Thus the ju axis of the s-plane is mapped on the unit circle of the z-plane, as shown in Figure 6.8. Obviously, the right-hand half plane (RHHP) of the s-plane is mapped to the outside of the unit cycle in the z-plane. A stable system means that for a given bounded input, the system output must be bounded. Similar to the analog system, the digital system requires that all poles plotted on the z-plane must be inside the unit circle