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VIDEO CONTROL AMPLIFIER (VCA) unit
The VIDEO CONTROL AMPLIFIER (VCA) unit is almost never used on our instrument, but a description
is provided here for completeness.
The VCA unit is a video control amplifier that is designed to filter the video signal (either secondary or
backscattered-electron) and to perform signal amplification with regard to the desired frequency, thus
enhancing the observation of structures in the sample that formerly were indistinct. The video signal is
attenuated to 1/10 of the original level by the VCA attenuator, then the attenuated signal is split, and one
copy of the signal is processed by a band-pass amplifier and processed according to the frequency range
selected on the VCA module. This processed signal is then mixed back with the other copy of the signal
from the attenuator, and the resulting composite signal is displayed on the DISPLAY MONITOR.
The operational interpretation of images processed by the VCA unit is that the peak to background of
features in the sample are enhanced using a frequency-dependent control, and therefore the enhancement is
not applied to the image brightness histogram, but rather to portions of the scanning image that satisfy the
frequency range selected. For example, the fine structural details of a sample are enhanced when the
FREQUENCY RANGE and BANDWIDTH knobs are set to a low value, and larger structural details are
enhanced when these knobs are set to a high value.
VCA
Frequency
Range
1
2
3
4
Frequency
Range Knob
Band Width
1
2
3
4
Band Width
Knob
Contrast
Contrast Knob
On
On / Off Switch
Off
Video Control Amplifier Unit
See the VCA instructions in the JEOL manual for more details.
1. FREQUENCY RANGE knob
The FREQUNCY RANGE knob of the VCA unit is used to select a frequency range for enhancement.
Frequency range 1 is used for the highest frequency, and range 4 is used for the lowest frequency to be used
for enhancement.
The frequency range selected is dependent on the scan rate and the nature of the sample being imaged.
2. BAND WIDTH knob
60