Crystal Mixer Circuit, Detailed Analysis

resistor. Coil L411, which is wound on re-

h e a t e r voltage from feedthrough

sistor R403, functions as a parasitic sup-

C411 to feedthrough capacitor

p r e s s o r . Plate voltage is filtered by re-

C417. Rf choke L413 and bypass ca-

s i s t e r s R404, R407, and R408, together

p a c i t o r C418 prevent rf currents

from circulating in the heater sup-

w i t h bypass capacitor C416. Feedthrough

p l y circuit.

c a p a c i t o r s C421 and C422 are best de-

(2) F e e d t h r o u g h capacitor C417 pene-

scribed by reference to the physical view

t r a t e s the upright center partition

of the rf tuner (fig. 69).

o n which tuning capacitors C402,

a. +105- Volt Dc Power Path. The dc path

C404, and C413 are mounted. The

for applying +105-volts to the plate of os-

inner conductor of the feedthrough

c i l l a t o r V402 consists of the following

capacitor connects the heater volt-

c i r c u i t elements:

age to oscillator V402 heater (pin

(1) The +105-volt bus is connected to

6 ) . The second heater (pin 1) is

the rear of the rf tuner (A, fig. 69)

connected to the rf chassis ground

b y a wire lead which connects to

to complete the heater circuit path.

t h e inner conductor of the feed-

The

c. Oscillator Power Signal Path.

t h r o u g h bypass capacitor, C422.

f r e q u e n c y generated across oscillator coil

(2) A n i n s u l a t e d l e a d c o n n e c t s t h e

L 4 0 9 (B, fig. 69) in V402 plate-to-grid

+105-volt bus from C422 to a sec-

circuit is inductively coupled to the crystal

o n d feedthrough capacitor, C421.

diode pickup coil L405. This coil is phys-

T h i s capacitor penetrates the up-

ically located so that it is also within the

right center partition which serves

m a g n e t i c field surrounding the rf ampli-

as a mounting surface for the three

fier plate circuit inductor loop, L404.

v a r i a b l e tuning capacitors, C402,

C404, and C413.

(3) D e c o u p l i n g resistor R408 (B, fig.

45. Crystal Mixer Circuit, Detailed

69) connects the dc voltage to the

Analysis

high-potential terminal of rf bypass

(fig. 28)

capacitor C416. The dc voltage

The incoming rf signal and the oscillator

continues through the series- con-

output signal are mixed in this stage. The

n e c t e d decoupling resistors, R407

s i g n a l from rf amplifier plate inductor

and R404, and rf choke L411, to the

L404 is inductively coupled to mixer pickup

l / 4 - i n c h wide loop of brass strap-

l o o p L405. The signal from oscillator-

ping mounted to the frame of C413.

tuned circuit L409, physically located

This strapping functions as a high

about 1 inch away from mixer pickup loop

Q inductor for the oscillator at the

L405 (B, fig. 69), is also within the mag-

ultra-high frequencies employed

netic field of L405 and inductively couples

in tuning unit 3.

t h e oscillator signal to the crystal mixer

b. 6.3 Volt Ac Path. T h e a c p a t h f o r

c i r c u i t . The nonlinear characteristics of

applying 6.3-volt ac voltage to the heater

a crystal diode, CR401, are untilized for

o f the oscillator tube consists of feed-

t h e heterodyning process because of the

t h r o u g h capacitor C417, rf choke L413,

f a v o r a b l e signal-to-noise ratio obtained

a n d bypass capacitor C418. This decou-

at the high frequencies encountered in the

p l i n g circuit is physically located within

200-400-mc frequency range of tuning

the rf tuner (B, fig. 69) as follows:

u n i t 3. The diode produces a number of

f r e q u e n c i e s at its output circuit. Two of

(1) A parallel path is provided inside

the rf tuner for heater voltage dis-

these are the sum and difference fre-

t r i b u t i o n . One path is through rf

quencies produced by the mixing process.

choke L408 for heater voltage (para

S i n c e pickup coil L405 and trimmer ca-

4 3 b ( 2 ) ) to rf amplifier V401. A

p a c i t o r C408 are resonated to 42-mc, the

difference frequency is passed through

second path is provided by the

output coils L4-15-L416 and coupling

I n s u l a t e d lead which connects the