The possibility of using larger delay time values in CPFSK systems with tight IF filtering to improve the bit error rate performance is investigated. The analysis uses a previously developed rigorous model for CPFSK systems with tight IF filtering. It is shown theoretically that a resulting system sensitivity of 36.2 photoelectrons per bit time for a 10−9bit error rate can be obtained for a modulation index of 0.8 and that the optimum system can be operated with a laser linewidth that is 0.87 times the linewidth tolerated by a DPSK system with tight IF filtering.