Nanosurf easyScan 2 AFM, Operating Instructions Manual

Page: 68 / 164

M
EASUREMENT MODES
68
- Ensure that you have selected the ‘Standard level’ or ‘Advanced level’ user
interface mode in the User Interface dialog.
- Click ‘Reference phase’: in the Operating mode panel.
If you do not want to automatically set the reference phase:
- In the Operating mode panel, deactivate Reference phase: ‘Auto set’.
- Enter the reference phase using the ‘Reference phase’ input.
Phase measurement
In Phase contrast mode, the phase of the measured cantilever vibration is
compared to the phase of a reference sine wave. The phase comparison is
performed by multiplying the measured vibration signal with the reference.
The measured phase,
φ
measured
, is the output of this multiplier, is related to
the actual phase shift between the measured vibration and the excitation
sine wave,
φ
actual
, as:
φ
measured
= 90° sin(
φ
actual
-
φ
reference
) ,
with
φ
reference
a reference phase that can be set by the user. This has two
important consequences:
• The measured phase shift lies between -90° and +90°, phase shifts outside
this range are folded back into the -90° to +90° range. The measurement
does not distinguish between phase
φ
and phase
φ
+ 180°.
• The phase shift measurement becomes less sensitive when the phase
appr90° and -90°.
Note that the phase of the cantilever vibration changes by 180° symmetri-
cally around its resonance frequency. This phase shift only fits into the -90°
to +90° range when the reference phase is set so that it is zero at the reso-
nance frequency. However, the operating frequency is generally set to be
different than the resonance frequency. Then, the reference phase is auto-
matically set so that the phase shift is zero at the operating frequency. Thus,
part of the frequency spectrum of the phase shift will be folded back into
the -90° to +90° range (figure
Folded back ).