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  Cathode Interface Symptoms
If you have a tube-type oscilloscope or plug-in that is getting old and can no longer be calibrated to meet gain or frequency response specs, you may be experiencing cathode interface. Cathode interface is caused by degradation of the actual vacuum tube cathode, as explained in detail on another page of this site: What Causes Cathode Interface?
On this page, we'll describe the symptoms of cathode interface, so that you can determine if it is a problem in your instrument(s). A third page What Can Be Done About Cathode Interface? describes possible remedies for this problem.
Checking for Cathode Interface
Connect the input of your instrument to its own square-wave calibrator output. Adjust the sweep speed and triggering to display a few cycles of the calibrator square wave. Carefully determine the displayed waveform amplitude, and compare that value with the expected amplitude (the calibrator output amplitude is specified in the calibrator section of the instrument manual, and is often shown on the instrument front panel itself.)
If the amplitude of the displayed signal is lower than expected, and if the signal also appears to have overshoot at the leading edge of each pulse, you probably have a cathode interface problem.
In this example, the vertical sensitivity was set to 0.2V per division. Knowing that the calibrator output was 1V amplitude, we expected to see a 5 division tall waveform (the dotted white waveform). The blue waveform was actually observed.
Rather than having overshoot on the leading edge (which would indicate excessive high frequency gain) the waveform shows droop on the trailing edge (indicating less than normal low frequency gain), a strong indication of cathode interface.
Cathode Interface Symtoms

For more information about the Tektronix instruments that are prone to cathode interface, consult Oscilloscopes - Selecting and Restoring a Classic by Stan Griffiths.

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Last modified on  2/29/00 10:08:28 PM