[SI-LIST] Getting radiation from SOC signal to an HDMI driver. Can't explain the mechanism.
- From: miner_tom Cipollone <tom_cip_11551@xxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 11 Oct 2012 18:54:43 +0000
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
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