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Frequently Asked Question

I changed the coarse phase adjust in the evaluation software, but nothing happened. What's going on?

The coarse phase offset adjust on the AD951x ICs depends on the divide ratio selected. It allows for the selection of which input clock edge is the trigger for the output from the divider. The number of edges which are available for selection is dependent upon the divide ratio. The phase offset is determined by 4 bits in the register for each divider. For a divide-by-2, there are only 2 choices. For a divide-by-15, there are 15 choices. By adding the use of the Start H/L bit, there are more choices, but it becomes more complex.

However, if the divider is bypassed there are no choices. The case of divide-by-1 (divider bypassed) does not allow for a phase adjust. Also, when divide-by-n is set, selecting a phase offset of n or 2n, etc. does not look like it does anything, because the phase offset selected is a whole number of cycles of the input clock signal. For example, when divide by 5 is selected, a phase offset of 5 looks the same as a phase offset of 0.

So, to answer your question: either the divider is bypassed (divide-by-1) or the selected phase offset is a multiple of the divide ratio.