This PSD is plotted in Fig. 3-16a, where 1/Tb = R, the bit rate of the line code. The disadvantage of unipolar NRZ is the waste of power due to the DC level and the fact that the spectrum is not approaching zero near DC. Consequently, DC-coupled circuits are needed. The advantages of unipolar signaling are that it is easy to generate using TTL and CMOS cir¬cuits and requires the use of only one power supply.