The modified gamma probe (GPROBEM) and gamma probe (GPROBE) elements are insertable elements that are placed on the input and output pins of active devices. When placed, they are invisible to the circuit so that the through signal path is not disturbed. They calculate the two reflection coefficients for the node into which they are inserted (Gamma 1 looks to the left and Gamma 2 looks to the right). The reflection coefficients themselves can be plotted, but the main thing to look at is the STAB_GPM measurement (with the GPROBEM element) and STAB_GP measurement (with the GPROBE element), which is the product of the two reflection coefficients. If this product is greater than 1 at any frequency or load / source pull point then the device is unstable. The biggest difference between the two probes is how they behave in circuits with feedback across the active devices that are being tested. As this example shows, the GPROBEM gives the correct answer, while the GPROBE does not. More information on the operation of these probes can be found in the Microwave Office Users Guide - in the Appendix sections. A paper on stability analysis, "Modified S-Probe Circuit Element For Stability Analysis", is available below under Downloads.