[SI-LIST] Re: Impedance Mismatch due to neck down

Quazi,

Try a web search for impedance matching tapers, there are many methods, 1/4wave 
transformers, bessel, expotential, binomial, klopfenstien, some of which can be 
designed with widebandwidths.


Regards

John

qazi@xxxxxxxxxx wrote:
> R: [SI-LIST] Impedance Mismatch due to neck downThanks guys for the
> responses.
> One reason for which we do not want to have 4-5 mil traces all the way is
> the resistive loss.
> 5 mils within the DUT and then gradual increase to 8 mils looks like a
> pretty good solution.
> Rgds,
> Qazi Iqbal
>   -----Original Message-----
>   From: Guasti Giovanni [mailto:Giovanni.Guasti@xxxxxxxxxx]
>   Sent: Friday, June 24, 2005 12:41 AM
>   To: 'qazi@xxxxxxxxxx'; si-list@xxxxxxxxxxxxx
>   Subject: R: [SI-LIST] Impedance Mismatch due to neck down
> 
> 
>   I agree with Paul, a slow signal would not notice the 1-2 cm long trace
> impedance mismatch.
>   But you could think to make 5 mils traces ewerywere too. 65 Ohm is not =
> so
> high but there are some simple ways to reduce it further.
> 
>   If you choose a dieletric thickness around  0.150mm you'll obtain an
> impedance of 50 ohm.
>   Isn't er=4 too low? with an er=4.3-4.4 the impedance reduces from 6=
> 5 to
> 60ohm.
>   With a 8 mil trace and er=4.4 -> Zo=47ohm. Is it what you would lik=
> e to
> obtain?
>   By
>           Giovanni
> 
>   -----Messaggio originale-----
>   Da: Qazi Arif Iqbal [mailto:qazi@xxxxxxxxxx]
>   Inviato: giovedì 23 giugno 2005 19.59
>   A: si-list@xxxxxxxxxxxxx
>   Oggetto: [SI-LIST] Impedance Mismatch due to neck down
> 
> 
> 
>   Hi Experts,
>   The board that I am using is of a stripline setup with 8mil traces (1 o=
> z
>   Cu), FR-4 (4.0) and 10 mil dielectric thickness which gives 50ohm
>   (relatively low speed). The BGA is tightly pitched and I have to neckdo=
> wn
>   traces around 4-5 mils which gives impedance mismatch (65.4 ohm). Neckd=
> own
>   is a must but I also want to avoid impedance mismatch, is there any way=
>  of
>   doing it? Does doubling help? (looks like this would give a parallel of=
>  2
>   65.4ohms = 32.7, worse than what I have).
> 
>   Thanks in advance,
>   Qazi
> 
> 
> 
> 
> 
> 
> 
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