In his paper “A Novel CAD Probe for Bidirectional Impedance and Stability Analysis” (A Novel CAD Probe for Bidirectional Impedance and Stability Analysis | IEEE Conference Publication | IEEE Xplore), Thomas A. Winslow presents an improved bidirectional impedance probe. This probe has been implemented as WSProbe in Keysight’s ADS (with some additional features).
Some time ago, I realized that the functionality described in the paper could easily be implemented by using the equations from Tian’s loop gain paper “Striving for Small-Signal Stability” (https://kenkundert.com/docs/cd2001-01.pdf), because Fig. 6 vom Winslow’s paper is equivalent to Fig. 7 from Tian’s paper.
I have now implemented the simple example from Winslow’s paper for QSPICE, using the same setup as in the example circuit Tian.qsch from the QSPICE distribution. The results match the plots in Fig. 10 and Fig. 11 of Winslow’s paper.
In addition to the impedances seen by a voltage source (same current on both sides) as presented in the paper, it is of course also possible to calculate the impedances seen by a current source (same voltage on both sides). I have called the corresponding admittances Ysrc and Yload and the resistances RsrcI and RloadI. Unlike Rsrc and Rload, they are almost constant over the simulated frequency range in this simple example.