Dc Vtvm

TM 11-6625 -537-14-1

b. Ac Measurements (fig. FO-2).

when the maximum voltage for each range is

(1) AC utum mode. The electronic voltmeter

applied to the input terminals. Levels B and C of

operates in the ac vtvm mode when the NULL

NULL switch S3 connect null and range divider

switch is set to VTVM and AC-de polarity switch

R301 through R31O and an opposing O- to 5-volt

is set to AC. Levels I and K of AC-de polarity

dc reference to the converter output. The O- to 5-

switch S5 connect the ac-to-dc converter to the

volt dc reference is obtained from the 500-volt dc

input terminals. The ac-to-dc converter provides

reference supply by levels C, D, G, and H of AC-

a n output of 5 volts dc when the maximum

dc polarity switch S5. The difference between the

voltage for each range is applied to the input

converter output and the reference voltage is

terminals. Levels B and C of NULL. switch S3

developed across the null and range divider.

apply the output of the ac-to-dc converter to null

NULL switch S3A selects the output of the null

and range divider R301 through R31O. A fixed

and range divider and applies it to the dc vtvm

output (1/500) of the null and range divider is

where the difference between the converter output

selected by S5L and applied to the dc vtvm,

and the reference is displayed on the VOLTS

VOLTS meter MI deflects to the right and in-

meter. Full scale deflection of the VOLTS meter

dicates the value of the unknown voltage directly.

corresponds to the setting of the NULL switch

Full scale deflection corresponds to the setting of

multiplied by the AC NULL MULT indicated by

the RANGE switch.

the position of the RANGE switch. The VOLTS

meter deflects to the right when the converter

(2) Ac differential mode. The electronic

voltmeter operates in the ac differential mode

output is greater than the reference voltage. The

when the NULL switch is set to 10, 1, ,1, or .01,

unknown voltage is measured by adjusting the

and the AC-de polarity switch is set to AC. Levels

value of the reference voltage until the VOLTS

I and K of AC-de polarity switch S5 connect the

meter indicates zero deflection. The unknown

ac-to-dc converter to the input terminals. The ac -

voltage is then equal to the reference voltage

to-dc converter provides an output of 5 volts dc

indicated on the front panel readout dials.

I. CIRCU

Section

T FUNCTIONING

5-3. Dc Vtvm

is developed across R206 and coupled to V203A

(fig. 5-3)

by C208. Resistor R212 is the plate load for

V203A. The output of V203A is coupled to V203B

a. General. The dc vtvm consists of a drift-free

by C21O and R214. Negative current feedback

dc amplifier and a meter. The input voltage is

from the cathode of V203B is coupled to the

converted to a square wave by a chopper and

cathode of V202 by R211. The output of V203B is

passed through three stages of voltage am-

developed across R215 and coupled to VOLTS

plification. The amplified square wave is syn-

meter Ml by C211. When chopper CK1 provides a

chronously rectified by the chopper and applied

contact across pins 1 and 7 during time tl, the

d i r e c t l y to the VOLTS meter. The amplifier

output of V203B flows through M 1 and feedback

stages use high negative current feedback to

resistors R218 and R219. The voltage actually

increase stability and to reduce changes in gain as

applied to the grid of V202 then is the difference

characteristics of individual tubes change due to

b e t w e e n the input voltage and the feedback

aging. For full-scale deflection, a 0.010 volt input

voltage. The amount of feedback voltage and the

s i g n a l will cause 100 microampere to flow

net gain of the three amplifier stages is adjusted

through the VOLTS meter.

by VTVM GAIN ADJ, R219. Resistor R230 is

b. Operation. The input is passed through

connected in parallel with R218 and R219 by S3D

double section low pass filter R201, C201, R202,

and S5J to increase the net gain of the three

a n d C202. Voltage limiter V201 prevents an

amplifiers in the .O1-ac null range. This com-

overvoltage from being applied to the dc vtvm.

pensates for the change in output impedance of

Resistors R225, R226, ZERO ADJ R227, and

the ac-to-dc converter when NULL switch S3C is

Capacitor C213 electrically zero VOLTS meter MI

set to .01. (See figure FO-2. ) When chopper CK1

by maintaining the input at zero potential with

provides a contact across pins 6 and 7 during time

respect to the filaments of V202 and V203. The

t2, R217 effectively shorts the output of V203B.

input voltage is applied to the grid of V202 by

The chopper therefore acts as a synchronous

R203, C203 and R204. Cathode bias for V202 is

rectifier by allowing only negative going pulses

developed by R207 and C205. The output of V202

5-4