how to find the phase of the signal in fft using system generator??

hello guys, im implementing a fft of frequency 4ghz sampled at a frequency of 10ghz, given a phase angle of "0" degree to real and "(pi/2)" degree to imag, using 512 point fft in matlab system generator,, my index(bin number) is proper i.e the highest peak is (@205) bin , so im able to find out the input frequency , but the phase angle is showing as 70 degree i did ((tan-1(imag/real))) number, the expected is 0 degree , kindly anyone help me!!!

[Sandeep Kumar]

how to check phase angle in a right way ???