[SI-LIST] Re: Getting radiation from SOC signal to an HDMI driver. Can't explain the mechanism.
- From: "Kenneth W. Egan" <kegan@xxxxxxxxxxxxx>
- To: "minertom11551@xxxxxxxxx" <minertom11551@xxxxxxxxx>
- Date: Thu, 11 Oct 2012 18:00:27 -0500
HDMI has a clock embedded during the video sync periods on the data channels.
This would be at 10x the HDMI_CLK line.
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On Oct 11, 2012, at 1:48 PM, Tom Cipollone <minertom11551@xxxxxxxxx> wrote:
> Hi,
> I am getting a fair amount of radiation from a point between a highly
> integrated system on a chip (SOC) and an HDMI driver IC. I have looked at
> the layout in this area and can't find anything that I can easily say is
> wrong. Microstrip over a ground plane. A bus carrying HDMI data (not TMDS
> yes) and a reference clock is being driven from the SOC to the HDMI
> converter/driver.
>
> I have run scans in an EMI lab many times and know that the frequencies are
> real, which relate to the HDMI video resolution of 1080i (approximately 741
> MHz and 370 MHz).
>
> "Near field" scans, done with an electric or magnetic probe attached to a
> spectrum analyzer, show that the *most* radiation comes from a series
> termination resistor on the clock line from the SOC to the HDMI driver. Of
> course, the clock is at 1/10 of the data switching frequency.
>
> Yes, this is odd. I have 741 MHz radiation from a clock running at 74 MHz.
> I can come up with some "mental" scenarios about crosstalk from the
> data lines getting onto the clock line but I have trouble believing them.
> The radiation has to be coming from something switching at 741 MHz and by
> some mechanism the high intensity point of that radiation is coming from
> the 74 MHz clock line, at the series termination resistor.
>
> Any ideas about this?
>
> Thank You
> Tom
>
>
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