Passband Tuning - Swann CUBIC ASTRO 102 BX Service Manual

- -
~~M~!~~TIONS
S""AItI
DIVISION
RI!lO
osc-
R214
TI06
RI!l1
CWH
LINE
~ ~
CI1I9
C/48
f
+IO.!lV
RI42
U04
+IO.!lV
I
!.'::
RI!!4
Rl!l'
-=
FROM
PITO
RI!l2
PST FILTER AND
DRIVERS
+IO.5V
2!!i1 PST MIXER
RI!l1
RIS.
FROM
QII2
()-"''oIV~~HI--'''''':''''l
PASSBAND TUNING
QI08, a dual-gate MOSFET, acts as a subtractive mixer, with the first
I.F. signal applied to gate one, and the passband tuning oscillator signal
applied to gate two. Bias for gate one is developed across RIZ7, while con-
necting gate two to the source via RIZ8 makes it operate without bias. Out-
put of Ql08, nowat the pass band tuning I.F. of 13.8 MHz, is taken from its
drain. 110) and CIZ4 form a parallel resonant drain load. C125 is a coup-
ling capacitor, routing the signal to the SSB filter via QIIO and to the CW
narrow filter via Ql09.
QlIO, conventionally biased,
acts as an emitter follower to
drive the SSB filter, FLI02. Af-
ter passing through the filter,
the signal is coupled to Qll2
via Cl)6. Qll2 is also
an
emit-
ter follower, with its output
developed across Rl44 and bias
developed
by
the Rl42-R143 com-
bination. Cl)8 is a blocking
capacitor.
Wh
en operating in the CW
narrow mode, voltage is applied
to the CWN line through R150 to
diode switch CRIO). This volt-
age reverse biases the base-emitter junction of Qll2, causing it to cease to
conduct, and thus breaking the signal path. Also, Cl39 shorts the signal
path through CRI03.
Signal from the CWN filter
amplifier Qlll (covered later)
and from the SSB filter are coup-
led to gate one of the second PET
mixer Qll). Rl48 and R151 are
isolation resistors, and Rl52 is
the gate return resistance for
QlIJ, which is also a subtractive
mixer. Oscillator energy is co-
upled to gate two of QlIJ in the
same manner as i t was to the first
PET mixer, and the bias arrange-
ment is also the same. Output is
taken via Cl46, now back at the
first I.F. frequency of 9 MHz.
C147 serves the dual function of
resonating with
TlO)
and serving
as the first element in a low pass
filter composed of C147 , 1104 and
C148 , which fllters out unwanted
mixer products. R157 terminates
the filter in its required load.
PBT oscillator Ql2) is a conventional Colpitts crystal controlled
illator, with C203 and CZ04 providing the feedback capacitive divider.
and R215 form the bias network, while Rl27 is the emitter resistance.
3-6
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·
,
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