[Elecraft] 1N34A-Diode,RF probe

[Jack Smith]

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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