The performance of the demod-remod technique is limited by additive white Gaussian noise, secondary signal interference, and phase noise on the primary signal.
Phase noise is caused by a random, time-varying carrier frequency and phase drift.
This drift originates in the transmitter, the receiver, or both, and exists on all real signals. Phase noise fundamentally limits the ability to remodulate and cancel a real FSK signal. The PLL in the post-m filter can, however, partially track this phase noise. Phase noise consists of both high and low frequency components. Since the secondary signal is uncorrelated with the primary signal, it appears as noise to the PLL and contributes to the jitter.