Fluke 123B/S Service Manual - Page 43

3 Circuit Descriptions 3.3 Detailed Circuit Descriptions electrical interference. In PCB versions 8 and newer R605 and R606 provide a more reliable startup of the backlight converter. Voltage at T600 pin 4 Voltage AOUT Voltage BOUT Voltage COUT zero detect zero detect Figure 3-7. Back Light Converter Voltages 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. CHANNEL ASIC OQ 0258 C HF IN HF-PATH OUTPUT STAGE ADC TRIGGER INPUT R AC LF IN DC GROUND PROTECT LF-PATH CONTROL SUPPLY CAL POS BUS SUPPLY Figure 3-8. C-ASIC Block Diagram 3-15