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
