Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 109

CIRCUIT DESCRIPTION
WJ-8718-19/FE HF RECEIVER
3.4.4
PRESELECTOR (FE-A1A1A3) (794274)
The Preselector mounts on the Frequency Extender, FE-Al. Assembly (see
paragraph 3.3 2). Figure 3-11 is a detailed block diagram of the Preselector which
should be referred to in the following circuit description. Figure 6-6, Preselector
Schematic Diagram, may be referred to for greater component level detail, if desired.
fut»,c *h= JJlf Preselefctor c°nsists of two "wow band voltage-tuneable bandpass
fin ^i mb i flrst.t"n,ng from 30-54 MHz, the second tuning from 54-100 MHz. Filter 1
9 (I!l\Z?ZLC7S1 °/ va/a0t°r di0d6S CR3"CR9. ^d inductorsTl, T2, L4, L8. Fitter
ThJS MHz =onSists of var«c'°^ diodes CR10-CR16, and inductors T3.T4, L5 L9
,13n'a6fe fr°m the DigUal Contro1 is imP^ssed on the varactor diodes and
tunes each filter from ,ts low end (tuning voltage low) to its high end (tuning voltage
CR17 «nd rRlSRWitThin^bHtWeen the fUterS iS «ecompIi«hed by PIN diodes CRl, CR2
bancI finPS mh? = dlodes "e controlled by the band switch data through U3. When
band 1 (30-54 MHz) is selected, E5 is high, U3-3 is 0 Vdc and U3-5 is + 15 Vdc This
forward biases CRl and CR17 and reverse biases CR2 and CR18. A low loss path is
created through filter 1, while filter 2 is cutoff. When band 2 (54-100 MHz is
selected E6 is high. U3-Slis +15 Vdc and U3-5 is 0 Vdc. This forward biases CM and
CR18 and reverse biases CRl and CR17. A low loss path is created through filter 2
while filter 1 is cutoff.
3-26
TUNING
VOLTAGE
RF IN
BAND 1-2
fr. RF OUT
1
—
'
'
INPUT
SWITCH
CR18
CRl 7
20-55 MHZ FILTER
T1,T2,CR3-CR9
OUTPUT
SWITCH
CRl
CR2
' •
1 e»
55-99 MHZ FILTER
T3,T4,CR10-CR16
1
>
1
i
BA
ND SEL
(21
/
.
.
\,
DECODER
U3
Figure 3-11. Preselector, FE-A1A1A3, Block Diagram