Abstract
A new simple and sensitive bridge method for the measurement of cathode interface impedance is reported in this paper. A tube with an interface impedance under test in this method forms a bridge together with a reference tube with no interface layer and with an external variable RC circuit in its cathode return. The bridge is excited by a set of three sinusoidal signals of 200 kc, 1.1 and 5 Mc. The unknown interface impedance is then measured by simultaneously balancing the bridge for these frequencies. The detector consists of amplifiers tuned to each frequency and a cathode-ray oscilloscope that displays each signal separately. A small wobbling of the transconductance in the reference tube is also provided for simplifying the measurement and overcoming drift of tube characteristics. This bridge has a sensitivity of the order of 0.002 rad as a minimum detectable phase shift, and allows a measurement of interface resistances as low as a few ohms with time constants of 0.01 μsec or more, even in ordinary tubes.
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