[SI-LIST] Re: Disadvantages of wider traces

Hi Simon,

The attenuation per unit length of a matched lossy trace can be
approximated as
attenuation[dB]= 4.35*(R(f)/Zo+G(f)*Zo), where R(f) and G(f) are the
frequency dependent conductor and dielectric losses.  The dielectric
loss can be approximated as G(f)=2*PI*f*C*tan_delta, where f is
frequency, C and tan_delta are the capacitance per unit length of trace
and loss tangent of the dielectric material. (both are slightly
frequency dependent, but that does not matter for this argument). the
capacitance per unit length of a trace is C=tpd/Zo, where tpd is the per
unit length propagation delay.  With all the above, the attenuation due
to dielectric losses on a matched lossy interconnect will become
4.35*2*PI*f*tpd*tan_delta, which is independent of the characteristic
impedance and independent of the trace cross section geometry.

the above is strictly speaking true for striplnes.  In microstrip the
picture is slightly different, and one can argue that in wider
microstrip traces more field goes through the dielectricmaterial, hence
dielectric losses in fact would increase slightly.

Regards,
Istvan



Partha Simon wrote On 04/20/07 12:14,:
> Hello Cheng,
> At higher frequencies we model transmission lines as Lossy Lines, along with
> defuault transmission line modeling, we consider Skin Effect as series
> component and Dielectric loss in parallel, which is dependent on the
> capacitance per length and which is highly influencied by loss tangent of
> the material, as we increase the trace with capacitance per length increases
> which causes Dielectric loss to increase.
> 
> Dielectric loss causes high frequency components attenuation which casues
> rise and fall time degradation, which leads to Inter Symbol Interference.
> 
> Regards,
> Simon
> 
> 
> On 20/04/07, Zhenggang Cheng <zcheng@xxxxxxxxxx> wrote:
> 
>>Hi Simon,
>>
>>What do you mean that wider trace will reduce skin-effect? And how is
>>that related to dielectric loss? To me, if skin-effect is reduced, it
>>means R is deceased, and then copper loss is decreased.
>>
>>Look forward to your reply.
>>
>>Thanks,
>>
>>Zhenggang
>>
>>-----Original Message-----
>>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>>On Behalf Of Partha Simon
>>Sent: Thursday, April 19, 2007 6:02 AM
>>To: babid_a@xxxxxxxxx
>>Cc: si-list@xxxxxxxxxxxxx
>>Subject: [SI-LIST] Re: Disadvantages of wider traces
>>
>>Hello,
>>Wider traces will lower the Z0 and may cause anti -reflection and it
>>depends
>>on what stack you are using, for typical 6-layer and nomial freqenciese
>>they
>>work well, and at higher higer frequencies they reduce skin-effect and
>>may
>>cause more dielectric loss.
>>
>>To work between these tradeoffs we need simulator and we cant generalise
>>the
>>case.
>>
>>Regards,
>>Simon
>>
>>
>>On 19/04/07, Babid A <babid_a@xxxxxxxxx> wrote:
>>
>>>Hello,
>>>Assuming that there is a chance for routing traces as wide as
>>
>>possible,
>>
>>>what are the disadvantages of having wider traces on the boards
>>
>>(>12mil) in
>>
>>>respect to noise, jitter etc?
>>>
>>>-Thanks,
>>>
>>>
>>>
>>>
>>>
>>>---------------------------------
>>>Ahhh...imagining that irresistible "new car" smell?
>>>Check outnew cars at Yahoo! Autos.
>>>
>
>>>
>>>
>>>
>>
>>
>>------------------------------------------------------------------


-- 
Istvan Novak            Sun Microsystems, Inc.
Istvan.Novak@xxxxxxx    Workgroup Servers, BDT Group,
                        One Network Drive, Burlington, MA 01803
                        Phone: (781) 442 0340
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