chopper frequency.
Since the voltage required for saturation is greater
than that required for beating, the null detector is most sensitive to an ac
component with a frequency that is a submultiple of the chopper frequency.
However, this is easy to detect because the meter will beat at the
difference frequency.
The low pass filter at the input of the chopper-
appearing at the output of the filter depends on both its amplitude and
frequency before filtering.
For all practical purposes, one should never
encounter any trouble above one hundred Hertz. Below this, the filter may
not attenuate the ac component enough. However, this is not as bad as it
appears. A 60-cycle ac voltage that is 1% of the input voltage will cause
an error of approximately 0.001% which is well within specifications. If ac
components that affect accuracy are ever encountered, additional filtering
as set forth in the operating instructions will eliminate the problem.
j. Variable resistor R239 in the feedback network provides a means of
adjusting the output current of the null detector to zero when there is no
input signal. The gain of the null detector is adjusted by means of R230 in
the feedback network for the 1-millivolt sensitivity and by means of R231
for the 100 microvolt sensitivity (fig 5-15 (fo)).
k. The recorder output is picked off divider string R226, R8, and R227
(fig 5-15 (fo)).
Recorder output AMP ADJ control R8 provides a means of
adjusting the output voltage up to a maximum of at least 20 millivolts at
full scale deflection (disregarding 10% over-range at end of scale).
The
voltage at the RECORDER OUTPUT terminals is proportional to the meter
reading.
l. When the 887A is used to make differential dc voltage measurements
between 0 and 11 volts, an internal voltage is nulled or matched against the
unknown voltage.
An extremely accurate reference is therefore required.
This is obtained from the 0- to 11-volt reference.
The 0- to 11-volt
reference is composed of a well-regulated -18-volt power supply, a range
divider, and a five decade Kelvin-Varley divider. The range divider reduces
the voltage from a pair of stable Zener diodes in the -18-volt reference
supply to 11 volts for the 10- and 1000-volt dc ranges and to 1.1 volts for
the 1- and 100-volt dc ranges before it is applied to the Kelvin-Varley
divider.
The Kelvin-Varley divider divides its input voltage (11 or 1.1
volts) more than 1,100,000 equal increments any number of which may be
selected by setting the five decades with the five-voltage readout dials.
The output of the Kelvin-Varley divider, therefore, provides an extremely
accurate reference voltage.
(1) The -18-volt power supply (fig 5-15 (fo)) uses diode CR102
and filter capacitor C101 to supply unregulated dc voltage to series
pass transistor Q101.
In the Model 887AB, unregulated dc voltage can
also be supplied by a set of batteries (BT1) in the BAT OPR and BAT
CHECK modes.
The -18 volts is regulated by comparing a sample of the
output
voltage,
tapped-off
divider
string
R109,
R110,
and
R111,
with the voltage from zener reference diodes CR 103 and CR 104 in
a
two-stage
differential
amplifier.
Transistor
Q103 is
a
dual
transistor, having matched current gain and matched Vbe, which ensures
minimum voltage change due to temperature in the -18-volt reference voltage.
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