|SPICE steup of FDA with BPF prior to ADC.|
|BPF Filter Response|
|Av=4, Filtered DDS source at 10.640Mhz, no FDA-ADC filtering.|
|Av=4, Filtered DDS source at 10.640MHz with shunt stage of FDA-ADC BPF populate.|
|Av=4, Filtered DDS source at 10.640 MHz with 3 stage BPF.|
At this point it was time to total up the bill for all of the filtering. I knew the insertion loss was creeping up since the DDS level had to be adjust up to hit nearly full scale on the ADC. I decided to return the FDA to a high gain previously used Rf=493 and evaluate the results.
|Rg=25, Rf=493, Filtered DDS source at 10.640MHz with 3 stage BPF.|
So in the end it appears the higher than expected HD2/HD3 response is predominantly deriving from the FDA and how it is being used. Taking them down to a reasonable level adds components and increased gain needs on the FDA. This seems like a losing game. I may have been better off placing all of the desired gain off board, then filtering, and then conducting the differential conversion on the board with a transformer.
This is still usable for my purposes, however, the spur clusters merit some attention. Originally I had attributed them to DDS artifacts, however, it became clear this was not the case when using a source comprised of a synthesizer and mixer. In both cases the spurs are identical in level and structure and follow the fundamental in frequency and level.