HP 3400B, User'S And Service Manual

Page: 42 / 77

Chapter
3
Theory
of
Operation
Block Diagram
A
signal being measured is applied to the input attenuator through the
Input connector, located on the voltmeter's front panel. The input
attenuator has an input impedance of
10 Mn
and provides two ranges
of attenuation. The output of the input attenuator is applied to the
impedance converter. The impedance converter is a non-inverting unity
voltage gain amplifier. It presents a high impedance to the input signal
and provides a low impedance output to drive the post attenuator. The
post attenuator provides six ranges in a
I, 3, 10
sequence. The two
attenuators
are
switched to provide
12
ranges of attenuation.
The output of the post attenuator is amplified by the wideband
amplifier. The wideband amplifier is a two-stage amplifier (U301 and
U302). The ac output of the wideband amplifier is amplified to drive one
of the thermocouples in the thermocouple pair (TC301).
The dc output ofTC3Ol is applied to the chopper op amp (U303) which
modulates the voltage. The resultant modulated signal is then
amplified and, after the amplification, the signal is demodulated back
to a dc voltage. This voltage is the output of the chopper op amp. The
value of the voltage is proportional to the dc input voltage.
The output of the chopper op amp is applied to a filter and also to the
second thermocouple of the thermocouple pair (TC30l). The output of
the filter is applied to a de output amplifier (U304C) which is used to
drive the meter (Ml).
The thermocouple pair (TC301) acts as a summing point for the ac
output of the wideband amplifier and the dc output of the chopper
op amp. The difference in the heating effect of these voltages is the dc
input to the chopper op amp. This difference input is amplified and is
fed to TC301 and also, after going through the output amplifier (U304C),
to the meter (Ml). This voltage represents the
RMS
value of the ac
signal applied to the Input connector. By using a "matched"
thermocouple pair and measuring the difference, the output of the
chopper op amp will be linear. Using a matched thermocouple pair also
provides temperature stability.
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