.. |MLA (TM)| unicode::  MLA U+2122
.. |(TM)| unicode::  U+2122

.. index:: Frequency Sweep, finding the resonance

.. _frequency-sweep-label:

Frequency Sweep
===============
.. image:: ../Icons/frequency_sweep.png
   :width: 20mm
   :align: left

After loading the cantilever and adjusting the detector, find the cantilever resonance by sweeping the frequency of the cantilever drive signal while measuring the detector response signal.  The ``Frequency Sweep`` panel opens when you click the top icon in the work-flow.     

``Run sweep``  starts the measurement between the ``Start frequency [kHz]`` and ``Stop frequency [kHz]``.  Edit these fields to change the sweep range.  The sweep range should include the likely resonance frequency of the cantilever, which should be provided by the probe manufacturer and is usually printed on the probe box.   Reducing the measurement ``Bandwidth [kHz]``  causes the sweep to be slower and the measured response to be more accurate. The sweep speed also depends on the ``Number of points`` recorded during the sweep.  The measurement time at each point is given by the inverse of the bandwidth.  You may need to adjust the ``Drive amplitude [V]`` in order to the right amount of excitation to excite the cantilever resonance.

Check boxes provide automatic features for finding the resonance and controlling the sweep: 

* ``Zoom peak``  performs a second sweep with finer detail, zoomed in on the peak response found in the sweep range.

* ``Transfer results to noise calibration`` transfers the center frequency and estimated sweep range to the next stage in the work flow.

* ``Reverse sweep direction`` gives a sweep with increasing and then decreasing frequency over the range specified.

..
    .. index:: Use Correction, de-embedding
    
    .. _use-correction-label:
    
    Correcting Driven Response
    ++++++++++++++++++++++++++
    
    ``Use Correction`` is an advanced function that can be left unchecked for simple resonance-finding.  This function is used to 'de-embedd' linear response from the measurement chain.  Sometimes electronic equipment between the output or input of the Frequency Lockin Amplifier\ |(TM)| (|MLA (TM)|), (see :ref:`intermodulation-measurement-label`) and the host AFM can cause frequency-dependent amplitude and phase shifts to the signals that are measured by the |MLA (TM)|.  Use Correction compensates for any undesirable linear transfer of the signal due to these components 'embedded' in the measurement chain.  The compensation is made using a procedure called de-embedding, performed in two steps:
    
    1. A frequency sweep is performed while by-passing the AFM (insert a short in place of the AFM).  After this sweep, press the ``Set Correction`` button, which transfers the result of this sweep to an internal correction file.
    
    2. Put the AFM back in to the measurement chain and check the ``Use Correction`` check-box. All subsequent sweeps performed with this box checked will correct for the amplitude and phase shifts measured in the first step.  
    
    In summary: 
    
    * ``Use Correction`` will correct the measurement for an arbitrary *linear*, frequency-dependant transfer function embedded between the output and the input points of the measurement chain.  
    
    * ``Set Correction`` stores the last-measured transfer function to an internal file which is used for the correction.