• Use iprobe component in the library to break the loop at a convenient point (where the effect of loading can be ignored). The probe is closed for dc analysis and open for stb analysis, where an input signal is injected and the loop-response is obtained.
• Perform stb analysis for the loop, and plot results using Direct Plot menu options (See slides on ECE 511 course site)

• Use cmdmprobe or diffprobe component from the library to break the loop at a convenient point (where all the loops are broken together). The probe settings are used to change between differential and common-mode loop analysis.
• Perform stb analysis for the loop, and plot results using Direct Plot menu options (See slides on ECE 511 course site)

Tutorial describes the loop-stability analysis method.

For stability analysis, these days in the new Spectre versions, you should use diffstbprobe rather than cmdmprobe. You point at that probe in the stb analysis form, and tell it whether you're analysing the differential or common mode loop gain. If using cmdmprobe (if you're using an older version) - then the cmdmprobe has a parameter to control whether you're analysing differential or common-mode loop gain (you set the parameter to +1 or -1; I forget which is which off the top of my head).