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LTC2000 3rd order intermodulation

Hi

I have a problem with 3rd order intermodulation in the LTC2000-14.

The problem is that when I output 2 carriers (sine waves generated in an FPGA) it looks very good when they are spaced a few MHz apart. But when I increase spacing between them to tens of MHz I get a very high 3rd order intermodulation.

For example:
1 carrier at 843.75 MHz and 1 carrier at 845 MHz. This is a setup that matches the graphs in the datasheet.
With DYNLIN_DIS = 1 I measure the 3rd order IMD at 842.5 MHz to < -75 dBc (I hit the noise level of the spectrum analyzer).
With DYNLIN_DIS = 0 and DYNLIN_GN = 75% I measure the 3rd order IMD at 842.5 MHz to -66 dBc.

1 carrier at 800 MHz and 1 carrier at 845 MHz. This is the worst case setup that I could find.
With DYNLIN_DIS = 1 I measure the 3rd order IMD at 755 MHz to -49 dBc.
With DYNLIN_DIS = 0 and DYNLIN_GN = 75% I measure the 3rd order IMD at 755 MHz to -48 dBc.

1 carrier at 755 MHz and 1 carrier at 845 MHz. Here you see that the IMD improves when the spacing increases.
With DYNLIN_DIS = 1 I measure the 3rd order IMD at 665 MHz to -59 dBc.
With DYNLIN_DIS = 0 and DYNLIN_GN = 75% I measure the 3rd order IMD at 665 MHz to -58 dBc.

Is the IM3 performance of the LTC2000 not better than this or is the problem in my design? Do you have any idea on what I should look into to improve the IM3?


Here is my setup:

Regards, Jonas