Thank you all for your valuable feedback. It is greatly appreciated.
Individual pair are shileded using foil and the braid coverage is more than
75%. The main problem frequency is 741 which is the harmonic of the main clock
frequency. The PCB layout seems good. I am fairly sure the issue is related to
source and sink. When I test the cable at a 3rd party lab, 741MHz is very low
but they see issue at higher frequency. If I can find a quiet source, I think I
am going to solve this. I am digging further.
Thank you againPeter
On Friday, May 16, 2025 at 03:30:23 AM PDT, Charlie Blackham
<[email protected]> wrote:
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 |[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
[email protected] or call 01274 911747. Our lead times for testing
and consultancy are typically 4-5 weeks.
From:[email protected] <[email protected]>
Sent: 15 May 2025 16:44
To: [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
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