[SI-LIST] Re: Differential line Via - affecting common mode noise
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Lakshmi Narayanan Sowrirajan, ERS-HCLTech" <lakshminarayanans@xxxxxxx>
- Date: Thu, 09 Jun 2011 06:00:55 -0700
There are few things that can be going on:
1. You are simply attenuating through the via so total energy reaching
the termination in the signal band has been reduced.
2. In your via case you have inadvertently done a better job matching
the electrical path length.
3. Your vias are long enough to cause substantial coupling between the
two lines of the pair, in effect building a CM choke.
Steve.
On 6/9/2011 4:28 AM, Lakshmi Narayanan Sowrirajan, ERS-HCLTech wrote:
> Hi Steve,
>
> Thanks for the reply, please find the details as below.
>
> 1) DRV---brkout---via---8inch routing-----via----conn---4inch
> routing----termination (x_p,x_n)
>
> 2) DRV---brkout--- 8inch routing-----conn---4inch routing----termination
> (x_p,x_n)
>
> Signal is probed near termination and we see less common mode noise in case
> 1, where we have via (modeled in HFSS as s4p)
>
> Common mode noise = (v(x_p)+V(x_n))/2
>
> Thanks& Regards
> Lakshminarayanan. S
> 9840250902
>
>
> -----Original Message-----
> From: steve weir [mailto:weirsi@xxxxxxxxxx]
> Sent: Thursday, June 09, 2011 4:32 PM
> To: Lakshmi Narayanan Sowrirajan, ERS-HCLTech
> Cc: si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Differential line Via - affecting common mode noise
>
> Where and how are you measuring your CM noise? Virtually all vias
> impose insertion loss. Non-transparent vias insert reactive
> discontinuities.
>
> Steve
> On 6/8/2011 11:49 PM, Lakshmi Narayanan Sowrirajan, ERS-HCLTech wrote:
>> Hi Experts,
>> How the via improve or degrade the differential lines
>> common mode noise and performance as I am getting more common mode noise for
>> a case where I don't have any via. If I include a transition via (modeled in
>> HFSS - s4p model) the common mode noise is getting reduced. Please direct me
>> to a good article/paper/book to understand the theory behind this.
>>
>> Thanks& Regards
>> Lakshminarayanan. S
>>
>>
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Steve Weir
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