Channel A - Channel B Measurement Circuits - Fluke ScopeMeter 123 Service Manual

In PCB versions 8 and newer R605 and R606 provide a more reliable startup of the
backlight converter.
zero
detect

3.3.2 Channel A - Channel B Measurement Circuits

The description below refers to circuit diagrams Figure 9-1 and Figure 9-2.
The Channel A and Channel B circuits are almost identical. Both channels can measure
voltage, and do time related measurements (frequency, pulse width, etc.). Channel A
also provides resistance, continuity, diode, and capacitance measurements.
The Channel A/B circuitry is built-up around a C-ASIC OQ0258. The C-ASIC is placed
directly behind the input connector and transforms the input signal to levels that are
suitable for the ADC and trigger circuits.
The C-ASIC
Figure 3-8 shows the simplified C-ASIC block diagram. The C-ASIC consists of
separate paths for HF and LF signals, an output stage that delivers signals to the trigger
and ADC circuits and a control block that allows software control of all modes and
adjustments. The transition frequency from the LF-path to the HF-path is approximately
20 kHz, but there is a large overlap.
C
R
INPUT
zero
detect
Figure 3-7. Back Light Converter Voltages
HF IN
AC
LF IN
DC
GROUND
PROTECT
Figure 3-8. C-ASIC Block Diagram
Voltage at T600 pin 4
Voltage AOUT
Voltage BOUT
Voltage COUT
CHANNEL ASIC OQ 0258
HF-PATH
LF-PATH
CONTROL
CAL POS BUS
Circuit Descriptions
3.3 Detailed Circuit Descriptions
ADC
OUTPUT
STAGE
TRIGGER
SUPPLY
SUPPLY
3
3-15