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_CUBIC '
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allWA", DIVISION
+8V
cause the
higher divider numbers also
reduce the loop gain.
,In this manner, the band VCOs are
constrained to
oscillate
on precise
multiples
of the
crystal
controlled
reference frequency
of 125 kHz.
The
exact multiple is the total
division
performed
on the VCO
frequency, in-
cluding that of the prescaler.
C248
128kHz
BAND REF
ose
8DIVIDER
PERMEABILITY TUNED OSCILLATORS
a positive voltage to offset this is developed
PEMlAalTY TUNED OSCILLATOR
10..
E~LINE
R612
R609
+.v
R235
FRO"
UI09
C24B
R241
R2U
+12
v o-_.J"'1,....,.""-....- .........
+~~
us.
There are
two identical permeab-
ility tuned
oscillators
in the ASTRO
l02BX.
Only one is described here.
The permeabili
ty
tuned oscillator
is the
primary means of
varying the
frequency
of the
transceiver.
It is
a Colpitts circuit, and the tuned fre-
quency is determined
by
L601
in par-
allel with the appropriate capacitors,
CR601, C603, C604, c605 and c606.
The
frequency tuned is 5.0 to 5.5 MHz, and
this combination
of capacitors is ch-
osen to
provide
temperature
compen-
sation for low drift,
and must be re-
placed. with
exactly the same types to
maintain this
stability,
if replace-
ment
becomes neces-
sary.
R606 provides a
De
return
for
the
gate
bias
of Q601,
which
is
developed
by
R609.
FeedMck
to sustain oscillat-
ion
is
achieved
by
the C609-C610 divid-
er combination.
The
drain
is
decoupled
to
ground
by R610
and C611,
while the
output is taken from
the
source to drive
FET buffer Q602.
The
bias gen-
erated
by
source re-
sistor R613 of Q602,
whose value is
set
by the
desired out-
put
impedance,
is
greater
than is de-
sirable for this FET; hence,
5-3
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