Generating Higher Frequencies with the N3ZI DDS.

All DDS Chips have a D/A converter at the output to generate the desired signal, a clean sign wave.
The process has some inherent features that can be used to generate frequencies higher than normally expected
Sin(x) / X
Normally there is a low pass filter at the output of the DDS chip.  The one included in the N3ZI DDS is a low pass filter that starts to roll off at 20MHz, and attenuates the first image by 30db. The software will allow you to tune up to 25Mhz, but the image would not be attenuated very much.  So to use frequencies in the 20 to 25Mhz you would have to design a better filter, or use it in a circuit that is insensitive to signals in the image frequency range.

However, Analog devices, the manufacture of the DDS chip has published some information that will allow you to utilize the images rather than the fundamental.  They are all precisely generated signals that are a result of the mathematics, not randomly generated spurs.
In the example shown above signals in the 70Mhz range are generated by using a bandpass filter.
If you are going to use this approach, you have to be careful regarding the filter designs. (My favorite filter design program is a free one from AADE)
Also you cannot get continuous coverage using this method, perhaps 10 or 20 MHz range, depending on the filter designs. i.e. 60 to 70MHz.

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