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About IP3 measurement (Signal Generator's influence)

Question asked by wangyiliu99 on Apr 3, 2011
Latest reply on Jun 2, 2011 by wangyiliu99
Hi, all,

Now I am doing the IP3 measurement of certain device. Please refer to the attached Setup.bmp file for the measurement setupImage. First, I wanted to confirm that the test system will not generate IP3. So I bypass the DUT and did the measurement. Unfortunately, 3rd order IMD signal was quite strong. With main signal (@f1, @f2) being 10 dBm, the IMD signals (@2f1 – f2, @2f2 – f1) are around -68 dBm. Since this power level is greater than the noise level, it is necessary to modify the test system. I found one reference by Agilent, as the attached Agilentsetup.bmpImage, low pass filter and/or isolator are added in the setup.

My questions are
1)     For the low pass filters connected to the SG1 and SG2, the cut-off frequency should be lower than the harmonics (say, < 780MHz), or it should be only a little bit greater than the f1 (=410 MHz). For the low pass filter connected to the SA, its cutoff should be < 780MHz? Would these newly added filters increase the intermodulations?
2)     In Agilent’s setup, it is mentioned that attenuator and/or isolator is preferable. In my case, since I don’t have isolators at hand, would there be any alternatives? Or purchasing the isolator is a must?

Your help would be really appreciated!

With my best wishes,