[ARC5] Shielding Those Neat DC-DC Converters

[Peter Gottlieb]

Excellent job!  I enjoyed your writeup and photos.

Peter
kb2vtl


On 3/13/2018 6:18 PM, David Stinson wrote:
> 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
>
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