Easypix microscope Instrukcje Operacyjne Pobierz pdf (Strona 57)

PHASE CONTRAST

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,

measured

, is the output of this multiplier, is related to

the actual phase shift between the measured vibration and the excitation

sine wave,

actual

, as:

measured

= 90° sin(

actual

reference

) ,

with

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

and phase

+ 180°.

• The phase shift measurement becomes less sensitive when the phase

approaches +90° 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).

Folded back: Frequency spectrum with phase shift folded back over the +90° limit

1 2 ... 52 53 54 55 56 57 58 59 60 61 62 ... 72 73

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Easypix microscope Instrukcje Operacyjne Strona 57