Chapter 4. CIRCUIT FUNCTIONING

11-6625-524-14-4

CHAPTER 4

CIRCUIT FUNCTIONING

4-1

GENERAL

T h e basic circuitry of the voltmeter is best understood by reference to the block

d i a g r a m , figure 4-1.

a . The input signal is sampled and detected by the rf probe. The direct current

( d c ) output signal from the probe, after passing through the attenuator section of

t h e range selector switch, goes through an input filter which rejects power line

frequency components which might cause a measuring error.

b. A single-pole, d o u b l e - t h r o w , solid-state dc modulator then converts the dc

signal into a nearly symmetrical square wave.

c. The square wave is impedance matched and amplified by a high impedance,

n a r r o w band preamplifier. A narrow band filter in the preamplifier's feedback

f u r t h e r improves the signal-to-noise ratio.

d. T h e preamplifier output is further amplified in the post amplifier stage.

e. T h e modulated alternating current (at) signal from the post amplifier stage

is rectified by a synchronous demodualtor and the resultant dc output is used to

provide heavy overall dc feedback and is temperature compensated. The dc output is

a l s o linearized and is connected to the indicating meter.

f . The dc modulator driver is connected to the dc modulator and to the reference

amplifier.

g . The output from the reference amplifier is fed to the synchronous demodu-

l a t o r . The phase relationship between the reference amplifier and the input signal

d e t e r m i n e s the polarity of the dc output of the synchronous demodulator.

h. A n electronically regulated power supply delivers dc voltages to the ampli-

fier.

4-2

DETAILED DESCRIPTION, VOLTMETER

F o r the following detailed descriptions of each circuit, the Schematic Di agrams

(F0-1 and F0-2) should be used as reference sources as well as the Block Diagram,

f i g u r e 4-1.

Due to the complexity of interconnections between the ranqe selector swit ch and

p o w e r selector" switch to their circuit connections, these-switches will not be de-

t a i l e d separately but will be integrated into the applicable circuit's description.

a. RF probe and probe input circuit: T h e diode cartridge assembly (1A1), part

o f probe (1A3), contains a coupling capacitor, 2 matched diodes and 2 filter capa-

c i t o r s . T h e probe also contains 2 10K filter resistors and 3 feedback resistors in

a sealed housing. The probe input connector (1J1) uses .001MF capacitors (1C1 thru

1 C 5 ) on its current carrying terminals to filter stray RF pick-ups which may enter

the probe cable.

b. Range attenuator: T h e range attenuator divider consists of resistors 1R3

thru 1R9. Depending upon the range switch position, the DC signal from the probe

w i l l either be sent directly to the input filter or will be attenuated prior to

entering the input filter.

c. A C l i n e r e j e c t i o n ( i n p u t ) f i l t e r : P r e c e d i n g the rejection filter is a R/C

f i l t e r ( c o n s i s t i n g of 3R1, 3R2 and 3C1) and a clipping diode (3CR5). This filter and

d i o d e aid in reducing and limiting spurious signals from entering the rejection

f i l t e r . The rejection filter consists of R/C network 3R3, 3R5, 3R64, 3R65 and 3C4

and a parallel "T" network consisting of 3C3, 3C5, 3R4 and 3R66. This filter is

tuned for a minimum of -60dB rejection at the power line frequency. Unless other-

w i s e specified, the filter is tuned for use on a power line frequency of 60Hz. It

i s advisable to retune the filter for line frequencies other than originally speci-

fied.

4-1