AppCAD coplanar waveguide calculations  RF Cafe Forums

PavelM

Post subject: AppCAD coplanar waveguide calculations
Posted: Wed Mar 31, 2010 3:08 am


Joined: Wed Mar 31, 2010
2:46 am Posts: 6 
Hi, When I am analyzing a coplanar waveguide
with and without ground plane I get some strange
results. Just for example:  dimensions are
in mils, Er=4.3 (FR4)  W=20, G=6, T=2.1, H=58,
L=1000 (L is not important for impedance calculations)
 with groundplane I get Zo=53.9 Ohm (1), and without
ground plane Zo=41.7 Ohm (2)  still "with groundplane",
when I change H to be H=58000 (equivalent to no
ground plane), I get Zo=54.7 (!!!, compare to (1)
and (2)). It means that groundplane below has almost
no influence on Zo, which is reasonable because
it is far from the signal line. But Zo is totally
different from "no groundplane" option. Looks strange.
Which one is correct and why there is a difference?
I guess that there is some problem with calculation
for "no groundplane".
Any comments will be
greatly appreciated.
Sincerely Pavel





Kirt Blattenberger

Post subject: Re: AppCAD coplanar waveguide calculations
Posted: Thu Apr 01, 2010 11:16 am



Site Admin 

Joined: Sun Aug 03, 2003
2:02 pm Posts: 701 Location: Erie, PA

Greetings PavelM:
I get the same results
as you do when using the latest version of AppCAD
(3.0.2).
AppCAD: Z = 53.9 ohms for H =
58 mil Z = 54.7 ohms for H = 1000 mils
I also plugged the numbers into AWR’s TXLine
program (http://web.awrcorp.com/Usa/Products/OptionalProducts/TXLine/)
and got these results:
TX Line: CPW w/ground
plane Z = 55.7 ohms for H = 58 mils Z = 56.6
ohms for H = 1000 mils (same as for 5800)
TXLine: CPW w/o ground plane
Z = 55.0
ohms for H = 58 mils Z = 56.9 ohms for H = 1000
mils
Yet another online calculator (http://chemandy.com/calculators/coplanarwaveguidewithgroundcalculator.htm)
gives the following (I converted from mils to mm
when entering):
chemandy.com: Z = 63.5
ohms for H = 58 mil Z = 64.7 ohms for H = 1000
mils
Part of the difficulty with calculating
an answer lies in the iterative approach needed
to calculate an effective dielectric value, as well
as simple models neglecting fringe fields, conductor
type and thickness, etc., as the chemandy.com model
evidently does. The others are pretty close to each
other.
Once the height is larger than the
gap/height ratio gets small, the results do not
change much with increasing height. The approximation
used in simple calculator test gap/height ratios
and apply different equations accordingly. It would
take a field solver to hone in a really precise
answer.
There is a good paper in the IEEE
Library, but you need to sign in to access it:
“Accurate and simple closedform formulas for coplanar
waveguide synthesis” T.Q. Deng, M.S. Leong and
P.S. Kooi
_________________  Kirt Blattenberger
RF Cafe Progenitor & Webmaster





PavelM 

Post subject: Re: AppCAD coplanar waveguide calculations
Posted: Sun Apr 04, 2010 4:11 am


Joined: Wed Mar 31, 2010
2:46 am Posts: 6 
Hello, Kirt Thank You for your time and informative
comments. I will try to explain my needs
and then to summarize my understanding of your comments.
I am developing a very low noise analog frontend
with low pass filter for high speed signal acquisition
board. The corner frequency is at few hundred
MHz (sorry, I can't be more specific). It is a Bessel
(constant group delay) filter. I try to do it with
discrete components (Ls and Cs). Those are quite
good components with high Q (>10) and low production
tolerances (<=2%). My problem is to take parasitics
of PCB layout into consideration in selection of
discrete L and C values. I plan to calculate the
L and C of PCB and reduce values of correspondent
discrete components by the same amount. Usually,
PCB calculators provide transmission line characteristic
inpedance Zo and wave propagation speed Vp. I use
them to calculate L=Zo/Vp and C=1/(Zo*Vp) per length's
unit. So, correct prediction of PCB absolute equivalent
electrical characteristics (Zo and Vp) is quite
critical for me. Let say, my design is sensitive
to difference in trace inductance of 0.5 nH vs.
0.7 nH and I want to be sure that my prediction
is reliable. Now, to your comments. 1.
As I understand, you agree that AppCAD is misleading
in prediction of impedance for CPW w/o GNDplane.
Quite surprising for me, but I will take it as it
is. 2. You provided results from two more calculators,
and they are quite different. Zo of 55.0 ohms vs
63.5 ohms is a big difference for me, as you already
understand. You also wrote that "chemandy" calculator
is "worse" (or not ?) by definition, because it
does not take (or does ?) in account the fringe
fields etc. etc. Which one would you use for
my specific need? Can you or anybody else
provide any suggestion, especially at the higher
level of the task. For example, may be, to use
significantly lower value of H will be better (i.e.
to put a GND plane closer to signal layer). It will
increase the Cpcb, which may be compensated by appropriate
selection of Cdiscrete with good !!! (0.1+0.05
pF) resolution, but, simultaneously, it will decrease
Lpcb, and it will make selection of Ldiscrete (which
are available with not so high resolution, but only
with 10+2 %) much easier. Any other proposals/advices
will be appreciated. Thank You for your time.
Sincerely Pavel





nubbage 
Post subject: Re: AppCAD coplanar waveguide calculations
Posted: Tue May 04, 2010 6:13 am



General 

Joined: Fri Feb 17, 2006
12:07 pm Posts: 312 Location: London UK

Hi pavel and Kirt (and anyone else) I have played
around modelling the closed and open stripline/microstrip
situation, and compared a number of formulae and
EM software. I ran the parameters through my
resulting excel calculator and for the closed case
and 58mil spacing i) 53.9 ohms ii) 53.38 ohms
iii) 53.28 ohms (I think linxtechnologies .com?)
iv) 53.41 ohms (Richardson's formula) v) 54.35
ohms (a rival website to this one) but for several
of those models I had to use a fudgefactor to allow
for the fringing fields. To derive that I used
fairly recent work by the University of Taiwan that
has resulted in design graphs from Finite Element
Analysis of the accurate fringing fields. All of
those checks were for a symetrical groundplane relative
to the center trace. For the open microstrip
case I get figures around 100 ohms for the width
stated (20 mil) and that agrees fairly closely across
a number of models.
_________________ At bottom, life is all
about Sucking in and blowing out.





PavelM 
Post subject: Re: AppCAD coplanar waveguide calculations
Posted: Fri Oct 29, 2010 3:31 pm


Joined: Wed Mar 31, 2010
2:46 am Posts: 6 
Hello, After long silence I think it will be
quite informative to provide an update about my
experience from building a practical circuit, using
of AWR TXLine. The circuit's purpose is to
work as high speed differential ADC driver with
differential antialiasing filter. After some advices
(see discussion above) about TL calculators, I used
an AWR TXLine in order to calculate the Zo and
Vp of transmission line, knowing the PCB stackup
and TL geometry. Then, having line fragment's length,
I calculated a fragment's equivalent L and C.
Then, I used those calculated L and C as parts of
the filter. Since filter has its pass band of few
hundreed MHz, each 0.1 pF of parasitic capacitance
and 0.2 nH of parasitic inductance may influence
circuit performance. Of course, filters configuration
and order were selected in order to use those parasitics
and not to fight against them. Finally, I made
PCB (FR4), assembled and measured it. It works
just fine from the first time, as it is designed
to do it. All characteristics are by design without
any need to change any component. So, looks too
good to be true. But two more circuits behaves the
same way. So, this way works. And I want you
to know it. May be, you will use this experience
in your work.
Good Luck! Pavel


Posted 11/12/2012


