Rf Tuner, 400-700 Mc, Introduction

transformer, with optimum coupling be-

c. V502 Oscillator Circuit. A capacity-

t w e e n primary and secondary windings.

l o a d e d half-wavelength pair of parallel

The lengths of the coaxial lines in the input

transmission lines is used in the oscillator

c i r c u i t are chosen to resonate (together

c i r c u i t . The lines are connected between

w i t h the tube interelectrode capacitances)

the plate and grid of V502 and are properly

in the 700- to 1,000-mc frequency range.

d i m e n s i o n e d to sustain oscillations over

Section A of tuning capacitor C510 loads

the required tuning range, which is 42-mc

t h e first coaxial line and provides the

higher than the incoming rf signal. Section

required tuning range; section B of capac-

D of tuning capacitor C510, mechanically

itor C510 serves the same function for the

g a n g e d with the amplifier sections, pro-

second cavity. For vernier tracking of the

v i d e s the tracking frequency range from

a m p l i f i e r with the oscillator, a metallic

7 4 2 - to 1,042-mc. Because of the con-

tab is positioned between the grounded wall

s t r u c t i o n a l difference between the ampli-

o f the coaxial-type cavity and the stator

fier and oscillator tuned lines, no metallic

p l a t e s of tuning capacitor C510, sections

t r i m m e r tab capacitor is required for the

oscillator circuit. A loop, positioned in the

A and B. Varying the position of the tab,

w i t h reference to the stator plates, per-

o s c i l l a t o r cavity, functions as an imped-

m i t s trimming the high-frequency end of

a n c e - m a t c h i n g transformer to inject os-

t h e tuning range during alignment proce-

c i l l a t o r voltage into the input circuit of

d u r e s . Capacitor C550 is the trimmer tab

c r y s t a l mixer CR501.

capacitor for the first input cavity; C560

d. Crystal Mixer Circuit. The nonlinear

is the trimmer for the second input cavity.

characteristics of a crystal diode, CR501,

Energy transfer from the second cavity to

a r e utilized for the heterodyning process

the cathode of the grounded-grid amplifier

b e c a u s e of the favorable signal-to-noise

i s obtained by coupling capacitor C501,

ratio obtained at the ultrahigh frequencies

which is connected between the stator

o f tuning unit 4. The diode produces a

plates of tuning capacitor C510B and the

number of frequencies at its output. These

conductive tube clip that supports the

a r e the two original signal and oscillator

cathode of amplifier V501.

f r e q u e n c i e s (tracked 42-mc apart), and

b. V501 Output Circuit. The amplified

their sum and difference frequencies.

rf signal at the plate of amplifier V501 is

C r y s t a l mixer coil L505 is resonated to

coupled from the conductive metallic tube

42 mc and the if, preamplifier circuit

c l i p that secures the plate of the tube

is pretuned to 42 mc; therefore, only

through a second tubular capacitor, C507,

the difference frequency is passed through

t o the stator plates of tuning capacitor

output termination CP501 and a short

C510, section C. This section of the cavity,

l e n g t h of coaxial cable to connector jack

w h i c h includes the plate of the amplifier

J507 on the 42-mc if. amplifier subassem-

a n d capacitor section C510C, constitutes

b l y . All other frequencies are bypassed

the third shorted coaxial line of 1/4-

t o ground through capacitor C508, which

w a v e l e n g t h tuning range. Similar to the

is connected to the input terminal of signal

first two tuned cavities (a above), C510C

peaking coil L508 within the crystal diode

l o a d s the single-tuned coaxial line and

compartment of the rf tuner.

provides a tuning range from 700- to

1,000-mc; C570 is the trimmer tab capac-

53. Rf Tuner, 400-700 Mc, Introduction

i t o r provided for alignment at the high-

(fig. 35)

frequency end of the band. A loop, posi-

t i o n e d near the low rf impedance area in

W i t h the front-panel band selector ro-

t h e tuned cavity, injects the amplified rf

tated to the 400-700-mc position, the signal

signal into the crystal mixer CR501 input

v o l t a g e from the main unit is connected

c i r c u i t . This loop functions as an im-

from coaxial switch S501 by cable assem-

p e d a n c e - m a t c h i n g transformer to match

bly W503 and its connector plug P503 to

the input impedance of diode CR501, which

m a t i n g jack J508 on the exterior of the

is in the order of 200- to 300-ohms.

4 0 0 - 7 0 0 - m c rf tuner.