Figure 22. Tuning unit 2, mixer and oscillator (20-70 mc), schematic diagram.

circuit of which is tuned to the difference

R e s i s t o r R 2 0 7 , the screen-dropping re-

f r e q u e n c y of 10.7 mc.

s i s t e r , is bypassed by C241. The rf output

v o l t a g e in the plate circuit is developed

a. Mixer V203A. The s i g n a l voltage

across the tuned plate circuit, which con-

f r o m second rf amplifier V202 is applied

s i s t s of tuning inductor L201C, capacitor

through coupling capacitor C219 to the grid

C 2 1 7 , coil L204, and trimmer capacitor

(pin 2) of the mixer. The voltage generated

C 2 1 8 . Resistors R208 and R211 are used

by oscillator V203B is also applied to the

t o load the plate tuned circuits. Resistor

g r i d of the mixer by coupling capacitor

R 2 1 4 and capacitor C216 provide plate

C 2 2 2 . The input voltages are developed

circuit decoupling. Bypass capacitor C259

a c r o s s grid resistor R212. Cathode bias

p r e v e n t s rf currents from circulating in

i s developed by the current flow through

t h e heater circuit. The voltage across the

R 2 1 3 , which is bypassed by C221. Re-

p l a t e tuned circuit is coupled to the grid

s i s t o r R219 and capacitor C229 provide

o f mixer stage V203A through capacitor

decoupling for the plate circuit. The plate

C219.

c i r c u i t load consists of the primary of rf

t r a n s f o r m e r T202 and its parallel capac-

36. Rf Tuner, Analysis of 20-70-Mc Mixer

i t o r C227. Resistor R217 functions as a

and Oscillator

p a r a s i t i c oscillation suppressor. Bypass

(fig. 22)

c a p a c i t o r C257 prevents Tf currents from

c i r c u l a t i n g in the 6.3-volt ac heater cir-

T h e mixer stage, V203A, receives sig-

cuit. The 10.7-mc output of the mixer

n a l s from second rf amplifier V202 and

voltage is fed through switch S201A to if.

from oscillator V203B. The oscillator

step attenuator A201 (fig. 25).

g e n e r a t e s a frequency which is 10.7 mc

b. Oscillator V203B. A p l a t e tuned

higher than the signal frequency. The two

o s c i l l a t o r is used to generate the local

signals are mixed within V203A, the plate

Figure 22. Tuning unit 2, mixer and oscillator (20-70 mc), schematic diagram.