[SI-LIST] Re: Return current of a trace in stripline
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 05 Aug 2013 10:02:08 -0700
"the current will take the path of least inductance"
Better to tweak that to "the /bulk of the/ current will take the path of
least /impedance/".
Thanks,
Vinu
On 08/04/2013 10:00 PM, Carrier, Patrick wrote:
> Hello Balaji--
> You are correct in that the current will take the path of least inductance at
> higher frequencies - which is precisely why it will couple energy to the
> planes through EM fields as opposed to just traveling down to the end of the
> trace and looping back. For this reason, it will couple onto both planes as
> it propagates. The amount of current in each plane will scale linearly with
> the distance from the trace.
> There is a great IEEE paper on this topic: IEEE TRANSACTIONS ON
> ELECTROMAGNETIC COMPATIBILITY, VOL. 49, NO. 1, FEBRUARY 2007
> So, if your planes are about the same distance from the trace (as in your
> case) and the trace travels over a split in one of them, you will end up
> radiating a lot of energy.
>
> However, you mention 3GHz as a frequency, which implies that you are running
> some kind of SERDES, which is likely differential. As has been discussed
> previously on this list, one of the possible advantages of differential
> signaling is less net return current in the reference plane.
>
> If you are interested, here is some more information on this topic:
> http://blogs.mentor.com/hyperblog/blog/2012/10/10/return-current-on-a-stripline/
>
> --Pat
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Balaji G
> Sent: Sunday, August 04, 2013 1:43 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Return current of a trace in stripline
>
> Hi Experts,
> We discussed a lot regarding path of return current before and this is
> regarding the path of return current in a stripline trace. As far I learnt,
> the return current will take the path of least resistance at low frequencies
> and path of less inductance at high frequency and hence the reason that
> return current travels in the plane directly under the signal's trace. My
> question is if we consider a signal travelling in a stripline which is
> sandwiched between the ground and split power plane where the signal to
> ground distance is 3.7mils and signal to split power plane distance is
> 4.3mils, should we worry about the split power plane at high frequency (say
> 3GHz) as the signal to ground distance is the path of least inductance and
> all the return current for high frequency signal trace flows in the ground
> plane causing no reflection/ EMI issues? Is my thinking right? Can you
> please provide your thoughts on this?
>
> Regards,
>
> Balaji
>
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