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Section 3-067-0700-00
THEORYOFOPERATION
INTRODUCTION
The following is a brief discussion of the basic operation of the major circuits in the 7D12 Calibration Fixture (067-
0700-00). The simplified block diagram in Figure 2 and on the schematic is provided to aid in understanding this discussion.
GATE WAVEFORM DIRECT INPUT
The gate waveform signal is routed directly from connector J1 to the 7D12 (50-ohm terminated input) gate waveform
display circuit.
VERTICAL DISPLAY DIRECT INPUT
The vertical display signal is routed from connector J4 to the 7D12 vertical display circuit. Resistor RBprovides
proper signal termination and R7 provides damping.
ATTENUATOR READOUT CHECK
This calibration fixture provides an interconnection from the 7D12 attenuator switch to the readout system. The
readout system displays the amount of attenuation (1, 2, or 5 times) as determined by the attenuator switch setting.
READOUT CALIBRATION
Integrated circuit U16 provides a precise 900 microampere current through the 7D 12 to the readout system. When
the proper calibration current is present, ten B's appear in the channel 2 readout position (lower readout).
U16 is an operational amplifier used as a current converter. Negative feedback, provided through R14, attempts to
stabilize the negative input with the positive input level. Loading variations (from about Oto -300 millivolts), control the
positive input level with positive feedback through R18.
A/D DIRECT INPUT
Input signals are routed to connectors J21 (positive) and J24 (negative). A 1 megohm input termination is provided
by R23 and R24. Resistor R22 protects the 7D12 input against excessive input voltage. Signal filtering is provided by C24
and C25. Decimal-point encodin9 for channel 1 is provided by R28 and R32. The V suffix is encoded by R30 and H34.
® 3-1
