５
MEASUREMENT OPERATION
TMPM5200-2
5-183
■
Filter/L
Set a
.
oop Gain
filter and gain amplifier for the feedback signal
Fback Fil
Specify t
from 16 f
slower w
observati
ssible. Too high a frequency causes the
f
cillation. So set a frequency that is high but does not
caus
selec
er used in this
s
nd the cutoff frequency indicates the beginning frequency of
gain
ters/Hz
he cutoff frequency of the feedback loop in this selection box. You can select
requencies between 0.01 Hz and 5 Hz. Response of the feedback loop becomes
hen a smaller frequency is selected, and vice versa. In ordinary topographical
on, select as high a frequency as po
eedback circuit to be prone to os
e oscillation. In the cases of the current image and force image, on the other hand,
t a lower frequency than that for topographical observation. The filt
ystem is a low-pass filter, a
decrease. Its characteristics are shown below:
Gain
Cutoff
frequency
Frequency
The sensitivity decreases gradually as the frequency becomes higher than the cutoff
frequency.
Loop Gain
You specify the loop gain of the feedback circuit. The gain becomes higher as the
number becomes larger. Usually, select a higher number (8 to 64) for the AFM mode,
and select a lower number (4 to 32) for the STM mode. Loop Gain and Filter/Hz are
closely related to each other. Even if the Loop Gain is small, a large Filter/Hz (faster
response) causes the feedback circuit to be prone to oscillation. Even if the Loop Gain is
large, a small Filter/Hz (slower response) causes the feedback circuit not to be prone to
oscillation.
During image observation, select about 8 for the Loop Gain; adjust Filter/Hz; then select
a one-step larger number for the Loop Gain if no oscillation occurs. When setting the
Loop Gain to 1, make sure that the Z position is near zero volts. If the Z position is far
from zero, the tip may collide with the specimen or go far away from the specimen
surface, because the Z position can follow only within
±
20 V to
±
30 V when the Loop
Gain is 1.
