[SI-LIST] Re: HDMI Frequency Failing RE Test by 7dB!!!!
- From: Ken Wyatt <ken@xxxxxxxxxxxxxxxx>
- To: doug@xxxxxxxxxx
- Date: Wed, 23 May 2012 14:05:55 -0600
Group...
Doug brings up a very good point. You'll want to be sure the image plane is not
quarter-wavelength resonant at 750 MHz or any other critical harmonic
frequencies. It would benefit you to measure the radiated emissions again to be
sure nothing else "pops up".
_______________________
Kenneth Wyatt
Wyatt Technical Services LLC
Woodland Park, CO
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On May 23, 2012, at 1:49 PM, Doug Smith wrote:
> Hi Ken and the group,
>
> One thing to watch out for is a parasitic resonance between metal planes
> close to each other (board and image plane) connected by inductors
> (wires). The capacitance between the planes and the inductance of the
> connection(s) between them form a nice high Q resonant circuit at some
> frequencies. You may be able to measure this frequency using the method
> in my articles:
>
> http://emcesd.com/tt2006/tt060306.htm
> http://emcesd.com/tt2008/tt010108.htm (if you can get a current probe
> around the wires)
> http://emcesd.com/tt2011/tt090611.htm
>
> If you manage to get energy into the structure at the resonant
> frequency, it will radiate nicely.
>
> Also, I have a new domain name! http://DesertLabRat.com points to my
> main site at this time ( http://emcesd.com). Out here in the desert my
> companions include the the camel spiders (Google it), scorpions,
> tarantulas, etc.
>
> Doug
>
>
> On 5/23/12 9:21 AM, Ken Wyatt wrote:
>> OK, your challenges will be greater with a plastic enclosure. Cable
>> radiation is almost certainly due to common-mode currents flowing on the
>> outside of the shield - especially with shielded HDMI cables. It doesn't
>> surprise me that the cables have differing emission levels. It simply means
>> there is a stronger common-mode source at the input cable (or that the
>> source might actually originating from the BlueRay drive.
>> There are only two things I can think of that would help you in this
>> situation (barring using a shielded enclosure):
>>
>> 1. Install ferrite chokes on each cable nearest the source (probably your
>> product).
>>
>> 2. Install an image plane under your PC board - essentially, a flat sheet of
>> metal. Then connect the HDMI connector shield to the image plane with a
>> SHORT wire (or better, multiple wires). Are the two HDMI connectors
>> positioned adjacent to one another (good practice) or are they on opposite
>> sides of the PC board (not good practice)? If they are adjacent, then
>> connect both connector shields to the image plane with multiple short wires.
>> The image plane should be mounted as close to the underside of the PC board
>> as possible. No other connections should be made to it. Here's the classic
>> article on image planes by one of the authors, Henry Ott:
>> http://www.hottconsultants.com/pdf_files/image_plane.pdf
>>
>> You may need a combination of image plane and ferrite chokes.
>>
>> Hope this helps, Ken
>> _______________________
>> Kenneth Wyatt
>> Wyatt Technical Services LLC
>> Woodland Park, CO
>> Email Me! | Web Site | Blog
>> Subscribe to Newsletter
>> Connect with me on LinkedIn
>>
>> On May 23, 2012, at 10:05 AM, vinod ah wrote:
>>
>>> Hi,
>>>
>>> I have enclosure which is plastic :-(. Can i connect the shield pins
>>> to ground plane as i don't have metal enclosure. will this help?
>>>
>>> Also i have a query on common mode current part of the analysis. As i
>>> mentioned earlier, the radiation level drops by 5dB when i remove the
>>> HDMI input cable (Blu ray output), while it only drops by 2dB when i
>>> remove HDMI out cable (TV input ). But since both the connectors are
>>> grounded in the same way and same cable is used, why is there a
>>> difference in radiation levels drops? Is it really common mode current
>>> issue?
>>>
>>> Regards
>>> Vinod A H
>>>
>>> On Wed, May 23, 2012 at 9:31 PM, Ken Wyatt<ken@xxxxxxxxxxxxxxxx> wrote:
>>>> Vinod,
>>>> The connector shield pins should all be connected to the signal return
>>>> plane. Don't use a ferrite as suggested. Then the connector shield itself
>>>> needs to be connected on all sides (as practical) directly to the metal
>>>> product enclosure. This will allow any common-mode currents to return to
>>>> their source(s) back INSIDE the product, rather than leaking out along the
>>>> outside of the cable shield, causing radiation. You may need a shim or
>>>> fingerstock to provide this bonding to the enclosure. If the enclosure is
>>>> non-conductive (plastic), then you are likely "hosed" and may have to
>>>> resort
>>>> to ferrite choke on each HDMI cable (at the product end).
>>>>
>>>> If you have a current probe, you'll be able to view these currents while
>>>> you
>>>> troubleshoot. Check my article on current probes in the March 2012 issue of
>>>> Interference Technology...
>>>>
>>>> http://www.interferencetechnology.com/the-hf-current-probe-theory-and-application/
>>>>
>>>> Cheers, Ken
>>>>
>>>> _______________________
>>>> Kenneth Wyatt
>>>> Wyatt Technical Services LLC
>>>> Woodland Park, CO
>>>> Email Me! | Web Site | Blog
>>>> Subscribe to Newsletter
>>>> Connect with me on LinkedIn
>>>>
>>>> On May 23, 2012, at 9:48 AM, vinod ah wrote:
>>>>
>>>> Hi Ken,
>>>>
>>>> Sorry the wording were not proper. Here is the corrected one.
>>>>
>>>> I just checked the layout and found that there are four shield pins
>>>> and we have used a zero ohms between shield pin and ground. Is this ok
>>>> to do so or should i have
>>>> solid ground connection to the shield pins. Also if i replace the zero
>>>> ohms with ferrite bead, will it help?
>>>>
>>>> Regards
>>>> Vinod A H
>>>>
>>>> On Wed, May 23, 2012 at 9:11 PM, vinod ah<ah.vinod@xxxxxxxxx> wrote:
>>>>
>>>> Hi Ken,
>>>>
>>>>
>>>> I just checked the layout and found that there are four pins for
>>>>
>>>> connecting the shield pins to ground and we have used a zero ohms
>>>>
>>>> between shield pin and ground. Is this ok to do so or should i have
>>>>
>>>> solid ground connection to the shield pins. Also if i replace the zero
>>>>
>>>> ohms with ferrite bead, will it help?
>>>>
>>>>
>>>> Regards
>>>>
>>>> Vinod A H
>>>>
>>>>
>>>> On Wed, May 23, 2012 at 8:56 PM, Ken Wyatt<ken@xxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> Vinod,
>>>>
>>>>
>>>> Is the HDMI connector shield -directly- connected to the (metallic)
>>>>
>>>> enclosure? This should ideally be multiple connections. If the shield is
>>>>
>>>> floating or poorly connected, you'll get common-mode currents flowing along
>>>>
>>>> the outside of the HDMI cable shield.
>>>>
>>>>
>>>> Have you measured the common-mode currents with a current probe?
>>>>
>>>> _______________________
>>>>
>>>> Kenneth Wyatt
>>>>
>>>> Wyatt Technical Services LLC
>>>>
>>>> Woodland Park, CO
>>>>
>>>> Email Me! | Web Site | Blog
>>>>
>>>> Subscribe to Newsletter
>>>>
>>>> Connect with me on LinkedIn
>>>>
>>>>
>>>> On May 23, 2012, at 9:17 AM, vinod ah wrote:
>>>>
>>>>
>>>> Hi Vijay,
>>>>
>>>>
>>>> We have option to control the current and voltage swing. We tried
>>>>
>>>> both, without violating eye test, but still the i am seeing radiation
>>>>
>>>> at 742.5MHz. The radiation level keeps chnaging as i try with multiple
>>>>
>>>> cables. I found the best cable i had to be failing FCC limits by 7dB.
>>>>
>>>> I used some HDMI cable which are sold by big brands, but i was failing
>>>>
>>>> FCC by 12dB !!!.
>>>>
>>>>
>>>> Hi Boris,
>>>>
>>>>
>>>> We tested the Blu ray player and TV with same HDMI cable used for our
>>>>
>>>> board testing. the Blu ray player used is from Sony while the TV is
>>>>
>>>> from LG, we connected the blu ray player to TV using same HDMI cable
>>>>
>>>> and took the readings.
>>>>
>>>>
>>>> Regards
>>>>
>>>> Vinod A H
>>>>
>>>>
>>>> On Wed, May 23, 2012 at 8:43 PM, Traa, Boris<boris.traa@xxxxxxxxxxx>
>>>> wrote:
>>>>
>>>>
>>>> Dear Venod,
>>>>
>>>>
>>>>
>>>> You said that TV is oke and Blu Ray is tested. Seperately or as a
>>>>
>>>> combination with the same cables you also used to attach your board?
>>>>
>>>>
>>>>
>>>> Kind regards
>>>>
>>>>
>>>> Boris Traa
>>>>
>>>>
>>>> System design engineer EMC
>>>>
>>>>
>>>>
>>>> It's the currents that make circuits work or fail.
>>>>
>>>>
>>>>
>>>> Philips Innovation Services/EMC center
>>>>
>>>>
>>>> Room 2.020
>>>>
>>>>
>>>> High Tech Campus 26
>>>>
>>>>
>>>> 5656AE Eindhoven, The Netherlands
>>>>
>>>>
>>>> Tel: ++ 31 40 27 43766
>>>>
>>>>
>>>> Fax: ++ 31 40 27 42224
>>>>
>>>>
>>>> E-mail: boris.traa@xxxxxxxxxxx
>>>>
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>>
>>>>
>>>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>>>>
>>>> Behalf Of vinod ah
>>>>
>>>>
>>>> Sent: Wednesday 23 May 2012 3:58 PM
>>>>
>>>>
>>>> To: SI-LIST
>>>>
>>>>
>>>> Subject: [SI-LIST] HDMI Frequency Failing RE Test by 7dB!!!!
>>>>
>>>>
>>>>
>>>> Hi All,
>>>>
>>>>
>>>>
>>>> I am designing a board with HDMI input and HDMI output. I am facing EMI
>>>>
>>>> issue at 742.5MHz (failing FCC limits by 7dB) when running 1080p30 on both
>>>>
>>>> Input and output HDMI i.e. BLu ray player connected to HDMI input connector
>>>>
>>>> of my board is giving out data at 1080p30 and HDMI out of my board is
>>>>
>>>> connected to TV which is also 1080p30 content. So i am unable to find out
>>>>
>>>> how is 742.5MHz coming out as radiation as it is 10th Harmonic of pixel
>>>>
>>>> clock 74.25MHz!!!!!!!. I am not facing any issues with other harmonics
>>>>
>>>> except for this. So i am totally confused on how to go about debugging this
>>>>
>>>> issue.
>>>>
>>>>
>>>>
>>>> Note: All the signals are routed on top layer without vias and all signals
>>>>
>>>> have solid ground plane reference. The blu ray player and TV used, is
>>>> tested
>>>>
>>>> for radiation and no significant levels seen.
>>>>
>>>>
>>>>
>>>> Regards
>>>>
>>>>
>>>> Vinod A H
>>>>
>>>>
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>
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