[SI-LIST] Re: Routing diff pair HDMI signals at 85 Ohms as opposed to 100 Ohms reduces radiation?
- From: Michael Vrbanac <vrbanacm@xxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx
- Date: Tue, 18 Dec 2012 11:26:28 -0600
HI, Lee,
Well, close but not quite but I'm guessing you are just trying to be
brief. Mismatches in impedance definitely have an effect on EMI due to
the reflections that result. Always. Whether or not they are presented
in a serious way is a system design issue and how it handles that
energy. And it would probably be good to specify you are assuming the
diff pairs represent a true "balanced system" so that efficient field
cancellation occurs. If the system is out of balance, whether or not
over a plane will increase common mode energy in the overall
interconnection system and thus increase EMI potential. I think you
noted that common mode issues are a significant issue in situations like
this in a later post so you have that right but that's how it happens.
It is exactly the same with fast Ethernet over unshielded twisted pairs.
The amount of spurious radiation is dependent on the pair imbalance. I
know you know that but not everyone does so this is just to clarify things.
Best Regards,
Michael Vrbanac
On 12/17/2012 12:44 PM, Lee wrote:
> Impedance does nor affect EMI nor does how your route over continuous
> planes. Diff pairs that are close to each other, whether or not over a
> plane are inherently quiet with respect to EMI. Think of fast Ethernet over
> unshielded twisted pairs.
>
> -----Original Message-----
> From: miner_tom Cipollone
> Sent: Sunday, December 16, 2012 4:14 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] (no subject)
>
> I was told recently that in order to help reduce EMI in HDMI signals, routed
> on a PCB from a driver IC to an HDMI connector, that it was better to route
> the signals at 85 Ohms differential as opposed to 100 Ohms differential.
>
>
> The contention was that with added connector L/C the overall average
> impedance normalized closer to 100 Ohms.
>
> Is there any credence to this? Is there any research material that agrees
> with this?
>
>
> (Hi Nazish)
>
> Thank You
> Tom
>
>
>
>
>
>
>
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