**Definition of ROC :**

The ROC stands for the region of convergence of X(Z) is set for all the values of Z for which X(Z) attains a finite value. Every time when we find Z transform we must indicate its ROC.

**The significance of ROC :**

- ROC will decide whether a system is stable or unstable
- ROC also determine the type of sequence that means: Causal or noncausal, Finite or infinite

**Properties of ROC for Z transform :**

- The ROC is a ring, whose center is at an origin
- ROC cannot contain any pole
- If x(n) is causal then ROC is exterior part of a circle of radius say ɖ
- If x(n) is anticausal then ROC is interior part of a circle of radius say ɖ
- If x(n) is two-sided sequence then ROC is the intersection of two circles of radii ɖ and β
- The ROC must be connected region
- If ROC of X(Z) include unit circle then and then only the Fourier transform of DT sequence x(n) converges
- For a finite duration sequence, x(n) the ROC is the entire Z plane except Z = 0 and Z = ∞

**Summary of ROC :**

- For Causal system, ROC is entire Z plane except |Z| = 0
- For the anti-causal system, ROC is the entire Z plane except |Z| = ∞
- For two-sided sequence ROC is entire Z plane except |Z| = 0 and |Z| = ∞