[ARC5] Diode Mystery

[Dennis Monticelli]

I had a 40 yr career in the semiconductor industry and can share some
knowledge.  Let's think about this diode noise aspect by examining how a
junction diode behaves in general.

Under forward bias it produces a very small amount of broadband noise; Shot
noise proportional to the square root of current plus some 1/f Excess noise
that depends largely upon how the diode was made.  It is not sufficient to
cause any problems, period.  There is only leakage under reverse bias.
Again, no noise.

A silicon diode is capable of relatively fast switching up through HF
frequencies.  So if presented with an abrupt change in excitation it will
follow it.  Hit it with an abrupt step in voltage drive and it will conduct
current equally abruptly.  That sharp change in current can produce a spurt
of RFI.  Turn off the voltage drive abruptly and the current will collapse
abruptly (somewhat shaped by the stored charge in the junction).  This can
also cause RFI.  A 60Hz sine wave is not abrupt excitation.  So basic
rectifier action will not cause RFI.

There is a related issue, though.  If the circuitry is exposed to strong
RF, such as your transmitter coupling into the AC mains, the RF can induce
mixer action within the diodes of an ON power supply as those diodes enter
into and out of their conduction cycles.  The diodes are non-linear during
those periods so harmonics can be produced, including BCB harmonics.  It
can also happen if the supply is OFF should the coupled RF be large
enough.  Placing 0.01uF caps across the diodes fixes that potential
problem.

I don't see how a 100% forward biased supply protection diode could produce
the noise problem theorized. Sounds more like a measurement problem of some
kind. DVMs can be fooled, particularly when in the presence of a strong RF
field.

Hope this helps with the understanding.

Dennis AE6C

On Mon, Nov 30, 2020 at 12:42 PM Owner <kargo_cult at msn.com> wrote:

> I think you want to take a good look at the output of the bench supply
> with a scope set to a low AC/RF voltage
> setting. I suspect the output has harmonic components from clipped
> sinewave somewhere in it. The DVOM
> that read high has for some reason an unusual susceptibility to this
> stray voltage component.
> Hue Miller
>
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