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AR# 22960

LogiCORE Direct Digital Synthesis (DDS) v5.0 - Why is my output not zero, when resetting the DDS with Noise Shaping or Phase Dithering is selected?

Description

Why is my output not zero, when resetting the DDS with Noise Shaping or Phase Dithering is selected?

Solution

The operation is as expected. When you use the Noise Shaping (i.e., Phase Dithering), you will not get a reset to zero. This is due to the way the DDS is constructed. If you reference Figure 8 on page 7 of the DDS v5.0 Data Sheet, you will see that the Phase Dithering of the LFSR is added to the output of the accumulator, and this is then used to generate the address for the wave table. All the registers are reset when a reset is applied, but you cannot reset the LFSR in the DDS to all zeroes, because it will not work correctly. Thus, there is no way to reset to zero when using the Phase Dithering.

This issue does not arise when Phase Dithering is not used, or if the output uses the Taylor Series implementation.

AR# 22960
Date Created 09/04/2007
Last Updated 12/15/2012
Status Active
Type General Article