure 2is used. Voltage at this frequency is obtained from that at any othersignal frequency by means of the heterodyne oscillator and converter,shown in Figure 3. This particular circuit is designed for operation atsignal frequencies in the 40-Mc range.The a-c peak-to-peak grid voltage applied to the 807 must be at least20 per cent greater than the value required to swing from zero biasto cut-off. The 3000-ohm series grid resistor in the 807 circuit aidsinmaintaining approximate equality of duration of the two halves ofthe plate-voltage wave, and also limits the peak grid current on positivegrid swings.IF. INPUTAP/>eM. ISfittCAT 20V. P~PFig. 2—90-volt and 150-volt supplies are regulated. One side of allheaters (supplied from 6.3-volt transformer) is grounded.(3) The 807 plate load is adjusted to produce plate-current saturationat a peak grid signal just short of grid current.(4) The value of capacitance C (in conjunction with other inherentcircuit capacitances) is determined by the maximum permissible timeconstants in the charge and discharge circuits. These should both besufficiently small compared to a half-period at the highest operatingfrequency to permit (a) e pto rise to within 0.1 per cent of E B in onehalfcycle and (b) to permit e v to fall to within 0.1 per cent of itsnormal minimum value, again within a half-cycle.(5) The lower limit of operating frequency is determined by thecut-off of the low-pass filter in shunt to R (Figure 1). This filter isnecessary both to remove undesired signal-frequency components fromthe output of the device and to allow the instantaneous potentials acrossR„ to rise and fall with each discharge-current pulse. If R were bypasseddirectly, to remove the signal-frequency components, the residualcharge on the bypass condenser would act as a diode bias, and preventcomplete discharge of condenser C. Then the discharge currents would