[SI-LIST] Re: High speed signal-return path
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: olaney@xxxxxxxx
- Date: Mon, 24 Sep 2007 11:21:23 -0700
3.125Gbps signals such as XAUI can have up to 75ps of differential skew.
This means (i) significant currents have to be supported by the
power/ground impedance (ii) any noise between power/ground will affect
the differential signal.
At 3.125Gbps edge rates, low power/ground impedance is difficult to
achieve. A high density of such signals can make it even more challenging.
A high power/ground impedance shared by multiple differential pairs
results in crosstalk between the pairs.
Thanks,
Vinu
olaney@xxxxxxxx wrote:
> That makes sense for DC current, but the point is that the power and
> ground planes are shorted together in the RF sense by a properly
> implemented bypass network. Therefore, one is as good as the other for
> RF. RF energy never reaches the power supply -- if it does, something is
> very wrong.
> Orin
>
> On Sat, 22 Sep 2007 22:11:06 -0700 (PDT) Jory McKinley
> <jory_mckinley@xxxxxxxxx> writes:
> Hello Orin,
> There is in that the return current has to do just that, return. So for
> single ended or common mode differential sourcing or sinking from power
> or ground could make a significant difference, somewhat regardless of
> edge rate. All mute for ideal differential of coarse.
> -Jory
>
>
>
>
> ----- Original Message ----
> From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
> To: jory_mckinley@xxxxxxxxx
> Cc: raja.anand@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Sent: Saturday, September 22, 2007 12:49:27 PM
> Subject: [SI-LIST] Re: High speed signal-return path
>
>
> We all know differential versus single ended, but "return current plane,
> not ground" needs explanation if you feel that there is some nontrivial
> difference at high frequencies between power and ground planes.
> Orin
>
> On Sat, 22 Sep 2007 10:36:14 -0700 (PDT) Jory McKinley
> <jory_mckinley@xxxxxxxxx> writes:
> Yes, however every plane is a potential return current plane, not ground,
> less important for true differential more important for single ended.
> How about any common mode noise on the differential signals, I would
> think Option 1 would allow for better potential isolation.
> -Jory
>
>
>
>
> ----- Original Message ----
> From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
> To: raja.anand@xxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Sent: Saturday, September 22, 2007 6:51:30 AM
> Subject: [SI-LIST] Re: High speed signal-return path
>
>
> Every power plane is a ground plane for high frequency purposes. There
> should be no material difference between the two implementations. If
> there is, something is really wrong with the power supply bypassing
> strategy. The choice between stripline and microstrip is generally
> driven by the particulars of the board layout. There is no definitive
> choice of one over the other based on general principles, only as
> circumstances dictate.
>
> Orin Laney
>
> On Sat, 22 Sep 2007 11:12:19 +0530 "Raja Anand" <raja.anand@xxxxxxxxx>
> writes:
>
>> Hi All,
>> For high speed differential signals (3.125Gbps ), is it really
>> required to
>> take these signals between Gnd Planes. Which option should I go for
>> from
>> below if my stack up is symmetrical?
>>
>> Option1:
>> L4 - Power
>> L5 - Signal
>> L6 -- GND
>>
>> Option2:
>> L4 - GND
>> L5 - Signal
>> L6 -- GND
>>
>> Regards,
>> Raja
>>
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