We develop a simple analytic gaussian model, predicting BER performance of optical DPSK receivers with high accuracy in the wake of all three phase noise sources impairing differential phase detection: linear phase noise (ASE-induced), nonlinear phase noise (the Gordon-Mollenauer effect), and laser phase noise Brownian fluctuations. We validate our analytical Q-factor based formulas using known analytical cases and importance-sampling based Monte-Carlo simulations.