[SI-LIST] Re: HDMI Frequency Failing RE Test by 7dB!!!!
- From: "alfred1520list" <alfred1520list@xxxxxxxxx>
- To: <ah.vinod@xxxxxxxxx>, "SI-LIST" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 23 May 2012 09:38:55 -0700
Theories abound but I'll share my recent experience.
I have a power supply board, an audio board, and HDMI board with
HDMI in and out. They are all interconnected with power connections
and ribbon cables to carry audio data. 5 things that we did solved
the problem (we also use a different test house:):
1) RF choke on all interconnecting power connections and
ribbon cables.
2) Ground the metal tab of the HDMI connectors directly to the chassis
thereby ground the HDMI cable shield directly to chassis.
3) Bypass audio cable to the chassis with capacitors.
4) Tightly RF seal all seams of the metal chassis to prevent any leak.
5) Use HDMI cable with built-in choke
Ken's current probe suggestions is excellent. That's the technique
I used to tackle my problem.
> One more observation, which I forgot to mention. When i remove only
> the HDMI input cable (Input to the board), the radiation level drops
> by 5 dB,
This might be a hint. Once the cable is disconnected, the HDMI Rx chip
will no longer be locked to incoming pixel clock, so the Source <=> Rx
path can no longer radiate. The fact that you still see a big peak suggest
the radiation isn't coming from the Source <=> Rx path, but rather is
some where else. Probably the shield ground isn't perfect and so the
Rx cable serves as an antenna. Disconnect the far end of the Rx cable and
plug it into the input HDMI and see what happen? If that affected the
EMI peak, then it is probably helping to radiate. A current probe on that
cable will reveal whether if it is aiding radiation.
> while if i remove only HDMI output cable the radiation level
> drops by 2dB. Both the HDMI input and output cables are same set of
> cables.
>
Seems like it is similar to the input cable.
I would use a coupler to connect the Source cable directly to the Sink
cable and see what happened to the EMI peaks, just to make sure there
is no surprise.
Remember that the 36-bit video bus still runs at 74.25 MHz and the noise
from the bus could leak to the ground plane and radiate either/both
through the power connections or other cabling to other modules.
Assuming your system will detect input disconnection and generate
valid test pattern, you might find out more by tuning to a says
30 kHz span centered on the EMI peak. Connect/Disconnect the input
cable and see if the peak shifts. Your system should be locking to the
Source if it is connected, and run on its own PLL clock when disconnected,
and more than likely the two clocks will not have the exact same frequency,
and you may be able to infer something.
Best Regards,
Alfred Lee
----- Original Message -----
From: "vinod ah" <ah.vinod@xxxxxxxxx>
To: "SI-LIST" <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, May 23, 2012 8:28 AM
Subject: [SI-LIST] Re: HDMI Frequency Failing RE Test by 7dB!!!!
> Hi All,
>
> One more observation, which I forgot to mention. When i remove only
> the HDMI input cable (Input to the board), the radiation level drops
> by 5 dB, while if i remove only HDMI output cable the radiation level
> drops by 2dB. Both the HDMI input and output cables are same set of
> cables.
>
> Regards
> Vinod A H
>
> On Wed, May 23, 2012 at 8:47 PM, vinod ah <ah.vinod@xxxxxxxxx> 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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