imd1 wrote on Sep 19th, 2008, 5:43am:Why do you think that eldo is using the old method ? I am curious how to go about testing for this.
I concluded this from one of the messages in
http://www.edaboard.com/ftopic265486.html :
Code:* .lstb This command improves the analysis of circuit stability.
* The .LSTB command measures the loop gain by successive injection (Middlebrook
* Technique). A zero voltage source is placed in series in the loop: the first pin of the voltage loop
* must be connected to the loop input, the other pin to the loop output.
It mentions Middlebrook but not his General Feedback Theorem.
Quote:So, AFAWK Spectre (and LTspice!) are the only simulators that support the (correct?) loop gain test fixture ?
The only simulator I know which has Tian's method directly built in is Spectre. In LTspice, I have implemented it by using some ideal sources together with a parameter sweep and an expression that calculates the loop gain from the simulation results. It should be possible to implement Tian's method in a similar way in most other Spice simulators.
The question regarding the correct loop gain is a bit more difficult to answer. From a usability standpoint, I like Tian's method best because of its symmetry. With respect to formal correctness, Middlebrook's General Feedback Theorem probably has the better arguments on its side. This question is somewhat academic, however, because for practical circuits, the results of all three methods discussed here are usually very close to each other. They also will all tell you correctly if the circuit is stable or not (see my webpage
http://www.geocities.com/frank_wiedmann/loopgain.html for details).