modeling with LAPLACE in G source value field doesn't seem to give the correct result

Hi ddurfeeengineer ,

Ah…

ddurfeeengineer said:

If it doesn't work in the simplest case to just match what a resistor does, then I can't expect it to work if I add a frequency dependence. 

I see what you want to show. Actually, the problem you have is quite simple:

ddurfeeengineer said:

Now when I run the test, the G-source compared to the standard resistor no longer give the same result as the frequency increases. 

That is because inductor values are different. I changed both inductors to .04 and the simulation results match. Also changed the G source"Laplace=1/250" to simply make the gain ".004" and that matched the resistor, too. Interesting aside: gain must be a value and not a fraction. You may have uncovered, or maybe simply revealed to me, a nice little hack if you want a calculated gain/transconductance.

mike

I can't believe I fat fingered the inductance and didn't notice.  I apologize for bothering you with this.  I will now try the original goal (which was to get a frequency dependent resistor) and see if it works ok.  If so, then I'll verify the answer.  dave

after that terrible mistake, I hope you are willing to entertain my followup question.  Why can't I make a resistor dependent on frequency.  (so no phase modification) so I can make a core resistance in an inductor.  Since the equation for the G source should be giving me 200 ohms at 20Kz, I would expect these two circuits to behave the same.laplace_test(2).asc

Hi ddurfeeengineer ,

Will take a look.

mike

hi ddurfeeengineer ,

Apologies, I misspoke. From the help:

s = sqrt(-1)*2*pi*f

You may want to check out this video:

www.youtube.com/watch

mike

MIke,

With this equation "LAPLACE=1/(.01*(((s*s)/(4*PI^2)))^.5)" I think I am taking the magnitude of s/(2*pi) which would be f.  With f at 20000, I should get 200 ohms for the denominator.

I squared and square rooted in case the phase due to the imaginary component was an issue.

Why wouldn't that work?

(the video is for voltage controlled voltage source.  I need a voltage controlled current source to make my resistor.  What exactly in the video should I look at to help figure out why the above equation doesn't work?)

MIke,

With this equation "LAPLACE=1/(.01*(((s*s)/(4*PI^2)))^.5)" I think I am taking the magnitude of s/(2*pi) which would be f.  With f at 20000, I should get 200 ohms for the denominator.

I squared and square rooted in case the phase due to the imaginary component was an issue.

Why wouldn't that work?

(the video is for voltage controlled voltage source.  I need a voltage controlled current source to make my resistor.  What exactly in the video should I look at to help figure out why the above equation doesn't work?)

curious if you have any insight into this.

The core loss in magnetic material is a bit more complicated then a simple 1/f relationship but I did some experimenting and created an R/f part using an undocumented version of the B part.  The attached schematic shows an example of it.

sqrt(abs(s**2))/6.2832 = freq

This value becomes a divisor of the initial 4meg to give 200 ohms at 20Khz. Plot I(B1)=V1/200=5ma. The phase of I(B1) is 0 so R is a real value.

I’m still using an older version so I don’t know if this feature is still viable in LTspice24.

TstBr.asc

Thanks for having an interest in my problem. 

I'm less concerned about the actual model for the core than I am that the LAPLACE function doesn't seem to be giving me a result that makes sense.  The change in resistance with respect to frequency (but with no phase difference) should match at a particular frequency and they do not seem to.  It is that question that is of most interest to me.

I hesitate to use the function if it isn't giving me answers that make sense to me in a fairly simple situation.

Here's a version using a G element. From what I can tell, the current equals the control voltage divided by the LAPLACE expression. 25.13274meg is a rounded version of 2xPIx4meg.

TstGr.asc

RetiredEE -- I wonder if you have any insight into the issue I brought up with the .asc file I provided above?

I ran your laplace_test(2) circuit using my LAPLACE expression and the current/phase of G2 matches the current/phase through R23 at 20Khz.  Using your expression, the equivalent resistance of G2 is |1/L(s)|. Testing shows it equals 200 ohms at 20Khz. However, it’s phase is 90 degrees not 0. I think the problem is in the s*s term.  This gives 4PI²eⁱ¹⁸⁰ . When the square root is taken, your left with 2PIeⁱ⁹⁰ which results in a 90 degree phase angle.  Change 4 to -4 which will divide out eⁱ¹⁸⁰ before the square root is taken. This gave the same results I saw by using my expression.