Band Voltage Controlled Oscillators - Swann CUBIC ASTRO 102 BX Service Manual

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eCUBIC

COMMUNICATIONS

SWAIfI DIVISIDN

BAND VOLTAGE CONTROLLED OSCILLATORS

UIOI

Rloe

1:.

CI09

BAND

veo

BUFFER AMPLIFIERS

BAND

veo

. - - - -....---4II--.... -"v'\/\""'--O

160

C102

CI61

RI44

RI43

R14:\

+8V

Each of the six bands has its own voltage

controlled

oscillator

(VCO)

that operates

at a frequency 14.5 MHz

higher than the low edge of the band

itself.

Each of these VCOs is the same

as the other,

with the exception of

the values of the

fre~ency

sensitive components, so only one will be discus-

sed here, that

for

the

160

meter band.

The oscillator circuit is

derived from the basic Colpit-

ts oscillator,

with CIOJ

and

Clo4 providing the tapped cap-

acitance for

feedback.

TIOl

and the

parallel

combination

of varicap CRIOl and CI02 pro-

vide the tuned circuit.

Volt-

age applied by the error line

through

isolation

resistor

RIOl tunes

the varicap.

CIOl

is a blocking

capacitor,

and

bias is

developed

by voltage

divider RI02-RIOJ.

Output is

taken through

blocking capac-

itor CI05 and series resistor

RI05, and is filtered by a low

pass filter composed of inductors LIOl and LI02, with CIO?, CI08, and CIIO.

The output of the

filter is

coupled through

series resistor RIO? to a

line common with the other five oscillators, and to the

base of summing amp-

lifier QIO?

Rl44 and c160 pro-

vide a

feedback path

from col-

lector to base of this stage, so

the gain

of QlO?

is determined

the

ratio of Rl44 to RIO?

(See the

"Servicing Hints" sec-

tion of this

manual

for a dis-

cussion of feedback amplifiers).

Bias for QlO?

conventional,

as is the emitter circuitry. The

collector of

QlO?

directly

coupled to

the

base

of Ql08,vCO

making

them

into

high-gain

compound

pair.

Output

the

L.O.

mixer

taken

from the

emitter of Ql08 through C169 and

R159 ,

while

an additional out-

put is

taken from the collector

through R150 and c162, and applied to the base of amplifier QI09.

This stage

then drives level translator QlIO through C164,

and bias is applied through

dropping resistor R156.

The output of QlIO, now a square wave at a

TTL-com-

patable level, is directly

coupled to the prescaler of the band phase locked

loop.

(See the "Servicing Hints" section for a discussion of the PLL.)

5-1

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