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The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Modeling >> Transmission Lines and Other Distributed Devices >> Transmission line modeling & Testing... https://designers-guide.org/forum/YaBB.pl?num=1145948515 Message started by ramana_akula on Apr 25th, 2006, 12:01am |
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Title: Transmission line modeling & Testing... Post by ramana_akula on Apr 25th, 2006, 12:01am Hi.. i want to model transmission line(verilog-A or AHDL). does any one of u had the idea??? generally we know RLGC model(this is for unit lenght).. i donno hw to get length parameter into picture.. i mean length need to be specified as parameter.. waiting for ur replies.. Thanks, Ramana... |
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Title: Re: Transmission line modeling & Testing... Post by Andrew Beckett on May 15th, 2006, 1:54am Why do this in Verilog-A when spectre has a built-in component (mtline) which will do this for you directly? Regards, Andrew. |
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Title: Re: Transmission line modeling & Testing... Post by ramana_akula on May 15th, 2006, 1:59am Hi Andrew.. Thanks for ur reply.. just i want to know hw spectre or spice implemented these transmission lines.. i just want to go through code for knowledge.. that's it.. |
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Title: Re: Transmission line modeling & Testing... Post by Andrew Beckett on May 15th, 2006, 7:03am Well, good luck. I'd say it would be pretty complex to model this in Verilog-A! Andrew. |
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Title: Re: Transmission line modeling & Testing... Post by Jess Chen on May 15th, 2006, 7:59am VerilogA is most functional in simulators like Spectre and SPICE. (VerilogA is somehwat crippled in all RF simulators I know because of the hidden state problem.) Those simulators were designed to solve ordinary differential equations, not partial differential equations. Transmission lines are described by partial differential equations. The only way I know of to make partial differential equations look like ordinary differential equations is with lumped approximations; chop the transmission line up into small segments, say of length delta. The trick is to find delta. The lumped approximation for a section of length delta is a pi or T or L network with RLGC scaled by delta, and perhaps 1/2, depending on which network you select. As I recall, I determined the number of segments by first determining the shortest delay of interest, then computing how far a pulse propagates in that time, and then dividing that distance by 20. I believe there's a similar frequency domain approach. The factor of 20 was just a rule of thumb based on trial and error. If you want more accuracy, you need small deltas. As for implementation, you need not write out a RLGC line for each segment, you can use the generate command. Since the segments are linear, you should not have any convergence issues but you may see slow run times if you have lots of lines and/or lots of segments. Harmonic balance based RF simulators can handle linear partial differential equations directly because they allow you to describe the system in the frequency domain. If you have access to one, you may be able to use an envelope option if your stimulus is not periodic. However, I don't think you will be able to use VerilogA with a harmonic balance simulator because to my knowledge, VerilogA does not let you describe behavior in the frequency domain, unless it is a noise look up table. I'm not 100% sure but I think you can enter look up table transfer functions in most hb simulators, without using VerilogA. -Jess |
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Title: Re: Transmission line modeling & Testing... Post by ramana_akula on May 15th, 2006, 11:09pm HI Jess Thanks alot for ur information... i was unable to understand the trick for determing delta.can u plz tell me wat will be general spec for shortest delay...can u plz explain further.. 1) suppose i want to have a model for 1m tx line.. hw many no.of segments(lumped) needed. 2) sud i need use ABCD parameters for interconnection of these segments.... u told about generate statement with suggestions,i want to use spectre AHDL...i was not restricted to verilog-A.. just i want to test my lumped model segment for tx.line.. Thanks Jess.. waiting for ur reply.. Thanks to Andrew for his caution.. -Ramana.. |
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Title: Re: Transmission line modeling & Testing... Post by Jess Chen on May 16th, 2006, 8:25am Hi Ramana, I use the following approach, which is empirical in nature. Let the maximum frequency of interest be fmax in Hz. Assume the line has L Henrys/meter and C Farads/meter. The propagation velocity, vp, is vp = 1/sqrt(LC). The minimum wavelength, lambda, is defined as lambda = vp/fmax. Let X be the length of your line. The number of segments, N, is N = 20*X/lambda. "20" is the empirical part. I found that 20 gives a pretty good match between lumped and distributed models. Your segment parameters will then be L*X/N Henrys C*X/N Farads G*X/N Mhos R*X/N Ohms I hope this helps. -Jess |
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Title: Re: Transmission line modeling & Testing... Post by Jess Chen on May 16th, 2006, 3:31pm I forgot to say something about the "generate" statement. For a good example of how to use the generate statement, look at some of the filters in the rfLib. For example, check out the filter.h file in the butterworth_lp directory. My only caution is that it will be hard to parameterize the number of segments because you cannot parameterize the number of internal nodes. It is for exactly that reason that the behavioral filters in rfLib do not follow the usual file structure. To parameterize the order of the filter, Cadence resorted to a netlisting procedure. It was messy but it worked. You can still make the lumped model somewhat flexible by using `define. With `define, at least you only have to change one line in the veriloga.va file. -Jess |
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Title: Re: Transmission line modeling & Testing... Post by mikki33 on May 21st, 2006, 9:32am Transmission line in HSPICE you may represent as a W-element. |
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Title: Re: Transmission line modeling & Testing... Post by jbdavid on Aug 3rd, 2006, 2:53am And in Verilog-AMS, I often want the line to be represented as discrete values (1/0) so I calculate only the delay from the length.. generally I can live with a single direction signal. |
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