Shown in this figure is a LINC2 analysis of a crystal oscillator circuit.  The graph shows the gain margin (magnitude of S21 in dB) at the zero phase crossover point  (P21 in degrees).  After selecting the crystal's series resistance from the "Tune" menu and adjusting it to 30 Ohms (originally 90), the dotted curve indicates that an additional 3 dB of gain margin is achieved.  In the physical circuit, connecting port 1 to port 2 closes the loop and starts oscillation at the design frequency (zero phase crossover point).