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I decided to check the linearity of my Yaesu FT-891 signal using an RF sampler and an oscilloscope. To generate two-tone (1550 Hz, 2150 Hz) signal I used Audacity. To connect the transceiver to the PC (MacBook Pro 2018) I used SignaLink.

Surprisingly turned out that the signal is not quite linear:

enter image description here

No matter how I change to output volume this is the best I can get. For a comparison, here is the exact same signal when another (DIY) digital interface is used:

enter image description here

Thus I'm quite sure that the problem is with SignaLink. However I have little idea what exactly is causing it or even where to start from. I wonder whether this is a known problem and maybe someone already described a mod to fix it (I didn't manage to find one). If there is no known solution a little piece of advice on how to debug the issue would be much appreciated.

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It might be the transformers frequency response is not flat. Try replacing them with reputable units that have a published frequency response such as here. See more signalink tweaks here.

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