Hi Karl,
Ok, the first thing is that you will not be able to synchronize to a sine wave. That is, if you want to use an external signal, that signal must be a square wave. All of the timing signals are square waves. Next, the internal timebase (20 MHz, 80 MHz, etc) is determined by DAQmx depending on the rate that you specify. It will choose the most appropriate timebase, given a specific rate. I hope this clears up your questions. Please let me know either way, and have a Great Day!
George