*From*: "Hassan O. Ali" <hassan@xxxxxxxx>*To*: mmoeller@xxxxxxxxxxxxxxx*Date*: Thu, 13 May 2004 00:36:36 -0400

Moeller, Merrick wrote: >Experts, > Are there any benchmarks that compare the performance between >HFSS and CST available? > I'm not aware of any public document that does that. However, I'm certain many users have made independent evaluation of the two tools for specific applications of their interest. The evaluation reports are usually not published in the public domain due to confidentiality and even legal reasons. > In going from a frequency domain solver to a time domain solver >and vise versa what are the trade offs? > Let's be specific here and consider CST Microwave Studio as the time domain solver and Ansoft HFSS as the frequency domain solver. (BTW, "CST" and "Ansoft" are company names, and "Microwave Studio" (MWS) and "High Frequency Structure Simulator" (HFSS) are the names of the 3D electromagnetic field solvers). MWS solves for transient (time-domain) electric field (E) directly (natively). That is, it directly solves for electric field in a 3D space as a function of time. It then indirectly, through Maxwell's equations, solves for magnetic field (H) as a function of time. Once E and H are known, many other related parameters (such as current distribution), can be computed (indirectly). Frequency domain parameters are indirectly obtained through Fourier transformation. HFSS solves natively for E field as a function of frequency. H field is indirectly obtained through Maxwell's equations. Other related parameters are indirectly computed just like in MWS. Time-domain parameter are indirectly obtained through inverse Fourier transformation. The keywords you need to observe from my two paragraphs above are "direct" and "indirect". Anything computed directly is "potentially" more accurate than the one computed indirectly. Therefore, transient parameters computed by MWS are "potentially" more accurate than transient parameters computed by HFSS. Likewise, frequency-domain parameters computed by HFSS are "potentially" more accurate than frequency-domain parameters computed by MWS. You have to meet certain Fourier transform conditions to obtain accurate results when you change parameters from one domain to the other. In most cases the conditions are hard to meet thereby limiting the accuracy. Any one field component, E or H, can be used for S-parameter computation. However, the two tools differ slightly in the way they compute S-parameters. MWS would need to solve the problem as many times as the number of ports to obtain all S-parameters, while HFSS solves the problem only once to obtain all the S-parameters. For high port count, MWS can potentially be much slower than HFSS due to this reason. For frequency-domain S-parameters to be valid, MWS requires the field at each port to settle to a particular (specified) level. The smaller the level the longer the simulation. The field in high Q structures take extremely long to die off. Such structures may take very long to solve using MWS transient solver. MWS uses a time stepping algorithm. That is, it solves for E field one time step at a time. Once all the time domain E field is computed, the fields at all frequencies are computed at once. On the other hand, HFSS solves for E field one frequency at a time. If you need time-domain results, you will have to solve for many frequencies. It offers at least two ways of obtaining fast frequency domain results : Fast Sweep and Interpolating Sweep. They are both "interpolating" schemes hence introduce their own errors. Generally you cannot really tell which tool will be faster in producing broadband results of a particular structure. Neither can you tell which one will be more accurate in that case. However, for narrowband, you are certainly better off with HFSS. The other issue is about scaling. Bigger size problem needs longer time to settle, which leads to longer MWS simulation time even for narrowband solution. Bigger problems also demand more memory and MWS, being a Windows software, can't solve problems requiring more than 2GB of memory. With HFSS, bigger size problems also means huge matrices to solve, and very long simulation time. In memory terms, HFSS is better as it can run on UNIX and use up to 8GB of memory (or more). > Is accuracy of the HFSS and CST at the same level? > This is a very important question! When I evaluate a tool I don't only check whether the tool is accurate, rather I check if it is *consistently accurate*. I like the tool to be predictable. When I use the tool to model a number of known structures I expect it to consistently give accurate results - for all the structures, all the time. Both MWS and HFSS can be setup to yield accurate results but the effort needed to achieve that differs considerably between the two tools. > Is there any type of problem that will not be accurate with one >or the other? > > Frequency-dependent loss is not handled well by MWS. However, you can do more kinds of analyses with MWS than HFSS. For example,TDR analysis with NRZ waveform. Good luck. Hassan. ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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

**Follow-Ups**:**[SI-LIST] Re: HFSS vs. CST***From:*Giancarlo Guida

**References**:**[SI-LIST] HFSS vs. CST***From:*Moeller, Merrick

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