[SI-LIST] Re: Characteristic Impedance

Hello Sunil,

The equation for characteristic impedance [Zo = sqrt(L/C)] - actually works
for higher frequencies, not lower ones. Its easy to get this confused with
lossy line models that include R or G components. The "loss" of the line
depends on R and G, but Zo will only depend on L and C as frequency
increases. 

The actual equation for characteristic impedance is sqrt[(R + jwL)/(G +
jwC)], where RLGC are the known circuit qualities and w is 'omega' which is
2+pi+frequency. So you can see that as frequency increases, the jwL and jwC
terms will being to dominate over the R and G terms (even if these terms
increase, they won't increase at the same slope). Zo will asymptotically
approach a value as frequency increases, which will be sqrt(L/C). 

At lower frequencies the Zo will actually increase quite dramatically. For a
visualization of this, see page 3 of this old presentation:
http://www.ansoft.com/hfworkshop03/Andy_Byers_Tektronix.pdf

Also check out the great book by Johnson and Graham - "High Speed Signal
Propagation - Advanced Black Magic". Plenty of other textbooks should cover
this theory too. 

I realize we can get into special cases or nuances when we talk about
frequency-dependent dielectric effects or surface roughness, but for most
cases this holds.  

Regards,
Andy 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Peterson, James F (EHCOE)
Sent: Wednesday, June 13, 2007 4:28 AM
To: SI LIST
Subject: [SI-LIST] Re: Characteristic Impedance

<snip>=20
"Sqrt (L/C)...it's a DC model"=20
<snip>

At DC, inductance looks like a short and capacitance looks open... :)

Actually the lossless model for transmission lines works quite well in a
frequency region between DC and gigabit frequencies (roughly between
1MHz to a few hundred MHz).

The best advice for Sunil has already been given (check out a good SI
book)...and note : at some point in our career we all asked these basic
questions (at least to ourselves), or we never learned.

Take care,
Jim Peterson
Honeywell

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Joseph Kao
Sent: Tuesday, June 12, 2007 10:16 PM
To: sunil_bharadwaz@xxxxxxxxx; SI LIST
Subject: [SI-LIST] Re: Characteristic Impedance

Dear Sunil,

Sqrt(L/C) means non-lossy, ideal status for trace,  so also no frequency
dependent.
it's DC model.

Regards,
Joseph Kao

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of sunil bharadwaz
Sent: Wednesday, June 13, 2007 12:38 AM
To: SI LIST
Subject: [SI-LIST] Characteristic Impedance

Hello Friends ,
What exactly does a 50 ohm line mean on a PCB ?
Z0 being Sqrt (L/C).Is Z0 frequency dependent?
How does a DC Power supply trace see this?

Thanks!!

Regards
Sunil.B

      =3D20
---------------------------------
Boardwalk for $500? In 2007? Ha!=3D20
Play Monopoly Here and Now (it's updated for today's economy) at Yahoo!
Games.

------------------------------------------------------------------
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 technical documents are available at:
                http://www.si-list.net

List archives are viewable at:    =3D20
                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
 =3D20

------------------------------------------------------------------
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 technical documents are available at:
                http://www.si-list.net

List archives are viewable at:    =20
                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
 =20
------------------------------------------------------------------
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 technical documents are available at:
                http://www.si-list.net

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 technical documents are available at:
                http://www.si-list.net

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

Other related posts: