５
MEASUREMENT OPERATION
5-214
TMPM5200-2
■
Filter/Loop Ga
This sets
in
the filter and gain amplifier for the feedback signal.
Filter [Hz]
Specify t
from 16
he cutoff frequency of the feedback loop in this selection box. You can select
frequencies between 0.01 Hz and 5 Hz when you click on the “ ” button.
of the feedback loop becomes slower when a smaller frequency is selected, and
a. In ordinary topograp
Response
vice vers
hy observation, select as high a frequency as possible.
T
uency causes the feedback circuit to be prone to oscillation. In the case of
the c
that u
The filter used in this system is a low-pass filter. Cutoff frequency indicates the starting
frequency that the gain decreases. Its characteristics are shown below:
oo high a freq
urrent image and the force image, on the other hand, select a lower frequency than
sed for topography observation.
Gain
Cutoff frequency
Frequency
itivity decreases gradually as the frequency becomes higher than the cutoff
ency.
Sens
frequ
Loop Gain
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 Fback Filter/Hz are closely
related to each other. Even if Loop Gain is small, a large Fback Filter/Hz (faster
response) causes the feedback circuit to be prone to oscillation. Even if Loop Gain is
large, a small Fback Filter/Hz (slower response) causes the feedback circuit not to be
prone to oscillation.
During image observation, select about “8” for Loop Gain; adjust Fback Filter/Hz; then
select a one-step larger number for Loop Gain if no oscillation occurs. When setting
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.
