Protecting your circuits from RF interference is getting to be a big deal. I calLED on a LAN VOIP phone maker when I worked for National S EMI . They had interference getting into their audio circuit from a cell phone set down a foot away from their offICe phone. It took some board redesign and the addition of small caPACitors on critical nodes in order to keep the cell phone RF out of the analog audio signal path. And one mistake engineers make is using big-value physically-big capacitors. You are trying to fiLTEr out RF at 900, 1,800 or 1,900 MHz. You need picofarads in a tiny 0402 or smaller case in order to have low impedance at those frequencies—be sure to look at the impedance plots for whatever cap you are thinking about using. Better yet, use parts like Semtech makes and rest assured that the parts will keep interfering RF signals out of your circuits.

　　I was trying to filter out RF form an office phone, if you are designing a cell phone or handhELD it is even tougher because then you have inches, not feet of separation. Your product might have Bluetooth, 802.11 as well as camera and cell phone systEMS. KeePINg the RF from all those interfering sources out of your circuitry CAN be tough. Like most EMI issues, this one comes up right before you want to ship, so be sure to have an understanding of these issues; they are coming up more and more often.

　　英文原文地址： http://www.edn.com/blog/1700000170/post/870011687.html

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