Chapter 5. CIRCUIT FUNCTIONING

TM 11-6625-524-14-2

CHAPTER 5

CIRCUIT FUNCTIONING

5-1. Block Diagram

e. The alternating current (ac) signal from the

o u t p u t stage is rectified by a synchronous

The basic circuitry of the voltmeter is best un-

d e m o d u l a t o r (3A2Q7 and 3A2Q8) and the

derstood by reference to the block diagram (fig. 5-

resultant dc output is used to provide heavy

1).

o v e r a l l dc feedback and temperature com-

a. The input signal is sampled and detected by

pensation. The dc output is also linearized in the

the rf probe. The direct current (dc) output signal

output circuit and connected to the indicating

from the probe, after passing through the at-

meter.

tenuator section of the range selector switch, goes

f. The dc modulator output is also connected to

through an input filter which rejects power line

a driver (3A2Q1 and 3A2Q2) which drives a

frequency components which might cause a

reference amplifier (3A2Q3 and 3A2Q4).

measuring error.

b. A single-pole, double-throw dc modulator

g. The output from the reference amplifier is

(3G1) then converts the dc signal into a nearly

fed to the synchronous demodulator. The phase

symmetrical square wave.

relationship between the reference amplifier and

c. The square wave is impedance matched and

the input signal determines the polarity of the dc

amplified by a high impedance, narrow band

output which is connected to the indicating

preamplifier (3A2Q9 through Q12). Its narrow

meter.

bandwidth also improves the signal-to-noise

h. An electronically regulated power supply

ratio.

(3A1CR1 and 3A1CR2) plus transistors delivers

d. The preamplifier output is further amplified

-14.5 volts to the amplifier. The amplifier

in the driver and output stage (3A2Q13 and

divides the power supply voltage to - 6 volts and

3A2Q14).

+8.5 volts.

5-1