HP 400E, Operating And Service Manual

Page: 17 / 57

Models 400E/400EL Section IV
SECTION IV
THEORY OF OPERATION
4-1.
GENERAL.
4-2. The 400E/EL is a solid state, average
responding, rms calibrated voltmeter. It also has
applications as
an
ac to dc converter and a wide band
amplifier. Figure 4-1 shows a simplified block
diagram of the instrument.
4-3. When relay
K
I
is closed, the input is not
attenuated; when
K
I
is open and
K2
is closed, the
input is attenuated by 50 dB. On the 0.001 through
1
volt ranges,
K I
is closed and
K2
is open. K2 is closed
and
K
I
is open on the 3 through 300 volt ranges. The
entire Input Attenuator assembly is shielded, and the
relays are operated remotely by voltages applied
through the RANGE switch. Variable capacitor Al C2
i s adjusted on the 3 volt range with a 3 volt 100 kHz
input in order to shape the frequency response of the
Input Attenuator.
44. The signal from the input attenuator is applied
to the impedance converter. The impedance converter
is a unity gain, feedback stabilized amplifier that
matches the high Impedance of the Input Attenuator
to the much lower impedance of the Post Attenuator.
4-5. The Post Attenuator attenuates the output of
the Impedance Converter b y 10dB for each step of
the RANGE switch. On the 3 volt range, the Post
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IMPEDANCE
I
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I
CONVERTER
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ATTEN I
I
I
I
I
I
I
I
I
I
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SHIELD
,
,
,
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POST
ATTEN
Attenuator is switched back to the 30dB position,
and then it attenuates lOdB per step on the higher
ranges. Variable capacitor S2C2 is adjusted on the
.003 volt range with a 3mV, 8MHz input to adjust the
8MHz response of the .003 volt range. With a full
scale input on any range except the .001 volt range,
the output of the Post Attenuator should be 3mV.
On the .001 volt range, the output should be
I
mV.
4-6. The Meter Amplifier is a four-stage, high-gain
amplifier utilizing both ac and dc feedback for gain
stabilization. The Meter Bridge, connected in the ac
feedback path of the meter amplifier, converts the ac
output of the amplifier to a dc voltage proportional
to its average value. This dc voltage drives the meter.
A2C28 and A2R38 adjust the gain of the amplifier so
that the meter will read rms volts. A2R38 is adjusted
at 400Hz, and A2C28 is adjusted at lOMHz.
4-7. The DC Output is a 0-1 volt level that is
proportional to meter deflection. R 2 is adjusted to
calibrate the de output. The AC Amplifier samples
the ac feedback and generates 0 to 150mV ac output
that is directly proportional to meter deflection.
4-8. SCHEMATIC DESCRIPTION.
(See Figure 7
- I).
4-9.
IMPEDANCE CONVERTER.
METER
AMPL
A2C28
A2R3B
METER
BRIDGE
DC
tOUTPUT
AC
�
OUTPUT
Figure 4-1. Simplified Block Diagram
4-1