LTSpice: One component value dependent on another's value

Question

I've asked this on the LTSpice group and am getting conflicting answers.

I have a tank circuit in a sim I'm working with, and I need to look at the effect of changing the L value across a range (using .step statement) and of course the companion capacitor value needs to be adjusted to match.

.param F 500000
.step param L 1uH 10uH 200nH
.param C (1/(2*Pi*F))**2/L

The capacitor value is set to {C} and the inductor set to {L}

I've been told that this "should work" and that it "will not work".

I suppose it comes down to when the capacitor value param statement is evaluated. When I run it, I'm told that it can't resolve the parameter C.

I'm not married to this approach, it simply seemed the most obvious way to proceed, but I do need to be able to adjust the C value to match the L in each run.

If this can't work, is there a way to get to what I need, having the C automatically adjusted to resonate with the L?

Spehro: I'm not sure exactly what you mean, and what I'm doing isn't working.

I have .param A = (2pi{F}) and .param B = {A}**2 and .param C = (1/{B})/{L} "Missing expression in **2" is the result.

The Photon:

This version: {(1/2piF))**2/L} gives me Error: undefined symbol in: "(1/2pi[f]))**2/l"

So I tried this version, setting the value in the component to:

{(1/2pi{F}))**2/{L}}} Error: undefined symbol in: "(1/2pi([f])))**2/(l)"

Answer 1

You should give us something reproducible. This may mean spending your time to produce a different schematic, one that does not disclose anything you care deeply about, but at the same time allows us to see the issue you are encountering. Surely, you are capable of take your existing schematic and, by hacking out parts of it while preserving the error, eventually have something to present to us for our examination.

As it stands (unless I'm mis-reading) none of us can replicate your problem. You won't share your netlist or errorlog or schematic. This pretty much shuts down any possibility of us finding your problem, except entirely by hunt-and-peck guessing about it. For now, this makes your question largely an unproductive waste of all our times, in my opinion.

This is your question. Why can't you afford a moment to produce a publishable reproduction of the problem you face, where a solution could be at least conceptually applied by you to your secret schematic?

That said, suppose I wanted to take a resistor divider and specify two things: \$R_\text{TH}\$ and the divider ratio at their shared node. Let's call the "top one" \$R_1\$ and the bottom one \$R_2\$ and the ratio as \$RATIO=\frac{R_2}{R_1+R_2}\$. Then in LTspice I may write:

.param RATIO={0.8}
.param RTH={5k}
.param R1X={RTH/RATIO}
.param R2X={RTH/(1-RATIO)}

Then I would use {R1X} for \$R_1\$'s value and {R2X} for \$R_2\$'s value in LTspice. It's as simple as that.

Skimming over what you've written I've no idea why you write, {A}**2, which is almost certainly an error. It probably should be either {A**2} or else {pow(A,2)}.

Since you won't share and won't work to produce something you can share with us, I think that's about all I can suggest. Go over your work with a jeweler's eye towards detail. See if you can come up with the problem on your own. None of us have a working crystal ball.

Answer 2

Works fine for me. Replace X with {X} where you want the value and write it as 3 separate statements.

Answer 3

You need to use .step to change the value of the capacitor for each .step'd value of L.. You will not succeed unless you do.

Above statement by John Woodgate in reply, when the same topic was asked as "Calculating the value of a component during a sweep" at [email protected]

Answer 4

I've solved it. The A^2 term requires NOT using curly brackets, and everywhere else requires using curly brackets on the variables. Using curly brackets around A breaks it in that one statement, and NOT using curly brackets around the variables anywhere else in this also breaks it.

This works, the values of the capacitor step along with the inductor, and the values then found to be optimal can be entered as fixed values in place of {C} in the capacitor for a final check, once the .step is commented out.

.step param L 39uH 68uH 1uH
.param A = (2*pi*{F})
.param B = A**2
.param C = (1/{B})/{L}

I was also told in the LTSpice google group that this is IMPOSSIBLE, and I would have to generate lists of values externally.