Hi Tom,
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Correct, things become unpredictable at lower frequencies. The original 346 diode was a special design in that, among other things, it had a p-doped guard ring to control the avalanche area as tightly as possible. The diode is in a hermetically sealed assembly and it was eutectically attached to the package to stabilize the temperature. Later, when HP obsoleted the Si process, the diodes came from Metelics, which unfortunately went downhill after being purchased by a conglomerate. I am not sure about the differences between old HP and Metelics diodes.
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Vladan
