# [SI-LIST] Re: Differential Pair - Impedance Question

• From: <J_Dong@xxxxxxxxxxxxxxxx>
• To: "'Rohit MISHRA'" <rohit.mishra@xxxxxx>, <si-list@xxxxxxxxxxxxx>
• Date: Fri, 1 Jul 2011 10:50:21 -0400

Here you can estimate the impedance of microstrip-type pcb:
http://www.microwaves101.com/encyclopedia/calmstrip.cfm

and stripline-type pcb:
http://www.microwaves101.com/encyclopedia/calsstrip.cfm

and coplanar-type pcb:
http://www.microwaves101.com/encyclopedia/calcpw.cfm

Note that all those calculations are approximated. If your parameters are
out of their range (e.g. dielectric constant too high, pcb too thick etc),
you have to use some full-wave simulators. Here is a free solution for that:
http://www.feko.info/ Their evaluation version of the software allows
simulation with limited number of unknowns, which should be more than enough
to handle this sort of problems.

-----------------------------------------------------

Junwei Dong, Ph.D.

Research Scientist

Microwave Engineering Corporation (MEC)

1551 Osgood Street, North Andover, MA 01845

Tel:  +1-978-685-2776 (Ext.12)

Fax: +1-978-975-4363

E-mail: j_dong@xxxxxxxxxxxxxxxx

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Rohit MISHRA
Sent: Friday, July 01, 2011 7:34 AM
To: si-list@xxxxxxxxxxxxx
Cc: suryan_tvm@xxxxxxxxxxx
Subject: [SI-LIST] Re: Differential Pair - Impedance Question

Jayasuryan,

Well I assume you are talking about pcb traces.

There are two sets of tools which is used in industry for calculating &
measuring characteristic and differential impedance. Field solver can be
used on pcb layout before pcb manufacturing while TDR or VNA is used on
manufactured pcb.

Since characteristic impedance and hence differential impedance depends on
geometry(space, width, thickness ) of traces, These geometry information you
get from pcb fabricator guy, when you provide him impedance information. PCB
layout engineer job is to use this geometry information and make a layout.

Once layout is ready, use field solver to calculate impedance and use TDR or
VNA to measure & ensure that pcb traces are manufactured with correct
impedance.

Hope it helps.

Rgds,
Rohit

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Friday, July 01, 2011 1:51 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Differential Pair - Impedance Question

Use a field solver.

Steve.
On 7/1/2011 1:04 AM, Jayasuryan KG wrote:
> Dear Rohit,
>
> Thanks for the detailed reply. Is there any tool to make sure that the
> impedance of the individual trace is 45 Ohm and it is 90 Ohm (no mutual
> coupling) when it will be differential?
> How can we make sure it is meeting the our requirement in manufacturing
too.
> Impedance coupons are the only way or do we have any other solution(s)?
>
> Best Regards,
> Jayasuryan
>
> -----Original Message-----
> From: Rohit MISHRA
> Sent: Thursday, June 30, 2011 7:06 PM
> To: si-list@xxxxxxxxxxxxx
> Cc: suryan_tvm@xxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: Differential Pair - Impedance Question
>
> Jayasuryan,
>
> When you talk about "45 Ohm impedance for individual", you are talking
> characteristic impedance(Z0) of individual or single ended line which is
> intrinsic to that line only and can't be changed by changing differential
> impedance actually it is other way around, once you have set the
> characteristic impedance of your single ended line then differential
> impedance between two lines depends on characteristic impedance(Z0) of
lines
> and mutual coupling between them.
>
> So if characteristic impedance of single ended line (D+&  D-) is Z0 and
> coupling coefficient between them is k then
>
> Zdiff = 2 * Z0 ( 1- k) or
>        = 2 (Z11 - Z12)
>
> ->  k = 0, Zdiff = 2 * Z0
>
> Hope it should be clear now.
>
> Rgds,
> Rohit
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
> Behalf Of steve weir
> Sent: Thursday, June 30, 2011 1:21 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Differential Pair - Impedance Question
>
> The single-ended impedance can be half the differential impedance when
> the lines have no mutual coupling.
>
> Steve
> On 6/30/2011 12:02 AM, Jayasuryan KG wrote:
>> Dear  Experts,
>>
>>
>> I have a question regarding differential pair.
>>
>> In my UBS differential pair, the impedance is 90 Ohm.
>>
>>
>>
>> Is it possible to achieve 45 Ohm impedance for individual (D+ and D+)
>> traces(45 Ohm+45 Ohm? Ohm) in the differential traces?
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Best Regards,
>>
>> - Jayasuryan
>>
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>

--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV  89501
www.ipblox.com

(866) 675-4630 Toll-free
(707) 780-1951 Fax

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