[SI-LIST] Re: HFSS vs. CST

HFSS v9 can do a step or impulse TDR transform, as Hassan mentioned (although 
I've never been able to get it to work), but can it do TDR with an NRZ waveform?

--
Daniel


From: "Giancarlo Guida" <gguida@xxxxxxxxxx>
> Dear Hassan,
> I think you are incorrect about the capability of  HFSS
> about TDR.
> 
> TDR can be easily obtained from HFSS results.
> 
> Regards
> Giancarlo
> ----- Original Message ----- 
> From: "Hassan O. Ali" <hassan@xxxxxxxx>
> To: <mmoeller@xxxxxxxxxxxxxxx>
> Cc: "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
> Sent: Thursday, May 13, 2004 6:36 AM
> Subject: [SI-LIST] Re: HFSS vs. CST
> 
> 
> 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.

-- 
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