6.
In order to provide for a more complete understanding of the 887A
Voltmeter, the following paragraphs describe each section of the circuit in
detail.
a. Since the instrument contains a 0- to 11-volt reference, the unknown
voltage is measured by comparing it to a known voltage with the aid of a
NULL detector only on the 1- and 10-volt range. On the 100- and 1000-volt
range, the dc input attenuator (R100 through R104)(fig 5-15 (fo)) divides
the unknown voltage by 100 and this attenuated voltage is then measured by
the potentiometric principle. Thus, after attenuation by 100, the 100- and
1000-volt ranges are reduced to the equivalent of 1- and 10-volt ranges.
The dc input attenuator is extremely accurate and has excellent long-term
stability.
The 10K variable resistor (R103) is used during factory
calibration to set up the proper division ratio. This adjustment can then
be performed as required at regular calibration intervals.
b. The DC Transistorized Voltmeter (DC TVM) is composed of an
attenuator, a null detector, and meter. The heart of the DC TVM is the null
detector in which the dc signal is modulated by an electromechanical
chopper, amplified by a 5-stage resistance-capacitance coupled amplifier,
rectified by a transistor switch, and finally filtered to produce a DC
output. The null detector has a high amount of negative current feedback.
This makes the proportion of the output current feedback approximately equal
to the signal voltage divided by the resistance of the feedback resistor,
regardless of the amplifier characteristics.
The high negative feedback
also makes the amplifier relatively insensitive to the gain changes in
individual transistors due to aging and replacement.
The output current
from the null detector is indicated on a meter that has tautband suspension.
This suspension does away with all friction associated with meter pivot
stickiness. Thus, any tendency for the meter pointer to stick at one point
of the scale and then jump to another point is eliminated.
The TVM
attenuator is used to reduce the voltage span of each range to a common
range usable by the null detector to produce proper meter deflection.
c. The null detector is a current feedback amplifier that drives a
meter. Any feedback amplifier is essentially a null-seeking device. That is,
it tends to make the voltage fed back to the input equal to the input voltage.
In a current feedback amplifier, the feedback voltage is equal to the voltage
drop across a fixed resistor caused by the output current or a portion of the
output current.
At the input to the 887A null detector, R201, C201, R202,
C202, R203, and C203 form a triple-section low pass filter that reduces any ac
component present on the dc voltage being measured (fig 5-15 (fo)).
The
difference between the voltage appearing at the output of the filter and the
voltage developed across feedback resistor R236 is converted to an alternating
voltage by C201, an 84 Hz chopper. The voltage across R236 is proportional to
the current flowing in the meter. The alternating voltage created by G201 is
amplified by a five-stage solid-state amplifier.
The first stage is a p-
channel field-effect transistor (Q201). The field-effect transistor provides
both high impedance and low noise input characteristics. The next four stages
consist of two match pair transistors (Q202 to Q205). During one portion of
the chopper cycle, the output of the amplifier is clamped to approximately
null detector common potential by Q206, a transistor switch. The transistor
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