SPICE - Frequency dependent resistor model

You can use Laplace transforms to make frequency dependent resistors for AC simulations. See

for ideas on making resistors out of voltage and current sources. I like using current sources to make variable resistors. Some flavors of Spice (LTspice and PSpice come to mind) allow you to use Laplace transforms in the E and G devices.

--
Mark

For an explanation from the PSpice folks, go to

and select Item 2. That example deals with frequency dependent inductors where Z = Ls. Since a resistor is simply Z = R without the trailing s, follow the example but exclude the trailing s. You will have to use the abs(s)/6.283185 operator as your frequency dependence since abs(s)/6.283185 = f. Good luck.

Thanks everybody for your replies. I guess my searches weren't comprehensive enough, and I apologise for not having checked out all the threads before posting a new one.

Jeff : Although I had already seen the post on modeling skin effect in resistors, I found the following relevant post when I searched in Google groups as you suggested. It refers to modeling the resistance as a function of frequency using the .PARAM command

qrk (Mark) : That was a great link that you posted. I found many useful ideas for simulating a resistor using controlled current and voltage sources. I will try incorporating Laplace transforms to model frequency dependence as you suggested.

Bob : This looks like a possible solution to the problem of incorporating frequency dependence. I think it can be combined along with Mark's idea above to do the job.

I will try out the various possibilities and post the results. Thanks again for all your help. It's been a great learning experience.

Aravind.

I think that only works for one specific value of "hertz", or stepping thru a .AC; certainly not for .TRAN

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.

I tried the .PARAM option without much sucess. I keep getting the error "illegal parameter value: hertz". I substituted HERTZ with other variants, like FREQ, but got the same error. Finally I tried OMEGA and got an error message saying that OMEGA is a state variable and cannot be used in a parameter expression.

The other solution of using voltage controlled current sources works beautifully. I used a GLAPLACE device and was successfully able to use it to replace the capacitor in an RC circuit. I guess it should work for any other element whose transfer function is known beforehand. My problem is solved !

Thanks again for all your help.

Regarding my earlier post in this thread: The technique for which I provided a link works fine for frequency domain simulations. No surprise there. But I haven't been able to use the same frequency-dependent resistor model in time domain simulations. Are you aware of how to use such a frequency-dependent resistor model in both the frequency and time domains?

As snipped-for-privacy@gmail.com discovered, it has to be a "GLAPLACE" or an "ELAPLACE" device. I know that this works in PSpice, but I don't know about the others.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.

ELAPLACE and GLAPLACE do not seem to be part of Berkeley Spice, but perhaps some may find a "B" non-linear dependant source source will do the job for them.

--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.  
--Schiller

perhaps

No that's not possible. Going from frequency domain to time domain requires a convolution and this is not supported by the Berkeley spice engine. This could be added with the XSPICE interface and would indeed be a nice add-on.

Maybe someone, someday...

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Thanks,
Fred.