Frequency interpretation of the z-domain transfer function

RecaU that we showed the relationship between the z-transform 

Given any z-domain expression, we can generate the equivalent frequency-domain expression by simply replacing z with eJ . In Section 10.2.6, we explained that the spectrum X(e ) of a digital signal is periodic with period Itt for normaUsed frequency. This can be thought of as a function that sweeps out a circle of radius 1 in the z-domain. The frequency domain is in effect the particular slice on the z-domain that this circle describes. 

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