General Description; Detailed Description; Impedance Converter Assembly A - HP 3400A Operating And Service Manual

Model 3400A
4-1. I NT�O DUCTI O N .
4-2. This section contains the theory of operation
of the Model 3400A R. 1\tIS Voltmeter.
general and detailed description of the theory of
operation.
4-3. G E N ER A L D ESCRIPTION.
4-4. The Model 3400A comprises two attenuators. an
impedance converter. a video amplifier. a photo­
conductor chopper (modulator/demodulator), a chop­
per amplifier, an emitter follower, a thermocouple
pair, and a direct reading meter.
4-5. A signal being measured with the Model 3400A
is applied to input attenuator At through the INPUT
jack, located on the Model 3400A front panel.
input attenuator has an input impedance of over 10
megohms and provides two ranges of attenuation.
The output of the input attenuator is applied to imped­
ance converter A2. The impedance converter is a
non-inverting unity voltage gain amplifier.
a high impedance to the input signal and provides a
low impedance output to drive the second attenuator
A3. The second attenuator provides 6 ranges in a 1,
3, 10 sequence. The two attenuators are switched to
provide 12 ranges of attenuation.
4-6. The output of the second attenuator is amplified
. by video amplifier A4. The video amplifier is a wide­
band, five stage amplifier.
video amplifier is controlled by an ac feedback loop.
The ac output of the amplifier is applied to TC401;
one of the thermocouples of the thermocouple pair.
4-7. The dc output of TC401 is modulated by mod­
ulator A5. The modulator comprises two photocells
which are alternately illuminated by two neon lamps
which in turn are controlled by the oscillator located
on power supply assembly A7.
modulator is a square wave whose amplitude is pro­
portional to the dc input level.
4-8. The square wave output of the modulator is
amplified by chopper amplifier A6. The chopper
amplifier is a high gain ac amplifi,�r. Its output is
applied to demodulator A5.
is a dc level whose magnitude is proportional to the
amplitude of the ac input. The demodulator output is
applied to two direct coupled emitter followers.
emitter follower is used to make the impedance trans­
formation from the high impedance output of the de­
modulator to the low impedance of the direct reading
meter M1 and TC402; the second thermocouple of the
thermocouple pair.
4-9. The thermocouple pair TC401 and TC402 acts
as a summing point for the ac output of the video
amplifier A4 and the dc output of the emitter followers.
SECTION
THEORY OF
Included is a
(See Figure 4-1.)
The
It presents
The overall gain of the
The output of the
The demodulator output
The
IV
OPERATION
The difference in the heating effect of these voltages
is felt as a dc input to modulator A5.
input is amplified and is fed to TC402 and to meter
M1. This amplified dc voltage represents the rms
value of the ac Signal applied at the INPUT jack.
using two "matched" thermocouples and measuring the
difference. the output to the modulator will be linear.
Using two thermocouples also provides temperature
stability.
4-10. The dc voltage driving meter M1 is also
available at the DC OUT jack, located at the rear of
the Model 3400A.
4-11. D E T AIL E D DESCRIPTI O N.
4-12. INPUT ATTENUATOR ASSEMBLY AI.
4-13. The input attenuator assembly is a capacit ive­
compensated attenuator which provides two ranges of
attenuation for the 12 positions of the RANGE switch.
See input attenuator schematic diagram illustrated on
Figure 6-1.
4-14. When the RANGE switch is positioned to one
of the six most sensitive ranges (,001 to . 3 VOLTS),
the attenuator output voltage is equal to the input
voltage. When the RANGE switc h is positioned to one
of six highest ranges (1 to 300 VOLTS), the input siJ{­
nal is attenuated 60 dB (1000: 1 voltage division) by
the reSistive voltage divider consisting of R101. R103,
and R104. Trimmer C102 is adjusted at 100 kHz, and
R104 is adjusted at 400 Hz to provide constant atten­
uation over the input frequency range.
4-15.

IMPEDANCE CONVERTER ASSEMBLY A2.

4-16. The impedance converter assembly utilizes a
nuvistor tube cathode follower circuit to match the
high output impedance of the input attenuator to the
low input impedance of the second attenuator,
cathode follower circuit preserves the phase relation­
ship of the input and output Signals while maintaining
a gain of unity. See impedance converter assembly
schematic diagram illustrated on Figure 6-1.
4-17. The ac signal input to the impedance converter
is RC coupled to the grid of cathode follower V201
through C201 and R203.
oped by Q201 which acts as a variable resistance in
the cathode circuit of V201. The bootstrap feedback
from the cathode of V201 to R203
effective resistance of R203 to the input signal.
prevents R203 from loading the input signal and pre­
serves the high input impedance of the Model 3400A.
The gain compensating feedback from the plate of
V201 to the base of Q201 compensates for any varying
gain in V201 due to age or replacement.
4-18. Breakdown diode CR201 controls the grid bias
Section IV
This difference
By
The
The output signal is devel­
increases the
This
4-1