# How do you find the phase lag of the current in this FM demodulation "discriminator" circuit?

I need some help understanding this discriminator circuit for FM demodulation.

(L1, C1) is the primary LC tank which resonates with the carrier frequency, whose FM signal is applied at V12. As the FM signal oscillates according to the signal, the instantaneous voltage and current amplitudes of the LC tank vary across the Bode plot for the circuit.

The RC filter then picks out the AF signal from the varying amplitude due to the signal's skipping around the amplitude. In particular it does so according to the voltage relations between that across L3, and the two halves of the centre tapped secondary.

Warning: I don't understand transformers comfortably yet.

Questions: Most important:

1. When the LC tank is above or below resonance, how do you find the phase of the current and voltage through and across L1?
2. When the LC tank is above or below resonance, what is the phase of the voltage across L3?

Just to verify:

1. Are the voltages and currents of L1 and L2 in phase / phase difference of 180 degrees?

2. Do the capacitors C and C4 cause a double 90 degree phase shift (180 degrees) on the voltage across L3?

I care about 1) because I don't know how to currently explain to myself why the phase of voltage across L1 and the current through L1 can be anything else than 90 degrees. Why? Because the voltage across L1 is di/dt which is a 90 degree phase shift off of the current through L1.

I care about 2) because in combination with 1), knowing the phase of the voltage across L3 would allow me to understand the voltage relations for the RC filter circuit. 3) Would allow me to understand the same. 4) Is supplementary.

• Bode plots don't deal with instantaneous voltages or currents. Jan 11, 2021 at 16:27
• @Andyaka aside from that? Jan 11, 2021 at 16:45
• I stopped reading after that. Jan 11, 2021 at 18:13
• @Andyaka Gee thanks. Jan 11, 2021 at 18:53