lpcware

LPC4357 and LCD radiated emissions

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Aug 9, 2016 by rawsewage
Content originally posted in LPCWare by briching on Mon Jan 25 13:20:55 MST 2016
Our product has made it into the first round of EMC (Electro-Magnetic Compatability) and has failed radiated emissions testing.  We think we have proven it is the LCD display, as once we disconnect and power-down the display, everything passes.  On a long-shot, we wanted to see if there is anyone out there who is also having a hard time getting through EMC, and perhaps learn from what they have done to get their LCD in the system to pass.

Our setup involves a Newhaven 7" capacitive touch display running at 60p 16bpp(565), driven by an LPC4357 module that the LCD signals come out of a SODIMM connector similar to the EA OEM board, which the lower board adapts to a 40-pin FPC connector for the LCD.  Any individual LCD signal spanning both boards is not longer than 3 inches before the ribbon cable.  All signals are matched in length to within several millimeters.  However, the traces have not been routed as controlled impedance.  Note that the ribbon cable is longer than 3 inches itself.

Our pixel clock is running at 29.1 MHz, and therefore the data bus is running at 14.55 MHz.  We see this 14.55 MHz fundamental, as well as odd harmonics (3n, 5n, 7n, etc.) on a spectrum analyzer (at 1 meter from the DUT) when the LCD data bus is toggling all common-mode (i.e. 0xFFFF to 0x0000).  Differential mode (0xAAAA to 0x5555) drastically tames the emissions, as we suspect return paths are utilized between every other bit.  Common mode datapath toggling by far exhibits the worst emissions.  We can tame the common-mode emissions down with a ferrite core on the LCD ribbon, but we are still a ways away from passing.  Adding some copper foil tape on the LCD ribbon helps even more, but we are wondering if there is something fundamentally wrong with the LCD ribbon not having enough ground return paths.  The Newhaven ribbon pinout is quite horrible from an EMI perspective, as the entire 24-bit data bus is referenced to only 2 ground pins, one on either side of the 24-bit bus.  It sure would have been nice of them to throw in several more ground pins.

All the usual precautions in the design were taken, like a full metal enclosure, embracing ground, careful floorplanning, decoupling capacitors where they should be, etc.  We can also verify power and signal integrity of many of the signals in question with a scope.  Overshoots and undershoots on the LCD databus are pretty minimal, and power supply integrity fluctuates by several 10's of millivolts in the worst case around the vicinity of our LPC4357.

Our next design cycle will likely involve moving the LCD connector directly up to the board that has the LPC4357.  But I thought I'd at least ask around to see if anyone has experience with a 7" display being the tall pole in their compliance testing. 

Anyone out there have any ideas on how we can lower our radiated emissions using NXP and Newhaven displays?

Thanks,
Brad

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