Re: MKIV CD noise analysis

[Tom Droege <droege@wwa.com>]

Herb and all,

When I took the data, there were two VGA monitors about 10' away.  There
are no switching power supplies except the ones in the computers, again
about 10' away.

I have now moved the Mark IV to the roof, where it is not 20' or so away
from everything.  Again, no switching power supplies near.  No computers
either.

The only noise makers on the pc boards are the scan clocks.  The main one,
the horizontal scan clock, runs at 3.088 MHz.  The vertical clock in not
synchronous with anything and runs in the 10-20 KHz region.  I will have to
measure it.  It is one of the more probable sources of noise.  Were I to do
all this again, I might count it down from the horizontal clock. 

Well, there is also the Stamp, but I have carefully isolated it.  

Perfection Mica Co.  makes some nice material for wrapping monitors and
stuff.  Mu metal foil.  Very fussy stuff but it can work wonders.

Tom Droege

At 12:13 PM 3/10/99 -0500, you wrote:
(snip)
>Tom, what kind of PC *monitor* are you using? Note these monitor
>frequencies:
>                          Horiz kHz   Vertical Hz
>MonoGraphics monitor:     18.432           50
>Color Graphics monitor:   15.750           60
>EGA monitor:              15.7 to 21.8     60
>VGA monitor:              31.5             50 to 70
>
>Another possible source would be the various switching power supplies.
>Tom could, I suppose, make a wire loop on his oscilloscope probe
>and hang it on his monitor or PC power supply and see what he can
>sync up as the strongest signal components. I don't think the switching
>supplies are running below a few hundred kiloHertz these days. The
>monitor's switching (?) supply could be most anything.
>
>As for the computer monitor, I suppose you can make a basket out of
>chicken wire, "enclose" the monitor, and ground the basket. Some of
>the old "Tempest" military equipment designed for minimum emissions was
>simply commercial equipment wrapped in copper screening, so this is
>not as hokey a solution as it sounds.
>
>One of the pleasures of this newsgroup is how well it explores these
>kinds of issues. Few books or articles even mention this stuff.
>
>Herb Johnson
>
>Herbert R. Johnson              http://pluto.njcc.com/~hjohnson
>hjohnson@pluto.njcc.com         voice 609-771-1503, New Jersey USA
>             amateur astronomer and astro-tour guide
>     S-100 computer restoration, parts, manuals as "Dr. S-100"
>   rebuilder/reseller of compact Macs for your computing pleasure
>
>
>