Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

:314 FREQUENCY MODULATION, Volume Iresponds to the average input conductance of the compensated tube.Since g itfor the frequencies investigated, varies almost as the squareof the frequency the value of R k is almost independent of frequency.It may be noted from the measurements in Figure 5 that at lowerfrequencies the optimum compensating value of R kruns somewhathigher. This is to be expected since at lower frequencies g variestmore slowly.The use of an unby-passed cathode resistor produces a loss in theeffective transconductance of a tube. This is expressed by1 +RliOnv i + « 2 c fc2 i? fc2(6)where gm is the nominal transconductance. Since the second term inthe denominator is usually small compared to unity the R (gm figure ofmerit is generally improved.It should be noted that in the case of the 6BA6 tube the resistorfor optimum compensation is somewhat larger than the normal cathodebias resistor. This makes it necessary to either operate the tube at alower than normal value of nominal transconductance or secure thebias from a point in the circuit which is positive with respect to thecathode return point. It is also conceivable, since the departure is nottoo great, that the optimum compensating resistor could be used bychanging the plate and screen voltages to shift the operating point.Input CapacitanceIt was found that for the frequencies and tubes investigated theinput capacitance was practically independent of frequency.The measurements presented in Figure 7 were made at 10 megacyclesusing the tube operating voltages in Table 1 and the circuit inFigure 1. If transconductance instead of plate current were used forabscissae the curve would be substantially linear so that the inputcapacitance could be expressed as:Ci = C + k 5gm (7)which bears similarity to (3) except in that the first term is notnegligible and the C's are independent of frequency.