Nick:
In the standard RF probe configuration, the diode carries little to no
current, as it works into a high impedance digital voltmeter, or, in the
old days, a 50 uA moving coil meter. My Vf/If diode measurements are for
currents quite a bit higher than involved in a typical RF probe.
If I have some time tomorrow, I'll run some automated measurements of DC
out voltage into a high Z load versus RF voltage.
Jack K8ZOA
www.cliftonlaboratories.com
Nick-WA5BDU wrote:
It's common to recommend a Schottky as a replacement for the 1N34A.
I've done it myself. That's why I was surprised to find this web page -
http://www.cliftonlaboratories.com/diode_vf_vs_if.htm
where this guy discovers that the Schottky diode (1N5711, to be
specific) can show some pretty high forward voltage drops. Turns out
that the low drop we're all familiar with exists at quite low values
of forward current -- less than 1 mA. At ~2 mA is a sort of
breakpoint where Vf begins to increase at a faster rate than in the
silicon or the germanium diodes. By ~10 mA, the 1N5711 has the
highest forward voltage of all three. Quite an eye-opener!
You might think that for the RF probe application, forward current
will be small. Maybe it is. But often times small average currents
are misleading if there's a short conduction angle (time).
72--Nick, WA5BDU
--back to counting screws
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