With the emergence of new high-speed data standards, such as the IEEE 802.3ba, the clock rate on individual channels can go up to 10 Gbps creating the need for highly efficient encoding schemes. One such scheme is the 64b/66b. An undesired effect of using encoding schemes is the spreading of signal power to higher harmonics, thereby requiring greater bandwidth to transmit the signal. A clear understanding of the exact effect of the encoding scheme on the PSD of the transmitted data will help in better understanding the bandwidth requirements associated with these encoding schemes. This will prevent over budgeting of the channel requirements and allow for more exact specifications to be developed. This paper analyzes the change in the power spectral density of random data when encoded using 64b/66b scheme. A comparison to similar effects caused by using 8b/10b is presented for reference. The results show consistent variations in PSD which allow us to characterize the encoding scheme.