http://www.ieee-pses.org/symposium http://www.emc2004.org/ -------------------------------------------------- On 7/8/2004, Howard Ji wrote:
I need to inject/induce a 60Hz common mode signal with a p-p voltage of around 100V onto a RJ11 cable. I have signal generator, amplifier and some coupling devices but none of my stuff works at 60 Hz. Any information would be appreciated. Hi Howard: I have done this type of test on analog modems and similar interfaces for voice circuits, and even some VOIP phones because there are some installation configurations that will present a common mode voltage equal to one half the AC mains supply voltage across the isolation barrier. This translates to 60 VRMS common mode in North America and 120 VRMS in countries with 240 VRMS mains. The result can be audible hum and/or impaired modem performance. For these situations there is no intentional path to ground through the line interface, so your generator does not have to deliver much current at all. In the case of an analog modem, my procedure is as follows: 1) Provide DC loop feed on the phone line using a feed circuit with an isolated DC supply. Some DC lab supplies are inherently isolated (my favorite is the old HP 6212 linear bench supply), but you can isolate any lab supply using an appropriate isolation transformer. 2) Connect a conventional variac to the AC mains through an isolation transformer, so that the output is completely balanced and floating with respect to earth. 3) Connect one side of the isolated variac output to earth ground. Connect the other side to the tip/ring pair of the phone line using a Y configuration consisting of a single 2.2 uF capacitor with a 10 K resistor to tip and a 10 K resistor to ring. Remember, the load impedance is typically in excess of a megohm, so your source impedance can be 10 K without causing problems. 4) Lastly, connect the low voltage side of the modem (the side with the RS-232 port) to earth ground. Now, when you gradually increase the output voltage of the variac, you are applying a balanced 60 Hz common mode voltage on tip/ring (common mode with respect to earth ground). The impedance you are bridging across tip/ring is 20 K, which is high enough not to disturb 600 ohm circuits. There are other ways to simulate the same effect, but this is the one that I use most often. This method would not work for a typical FXS or SLIC type feed circuit, since these circuits are typically ground referenced and significant common mode current would flow. However, when the interfaces being tested have an intentional isolation barrier, this method works well. If you decide to try something like this, be sure to draw an equivalent schematic of the complete system and think through how your setup will work. It is easy to overlook something and end up with a test that doesn't really do what you want. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com <http://www.randolph-telecom.com/> ------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. IEEE PSES Main Website: http://www.ieee-pses.org/ To post a message send your e-mail to [email protected] Instructions for use of the list server: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
