Cathode Activity Measurement: A Modification of the Dip Test

The activity measurement technique to be described, was developed to fill the need for a standard technique to be used on a rather large diode program. The requirements of an activity measurement for this program which are common to any tube development, may be stated briefly as follows: (i) The measurement must cause a minimum disturbance to the chemical equilibria existing in the tube, in other words not interrupt the life processes. (ii) The measurement should be rapid, to permit regular study of a large number of tubes. (iu) The parameter (s) measured should be related to some fundamental property of the cathode, independent of the tube. (iv) The technique should not require complicated or specialized electronics which are subject to break down or drift. A review was made of the currently available techniques. All of them suffer from one or more shortcomings when evaluated by the above 2363 2366 T H E BELL SYSTEM TECHNICAL J O U R N A L , DECEMBER 19(57 requirements. Child's law plots or perveance measurements arc useful but tend to substantially upset the tube equilibrium especially when data are taken at higher cathode loading than the normal operating point of the tube. The higher power invariably leads to deactivation, especially in close spaced diodes. If data above the operating point are taken with short (1 to 2 /xsec.) pulses this problem is eliminated but the process is very time consuming and involves complex electronics. Use of single high-voltage pulses to obtain the current at a fixed point in the Schottky region yields good information so long as equipment is stable and capable of precise calibration.