[SI-LIST] : EMC quiet serial device: Followup???

Roland F. Portman ([email protected])
Tue, 09 Feb 1999 15:58:26 -0800


This is to add some clarification to my earlier posting.

The modem is the problem. When powered off the emissions went away.
We have -13dB with the modem off, +7dB with the modem on.
We tried different serial cables with no change. We disconnected
the serial cable and powered up the modem - the problem persisted.
The noise is coming from modem and coupling onto
the small twin-lead power cord to the power brick (120VAC to 9VAC)

1. The modem is marked FCC Class B compliant. (sure ;-)
2. A mouse is required in addition to a separate serial device.
3. We can't add ferite beads to the modem power cord to try and make the
system pass.
4. Without the modem we have lots of margin even with our product installed in
the PC.

What I really need is some suggestions for replacement serial devices.
So far I have received

Palm III
Wacom Intuos graphics tablet
Aiptek Hyperpen 5000 tablet

Other suggestions would be greatly appreciated.



This group may not be the best place to ask for this type of information, but
I am sure many of you
are involved in the area of EMC besides SI.

I have a new product which resides inside a workstation (mini-tower). The PC
and our product within
passes FCC/CE class B with plenty of margin, however the standard requires
that a 'Typical System' pass Class B.
This system includes not only the mini-tower, monitor, keyboard, and mouse,
but also a perpherial connected
to the parallel port (a printer in our case) and a perpherial connected to the
serial port (an external modem in our case). We have tried two different
modems. Both cause us to fail Class-B. If we turn them off the problem
goes away.

Can someone suggest a serial device (mfg and model) which they know to be EMC

Your help is greatly appreciated. For those of you who are similarly
interested, I will post a single summary
to this new-group.

Thanks in advance,


Roland F. Portman
BiT Microsystems, Inc.
48499 Milmont Drive
Fremont, CA  94538

(510) 623-2341 Ext 129 (510) 623-2342 Fax

[email protected] http://www.bitmicro.com

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