Dear Sirs,
thank You for all the support You give with application notes, user guides and dedicated answers to people around the world.
Sorry for my lack in English syntax, I'm Italian and live near Milan.
I think I did not find a similar question about this component, please forgive me if a similar discussion already exists.
Our working group developed a DVB modulator with AD9747.
I personally tested these modulator boards with a demodulation instrument capable to show the EVM / MER of the constellation. Even the MER as a function of subcarrier, a feature extremely useful to debug the boards.
The application involves the use of the modulator in a high vibration, high temperature and very compact environment. Overheating can happen, we are not concerded to sell a indestructible board, but we would like to understand the causes of reduced RF performance.
In our board a FPGA gives the I/Q samples to the DAC AD9747. Then, a direct conversion modulator translates the spectrum in the RF band of the application.
After some use in the field (about 100 hours of operation), we had issues in modulation quality. The brand new board has a MER of 35 dB in our reference frequency, but after use in this harsh environment, the MER became 10 dB, so it hardly could be locked by the receiver. This is the strongest example of degradation experienced by us.
We know our application is extreme, however some boards show the issue described, while some others do not. Moreover, one board had this MER distortion even at the very first Test after production.
The other parts of the board work perfectly, even the FPGA seems to work correctly. The comparison between a board that works correctly and a faulty one shows that the problem seems limited in the DAC zone. The PLL is ok, the amplifiers are good too, and the mixer does not show any strange behaviour at our knowledge.
Moreover in other faulty boards we see that if we cool the board, the EVM / MER increases, so the modulation seems to improve significantly and the board stabilizes when it is cooled. I have a range of behaviours from different boards. Some boards are in the total failure state (no DVB lock), some others have a degradation on every subcarrier (similar to a sort of wide band noise), some others have holes that randomly show on different subcarriers. The issue becomes more and more evident when the board temperature increases, while at lower temperature it seems that the MER gets better.
In Your opinion what whould we need to check on our boards? Is there any document about degradation of MER due to overheating of DAC IC or of some component in the nearby (we followed the reference design of AD9747).
Do You think the mounting process could be faulty after some use? We recently supposed the exposed pad of the DAC could have "continuity problems" with the ground plane on the bottom. What signals should You monitor? And an X-ray analysis and comparison between a faulty and a working board could show some evidence of the problem?
Would You suggest the substituition of the component? Do You feel this rework could fix the problem?
Thanks a lot for Your kind attention
Best Regards
