Intermodulation distorsion is longer time matter of my interest. My IMD kit using N2PK VNA is published on this web. It is suitable for both active and passive circuits IMD measuring. But I see some disadvantages when using it. First, VNA is using two DDS placed on the same board, close to each other. Second, both DDS are supplying from the same source. Third, amplifiers they have not good enough gain, output level should be +30 or more dBm with the level adjustment possibility.
I had to solve several problems. To choose DDS type, to suggest reference signal source, select the appropriate uP and to write the software. First I was looking for solutions on the web (why unnecessarily invent what was invented?).
I found very interresting DDS project on the I0CG webside. Giuliano developed double DDS using AD9912 from Analog Devices.
He wrote: "My laboratory tests demonstrate that spurious performance and frequency extension are superior to all others actual DDS, including the AD9951. In a few words this new AD9912 DDS is the best solution available today for amateur radio applications”.
Giuliano wrote software for PIC 18F6720, which controls both DDS circuits. After friendly discussion Giuliano was so kind and sent me lot of materials regarding to double DDS plus his tested software. Thanks again, Giuliano! It was a turning point for me.
I started to prepare my own PCBs for DDS, Reference 1 GHz clock and the Control board. The result of my several months job is visible in the following pictures. Double DDS is the first part of whole project, which includes two amplifiers plus LPF for every band, too.