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NULL DETECTOR

 
  
 
TM 9-4935-282-34
803D
finally filtered to produce a DC output. The chopper amplifier
through R310) for the 0.001 volt null range; this is not the input
has a high amount of negative current feedback. This makes
resistance of the 803D. The input resistance is determined by
the output current approximately equal to the signal voltage
dividing the unknown terminal voltage by the current drawn
divided by the impedance of the feedback network, regardless
from the unknown. The current drawn from the unknown is
of the amplifier characteristics. The high negative feedback
equal to the difference between the unknown terminal voltage
also makes the amplifier relatively insensitive to the gain
and the internally known voltage divided by the resistance of
changes in individual tubes due to aging and replacement.
the input attenuator. The equation for input resistance can be
The output current from the null detector is indicated on a
hence written as:
meter that has taut-band suspension. This suspension does
away with a friction associated with meter pivot stickiness.
Rin = Eu = Eu Ra = Es (Ra+ Rs ) -Rs where:
Thus, any tendency for the meter pointer to stick at one point
Iu  Eu-E  Es -E
of the scale and then jump to another point is completely
eliminated. The attenuator is used to reduce the voltage span
Rin
= input resistance of voltmeter
of each range to a common range usable by the chopper
Eu
= Es - Iu Rs = terminal voltage of unknown
amplifier to produce proper meter deflection.
b. NULL DETECTOR. At the input to the null detector,
Iu
= current drawn from unknown
R201, C201, R202, and C202 form a double section low pass
Es
= source voltage of unknown
filter that reduces any AC component present on the DC
Rs
= source resistance of unknown
voltage being measured. The difference between the voltage
appearing at the output of the filter and the voltage developed
Ra
= resistance of input attenuator
across the feedback network is converted to an alternating
E
= voltage indicated by voltage readout dials
voltage by G1, an 83 cycle chopper. This chopped voltage is

= absolute value (magnitude only)
amplified by V202A, V202B, and V03A before passing through
cathode follower V203B. During half the chopper cycle the
Since the reference voltage (E) is equal to the unknown
output of the amplifier is clamped to approximate null detector
voltage (Eu) and the source voltage (Es) at null, no current is
common potential by G1 while during the other half the output
drawn from the unknown and the input resistance is therefore
is filtered by C212 to provide a DC current for the meter. The
infinite.
voltage developed across feedback network R220, R221, and
d. In the AC vtvm mode, null switch sections S3C and
R222 is proportional to the meter (output current. When the
S3D and AC - DC switch section S5E provide connection to
chopper provides connection between contacts 5 and 7, this
only one position on the vtvm attenuator regardless of where
feedback voltage effectively reduces the magnitude of the
the range switch is set. This is because the output of the AC to
voltage that is chopped and applied to the input of the
DC converter is 5 volts DC for full input on each range. In the
amplifier.  The impedance of the feedback network (R220,
AC differential mode, the voltage difference (converter output
R221, and R222) is adjustable between 8.82 and 9.83 ohms.
voltage minus reference voltage) is reduced by the same
Since the output current is approximately equal to the signal
positions on the vtvm attenuator as for DC differential
voltage divided by the impedance of the feedback network, a 1
measurements. Because of this and the fact that the converter
my signal voltage indicates an output current of 101.7 to 113.4
puts out 5 volts DC for full input on each range, the null range
ua. However, there is a loss due to finite amplifier gain and
used must be multiplied by the AC null multiplier indicated by
filtering that leaves the output current around 100 ua which can
the range switch to find the full scale difference between the
be set accurately by means of the feedback network. Thus,
unknown voltage and the reference voltage.  The input
current feedback makes the output current essentially
impedance for the AC vtvm and AC differential mode depends
proportional to the signal voltage. For full scale deflection, a 1
on the input impedance of the AC to DC converter and its
mv signal voltage will cause 100 ua to flow through the meter.
attenuator. The input impedance is thus dependent on the
c. INPUT ATTENUATOR. In the DC vtvm mode, four
setting of the range switch and is 1 megohm 35 uuf for the 500
positions on the vtvm attenuator selected by range switch
volt AC range, 1.1 megohm 35 uuf for the 50 volt AC range,
section S2C provide the necessary reduction of the 500, 50, 5,
and 1 megohm 50 uuf for the 5 and 0.5 volt AC ranges.
and 0.5 volt ranges for proper chopper-amplifier input. For this
e. NULL DETECTOR POWER SUPPLY. The B+ supply
mode, the resistance of the attenuator and thus the input
for the null detector is obtained from a half-wave rectifier
resistance of the 803D is 50 megohms (R301 through R310).
consisting of diode CR201 and a filter network (C213A, R227,
In the DC differential mode, the voltage difference (unknown
and C213B) that is regulated by an OA2 tube (V204) and
voltage minus reference voltage) is reduced by four positions
series dropping resistor R230. Divider resistors R228, R229,
on the vtvm attenuator selected by null switch sections S3C
R231, and R232 and diode CR202 provide a compensating
and S3D to give full scale deflections corresponding to inputs
voltage for the purpose of adjusting the null detector to zero
of 10, 1, 0.1, and 0.01 volts.  For full scale deflection
with R232 when there is no signal input. Diode CR202 keeps
corresponding to 0.001 volt, the voltage across the attenuator
one side of R232 at approximately -0.6 volts DC with respect to
is fed directly to the chopper amplifier. Although the resistance
the null detector common.
of the vtvm attenuator is 10 megohms (R305 through R310) for
the 10, 1, 0.1, and 0.01 volt null ranges and 1 megohm (R306
3-2

   


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