Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

INPUT IMPEDANCE OF SEVERAL RECEIVING-TYPEPENTODES AT FM AND TELEVISIONFREQUENCIES*ByF. MuralIndustry Service Laboratory, RCA Laboratories Division,New York, N. Y.Summary— The input impedance of vacuum tubes is an importantcircuit design consideration at high frequencies. This report includesinformation on the input impedance of a number of currently availabler-f pentodes.Measurements are given on the variation of input resistance withfrequency, and on the variation of input resistance and capacitance withplate current. Measurements are also given for the compensation of inputresistance and capacitance variation with plate current by means of unbypassedcathode resistance.The frequency range of measurement was chosen roughly to cover thefrequency modulation and television transmission assignments as well asthe recommended intermediate frequencies of receivers for these services.Method of MeasurementT^HE resistance measurements were made by observing the changein Q when the vacuum tube input was shunted across a tunedcircuit. For this purpose two standard commercial Q-meterswere used to cover the desired frequency range. An important considerationin these measurements, particularly for low values of biasvoltage and for high values of measured resistance, was to keep the r-fvoltage across the tuned circuit to as low a value as possible. This wasdone not only to prevent grid current but also to avoid a shifttube's operating point.of theSince this difficulty was encountered primarilyat the lower frequencies the Q-meter used for this work was equippedwith an external 30-microampere d-c meter connected in series withthe Q-indicating meter of the instrument.This meter was calibratedby connecting a standard signal generator with calibrated attenuatoracross the Q-meter terminals and using the 1-volt point on the Q-meteras reference.values.This arrangement also extends Q measurements to lowerIt was also found necessary to place a shield plate,electrically connectedto the case of the Q-meter, between the coil of the tuned circuitand the tube being measured. This was done to eliminate the effects* Decimal Classification: R330 X R630.305