I ran into this a couple years ago when a customer of my employer was having trouble with EMI testing of a product we manufactured. Their cables did not have the external shield on the HDMI cable bonded to the connector. A little copper tape proved the source of the problem.
Now, for a kicker… The HDMI committee specification for the cables does not address termination of the outer shield of the cable. These faulty (from an EMC perspective) cables were entirely compliant with the HDMI cable specification. The customer went to a local computer store and bought a selection of cables. Some had the outer shield terminated properly (from an EMC perspective) and some did not. Price was no indicator of the termination status of the shield. I pushed back through our product team, but as of when I retired in June of last year I had heard of no resolution to this issue. Buyer beware… Ghery S. Pettit, NCE Pettit EMC Consulting LLC [email protected] From: Ken Wyatt [mailto:[email protected]] Sent: Monday, October 24, 2016 7:29 AM To: [email protected] Subject: Re: [PSES] HDMI questions I’d like to reinforce James’ point on the poor shield bonding on many mid- to low-priced HDMI cables. Usually. it’s just a 1” pigtail, which can easily cause EMI failures. Because brands and cable models come and go, I always suggest clients buy at least two of everything and cut one apart to physically inspect the shield termination. Ken _______________________ I'm here to help you succeed! Feel free to call or email with any questions related to EMC or EMI troubleshooting - at no obligation. I'm always happy to help! Kenneth Wyatt Wyatt Technical Services LLC 56 Aspen Dr. Woodland Park, CO 80863 Phone: (719) 310-5418 Email Me! <mailto:[email protected]> | Web Site <http://www.emc-seminars.com> | Blog <http://design-4-emc.com/> The EMC Blog (EDN) <http://www.edn.com/blog/The-EMC-Blog> Subscribe to Newsletter <http://www.emc-seminars.com/Newsletter/Newsletter.html> Connect with me on LinkedIn <http://www.linkedin.com/in/kennethwyatt> On Oct 24, 2016, at 7:16 AM, Pawson, James <[email protected]> wrote: Ah, hello 742.5MHz my old friend. Alfred’s analysis in his email is good especially regarding frame rate. If you look in the 1.4 HDMI standard in section 5.1.2 you can see that the “vertical blanking” period consists of 45 lines. A breakdown of what’s contained within follows below that section. You can conduct further analysis of this period by using an FFT function on a high bandwidth digital scope (although you may struggle with the lower number of samples available in this target block) to narrow in on which part of the signal is causing you issues. As for the frequency: 1080p/50Hz resolution uses a clock frequency of 148.5MHz. The three data pairs frequency is 5 x f_clk = 742.5MHz. Expect to see multiples of the clock at 445.5, 594 and 891MHz and plenty of emissions above 1GHz at 1485, 2227.5, 2970MHz, etc. 1080i/50Hz and 720p/50Hz resolution uses clock of 74.25MHz and data of 371.25 MHz It is this data fundamental that causes many of the radiated emissions problems associated with HDMI. This can be caused by poor signal integrity, mainly uneven tr/tf within each half of the differential pair. Possible solutions involve playing around with the pre-emphasis and drive strength register settings within the silicon, remembering to verify the signal integrity if you’ve found a combination that seems to work. Inline common mode chokes sometimes help at frequencies above 1GHz but more often not at these “lower” frequencies in the HDMI interface. Your best bet is carefully selecting a HDMI cable that has good shielding termination between cable screen and backshell. This is mostly a case of trial and error when purchasing from catalogs (forget eBay or Amazon here) or working with the cable vendor (e.g. Molex, Amphenol, Palpilot – other reputable cable vendors are available) to specify a decent quality cable. Cut open a $1 cable from Amazon and marvel at the internal construction! I hope this helps, Best of luck James From: Brent DeWitt [mailto:[email protected]] Sent: 15 October 2016 23:58 To: [email protected] Subject: [PSES] HDMI questions I’ve been working with some Silicon Images (Lattice Semi) 9777 multiplexer chips lately and would appreciate any insight list members might have. When used at any resolution below 4k, there seems to be a 10 dB emission “pedestal” that stands out of the baseline emission at several frequencies, 742 MHz in particular. The pedestal is 666 usec long and repeats at whatever frame rate is selected. At first, I thought it was correlated with the SPI bus activity, since the timing was identical, but further experiments show that not to be true. My question is, is this inherent to HDMI, or to the 9777? As is probably obvious, I don’t have a whole lot of experience with HDMI video. Thanks all! 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