Two of these resistors are shunted by the 10K total resistance of the second
decade.
Between the two wipers of S5 (voltage dial A) then, there is a
total resistance of 5K (10K paralleled by 10K).
(1) Thus, the first decade divides the voltage across it into eleven
equal parts with one of the equal parts appearing across the two shunted
resistors.
Similarly, the second, third, and fourth decades divide the
voltage across them into ten equal parts. Note that the second, third, and
fourth decades each have eleven 1K resistors. The resistors may have the
same value because padding resistors R328 - R329 and R315 - R316 are used
across the third and fourth decades respectively to keep the proper
resistance matching. The last decade, with its associated shunt resistors
to keep the proper matching, is a variable resistor which can be set to pick
off increments equal to less than 1/100 times the voltage across its input.
The Kelvin-Varley resistors are matched for both temperature coefficient and
tolerance, thus providing an overall accuracy of 0.002% absolute from 1/11
of full scale to full scale. With the null switch in any null range, the
output of the Kelvin-Varley divider is connected in series with the TVM
attenuator, thus providing the accurate 0- to 11-volt or 0- to 1.1-volt
reference voltage required.
(2) Variable resistor R111 is used during final factory calibration
to set the reference supply to -18-volts.
This adjustment is not
exceedingly critical and should have to be done only when a component of the
reference supply has been replaced.
The voltage from the Zener reference
diodes is reduced to 11 volts at the input to the Kelvin-Varley divider by
adjusting variable resistor R120 during calibration. Range-divider variable
resistor R122 may then be adjusted for 1.1 volts at the input to the Kelvin-
Varley divider.
The 2 ohm trimmer resistors (odd resistors from R301 to
R325) and variable padding resistors R338, R351, and R364 should require
adjustment only after a component of the Kelvin-Varley divider has been
replaced.
n. The ac to dc converter (fig 5-15 (fo)) is composed of an attenuator,
an operational amplifier, and a rectifier-filter circuit. A pair of diodes
in the rectifier-filter circuit are used to convert the unknown ac into
pulsating dc.
This pulsating dc is then filtered to obtain a dc voltage
that is proportional to the average value of the ac input voltage.
The
output, however, is calibrated to indicate the RMS value of a pure sine
wave. An operational amplifier with high negative feedback is used to make
the rectification characteristics of the diodes linear and stable.
The
first stage is an n-channel field effect transistor (Q501).
The field-
effect transistor provides both high impedance and low noise input
characteristics. The next four stages consist of two transistor doubletons
(Q502, Q503, Q504, and Q506).
Transistor Q505 acts as a dynamic load and
thus increases the output impedance of the amplifier.
The amplifier
achieves a midband loop gain of approximately 70 dB with a virtually flat
frequency response from 20 Hz to 20 kHz. At the output of the amplifier,
full-wave rectification is used to return negative feedback to the gate of
the field-effect transistor. The high negative feedback makes the amplifier
practically noise free and relatively insensitive to gain changes in
individual stages due to aging and transistor replacement. An attenuator is
used to reduce the ac input voltage on the higher ranges to within the
operating level of the converter amplifier.
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