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Chapter 3: Service
Troubleshooting and Repair
High-Frequency Path
The 10 signal is ac coupled to a series of discrete
emitter followers, Amp 1. Operational amplifier U2 sets the bias at the input
of the emitter follower amplifier. The high-frequency signal drives the emitter
of a common base amplifier. The common base amplifier drives the output
cable.
Low-Frequency Path
U1 provides the low-frequency path. One input to
U1 is 4% of the signal to the high-frequency amplifier. The other input to U1 is
4% of the probe output voltage, summed with the offset voltage from the
probe power box. The gain/bandwidth product of U1 limits the frequency
response of the low-frequency amplifier to 400kHz. U1 drives the base of the
common base stage.
Power Box Circuitry
Power Box CircuitryThe probe signal is fed via the coaxial cable directly
through the power box to the Type-N connector.
The power box takes five inputs from the probe power connector and
conditions them for the probe. The probe power inputs are:
• Two probe ID lines
• Two supplies, +17 Vdc and -17 Vdc
• Offset current of 5 mA
The probe ID lines are pulled to ground and identify the probe when it is used
with oscilloscope probe power.
• The probe power box provides to the probe:
• Two voltage supplies, +14 Vdc and +4 Vdc
• A current source of -86 mA
• An offset voltage of 10 V
An adjustment in the offset circuitry trims any offset error when there is no
offset input.
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