Section III. THEORY OF TUNING UNIT 1

calibrating signal path cannot be

which may be present at the front-

disturbed by any rf potential

panel IMPULSE OUTPUT jack.

Section Ill. THEORY OF TUNING UNIT 1

25. Block Diagram of Tuning Unit 1

g. 1,600-Kc If. Amplifier. The

1,600-

k c if. amplifier consists of four cascaded

(fig. 15)

double-tuned if. amplifier stages: V4, V5,

The block diagram for tuning unit 1 is

V 6 , and V7. These stages provide addi-

s h o w n in figure 15. For complete circuit

tional gain and selectivity for the 1,600-kc

d e t a i l s , refer to the schematic diagram,

if. signal. To vary the signal amplification,

figure 99.

a front-panel GAIN control is provided.

h. 455-Kc If. Amplifier. The 455-kc if.

a. Six-Position Turret Tuner. The six-

position turret tuner receives the rf signal

a m p l i f i e r is similar to the 1,600 kc, ex-

f r o m the main unit, and provides appro-

cept that its center operating frequency is

priate tuned circuits for any of the six rf

455 kc, instead of 1,600 kc. It also has four

b a n d s selected by MEGACYCLES band

s e p a r a t e stages: V8, V9, V10, and V1l.

s w i t c h S1. The rf signal is then coupled

The front-panel GAIN control, which varies

to the rf amplifier.

t h e signal amplification, is common with

b. Rf Amplifier Stage, V1. The rf am-

t h e 1,600-kc if. amplifier.

plifier stage, V1, amplifies the signal and

i. If. Output Selector. T h e i f . o u t p u t

c o u p l e s it to mixer stage V2. The front-

s e l e c t o r is a relay-operated switch which

panel TUNING control tracks the rf ampli-

allows the output from the if. amplifier in

f i e r and mix e r tuned circuits with the

use to be coupled to the detector and

output of the oscillator.

e l e c t r o m e t e r assembly, Al. Selection of

c. Oscillator, V3. O s c i l l a t o r V 3 g e n -

the proper relay-energizing voltage is

e r a t e s a signal which is 455 kc higher

a u t o m a t i c , depending. on the position of

t h a n the incoming rf signal for bands 1

the MEGACYCLES band switch S1.

a n d 3 (para 26), or 1,600 kc higher than

j. Detector and Electrometer Assembly

t h e incoming rf signal for bands 2, 4, 5,

A1. T h e d e t e c t o r a n d e l e c t r o m e t e r a s -

a n d 6. This signal is injected into mixer

s e m b l y receives the output from the if.

stage V2.

amplifier and performs two main functions.

d. Mixer Stage V2. M i x e r s t a g e V 2

F i r s t , the audio detector demodulates the

h e t e r o d y n e s the incoming rf signal from

i f . signal and feeds the resultant output

rf amplifier with the oscillator frequency,

signal to audio amplifier V701 in the main

a n d produces an output difference fre-

u n i t . Secondly, dual meter detectors are

q u e n c y . This difference frequency, which

used in a voltage-divider network to sup-

may be either 455 kc or 1,600 kc, depend-

p l y driving voltage to two separate elec-

ing on the band selected by the setting of

trometer stages. These are extremely

the MEGACYCLES band switch, is coupled

high-impedance electron tubes which faith-

to the if. input selector.

f u l l y reproduce the applied input signal,

e. If. Input Selector. The if. input se-

with no degradation or amplification, in its

l e c t o r allows for automatic relay-operated

amplifier. Two additional steps of attenua-

selection of either the 455-kc or the 1,600

t i o n , 10 db or 20 db, are inserted in the

kc output from the mixer stage. The proper

s i g n a l path by rotating SIGNAL ATTEN-

s i g n a l path for the selected if. signal is

UATOR DB switch S701 to its various posi-

c o m p l e t e d together with a second relay

t i o n s . In the 0 SUBST ONLY and 0 CW

housed in if. step attenuator AT1.

ONLY positions of S701, 6 db attenuation

f. If. Step Attenuator AT1. If. step at-

i s in the if. signal path. In the 20 DB

t e n u a t o r AT1 is a ladder network which

position, 16 db of attenuation is in the path;

provides an inherent 6-db loss in the sig-

in the 40, 60, and 80 DB positions, 26 db

n a l path, before the signal enters the it.

of attenuation is in the path.