Just to add to this discussion, I’ve found that bought cables have a wide variety of end-connector to end-connector resistances – anywhere between 4-100M ohm down to 30-50 milli ohm.
Assuming the cable is 1-2 m long, 30-50 milliohm cable impedance would rule the cable out from being the problem. Measure resistance using a 4-port meter, which can be bought quite cheaply and this one works well, https://www.amazon.co.uk/Milliohm-Milli-ohm-Resistance-Multimeter-Accuracy/dp/B08WTQRCF5 Best regards Charlie Charlie Blackham Sulis Consultants Limited Tel: +44 (0)7946 624317 Web: https://sulisconsultants.com/ Registered in England and Wales, number 05466247 From: James Pawson (U3C) <[email protected]> Sent: 16 May 2025 09:25 To: [email protected] Subject: Re: [PSES] HDMI cable emissions Hi Peter, Cable quality is important, mostly in terms of terminating the shield well to the connector backshell. Good quality off the shelf cables are hard to find, particularly from low cost online re-sellers. We went through many samples of trial and error before we found some that worked well. What is the source of the noise? Mostly imbalance in the differential signal. Uneven rise/fall times, unequal duty cycles within the pairs, amplitude differences, impedance differences on lines… it all accrues to give pulses of common mode current mostly aligned with the rising / falling edges of the signals. You’ll see harmonics of the clock signal, I remember 741.16MHz being particularly troublesome. Frequencies will change significantly with mode (480i/576i, 720p, 1080i, 1080p) and slightly with frame rate (50/60Hz) Fixing the imbalance can be very difficult. In many cases it is baked in to the driver silicon and there is not much that can be done. Ted Eckert’s comment about PCB layout is worth taking note of. The quality of the sink and how it handles the return of the CM currents is also key. Try removing the sink from the test chamber via a long cable or wrapping it in tinfoil and terminating the shield to the cable shield. I did make a proper self powered TMDS + EDID termination for EMC testing of HDMI signals and it still does occasional duty in our lab. It pretended to be a 4k Samsung 5” TV but in reality only had one pixel – the little light to tell you it was on. I really should redesign it and make them more widely available. Connecting cable screen with a low impedance (not just resistance) bond from connector shell to metal chassis if available (ref Karen B’s comment) can be very useful. Since this isn’t always available, making sure there is a low impedance connection from connector shell to PCB 0V plane should also be implemented regardless of chassis connection. Try other sources and sinks, try lots of cables, try connection of cable shields to PCB and chassis, if you have high speed scopes try quantifying the signal imbalance and CM component (difficult), try ferrite cores (YMMV), go over your layout with a fine tooth comb. Best of luck, I spent years working on exactly this problem. With careful layout and cable selection I have achieved a Class B emissions pass at 1080p/60 with a single 4 layer PCB in a plastic chassis with a 3dB margin. All the best James James Pawson Managing Director & EMC Problem Solver Unit 3 Compliance Ltd EMC : Environmental & Vibration : Electrical Safety : CE & UKCA : Consultancy www.unit3compliance.co.uk<http://www.unit3compliance.co.uk/> | [email protected]<mailto:[email protected]> +44(0)1274 911747 | +44(0)7811 139957 2 Wellington Business Park, New Lane, Bradford, BD4 8AL Registered in England and Wales # 10574298 For inquiries, bookings, and testing updates please send us an email on [email protected]<mailto:[email protected]> or call 01274 911747. Our lead times for testing and consultancy are typically 4-5 weeks. From: [email protected]<mailto:[email protected]> <[email protected]<mailto:[email protected]>> Sent: 15 May 2025 16:44 To: [email protected]<mailto:[email protected]> Subject: [PSES] HDMI cable emissions Hello group, I am working on a HDMI cable where it does not pass radiated emissions with good margin. I have great braid coverage and have soldered the braid to the connector shell and have fully sealed and covered it with copper tape. Is there anything else that I need to do? My issue is mainly at below GHz when I use 1080P resolution. Thank you Peter ________________________________ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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