Impedance plot trick in LTspice

I'm looking for a way to plot the impedance curve of a loudspeaker without having to enter an expression in the LT spice graphical post processor, but simply probing on a net label. It does not matter if the value is returned in volts instead of ohms. I was thinking of entering the transfer function in a behavioural source, but BS return zero current value in case of AC analysis. Thanks to those who can help me.

• Are you saying that you have the impedance data for the loudspeaker and want to import that in to LTspice? There is a way of doing this but it only works for .AC analysis. If this is what you want, I can write up the technique for this. If you want to do transient analysis, you need to synthesize the impedance with lumped RLC elements.
– qrk
Commented Jan 11, 2023 at 22:28
• I already have a loudspeaker model perfectly working. Commented Jan 12, 2023 at 10:23
• @AndresRu This is what I don't understand... if you have a perfectly working loudspeaker model why would you need to plot it's impedance more than once? Commented Jan 12, 2023 at 16:09
• The model is intended for the design of loudspeaker systems, acoustic transmission lines in particular. Thus it is very convenient to see how changes to the system (e.g. the addition of damping material or a dimensional change) affect the impedance curve. Commented Jan 12, 2023 at 16:31

Add a current source to your schematic. Set it's AC amplitude to 1.

Connect it to the speaker circuit and run an AC analysis. Set it to something like Octave, 200 points per octave, start frequency 1, stop frequency 100k.

Plot the voltage where the current source connect to the speaker.

Right click on the left axis of the graph, change setting to logarithmic.

Left axis should now read directly in ohms, bottom axis should be frequency.

• Thank you, I was aware of this method. It was already discussed in another thread. I have an amplifier connected to the loudspeaker and the idea is to have a test point always ready to simulate the impedance curve. Commented Jan 11, 2023 at 22:16
• @AndresRu I suppose that could be done, I'd have to think about it some more. In my amp sims I use jumpers to select different sources and loads, I can move the jumpers to put in a current source to check input or output impedance but I never had a reason to do that with the load as it's generally not going to change it's impedance very often. Commented Jan 11, 2023 at 22:28
• At the moment I'm using the current source technique. I've got the circuit integrated in the amplifier subcircuit and if necessary I rotate the amplifier block and connect it to the speaker through the 1A AC source instead of the amplifier output. The challenge is to find an even more user friendly solution: a test point always working that does not interfere with the system. Commented Jan 12, 2023 at 11:25