HP 740B Operating And Service Manual page 34

Model 740B
4-95. TheX10divider is arranged in the same manner
as the Xl divider. The X10 divider input resistance is
10 kn: 9 kn within the divider and 1 kn from the ex-
terior combination of S5R5 and the X10 2 divider. The
X10 divider divides the input voltage in 0.01 V steps
resulting in an output across the entire Reference Di-
vider of 0 to +0.99 V. The X102, X103 and X104
dividers operate in the same manner. Each divider
adds resolution to the output resulting in a 0 to
+0.99999 V output when all five VOLTAGE SET Con-
trols are rotated.
4-96. In Standard mode, the output voltage is taken
from the Standard Vernier Divider which drops the
remaining 0.00001 V in steps of 1
/lV,
from 0 to +10
/l
V. The resultant Reference Divider output in Stan-
dard mode is 0 to + 1. 00000 V.
4-97. Each decade of the Reference Divider utilizes
4 resistors to derive the 1 to 0.1 division ratios. The
resistor values are 1 kn (R1), 2 kn (R2), 2 kn (R3)
and 4 kn (R4). The VOLTAGE SET switches, S4
thru S8, rearrange the connections between the four
resistors to obtain nine combinations, each combina-
tion producing a different division ratio, yet main-
taining a resistance of 9 kn within each divider. The
output of each decade divider is paralleled with the
resistance of an external resistor (S4R5, S5R5, etc.)
and the complex resistance of the following decade
dividers. The entire series -parallel resistance com-
bination remains at 1 kn, regardless of the settings of
the VOLTAGE SET switches.
4-98. Resistors in the Xl and X10 dividers are in-
dividually matched by adjusting variable padding re-
sistors on Al (Figure 7 -5). The Al padding resistors
allow compensation to be made for aging of the re-
sistors on S4 and S5. The padding resistors are ad-
justed when performing the Internal Alignment Pro-
cedure (Paragraph 5-57).
4-99. METER LOOP.
4-100. The Meter Loop consists of the Meter Filter,
the Meter Amplifier, the front panel meter and part
of the Sensitivity switching. The meter displays the
input voltage in Voltmeter mode, the 6th digit of the
input voltage in Differential Voltmeter mode and the
6th digit of the output voltage in Standard mode.
4-101. The input to the Meter Loop comes from the
Main Loop voltage feedback path in Voltmeter mode.
On the 1 mV to 1 V ranges, the Meter Loop input is
o
to 1 Vdc -- the same voltage fed back from the
output of the Main Loop to the Main Loop Feedback
Divider. On the 10 V, 100 V and 1000 V ranges, the
Main Loop Feedback Divider attenuates the Meter Loop
inputbya factor of 0.1, 0.01 and 0.001 respectively,
resulting in a constant 0 to 1 V input on all ranges
in Voltmeter mode.
4-102. In Differential Voltmeter mode, the Meter Loop
input is connected to
W'
the Reference Supply common.
The Meter Loop is referenced to
W,
the grolU1d re-
ference for all of the critical circuits in the Main Loop.
Whenever the Reference Loop output differs from the
Main Loopfeedback voltage, current flows through the
01794-1
Section IV
Meter Loop, proportional to the error between the
Main Loop feedback and Reference Loop output volt-
ages. The Meter Loop detects and displays the error
voltage, allowing a differential measurement to be
made. TheXl throughX10 4 SENSITIVITY pushbuttons
control the Meter Loop sensitivity.
4-103. In Standard Mode, the Meter Loop input comes
from the Standard Vernier Divider, p/o S9. The input
varies from 0 to + O. 1 V according to the position of
the STANDARD VERNIER Control. Meter Loop gain
is 10 in Standard mode resulting in a 0 to + end-scale
deflection as the STANDARD VERNIER Control is
turned clockwise.
4-104. METER FILTER AND AC AMPLIFIER (Fig-
ure 7-6).
4-105. The input to the Meter Loop is applied to the
Meter Filter, C8, C9, C10 and R6. The Meter Fil-
ter removes noise from the dc input. The dc output
of the Meter Filter is applied to the Modulator, A16V1
and V2.
4-106. The Modulator consists of two photocells that
are lighted alternately by neon lamps A16DS1 and
A16DS2. The lamps are driven by a 95 Hz square
wave from A8. The modulated output of A16V1/V2
is a 95 Hz square wave proportional to the difference
between the Meter Loop input and degenerative feed-
back voltage from the output. After the Meter Loop
responds to an input, the amplitude of the square wave
is very small due to the high gain of the Meter Amp-
lifier.
4-107. The ac error signal from the Modulator is
applied through A2 pin 1 and A2C 1 to the base of A2Q1.
A2C1 decouples the ac error signal from the input dc
level. A2CR1 limits large negative spikes that may
occur when changing ranges or functions. A2Q1
through A2Q3 comprise a cascade -coupled, three-
stage amplifier. Degenerative feedback within the
three-stage amplifier ensures gain stability and pro-
vides some gain correction for varying input voltages.
The output of the three -stage amplifier is coupled
through A2C4 to A2Q4.
4-108. A2Q4 and A2Q5 comprise a differential amp-
lifier. Gain of the amplifier is affected by feedback
voltage from the collector of A2Q16 through A2R15 to
A2Q5.
When the X102. X10 3 orX10 4 SENSITIVITY
pushbutton is depressed, the degenerative feedback is
reduced by switching A2Rll in parallel with A2R12,
reducing the feedback impedance to ground. This
results in increased gain for the A2Q4/A2Q5 differ-
ential amplifier. The output of the differential amp-
lifier is am plified by A2Q16 and coupled through A2C 7
and A2 pin 8 to the Demodulator. A2TP8 provides a
convenient point to monitor the operation of the Meter
Amplifier AC Section.
4-109. The amplified ac error signal is demodulated
by A16V3 and A16V4. The Demodulator photocells
are lighted by A16DSI and AI6DS2, the same neons
that light the Modulator photocells. The Demodulator
output is a dc level proportional to the input ac error
signal. The Demodulator output voltage is applied to
the Meter Driver through A2 pin 10.
4-9