Max,
PCs have been subject to close US and German scrutiny since at least 1980.
That has forced them to adopt very good shielding practices including
direct connection of the shields to the cabinet in a low inpedance
fashion. Anyone in the industrial controls area who needs to get up to
speed on how to meet EMC requirements on products which incorporate high
speed digital electronic processing capabilities should go to school on
the PC on thier desk. Examining a name brand PC is one way to get up to
speed on good shielding technique real fast.
Is good shielding enough? If you are using digital signals - 0-5V - 1's
and 0's - then it's pretty tough for the continuous RF tests to disturb
you with reasonable shielding technique. ESD and EFT may still cause a
problem, but usually not if the shieling is continuous. Shielding is
often not enough though when trying to protect thermocouple, strain gages,
or RTDs. With these devices a driving circuit applies a low level signal
to the transducer and a receiving portion looks for a very small change in
the resulting signal (mV) to indicate temperture rise or whatever. These
receivers and drivers are typically op amp circuits with multiple stages
to correct for non-linearities before the final A/D stage. These must be
designed to maximize common mode rejection and their ground reference must
be designed to be well behaved. A good shield will give you about 40dB of
surpression and that is frequently just not enough.
Jon D. Curtis, PE
Curtis-Straus LLC j...@world.std.com
One-Stop Laboratory for EMC, Product Safety and Telecom
527 Great Road voice (508) 486-8880
Littleton, MA 01460 fax (508) 486-8828
http://world.std.com/~csweb
On Thu, 13 Feb 1997, Max wrote:
>
> Jon,
>
> That's great information--I also anticipate a requirement for heavy
> industrial immunity in the future and have been wondering what problems I
> might be in for.
>
> With PCs (and computers in general), isn't it the case that if the cables are
> shielded and grounded to the cabinet there isn't likely to be a problem?
>
> For emissions, BTW, I have also had good luck with DEC.
>
> Max Kelson
> mkel...@es.com
>
>
> %>
> %>I have tested systems to the heavy industrial immunity specification which
> %>included class B PCs. Both HP Vectra computers and Dell computers faired
> %>well. Ocassionally the monitors sold with these systems are disturbed to
> %>the point of turning themselves off (a failure in most books). To date
> %>I've always been able to solve this problem by upgrading to an NEC
> %>multisync monitor. The key distinquinction of all these products is that
> %>they really do meet class B by wide margins and use very good shielding to
> %>get to that level. Once you have shielding that good and use digital
> %>techniques inside (as opposed to small signal, high impedance analog
> %>signals - thermocouples, etc.) heavy industrial immunity compliance is
> %>usually a given.
> %>
> %>Jon D. Curtis, PE
> %>
> %>Curtis-Straus LLC j...@world.std.com
> %>One-Stop Laboratory for EMC, Product Safety and Telecom
> %>527 Great Road voice (508) 486-8880
> %>Littleton, MA 01460 fax (508) 486-8828
> %>http://world.std.com/~csweb
> %>On Wed, 12 Feb 1997, Tony Fredriksson wrote:
> %>
> %>>
>
