Close to the carrier noise in DDS

In the present paper we discuss the close-to-the-carrier noise in direct digital frequency synthesizers (DDFS). Large spurious signals due to the accumulator truncation are well understood. The same might be said about the background noise due to the finite bit length of the sine words in look-up tables and to some extent also about the noise caused by DA convertors. However, the same cannot be said about the close-to-the-carrier noise. We often encounter the statement that the output phase noise of DDFS is that of the reference oscillator reduced by the square of the normalized frequency. But our investigations proved that this need not be generally true. In the following sections we provide new insight both from the theoretical and experimental point of view.