I find this a bit amusing, as we have a desktop PC that we use to test our modems, and the PC emissions actually go down when we put the monitor on top of the PC! This is the only way that this PC would pass Class B emissions. We believe that the monitor puts some weight onto the finger stock to create a better connection.
Wayne Cooke Regulatory Engineer, 3Com Corp. email: [email protected] "Allan, James" <[email protected]> on 09/03/98 01:00:43 PM Please respond to "Allan, James" <[email protected]> To: "'emc-pstc @ieee.org'" <[email protected]> cc: (Wayne Cooke/MW/US/3Com) Subject: RE: Recommendations for PCs Funny how the same experience can lead to different conclusions. Like you, I have found that desktop systems do radiated more with the monitor on top than with it beside the unit. At that time we did extensive tests to find out why because our pc was well designed with lots of conductive fingerstock at strategic points. By carefully substituting passive weights and non conductive platforms into the test we found that the problem had nothing to do with seams opening up. The problem was ordinary old di/dt on the monitor signal cable. As the monitor came into close proximity to the pc the capacitive coupling between the monitor and pc increased and thus the common mode current on the video cable increased. The cable made a real good loop antenna. I'll bet if you look closely at your results you will find that the problem frequencies were video dot rate related. I might add that this was several years ago and things might be different in pc land today. I only present this to show that popular beliefs may or may not ALWAYS hold up to scientific examination. Jim -----Original Message----- From: CTL [SMTP:[email protected]] Sent: Thursday, September 03, 1998 9:25 AM To: Mark Briggs; emc-pstc Subject: Re: Recommendations for PCs I have had the same experience with desktops. All desktop cabinets cannot support the weight of monitors, especially 15" and larger. -----Original Message----- From: Mark Briggs <[email protected]> To: emc-pstc <[email protected]> Date: Wednesday, September 02, 1998 7:48 PM Subject: RE: Recommendations for PCs >One other point to bear in mind - towers often prove to be better hosts >than an equivalent desktop version. The FCC requires that the monitor be >placed on top of the PC if it is a desktop unit in order to re-create a >typical operating condition (refer fig 11 of ANSI C63.4). I have seen >several PCs which are ok UNTIL the monitor is placed on top of the host >and causes the chassis to flex, reducing the shielding effectiveness of >the host and letting the higher frequencies out. > >Mark > > >> -----Original Message----- >>From: rbusche [SMTP:[email protected]] >>Sent: Wednesday, September 02, 1998 8:01 AM >>To: emc-pstc >>Subject: Recommendations for PCs >> >> ---------------------------------------------------- >>We are in need of a PC to perform EMC testing in support of FCC and >>CISPR limits. We are looking for the latest, state of the art, system >>with at least 400 MHz processors. Can someone suggest to me (off line if >>desired) a good candidate for this application? It is surprising to me >>the number of quality systems that fail even BEFORE additional cards are >>added. >> >>Thanks in advance >> >>Rick Busche >>Evans & Sutherland >>Salt Lake City, Utah >>[email protected] >> >> --------- >>This message is coming from the emc-pstc discussion list. >>To cancel your subscription, send mail to [email protected] >>with the single line: "unsubscribe emc-pstc" (without the >>quotes). For help, send mail to [email protected], >>[email protected], or [email protected] (the list >>administrators). >> >> >>--------- >>This message is coming from the emc-pstc discussion list. >>To cancel your subscription, send mail to [email protected] >>with the single line: "unsubscribe emc-pstc" (without the >>quotes). For help, send mail to [email protected], >>[email protected], or [email protected] (the list >>administrators). >> > > >Mark Briggs >[email protected] > > >--------- >This message is coming from the emc-pstc discussion list. >To cancel your subscription, send mail to [email protected] >with the single line: "unsubscribe emc-pstc" (without the >quotes). For help, send mail to [email protected], >[email protected], or [email protected] (the list >administrators). > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], or [email protected] (the list administrators).
