Circuit Description - HP 400D Organizational, Ds, Gs, And Depot Maintenance Manual

4-1. BLOCK DIAGRAM.
4-2. The electrical circuits of the voltmeter are shown
in the block diagram in figure 4-1; they consist of an input
voltage divider controlled by the RANGE switch, a cathode
follower input tube, a precision step attenuator controlled
by the RANGE switch, a broadband amplifier, an indicat-
ing meter, and a regulated power supply. The voltage
applied to the INPUT terminals for measurement is
divided by 1000 before application to the input cathode
follower when the RANGE switch is set to the 1-volt
range and higher; the input voltage is applied directly
to the cathode follower on the lower ranges. The voltage
from the cathode follower is divided in the precision
attenuator to be less than 1 millivolt for application to
the voltmeter amplifier. The output of the amplifier
is rectified in a full-wave bridge rectifier with a d-c
milliammeter across its midpoints. The resultant
direct current through the meter is directly proportional
to the input voltage.
4-3. INPUT VOLTAGE DIVIDER AND STEP
ATTENUATOR.
4-4. The input voltage divider limits the signal level
applied to the input cathode follower to less than 0.3
volt rms when voltages above this level are measured
with the RANGE switch set at the 1-volt range or above.
The divider consists of a resistive branch with one
element made adjustable to obtain exact 1000:1 division
at middle frequencies and a parallel capacitive branch
with one element made adjustable to maintain exact
1000:1 division to beyond 4 megacycles. The input
impedance of the voltmeter is established by this divider
and is the same for all positions of the RANGE switch.
On the six low-voltage positions of the RANGE switch,
the input divider provides no attenuation of the input
voltage. (See figure 5-10 for the complete schematic.)
4-5. The step attenuator in the cathode circuit of the
input cathode follower reduces the voltage to be measured
to 1 millivolt or less for application to the voltmeter
amplifier. Each step of the attenuator lowers the signal
level by exactly 10 db (1:
of six precision wirewound resistors which are selected
to very high accuracy and carefully mounted on a 12-
position rotary switch. The RANGE switch rotor has two
contractors (see figures 5-9 and 5-10); the first contacts
each resistor in turn while the input divider is in the
non-attenuating position; the second rotor finger repeats
these contacts while the input attenuator is in the attenu-
ating position. On the .001-volt range a fixed capacitor
(C15) is automatically connected to provide flat frequency
response beyond 4 megacycles. In the .003- and the .01-
volt ranges, separate adjustable capacitors (C14, C16)
are automatically connected to the attenuator to permit
setting the frequency response at 4 megacycles. C14 and
C16 are also connected to the attenuator on the 3- and
10-volt ranges. Fixed capacitor C106 (permanently
connected) flattens frequency response on the .03- and
30-volt ranges.
00102-2
SECTION IV

CIRCUIT DESCRIPTION

10). The attenuator consists
4-6. Cathode follower V1 provides a constant, high input
impedance to the input voltage divider and INPUT ter-
minals of the voltmeter and provides a relatively low
impedance in its cathode circuit to drive the step at-
tenuator. The voltage gain factor across V1 is 0.95.
4-7. BROADBAND VOLTMETER AMPLIFlER.
4-8. Amplification of the signal voltage is provided
by a four-stage stabilized amplifier consisting of tubes
V2 through V5 and associated circuits. The amplifier
provides between 55- and 60-db gain with about 55 db
of negative feedback at mid-frequencies. The feedback
signal is taken from the plate of the output amplifier (V5)
through the meter rectifiers and gain-adjusting circuit
to the cathode of the input amplifier (V2). Variable
resistor R107 in the feedback network adjusts the negative
feedback level to set the basic gain of the amplifier at
mid-frequencies, while adjustable capacitor C102 permits
setting amplifier gain at 4 megacycles. Variable resistor
R118 in the coupling circuit between V4 and V5 permits
adjusting the gain of the amplifier at 10 cycles per
second by controlling the phase shift of low-frequency
signals between these two stages (increasing phase
shift decreases degeneration and increases gain).
4-9. Variable resistor R119 in the grid return path for
V3, V4, and V5 adjusts the total transconductance of
these tubes in order to restrict the maximum gain-
bandwidth product of the amplifier. The gain-bandwidth
product must be restricted to give a smooth frequency
response rolloff above 4 megacycles and to prevent
possible unstable operation at frequencies far above
4 megacycles when tubes having unusually high trans-
conductance are used (tube transconductance tolerances
during manufacture permit wide variations in new tubes;
the adjustment permits the use of such tubes). The
plate voltage from V5 is rectified by the meter rec-
tifiers and drives the feedback network. The cathode
voltage of V5 is fed to the meter OUTPUT terminals
for monitoring purposes. The current through V5, and
thus the signal voltage at the cathode, is affected by
the loading of the meter rectifiers. For signal levels
causing third- scale or more meter deflection, this dis-
tortion consists of a very small irregularity near 0 volts
on the waveform as each diode begins conduction.
4-10. INDICATING METER CIRCUIT.
4-11. The meter rectifier circuit consists of two silicon
diodes and two capacitors connected as a bridge with the
indicating meter across the mid-points as shown in
figure 4-2. The diodes provide full-wave rectification
of the signal current for operating the meter. Electron
flow through the meter is supplied in the following manner
(see figure 4-2). During the positive-going half cycle
of plate voltage on V5, rectifier CR1 conducts electrons
from both C32 and C33 back to the B+ buss. The portion
of electrons from C33 flows through the meter on the way
to B+. At this point in the cycle, both C32 and C33 are
charged to the potential of B+ less some small drop in
R51 and R52.
TM 11-6625-1514-15
Section IV
Paragraphs 4-1 to 4-11
4-1