Allpass Transfer Function Now. the poles of a causal stable transfer function must lie inside the unit circle in the z-plane Hence, all zeros of a causal stable allpass transfer function must lie outside the unit circle in a mirror-image symmetry with its poles situated inside the unit circle Copyright C 2001, S K Mitra8 Copyright © 2001, S. K. Mitra Allpass Transfer Function • Now, the poles of a causal stable transfer function must lie inside the unit circle in the z-plane • Hence, all zeros of a causal stable allpass transfer function must lie outside the unit circle in a mirror-image symmetry with its poles situated inside the unit circle