Capacitive coupling phase frequency discriminator circuit

Capacitively coupled phase discriminator Figure 7-49 (a) is the basic circuit of a capacitively coupled phase discriminator. The two circuits are shielded from each other. In the figure, Cm is the coupling capacitance between the two circuits, and its value is very small, generally only a few picofarads to a dozen picofarads.
The coupling loop part is shown separately in Figure 7-49 (b), set C1 = C2 = C, L1 = L2 = L, and its equivalent circuit is shown in Figure 7-49 (c). According to the coupling circuit theory, the coupling coefficient of this circuit can be obtained as: