Thanks Ken, I do vaguely remember reading that normative section before but I've always had products where the cables have included an AC port (or connection to a power plant) and/or 'long' IO metallic interfaces which in essence mandates the 150kHz start frequency. Andy
___________________________ From: ext Ken Javor [mailto:[email protected]] Sent: Wednesday, November 19, 2003 8:59 AM To: White Andy (NMP/SanDiego); [email protected]; [email protected] Subject: Re: Conducted immunity of a USB port My 1996 release of IEC 1000-4-6 has an informative Appendix B that states the start frequency is the speed of light divided by the sum of the cable length and EUT size. If people are not doing this for short cable runs, they are over-testing at low frequencies by orders of magnitude. From: [email protected] List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Wed, 19 Nov 2003 08:20:50 -0800 To: <[email protected]>, <[email protected]>, <[email protected]> Subject: RE: Conducted immunity of a USB port The EM clamp can be used but the preferred method would be using a CDN or direct injection to the screen. I could not find the reference to start the testing at higher than 150kHz within 61000-4-6. [Perhaps I missed that bit?] I am surprised that the CDN for the screen did not work as I have tested 100bT Ethernet screened cable and T1 screened cables using suitable CDNs to inject on to the screen. The functionality of the port was not degraded by the CDN. I would verify/re-check your 'home made' adapters. Andy ___________________________ Andy White EMC Engineer Nokia, San Diego ___________________________ From: [email protected] [mailto:[email protected]]On Behalf Of ext Ken Javor Sent: Tuesday, November 18, 2003 7:32 PM To: Ton Bouw; [email protected] Subject: Re: Conducted immunity of a USB port I can only answer parts of this question. You should test for the maximum length the USB interface could attain, but since that length is 5 meters in this case, the limit lower frequency should be tailored upwards to account for that. There is no use testing at 150 kHz on such a short cable. This is right out of IEC 61000-4-6. If CDN design presents obstacles to testing high speed USB, I think an EM clamp is a better approach. In fact, I believe an EM clamp is a better approach for any shielded cable. The point of a CDN is to inject a signal into an equipment connector pin. When a cable is shielded, the mechanism for getting the rf signal on a pin is the transfer impedance of the shield. In order to test that, you need to get current flowing down the length of the shield. I think an EM clamp is a better mechanism for achieving that. From: Ton Bouw <[email protected]> Reply-To: Ton Bouw <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Tue, 18 Nov 2003 23:51:07 +0100 To: [email protected] Subject: Conducted immunity of a USB port Dear group, I have the following questions on testing a USB port for conducted immunity 1) USB extension cables are available in the consumer market. The USB 1.0 spec limits the cable length to 5.0m. Therefor I believe that the conducted immunity test is required even if the cable length of the DUT is less than 3m, unles a restriction is made in the user manual.. Is that correct? 2) I tested conducted immunity using a CDN-S9 with home-made adaptors from DB9 to USB using 4 pins of the DB-9 and the screen connection. I assume that this should give a representative result? For information: this does not work for hi-speed USB because of the LPF in the CDN. 3) Are CDN's for USB available? Or is it better to use an EM clamp? kind regards, Ton Bouw
