  We apply the central limit theorem for deriving approximations to the
auto-correlation function and power density function (spectrum) of second-order
spectral null (dc2-balanced) codes.We show that the auto-correlation function
of dc2-balanced codes can be accurately approximated by a cubic function. We
show that the difference between the approximate and exact spectrum is less
than 0.04 dB for codeword length n = 256.
