**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**