In a message dated 10/8/2002, Alex McNeil writes:
> As I am in panic mode, is there some guidance out there for me e.g. is it > usually a matter of changing the interface IC's or hanging special > components onto the lines? > Hi Alex: This test is indeed a difficult one to pass, but in general the transformer-type interfaces that you are dealing with are less susceptible than some other types. I have seen ISDN and Ethernet interfaces pass this test without the need for extreme measures. In the absence of more specific information about your situation, I can offer just a few general guidelines: 1) EN 55024 allows you to use either a CDN or a clamp to induce the RF. Some people find that they get more favorable results with the clamp than with the CDN. 2) The interference is applied common mode. In *theory* your transformer coupled interfaces will have fairly good common mode rejection, but this can be compromised by a bad layout or by some types of surge protection devices. 3) If you have to add some filtering to pass, adding a common mode choke is a good first step toward reducing susceptibility. Adding caps from each conductor to chassis ground also helps, especially if used in conjunction with a common mode choke, with the choke on the line side of the caps. Note, however, that high value caps can interfere with proper operation of the circuit. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
