# [SI-LIST] Re: Power planes

• From: Geoff Stokes <gstokes@xxxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Fri, 25 Jul 2003 13:00:03 +0100

```You can get accurate values of Zo and delay from TXLINE a free calculator
available from AWR, associated with their main product, Microwave Office:

http://mwoffice.com/products/

In this you use the bulk Er (at high frequency).  For microstrip, you don't
need to know the effective Er but it's calculated for you anyway.

The Rogers web site also gives a useful calculator.  Isn't this fun!  The
computer people have turned into microwave engineers!

Cheers
Geoff

> -----Original Message-----
> From: jeff_latourrette@xxxxxxxxxxx
> [mailto:jeff_latourrette@xxxxxxxxxxx]
> Sent: 24 July 2003 18:43
> To: Raymond.Anderson@xxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Power planes
>
>
> Hi Karan, Ray:
>
> I think comments below referenced to Geoff were from my
> posting of 7/18.  As Ray points out velocity in microstrip is
> only dependent on effective dielectric constant and given you
> scale line for same impedance when changing the dielectric
> thickness, velocity shouldn't change.  So this effect is moot
> if you are maintaining line impedance.  Epsilon effective
> will be slightly larger (& velocity slightly slower) for low
> impedance lines, than for high imedance lines.
>
> According to equation previously referenced, Epsilon
> effective is slightly dependent on W/h and even less on
> t/h-in many stack-ups, we can often ignore thickness.
> Although this is equation-based and probably limited in
> accuracy to reasonable values of W/h & t/h intuitively, one
> should be able to imagine the difference in field
> distribution between a narrow, high-Z line and that of a wide
> low-Z line.  Ray also correctly points out that stripline's
> velocity is solely dependent on material dielectric constant
> and W/h, t/h doesn't matter because all fields are contained
> in the dielectric.
>
>
> OK-so how bad is this effect for microstrip??  I dug and
> found a very old design guideline, plotting Epsilon effective vs W/h.
>
> For Alumina (thin-film), it varies from 5.9 (W/h=0.1, Z=130
> ohms) to 6.5 (W/h=1.0, Z=50 ohms) to 7.4 (W/h=3.0, Z=26 ohms)
>
> The only reference on the page is "Report No. 03-68-85",
> probably funded by (D)ARPA.
>
>
> Hope this can help,
>
> Jeff LaT.
>
>
>
> -----Original Message-----
> From: Ray Anderson [mailto:Raymond.Anderson@xxxxxxx]
> Sent: Thursday, July 24, 2003 9:28 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Power planes
>
>
>
> >
> >This is quite a general question.
> >
> >---------------------------------------------
> >
> >If I place the GND plane too close to the track wouldn't it
> affect the velocity
> of signal on track ?
> >
> >Possible reason is the capacitance b/w track and GND will
> increase which will
> cause the delay in changing signal(rising edges) as it will
> need more time to
> charge the increased capacitor value.Geoff just told us that
> placement of GND
> affects the velocity of Microstrip and Not Strip Line. How
> far is it true ?
> >
> >
>
> The distance between a signal trace and a reference plane in a
> stack-up does not effect the velocity of the signal propagating
> on the signal trace. It can effect the impedance of the signal
> trace. The velocity of propagation is dependent only on the effective
> dielectric constant (Er) or electric permittivity of the
> substrate material.
>
>
>
> Yes, there will be more distributed capacitance between the signal
> trace and the plane for small spacings, but this does not effect
> the velocity of propagation as the PUL inductance of the signal
> trace will decrease as well.
>
> some relevant equations:
> ------------------------
>
>       Velocity = C/sqrt(Er)    <= velocity is a function of Er
>               (where C is speed of light in this equation)
>
>       Velocity = 11.8/sqrt(Er) inches/nSec  <= numeric
> version of above
>
>       Velocity = 1/sqrt(L*C)  hence:
>          L = (delay^2)/C  and C = (delay^2)/L
>
>
>
> -Ray Anderson
> Sun Microsystems
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>
>

_________________________________________________________

Zetex Semiconductors - Solutions for an analog world

http://www.zetex.com
_________________________________________________________

######################################################################
E-MAILS are susceptible to interference.  You should not assume that
the contents originated from the sender or the Zetex Group or that they
have been accurately reproduced from their original form.
Zetex accepts no responsibility for information, errors or omissions in
this e-mail nor for its use or misuse nor for any act committed or
omitted in connection with this communication.
If in doubt, please verify the authenticity with the sender.
######################################################################

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:
http://www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu

```