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
.
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.
