Trigger Circuit - Fluke ScopeMeter 123 Service Manual

123
Service Manual
Range
Current µA
Pulse width at Full Scale
To protect the current source if a voltage is applied to the input, a PTC resistor R172,
and a protection circuit on the TRIGGER part, are provided (see Section 3.3.3).
Frequency & Pulse Width Measurements
The input voltage is measured as described above. From the ADC samples to built the
trace, also the frequency, pulse width, and duty cycle of the input signal are calculated.
Probe Detection
The Input A and Input B safety banana jacks are provided with a ground shield,
consisting of two separated half round parts. One half is connected to ground via the
protection PTC resistor R106/R206. Via a 220K resistor installed on the input block, the
other half is connected to the probe input of the D-ASIC (pin 54, 55). If the shielded
STL120 test lead, or a BB120 shielded banana-to-BNC adapter, is inserted in Input A or
Input B, it will short the two ground shield halves This can be detected by the D-ASIC.
Supply Voltages
The +5VA, +3V3A, and -3V3A supply voltages are supplied by the Fly Back Converter
on the POWER part. The voltages are present only if the test tool is turned on.

3.3.3 Trigger Circuit

The description refers to circuit diagram Figure 9-3. The trigger section is built up
around the T-ASIC OQ0257. It provides the following functions:
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3-20
Table 3-5. Capacitance Ranges, Current, and Pulse Width
Triggering: trigger source selection, trigger signal conditioning, and generation of
trigger information to be supplied to the D-ASIC.
Current source for resistance and capacitance measurements.
Voltage reference source: buffering and generation of reference voltages.
AC/DC relay and Resistance/Capacitance (Ω/F) relay control.
50 nF 500 nF
0.5 µA 5 µA
25 ms 25 ms
5000 nF 50 µF
50 µA 500 µA
25 ms 25 ms
500 µF
500 µA
250 ms