Hi, I want to find out if there is a possibility to generate the Bode plot of a plant transfer function by using .bode. If the .bode works well in an open loop plant + regulator setup, there is a difficulty if, for instance, I want to perturb the reference (V2 in the picture) to get only the plant transfer function.
Of course, probably the trick with a sine voltage source and the set of equations as described in LTspice help file will work. I also tried with two .bode directives (one to be just the injected signal and another one to be just like a probe in a similar manner I was able to use the @ fra device in 17.1 release) without success.
Any ideas?
The .bode command is for extracting the open loop gain from the circuit while it’s operating closed loop.
–Mike
In latest Qspice update, Mike added feature to allow define of input and output nodes for .bode. Here is his comment in revision file, corresponding help in .option also updated.
09/11/2023 Implemented .options bodeinput=… and .options bodeout=… to allow override the input and output nodes used for .bode analysis.
With this update, you can add perturbing source on top of reference source in open loop setup (both end of perturbing source must not ground), this equivalent to add a perturbing signal in reference. By assigning bodein and bodeout in .option to reference and output net name, bode plot of plant in a open loop setup can be obtained.
6 Likes
That’s great! Thank you for posting.