I am reading this paper titled A Versatile Digital GHz Phase Lock for External Cavity Diode Lasers. This is part of the circuit.
I am confused about the functionality of the part labeled "Phase Advance Filter."
The paper says:
two mechanisms oppose each other, which leads to a phase shift of 180◦ at modulation frequencies typically between 1 and 10 MHz . In order to partly compensate for this effect, a phase-advance loop filter is used in the fast feedback path, followed by a buffer stage (IC1D) with an adjustable gain to drive the laser diode current modulator.
The two mechanisms being at low frequencies the lasing medium temperature changes so the lasing medium's refractive index changes, and at high frequencies there are current-induced charge densities which affect the refractive index of the medium.
How does this filter (at least partly) oppose these mechanisms?
When I tried to solve the transfer function for the phase advance loop filter, I find this as a Bode Plot. I don't think this is right however, because it says the phase angle doesn't change in frequency when it should since frequencies between 1MHz and 10Mhz should be 180 degrees out of phase. Can someone help me to derive the transfer function?