Is it possible to get a higher frequency out of a DDS than I put into it?
While the DDS function is inherently a frequency division, there are a couple of approaches that can be used to get a higher output frequency than the reference frequency you provide into the device.
1) Some devices have internal reference clock multipliers. These are PLL devices that multiply the reference input up to a higher frequency which is then used as the system/sampling clock for the DDS engine and DAC. Using a PLL to multiply up the reference input will, however, negatively impact your output performance
2) The DDS is a sampled system and Nyquist theory applies. In addition to the fundamental output, there is a rich spectrum of output harmonics. It is possible, by using a band pass reconstruction filter, to lock onto a signal in the 2nd, 3rd, or even higher Nyquist zones. The strength of the signal will be lower than the fundamental is, for more on that look for discussions. articles, and white papers about the sinc (sin(x)/x) response.
3) It is also possible to take the output of from a DDS and feed that into an PLL or mixer to 'up convert' the output to a higher frequency.
Each of these approaches has some drawbacks, but they are all worth exploring and good to understand.
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