ads cpw

Hi everybody!

I need to simulate some microwave circuits in coplanar waveguides by using ADS Momentum 2006. I prefer to use STRIPs instead of SLOTs even thought simulation is demonstrated to be more efficient with SLOTs, since I generally start from masks or .gds file.Then, I use 2 metallic strips as ground planes and one strip between them as RF line. I set two opposite ports as "internal" and the other two couples of ports as "ground reference", obviously referred to the rispective internal port.

I noticed that S-parameters results from a full wave simulation of a simple 50 ohm CPW (for istance, 1 mm of length) are different from the CPW lumped element provided by the schematic tool of ADS. These results are different also from measurement results! In fact, I usually try to simulate a complex structure in Momentum (i.e. suspended bridge, discountinuities, transitions, ecc..) and it seems to be coherent with measured data, but if there are some simple CPW path, I need to simulate the whole structure by exporting full-wave S-parameters in the Schematic and by adding CPW lumped elements where needed, rather than simulating CPW path directly in Momentum. Otherwise, simulated data are different from measured data! You can verify my assertion by designing a simple CPW line (using Strip and not Slot!) in Momentum and comparing it with an equal length CPW lumped element.

Did I make some mistakes in preparing the model? Is there anyone among you who has ever encountered this kind of problem?

thank you in advance!

Hi sool83,

My question is why you use internal ports instead of single ports? Furthermore, as I remember it is not necessary to use two ports as "ground reference" for a CPW simulation. Generally, if you made the right set up for momentum the results are really close to ADS schematic simulation.

So just check the setup of your simulation (substrate, meshing and so on), then try with single ports only, then compare the results with schematic.

B.R.
Kai

Hi sool83,

I think that I can help you on CPW, I have already faced the same problem, actually it is a little bit difficult to do it in ADS, so send give me your email in order to send you some files. It is a little bit big and I can not upload it here.

Hi,

did you solve your problem, because Im having the same problem? But i am simulating CPW with gnd plane.

What was the fault?

You need to setup signal ports on the signal trace and ground referecne ports on the ground traces

;;

Could somebody upload and GCPW and/or CPW ADS model?

I have done some work about simulating CPW lines in Momentum. I think for CPW simulation in momentum, Single Mode port with a Reference Offset maybe needed to get a more accurate result.

Be careful of the backvia, you may need to via properly as NOT to general unwant mode at the input port, or it could affect the calibration work. In Sonnet, the box are all grounded, but in momentum, you should ground the ground plan carefully first.

Can anyone tell me which port is more accurate?
single port or internal port?

Hi, so I'm trying to understand this -- using ADS/Momentum and CWP lines, I'm starting in the schematic editor, adding a CPWSubstrate and a CPW line of a certain width/gap/length which is matched to that substrate.

I then simulate this and am trying to see/optimzie the CPW width/gap size(s) in order to obtain 50 ohm matching.

This doesn't work; plotting PortZ(1), and PortZ(2) for the two ports results in 50 Ohm exactly, which is the impedance of the termination ports.

Then I try to use this and dump it into Momentum and do the same simulation; I get a warning about port impedance:

"WARNING: The 2D port solver data is not available. Using default values (50 Ohm for the transmission line parameters".

and

"WARNING: Port Solver failed to compute propagation constant and characteristic impedance. 50 Ohm default impedance will appear in Dataset"


Any thoughts? Can somebody please send me a sample DSN file which shows port impedance calculations?

Thanks!

As a separate question -- how do you design bends/etc. in CPW?


Thx!

Hi,

to simulate a CPW you can see also here:

http://www.uwaverf.altervista.org/fo....php?f=19&t=19

bye

hi Dan,
I also have the same question 'how do you design bends/ arc and circle'
have you figured it out?

Thanks

Stripline Circuit Design, Harlan Howe is a useful reference.

Regarding the previous link i posted it was for HFSS simulations, the following link regards the CPW simulation in momentum:

uWaveRF - View topic - CPW Wilkinson Power Divider in ADS

Hi All,

First of all I recognize that this is a thread about an ADS CPW simulation.

I have had good luck with HFSS and wave ports exciting GCPW transmission lines. There are several subtleties that need to be addressed but they are discussed in the HFSS help files. Look in the impedance calculation sections of the documentation. Basically, CPW lines are "mode rich" and the situation changes with the relative sizes and spacings of the lines. I suspect that this is one reason for the popularity of microstrip and striplines over the various CPW and GCPW configurations. CPW lines are attractive when components are placed on the line because a ground is readily available without the added confusion of pads and vias. It wouldn't surprise me if you were modeling an unexpected mode with ADS. You might look at the fields at your port to see if you are getting the excitation you expect.

Reference books by Simon and Waddel are also useful references. I have cited them in numerous post (here, EDA board) in the past.

I've also seen a similar behavior (discrepancy between Z0 in Momentum and ADS components in schematic for CPW lines).

From my observations, the discrepancy was larger for larger width of the signal trace. In these cases, the s-parameter and consequently Z0 of the line were dependent on where I placed the Internal ports (where across the width of line). But when I used single-ports, the results matched between Momentum and ADS. This discrepancy reduced for smaller width of the lines and disappeared for width smaller than about lambda/20, but existed for larger width.

I thought the reason that this discrepancy wasn't seen with single-ports was that in single-ports the whole edge will be equipotential, but in internal ports, the excitation depends on where the input lumped source is applied.

For very large widths of signal lines, this makes sense to be true, but I wonder whether this discrepancy (dependence of Z0 on where the internal ports are located across the width of the line) exists also in practice (i.e., widths larger than lambda/20 or so shouldn't be used) or it's more simulation inaccuracy for these range of width.

It would be great if you could share your experience.

Thanks a lot.

hi friends
i have a problem in licence file how to solve this problem in cst studio can any one suggest me.........
gopigopinath10@gmail.com

Hello Asturias!

I myself am right now experiencing the same related issues regarding the difference between schematic and MoM co-simulation of a simple CPW structure. I tried with with both internal and single port on the conducting strip and still I have differences. Not to mention that what I plan to do is using a couple of such CPW Layout created components for adding them in cascade in order to obtain a larger structure that can be later width-modulated.

Could you please send me the info.

Thanks in advance.