HP 400D Operating And Servicing Manual page 16

III
Sect. Page 2
SlA of the range switch sets up connections in such
a manner that the input voltage Is applied directly
to the grid of cathode follower VI without being re­
by
duced the 1000: 1 divider. With the range switch
on any one of the six higher positions (1 volt to
300 volts), section SlA establishes the connection
Vl
between the grid of and the resistive divider at
the junction of resistors R3 and R4, and the input
voltage is reduced a thousand-to-one before it is
applied to the grid of Vl.
b. Six-Step Voltage Divider
Two series-connected wirewound resistance
c.
(RIOA, B and RllA, B.
of cathode follower VI constitute a r - e sistance di­
vider tapped for six output voltages.
The six taps are brought out to contacts on section
SIB of the DB VOLTS switch. The movable member
of with two
SIB is fashioned
at 1800• Thus as the switch is moved through the
full range its rotary travel. contact is made twIce
of
of
with each the six taps, once on the travel through
the six lower ranges and once on the travel through
the six higher ranges. Since on the six higher ranges
the input voltage is divided by a thousand before it
is applied to the 6 step divider, each tap
sistive divider serves two ranges, thus making
available a total of twelve ranges.
The output from VI is applied to the grid
of
first the
stage amplifier.
of the meter, the maximum voltage that can be
applied to the grid of V2 is .001 volt. The resistance
divider in the cathode circuit of VI provides such
reduction on each range that for full scale voltage
at the ffiPUT terminals, the voltage applied to the
grid of V2 will not exceed . 001 volt.
The rc networks in the cathode circuit of VI mini­
mize dc switching transients while the ranges are
being changed. The variable capacitors switched
into the circuit on the . 01/10 volt and. 003/3 volt
ranges are provided for adjustment of the high­
frequency response. The trimmer capaCitor con­
nected across the 1000: I divider compensates (or
stray capacity to keep the division ratio constant
over the lull frequency range.
3-3 AMPLIFIER, RECTIFIER AND METER
The four-stage amplifier provides high gain over
a wide frequency range.
applied to a full-wave rectifier actuating a I-milli­
ampere meter movement. The amplifier-rectifier
system is stabilized with an overall feedback loop.
At the edges of the frequency range, the amount of
negative feedback is controlled to provide the maxi­
mum stability consistent with the gain available.
D) in the cathode circuit
contacting arms. spaced
in
the re­
of
V2, the
F01' scalc dcflcction
full
The amplifier output is
a. Amplifier -
Between the grids of V2 and V5 the amplifier
yields a net gain of approximately 55 to 60 db over
a ID-cycle to 4-megacycle band.
negative feedback, frequency compensating networks
in
the plate circuit of each stage, and cathode de­
generation at low lrequencies (the cathode resistors
are not bypassed at low frequencies) provide an
amplifier of high stability which can operate over
an extremely wide (lO-cycle to 4-mc) band.
feedback of the amplifier is returned from the plate
of the last stage (V5), through the rectifier-meter
circuit, to the cathode of first stage (V2) in such
phase as to be degenerative in effect.
adjusted by means of variable resistor R29 in the
feedback loop. Another adjustment in the feedback
loop, variable capaCitor C21, is used for adjusting
the frequency response of the amplifier at high
frequencies.
of
The stages the amplifier are resistance-capacitance
coupled. The coupling circuitry between each stage
is frequency-compensating, and provides separate
coupling for low and high frequencies. This feature
of the circuit design contributes to the stability of
the amplifier across the wide band over which the
voltmeter is rated to operate.
When the instrument is used
V5 is operated as a cathode follower and supplies
voltage at the OUTPUT terminals. The impedance,
OUTPUT
looking he
into t
mately 50 ohms.
The output from the plate of V5 is delivered to a
fullwave rectifier.
b. Rectifier and Meter -
The rectifier-meter Circuit is arranged in a
bridge-type configuration, with a crystal diode and
a capaCitor in each branch and a dc milliammeter
connected across its midpoints.
nection provides fuUwave rectification of the input
current. The design of the bridge is such that
pulsating direct current is delivered to the meter
circuit and 2) an alternating current of the same
frequency as the current at the rectifier input is
delivered to the output of the bridge.
rectifier-bridge output, the ac flows through the
feedback loop to the cathode of V2.
The current through the meter is proportional to
the average value of the waveform of the voltage
of
applied to the input the rectifier. Since calibration
of the meter in rms volts is based on the ratio that
exists between the average and effective values of
a voltage that is a true sine wave, deviation in a
waveform from that of a true sine wave may cause
meter measurements to be in error.
gives an indication of the Umits of possible error
of
A high level
The
The gain is
as
an amplifier, pentode
terminals,
is appJ'oxi­
The diode con­
1)
a
From the
Table 2-1