RHK Support FAQ

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On a Non-Contact AFM setup in PLL mode, why is the image of the Phase data channel much sharper than the df Image. Can’t I use the Phase data to feed the Z controller?

In PLL mode the Phase Locked Loop (PLL) will try to keep the phase constant. So what you see in the phase image is all the high frequency components that the PLL was not able to follow. A perfect PLL (which is not possible, of course) would give you a perfectly flat phase signal. What you see in the phase channel are just high frequency components, which make a much sharper image. By the way you cannot use this phase signal as input for the Z feedback because it misses all the DC components.

We therefore recommended disabling the PLL for the phase imaging. Now the phase image contains all frequency components unless you use it as input for the Z loop.

On any microscope (STM/AFM/NCAFM), if I image the data stream that feeds the input of the Z PI controller I only see high frequency components. Why?

Any signal that you feed into the Z PI controller will be kept constant by this controller. You will only see the high frequency components that the controller was not able to follow. This is the same story as in the last question, just on another PI controller.

In general it is only useful to image the data that feeds a PI controller if you want to judge that PI controller’s operation. The data of this channel is usually meaningless because it is high-pass filtered by the PI controller. This is the price to pay for getting a topography signal.

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