[SI-LIST] Re: FWD: PCB Trace Z, C, & L relationship
- From: "Peters, Stephen" <stephen.peters@xxxxxxxxx>
- To: "'Larry.Smith@xxxxxxx'" <Larry.Smith@xxxxxxx>,si-list@xxxxxxxxxxxxx, Raymond.Anderson@xxxxxxx
- Date: Wed, 22 Jan 2003 15:27:03 -0800
Hi Larry, all:
FWIW -- A mnemonic device for remembering which is which:
A conductor sandwiched between two planes in a PCB is a stripline structure
because one has to 'strip' away the layers to get to the conductor.
Regards,
Stephen Peters
Intel Corp.
-----Original Message-----
From: Larry Smith [mailto:Larry.Smith@xxxxxxx]
Sent: Wednesday, January 22, 2003 3:10 PM
To: si-list@xxxxxxxxxxxxx; Raymond.Anderson@xxxxxxx; Larry.Smith@xxxxxxx
Subject: [SI-LIST] Re: FWD: PCB Trace Z, C, & L relationship
Ok, maybe someday I will learn the difference between microstrip and
stripline... Somewhere along the way, I got a mental block on this and
can never keep them straight. I usually draw pictures...
regards,
Larry
> Delivered-To: si-list@xxxxxxxxxxxxx
> Date: Wed, 22 Jan 2003 14:59:33 -0800 (PST)
> From: Larry Smith <Larry.Smith@xxxxxxx>
> Subject: [SI-LIST] Re: FWD: PCB Trace Z, C, & L relationship
> To: si-list@xxxxxxxxxxxxx, Raymond.Anderson@xxxxxxx
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>
> Bob - There are four important parameters for ideal transmission lines:
>
> L - inductance/length
> C - capacitance/length
> Vel - velocity (length/time)
> Z0 - Impedance
>
> Of the four parameters, only two are independent. Given any two
> parameters, the other two can be calculated. Here here are the basic
> equations to convert one parameter to another. By manipulating these
> equations, you can find any parameter in terms of any two of the other
> parameters.
>
> 1) Vel = 1/sqrt(LC) = 1/delay
> 2) Z0 = sqrt(L/C)
>
> Other useful forms of the equations are:
>
> 3) L = delay^2/C = delay * Z0
> 4) C = delay^2/L = delay / Z0
>
> One interesting outcome of these equations is that all 50 Ohm
> microstrip in FR4 has 3.39 pF/inch and 8.47nH/inch regardless of the
> dimensions of the trace. The velocity is determined (half the speed of
> light) by the dielectric constant of FR4 (assume eR=4). The velocity
> and impedance are all you need to calculate the inductance and
> capacitance per inch. (Note, these numbers will be a little different
> if the eR is not exactly 4).
>
> Your equation #2 below is a lot like my equation 1) above. It should
> hold for both microstrip and stripline. Microstrip is a little cleaner
> because it is a homogeneous environment (all fields are in the
> substrate between the current and return current). Stripline is a
> little messier because fields are in both the substrate and air. This
> leads to an effective dielectric constant that may be frequency
> dependent.
>
> Anytime an impedance calculator reports L, C and Z0, you can check to
> see if the numbers are self consistent. They are not independent. Of
> couse, things get more complicated with non-ideal transmission lines that
> have loss. The L and C parameters become frequency dependent, but
> at each frequency point, I believe these relationships still hold.
>
> regards,
> Larry Smith
> Sun Microsystems
>
> > >From btarasew@xxxxxxxxxxx Fri Jan 17 15:05:40 2003
> > To: si-list@xxxxxxxxxxxxx
> > Subject: PCB Trace Z, C, & L relationship
> > MIME-Version: 1.0
> >
> >
> > I have a few EM textbooks that describe the Transmission Line
> > relationship between Z, C, & L:
> >
> > (#1) Z aprx= sqrt(L/C) [ignores R & G]
> > (#2) L*C = (permeability)*(permittivity) = 1/(velocity)^2
> > resulting in
> > (#3) Z aprx= 1/(C*velocity) So you can calculate Z without
> > determining L
> >
> > Question: When I use a pcb trace impedance calcuator that
> > calculates Zo along with Co & Lo, the above relationship (#2) seems to
> > hold for the Microstrip configuration, but it does not seem to hold for
> > the Stripline configuration.
> >
> > Can someone please shed light on this? Does the relationship
> > hold only for a single trace & ref plane (Microstrip) and not for a
trace
> > with two ref planes (Stripline). Does it have something to do with two
> > return paths instead of one in the Stripline configuration? Is the
> > impedance calculator reporting incorrect Co & Lo?
> >
> > Thanks in advance,
> > Bob Tarasewicz
> > Westell
> >
> >
> >
>
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Other related posts:
- » [SI-LIST] FWD: PCB Trace Z, C, & L relationship
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- » [SI-LIST] Re: FWD: PCB Trace Z, C, & L relationship
- » [SI-LIST] Re: FWD: PCB Trace Z, C, & L relationship
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