New methods for the measurement of cathode interface impedance

Two improved methods for the measurement of cathode interface impedance have been developed, and their limitations have been analyzed. One of these, the complementary network method, is an improvement of a technique disclosed at the IRE National Convention in 1952. The other, the shunt admittance bridge, has not been described previously. Both methods allow the measurement of impedance with both small resistance and short time constant, well below the limit, 50 ohms at 0.1 µsec, of most present equipments. With the development of improved cathode alloys, the measurement of interface impedances having short time constants and low resistances has become important to control this parameter in manufacture and to obtain further improvement. For the complementary-network bridge, the theoretical analysis has shown the extreme importance of minimizing stray inductance in the complementary network. When corrections are applied, the complementary-network bridge has good accuracy, with less than one-ohm error at 10 ohms and 0.05 µsec and lower relative errors for higher resistances. The shunt admittance bridge is most satisfactory when tubes with transconductances greater than 10,000 µmho are to be measured. An impedance transformation is used which allows much easier physical realization of the measurement network than in other interface measurement methods. For tubes with transconductances greater than 10,000 µmho, the shunt admittance bridge will provide accurate time constant and resistance data down to 0.02 µsec and 5 ohms.