[RFI] Car laptop adaptors

[Bryan Swadener]

Chuck Hutton wrote:
> I suppose this is as good a place as any to ask
> whether anyone knows of a
> particular model of adaptor that will allow my 18V
> laptop to run off of 12
> VDC without creating RFI. A few friends have had RFI
> trouble on DXpeditions
> where a laptop was a necessity to run an SDR. Rather
> than playing hit and
> miss on eBay, I thought I'd ask here.
> 
> Chuck
> WD4ELO

Hi Chuck,

I know of no commercially available DC-DC boost
converters that claim to be quiet and produce the
voltage/current you need.  We've been using this
(http://tinyurl.com/2hv8dz) unit that was designed and
built by a coworker who is very good with switching
power supplies. It's housed in a cast aluminum box and
as you can see from the diagram, it utilizes
decoupling on the I/O paths. It was sniffed w/ a
spectrum analyzer and though I don't recall the
figures, the switching noise was very low.  Use of it
confirmed it appeared to be dead-quiet.  Though it
produces 16V (4.5A max), it'd be easy enough to tweak
the feedback resistors (R10/R11) to get 18V.

My coworker also came up with a boost converter to get
13.8V @ 20A from the 4 x Group27 car battery "pack" we
were using.  It also works flawlessly (I see MFJ now
produces a MFJ4416.)  I don't see that they produce a
unit suitable for your needs.

73,
Bryan WA7PRC

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