It is shown that the channel crosstalk limit of -30dB for the recently developed 100GHz-spaced silica-based arrayed-waveguide grating multiplexer is determined by the optical path phase error distribution and that the crosstalk caused by the deformation of the light power distribution in the multiplexer is -39dB. The high-frequency components of the phase error distribution should be reduced by al least two-thirds to achieve a crosstalk of -39dB.