CHAPTER 5. FUNCTIONING OF EQUIPMENT

TM 11-6625-444-14-2

CHAPTER 5

FUNCTIONING OF EQUIPMENT

Section I. BLOCK DIAGRAM ANALYSIS

5-1. Introduction

buffer amplifier so that when the input signal is

z e r o (input terminals shorted); there is 0 V

output. U1 is overload protected to 1000 V dc

This section describes the operation of the maj-

or rms.

or circuit functions of the voltmeter. First the dc

voltage measurement mode is discussed; fol-

l o w e d by ac voltage, dc ratio and the ana-

d. Active Filter. The dc output voltage of U1

log-to-digital (A/D) converter. For this discus-

sion refer to the block diagram (figure FO-1).

is then applied to an active filter, A1-U100, that

attenuates undesirable ac components of the dc

s i g n a l . Two ranges of filtering are provided;

5-2. Dc Measurement Mode

when the FILTER push button switch is in the out

(normal) position, the active filter attenuates 60

a. General. The voltmeter measures dc volt-

H z ac components about 60 dB and has a

age up to ± 1000 V and presents the measured

response time of approximately 60 milliseconds

voltage on a 5% digit display. Four ranges are

(ins). With the switch pushed in, 60 Hz ac sig-

provided; 2, 20, 200 and 1000 volts full scale.

nals are attenuated approximately 100 dB but

The major circuit blocks discussed are: dc volts

the response time is increased (slowed) to 600

attenuator, dc buffer amplifier, active filter and

ms.

the A/D buffer.

e. A/D Buffer. The high impedance signal

b. Dc Volts Attenuator. When the voltmeter is

from the dc buffer amplifier is applied to the A/D

in the dc mode, signals are applied to the dc

buffer, A1-U104, which has a gain of unity and

volts attenuator which provides an input imped-

provides a low source impedance to the A/D

on the 20 V, 200 V and 1000 V

ance of 10 M

converter input.

ranges.

W h e n the 2V full scale range is

selected, signals are applied directly to the input

of the dc buffer amplifier, A1-U1, and the input

5-3. Ac Measurement Mode

. The 20V full scale

resistance is 10,000 M

range divides the input by a factor of 10 so that

a. General. The voltmeter measures the true

an input signal of 20 Vdc results in 2 Vdc applied

rms of ac voltages over a frequency range of

to the input of A1-U1. The 200V full scale range

0.5 Hz to 1 MHz. The voltage ranges are identi-

divides the input voltage by 100 and the 1000V

cal to those of the dc voltage mode. However,

f u l l scale range divides the signal by 1000.

there are two modes of ac voltage measure-

Thus, the maximum voltage presented to the dc

ment; ac and ac+dc. The major circuit blocks

comprising the ac measurement mode are the

buffer amplifier never exceeds 2 volts.

preamplifier/scaler and true rms converter. Af-

ter the true rms converter, the processing of the

c. Dc Buffer Amplifier. The dc buffer amplifi-

ac signal is identical to that of the dc signal

er (A1-U1) has an extremely high input

(which was previously discussed in paragraph

impedance and has virtually no loading effect on

5-21.

the dc volts attenuator. The gain of U1 is 5X so

that an input signal of 2, 20 or 200 V full scale

results in a 10 V output signal. An input signal of

b. Preamplifier/Scaler. When the voltmeter is

in the ac mode, signals are coupled through ca-

1000 V produces an output voltage of 5 V from

A7-C4 to

U1.

The DC ZERO adjustment balances the

pacitor

the

preamplifier/scaler

5-1