Hi Leong,
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OK thanks for that.
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From experience, I don't think you will be able to achieve the required performance of your test setup without building fundamental frequency notch filters to place on you analyser input.
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Ideally you need to be able to measure signal levels below -100dBm, in the presence of test signals at 0dBm or more. Even with a high end SA, this is asking a lot.
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Steve and Everett wrote this useful practical guide for Siglent.
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https://www.siglenteu.com/application-note/inter-modulation-distortion-imd-testing/
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I think the variation you are seeing is due to the external and internal IP generation aiding and abetting each other as the phase and amplitude relationships vary with frequency.
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In the old days of analogue TV transmitters, we used to deliberately generate IP products at low level, and inject them into the RF transmission chain, to null out IP's that were being produced in the high power amplifier stages. We called this feed-forward correction.
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Regards,
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Martin
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