[Milsurplus] Shielding Those Neat DC-DC Converters

[David Stinson]

DC to DC Converter technology continues to
advance.  These tiny devices have the potential to
power our boatanchor radios without heavy iron and
rectifier heat.  Very handy devices, but with one
big down-side:  RF Noise.  These are switching
supplies and the switching operation creates a lot
of RF hash; you can't deploy one "naked."  But
defeating the noise is not as hard as some might
think.

Some folks have asked me to write in more detail
about using the inexpensive little DC-DC
converters from "the 'Bay" as regulated receiver
B+ supplies.   I like those similar to this one:
https://photos.app.goo.gl/cCfkeOz0l7qUUDsi1
which can input 8 to 32 Volts and deliver 45-390
Volts at 40 Watts continuous (need cooling at that
level) and 70 Watts intermittent, peak about 200
milliAmps.   When used at 50-70 mills for a
receiver, I haven't found cooling necessary.  Shop
around for the best price plus shipping;  prices
for them are all over the place.  There's a new
type that provides both regulated B+ and regulated
filament voltages, but I haven't tried that one
yet.  To find them, search your favorite auction
place for "DC-DC High Voltage."

I put one "naked" on my bench with an Anritsu
service monitor to take a look at the noise.  28 V
in, 110 V out, with the probe loose-coupled an
inch away from the supply.  The Noise Floor of the
Spectrum Analyzer in this configuration is about
-90 dBm.  The sweep is from 500KC to 18MC.  The
yellow trace is instantaneous level.  The green
trace is peak-hold.   You can see the big, square
switching pulses creating a noise level of -50 to
-60 in this configuration:
https://photos.app.goo.gl/etvGkuVLraXA9lJM2

Used an inexpensive "cookie" tin to build a simple
shielding box.  Cleaned paint and coatings away
with a rotary wire brush on my drill.  Mounted
three feed-through capacitors: A+ In, B+ Out and
A-B- Common:
https://photos.app.goo.gl/FvpEf9ydWceL84Wf1

Here's a diagram of  the build, using "Pi"
low-pass filtering to kill the noise.  Be careful
that your connections and polarity are correct:
https://photos.app.goo.gl/ieoYogAz8agg6jkP2
Yes; I recommend the A-B- Common connection also
be filtered, even though it gets connected to
ground.  I got away with not doing that on my
first build and it worked well, but this one is
even quieter so it's worth the extra parts IMHO.
The coils are unknown toroids from the junk box
and I used the Anritsu to sort "noise ferrites"
from "RF ferrites;"  the "noise" type kill noise
better, of course:
https://photos.app.goo.gl/VclxSUC11QtoBeIU2
The caps are also from the junk box.   .1 to .05
uFd worked fine .  The feed-thru caps help a lot
in keeping the "crud" in the box.

Sat the little board on a piece of plastic to
insulate it and secured the stack to the can with
a dollop of "Shoe Goo" glue (I think you could
patch a torpedo hole in a battleship with that
stuff). Here's the complete build:
https://photos.app.goo.gl/Nnedsb3xwrDCNpTi2
No; it's not pretty, but at my age, time is
precious and "function" trumps "form."  

I powered the supply *without closing* the top of
the box and looked at the spectrum, coupled as it
was in the first test.   The noise pulses over the
mid-HF band are down 20 dBm vs. the "naked" test:
https://photos.app.goo.gl/DPP1UmaqasKxOh8D3
With the lid installed to complete the shield and
the probe placed right on top of the box over the
switching inductor, coupled closer than the other
two tests, the RF noise has disappeared into the
noise floor under -90:
https://photos.app.goo.gl/Oq7hg93idZGgVjOs1
Ready to go power an RAX receiver for me.

The whole project took about an hour to build.
$7-10 bucks and a handful of junk box parts, plus
a "scrap" box and the RAX is happy, and stable.
Give this a try- I think you'll like it.

GL OM ES 73 DE Dave AB5S