I need to generate a quadrature signal on it, AD9958 like this is allowed because DDS has two parallel channels. I can form a static complex signal (at a constant signal frequency). The phase shift is honest 90 degrees. But if, for example, to try to form a complex chirp signal, then the phase between the components begins to behave unpredictably. I measure the phases with a double-beam oscillograph at a low frequency (LFM 1 ... 5 MHz) in the single-shot mode. What advise, colleagues? Did I not take into account something?