The Effect of a Cathode Impedance on the Frequency Stability of Linear Oscillators

High stability oscillators usually employ tubes having rather high values of transconductance. Excellent frequency stability as to valve capacitance and supply voltage changes results, but a change of the cathode impedance produces large frequency changes. Cathode impedance changes in a valve may reach 100 ohms during its lifetime because of the growth of an interface layer between the oxide coating and the cathode sleeve. In order to reduce the effect of the cathode impedance, the gm value must be reduced considerably, the optimum being near 1/Xg, where Xg is the grid-cathode reactance. The ratio of the anode/grid RF voltages should be made equal to the ratio of grid/anode capacitances, or equal to unity. Full advantage cannot always be taken of the recommended gm value, as this makes the oscillator more susceptible to random capacitance variations and to changes of supply voltage, but a considerable reduction below customary values is possible. In this way, the longterm stability can be improved at least ten times.