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3-2. INPUT ATTENUATOR
a.
In
the VTVM mode, the resistance of the input
attenuator and thus the input resistance of the voltmeter
is 50 megohms (R301 through R310).
For full scale
deflection, 1 mv must be applied to the null detector.
The necessary reduction is provided by four positions
on the input attenuator that are selected by range switch
section S2C.
b
In the differential mode, the resistance of the input
attenuator is 10 megohms (R305 through R310) for the
10, 1, 0. 1 and 0. 01 null ranges and 1 megohm (R306
through R310) for the 0. 001 volt null range.
However,
this is not the input resistance of the voltmeter. The in-
put resistance is determined by dividing the voltage of
the unlrnown by the a:m:mnt of current drawn from the
un-
known. The current drawn from the unknown is equal to
the difference in voltage between the unknown and the in-
ternally known voltage divided by the resistance of the
in-
put attenuator. The equation for input resistance can be
hence written
as
Eu
~n= ~
Eu Ra
!Eu-El , where
Rin
=
input resistance of voltmeter
Eu
=
voltage of unknown
lu
current drawn from unknown
Ra
=
resistance of input attenuator
E
voltage indicated by voltage
readout dials
I
=
absolute value (magnitude only}
Thus, the input resistance is infinite at null.
c.
For full scale deflection of 10, 1, 0. 1, and 0. 01
volts, the voltage difference (unknown voltage minus re-
ference voltage) must be reduced by an attenuator. This
reduction is provided by four positions on the input at-
tenuator that are selected by null switch section S3D.
On the 0. 001 volt null range, the voltage across the
input attenuator is fed directly to the null detector.
3-3.
NULL DETECTOR
a. GENERAL. The null detector is a fixed gain device
containing four resistance-capacitance coupled voltage
amplifier stages with a high amount of negative current
feedback. With high negative current feedback, the out-
put current is approximately equal to the signal voltage
divided by the impedance of the feedback network regard-
less of the amplifier characteristics or even the load
impedance.
The high negative feedback also makes the
amplifier relatively insensitive to the gain changes in
individual tubes due to aging and replacement. The out-
put current from the null detector is indicated on a meter
that has taut-band suspension.
This suspension does
away with all friction associated with meter pivot sticki-
ness. Thus, any tendency for the meter pointer to stick
at one point of the scale and then jump to another point is
completely eliminated.
3-2
b.
OPERATION.
At the input to the null detector,
R201, C201, R202, and C202 form a double section low
pass filter that reduces any AC component present on the
DC voltage being measured. The difference between the
voltage appearing at the output of the filter and the voltage
developed across the feedback network is converted to an
alternating voltage by Gl, a 94 cycle chopper.
This
chopped voltage is amplified by V202A, V202B, and
V203A before passing through cathode follower V203B.
During half the chopper cycle the output of the amplifier
is clamped to approximate null detector common poten-
tial by
Gl
while during the other half the output
i~
filter-
ed by C212 to provide a DC current for the meter. When
the chopper provides connection between contacts 1 and
9, a voltage is developed across feedback network R220,
R221, and R222 that is proportional to the meter (output)
current. This feedback voltage reduces the filter output
voltage with respect to the four stage amplifier.
The
impedance of the feedback network (R220, R221, and
R.222) is adjustable between 8. 82 and 9. 83 ohms. Since
the output current is approximately equal to the signal
voltage divided by the impedance of the feedback network,
a 1 mv signal voltage indicates an output current of
101.
7
to 113. 4 ua.
However, there is a loss due to
finite amplifier gain and filtering that leaves the output
current around 100 ua which can be set accurately by
means of the feedback network. Thus, current feedback
makes the output current essentially proportional to the
signal voltage. For full scale deflection, a 1 mv signal
voltage will cause 100 ua to flow through the meter.
c. EFFECT OF AC COMPONENTS. The only AC vol-
tage component that will reduce the accuracy of the 825A
is one that either saturates the chopper-amplifier or one
that beats with the chopper frequency. Since the voltage
required for saturation is greater than that re-iuired for
beating, the null detector is most sensitive to an AC
component with a frequency that is a submultiple or a low
multiple of the chopper frequency.
However, this is
easy to detect because the meter will beat at the differ-
ence frequency.
The low pass filter at the input of the
chopper-amplifier will attenuate any AC component.
The magnitude of the AC voltage 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 a few hundred cycles.
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
10%
of the input voltage will
cause an error of approximately 0. 01
%
which is well
within specifications.
If
AC components that affect
accuracy are ever encountered, additional filtering as set
forth in paragraph 2-9 will eliminate the problem.
d. ADJUSTMENTS. Variable resistor R232 in the null
detector power supply provides a means of adjusting the
output current of the amplifier to zero when there is no
input signal.
The gain of the amplifier is adjusted by
means of R222 in the feedback circuit.
e.
RECORDER OUTPUT.
The recorder output is
picked off divider string R225, Rl, and R226.
Output
level control Rl provides a means of adjusting the out-
put voltage up to a maximum of approximately 23 milli-
volts at full scale deflection. The voltage at the output
terminals is proportional to the meter reading.
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