[email protected] wrote (in <OFDAC57894.50D17ACD- [email protected]>) about 'What is wrong in my 3 meters FAC measurement', on Thu, 25 Mar 2004: >You wrote "The bottom line is the common mode RF on the power leads. >This needs to be fixed. Bypassing the + and - directly to the chassis at >the chassis wall may help." > 'By passing' means connecting the + & - of the power supply to >the shield of the cable through suitable RC combination near to the EUT?
No. See below. > The EUT chasis is of plastic material coated with metal. This >metalic coating is not having any electrical connection with the EUT. Well it should have. Being not grounded, it acts as an antenna for any r.f that is generated internally. >When the shielded cable was used to power the EUT, d.c negative (digital >ground) of the EUT was connected to the shield of the cable to get a >pass result. In the power supply d.c negative is connected to chasis >earth. Keep that connection between d.c. negative and shield, and also connect the metal coating to d.c. negative. Then put a *low-inductance* capacitor from the + of the power supply to d.c. negative at the point where the + supply enters the enclosure. You may need more than one capacitor in parallel, depending on what frequencies are being emitted by the cable, but you should try single values from 1 nF to 10 nF initially. You probably don't need to put any resistor in series with the capacitor, but sometimes 50 to 150 ohms does make an improvement. This is not predictable; you have to experiment. Make sure, from the capacitor data sheets, that the self-resonant frequencies of your capacitors are much higher than the frequencies being emitted. -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
