I am using the parallel port of the AD9910 (eval board) to modulate the output frequency. My destination bits are fixed and the TxEnable pin is always on. I am using a FPGa to send the data to the parallel port. The amplitude of the output changes with change in frequency. I don't understand why this happens. I am only modulating the frequency. To make sure it wasn't a problem with my oscilloscope I used a waveform generator to generate the same set of frequencies and the amplitude remained the same. A change in amplitude does't suit my needs. Any help would be great. Thanks!