TM 11-6625-444-14-2
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;
when the FILTER push button switch is in the out
(normal) position, the active filter attenuates 60
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
. 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
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
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
