Several Questions about MK2-A810 DSP Code

Di Huang
  • Di Huang

    Di Huang - 2014-04-22


    I read DSP code of MK2-A810 through recently, the code is really great, but several questions confused me a lot, they are:

    1. In auto-approach, the mover wave patterns are generated by GXSM and then store in waveptr of DSP. However, I cannot find the code that generating Parabolic waveform, nor can I observe parabolic waveform in osciloscope while run mover and choose waveform "Parabolic". Moreover, is the IW mode prepared for Besocke scanner?
    2. Are QEP counters and Counters on front panel the same thing?
    3. I noticed that while doing area scan in ch2nd const height mode, Z profile got from AS_SCAN_XP is stored in Z_delta_buffer, and Z_delta_buffer locates in probe datafifo. My question is, if I scan in constant height mode, and use gridded probe or triggered probe at the same time, will they distrub each other?
    4. I cannot understand the function of self adaptive IIR filter for feedback ADC0, why cannot feedback at a constant high feedback speed?
    5. I wish to use a stand-alone Lock-in to get dIdV, how can I achieve this goal? I wish to do gridded probe as well.

    Thanks a lot! :-)


  • Percy Zahl

    Percy Zahl - 2014-04-22

    1) not sure what code revision you are using, seams old: There is no "parabolic" wave form any more (this parabolic shape was numerically generated by obsoleted DSP code we got rid of now). Today there are the optimized "cycloid" shapes and a few other simple wave forms. Mostly the trivial saw tooth works just fine. The "IW = Ichworm" mode is a special more with a custom phase shift between X/Y-wave signals.

    2) The QEP counter mode is a special form -- refer to the MK2 GPIO manual for the pinouts at the 25pin SubD (back of the box)
    The front Counter1/2 break outs are are for simple pulse counting as used by GXSM normally.

    3) obsoleted/rework in progress. Don't use for now.

    4) not sure what your Q is exactly. You can turn it off. The idea is to "simulate" the benefits of a classic analog log-amplifier which has a decreasing bandwidth with signal magnitude and such a higher noise rejection. Often beneficial for STM.

    5) If you think a external LockIn will work better, just take care of all modulations, etc. and hook the LockIn output signal(s) to any of the free A810 inputs for reading -- done.

    • Di Huang

      Di Huang - 2014-04-23

      Thanks a lot!
      But to question 5, I am still confused. For example, if I want to do area scan, and want to get dIdV spectroscopy on some points, with internal lock-in, I can realize this by using trigger, but how to achieve my goal with external lock-in?
      Here are another questions:
      6. Can Bias DAC6 work in HR mode? Our future STM's temperature may be lower than 1K, so the Bias resolution will become a limit. Moreover, if my XYZ offset and scan channels are working in HR mode, do I need a low pass filter before input them into my HV-amplifier?
      7. How to realize tip retract function, for, say, leave a short while after tip approach and find a great place on sample?

      Last edit: Di Huang 2014-04-23
  • Di Huang

    Di Huang - 2014-05-15

    Is there any one here can answer my questions above? Thanks a lot!

  • Percy Zahl

    Percy Zahl - 2014-05-15

    5 -- leave the lockin running all time and just acquire the data, don't see the problem.
    6 -- I won't rely on the HR for such a resolution in particual as the DACs are possible not as precise (you will work beyond specs) and may have an offset as well. Consider an external 1:20 voltage divider/attenuator for example.
    I doubt you will need an low pass for offset as the all over system bandwidth (amplifier/piezo...) will act as one in a way, but it wont hurt.
    7 -- two options: a) just set the current set point to ZERO, as there is always some noise it will retract your tip. b) use the "Z" control and manually retract via Z-offset (manually offset know or via software Z-offset-control with ANALOG adding (or via the MK3/MK3-SPD digital GXSM offset link)) as far that the Z-servo goes out of range and you will further retract the tip.



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