Newsgroups: rec.radio.amateur.misc

> While clocks and other digital hash are a problem with computers, the
> primary culprits are usually the very strong VLF deflection *pulses* that
> the monitor uses to scan the screen. These several *watt* pulses radiate
> harmonics across a broad spectrum. Thus the new low field monitors *do*
> make a marked difference in interference levels since the way they reduce
> the fields is to increase the overall effectiveness of the shielding.
> 

Sometimes.  However, if you're getting interference in the VHF regions or
above (basically anything above 30 MHz or so), I would very much doubt that
the monitor deflection is the culprit.  (Yes, these are some very strong
pulses, but their fundamental frequency is typically around 30-40 kHz, and
is VERY rarely higher than 80 kHz - and that only for high-res (1280x1024)
displays.  Also, the primary mode of radiation here is pure magnetic field -
meaning that it's coupling into the radio inductively, and also that 
"shielding" in the conventional sense will do very little.

The new "low field" monitors (look for units which are labelled or advertised
as complying with the "MPR" requirements - which is the Swedish ELF/VLF 
electric and magnetic field standard, among other things) do NOT obtain this
improved performance in low-frequency E and M fields through conventional
shielding.  Magnetic emissions are generally held down by better yoke designs
and the use of cancellation coils, while electric fields - usally a problem
at the CRT face - are controlled by grounded conductive coatings (usually
an indium/tin oxide coating, or "ITO") on the CRT face.   The degree of
high-frequency EMI control (via shields or whatever) in a monitor does NOT
indicate how well it performs in these lower-frequency aspects.

> >Fortunately, Macs are best-sheilded and filtered computers available. Perfect
> >for the ham radio user. 
> 
> The etch-a-sketch, er Mac, is fairly well shielded, but is not anywhere as
> effectively shielded as a Sun or AT&T 386WGS. The latter has a double metal
> case and finger stock at all the joins. The Sun 386i is also double shielded.
> I keep my scanner sitting on the Sun with no hint of interference. The Mac
> depends on coated plastic and a couple of flimsy shield plates. It *is* a
> tight box though, as many people who have had them overheat will attest.

Are you talking about monitors here or the shielding of the "processor box"
itself?  With the Mac, these two aren't separable.  Keeping a scanner "sitting
on the Sun" without interference might not be the same as "sitting on the Sun's
monitor" without interference.  Also, "flimsy" is not a concern in "shield
plates," assuming that they're not also structural parts.  In addition, I'm
not sure that I'd want to predict EMI performance based on a visual inspection;
there have been a number of boxes in my experience which sure LOOKED good -
heavy metal boxes, etc. - but which were sure lousy on the range.  As an
additional example - I have recently come across a monitor design which uses
no sheet metal at all, except around the video amplifier board (which is where
all the nasty HF/VHF stuff happens), and did quite well in EMI checks.  I'd
tend to rely more on the manufacturer's certification (in absence of the
ability to haul the thing out to a test range myself); if, for example,
the manufacturer claims compliance with the new European (CISPR) "B" levels,
it's a pretty quiet system.


Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
                   User Interface Tech. Div.|those of my employer or any other
myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.


Newsgroups: rec.radio.amateur.misc