L1 and L2 are magnetically tightly coupled and by adding C3 across the center- tapped coil they will form a parallel tuned circuit with a resonance frequency equal to the unmodulated carrier frequency Capacitor C5 will shift the phase of the input signal by 90o with reference to the voltage across L1 (and L2). These voltages are shown as Va and Vb in the phasor diagram given in Figure 119. Using the input signal Vfm as the reference, the phasor diagrams now look the way shown in Figure 119