Introduction to Unit Theory

networks as shown in the following chart.

panel c o n t r o 1 s of the impulse

generator are set so that the meter

After the meter has been calibrated by the

indicates the same amplitude as

use of the self-contained calibrating signal,

t h e impulse generator is disabled and the

t h a t produced by the incoming rf

i n c o m i n g rf Signal is applied to the re-

signal.

c e i v e r . T h e meter pointer deflection is

(4) The value of the incoming rf signal

read d i r e c t 1 y, taking into account the

i s read from the settings of the

setting of the SIGNAL ATTENUATOR DB

i m p u l s e generator coarse and fine

control. For precise measurements, the

output front -panel controls.

l o s s in the 30-foot length of rf cable is

a d d e d to the meter indication. Figures 1

7. Introduction to Unit Theory

and 2 show the signal paths for this method

The signal flow and basic functioning of

of measurement, `when the shunt method of

the test set in the shunt and series methods

c a l i b r a t i o n is used.

of calibration have been discussed in this

section. The following sections cover a de-

Full-scale meter

indication (in

tailed analysis of the test set on a unit-by

Indication (in

SIGNAL ATTENUATOR DB

microvolts unless

db above 1

control

setting

otherwise indi-

u n i t description.

microvolt)

cated)

a. Section II analyzes the circuits housed

1,000

in the main unit, which consists of rf out-

10,000

p u t switching and attenuation, metering,

100

100,000

audio amplification, and power supply.

80 (plus the inser-

140

10 volts

b. S e c t i o n I I I d e s c r i b e s t h e d e t a i l e d

tion of Fixed

f u n c t i o n i n g of tuning unit 1. This unit

Attenuator CN-

h o u s e s a six-position turret tuner and rf

7 2 1 / U R M - 4 5 be-

tween the rf

c i r c u i t , i f . input, attenuation, o u t p u t

cable and the

s w i t c h i n g circuits, two separate if. ampli-

SIGNAL INPUT

f i e r subassemblies (one tuned to 455 kc;

jack).

t h e other tuned to 1,600 kc), and the de-

tector-electrometer assembly.

b . Substitution Method. A second basic

c. Section IV presents a description of

method of operating the test set is through

the circuits housed in tuning unit 2. These

t h e substitution method of measurement.

are a two-band rf section whose resonance

This technique is used when the loop, verti-

to incoming rf signals is varied by chang-

cal, or discone antenna is the signal pickup

i n g the setting of an i n d u c t u n e r c i r c u i t ,

d e v i c e for broadband (noise) signals. In

a three-step if. attenuator, a single if.

this method of procedure, the known ampli-

a m p l i f i e r subassembly (operating at 10.7-

t u d e output of the impulse generator is

mc center frequency), and a detector-

substituted for the previously unknown

e l e c t r o m e t e r assembly which is identical

a m p l i t u d e of the signal under measure-

in all four tuning units.

m e n t . Figure 3 shows the signal path for

d. Section V follows the detailed signal

t h i s method of measurement, when using

path in tuning unit 3. This unit covers its

the series method of calibrating the meter;

complete frequency spectrum in one band,

E s s e n t i a l l y , this method reverses the se-

u s i n g an rf tuner whose resonance to in-

quence of operations described in a above

coming rf signals is varied by gear-driving

in the following manner:

a three-section air-dielectric capacitor.

(1) T h e incoming rf signal is applied

The if. signal path consists of an if. pre-

to the receiver without initially

a m p l i f i e r housed in one subassembly, a

c a l i b r a t i n g the meter. The meter

t h r e e - s t e p if. attenuator housed in another

p o i n t e r deflection and the setting

subassembly, and a main if. amplifier

of the SIGNAL ATTENUATOR DB

housed in a third subassembly. Both the if.

c o n t r o l are noted.

p r e a m p l i f i e r and main amplifier operate

(2) The incoming rf signal is disabled.

a t a center frequency of 42 mc. T h e

(3) T h e coarse and fine output front-