HP 3400B, Руководство пользователя и сервисное руководство

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Chapter
3
Theory of
Operation
Input Attenuator
•
Input Attenuator
The input attenuator is a capacitive-compensated attenuator which
provides two ranges of attenuation for the
12
positions of the Range
selector switch. When the selector switch is positioned to one of the six
most sensitive ranges
(1
m
V
to
300
m V), the attenuator output voltage
is equal to the input voltage. When the selector switch is positioned to
one of the six higher ranges
(1 V
to
300
V), the input signal is
attenuated
60 dB (1000: 1
voltage division) by the resistive voltage
divider consisting ofR121, R l22, and Rl23. Trimmer C134 is adjusted at
100
kHz, and Rl23 is adjusted at
50
Hz to provide constant attenuation
over the input frequency range.
•
•
•
•
•
Impedance Converter
The impedance converter uses a FET-follower circuit to match the high
output impedance of the input attenuator to the low input impedance of
the post attenuator. The FET-follower circuitry has unity gain with no
phase inversion between the input and output signals.
The ac input signal to the impedance converter is resistor/capacitor
coupled via R 127 and C149 to the gate ofFET Q121. The output of the
converter is developed across Q122 which acts like a variable load
resistor for the FET.
The bootstrap circuitry, consisting of C152 and R130, increases the input
impedance of the converter circuitry and the compensating feedback
circuitry, consisting of C151, decreases the output impedance of the
converter. The bias voltage for Ql22 is developed by resistors R133,
CR125, R134, and R l40 .
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