Filter
Top Products
Theory of
Operaton
-
494A/4g4Ap
Service,
Vol.
1
Since
the
amplifiers
are
nearly
identical,
the
follow-
ing
description
applies
to
both amptifiers.
The
ac and
dc signal
paths
are
treated
separat€ly.
Figures
74
and
7-5 are
simplified
diagrams
of
the
ac
and
dc signal
paths.
=
'PRINTED
COMPONENT
Figure 7-4.
Equivalent
ac
circuit
of
an B2g MH:
amptifier.
f n
th€ ac circuit (Figure
7-41,
C1, L1,
and
L2 form
the
input
matching
network.
(tn
the first
stag€,
L1
is actu-
ally
the series
inductance
of
dc-blocking capacitor
cl016 at
the input of
the amplifier.)
The coilector
circuit
is matched
to
50o
by
L4
and
c2.
To
a large
extent,
L3
controls
the
gain
of
the
stage.
High frequency
stability
is enhanced
by Rl and
R2.
In
the dc circuit
(Figure
7-5),
negative feedback
through the voltage
divider,
consisting of
RO
and
R4,
sets
the collector
voltage
as
a fixed
proportion
of
the
-12
V reference
supply.
Collector
current is
deter-
mined by R5.
Current
requirements
for the first
stage
are
less
than the
requirements
for
the second
because
the first stage
requires
less
intermodulation
distortion
performance.
Diode
clamps
are
provided
for
each
amplifier
(CR1013
and
CR1022 at
the bases of
the
amplifiers
in
the actual
circuits)
to
protect
the
transistor
against
reverse
breakdown
of
the
base-emitter
junction
in
case
the +12
V suppty
faits.
In
the
actual
amplifiers
(not
shown in
Figures
7-4
and 7-5)
Ll 014
and
Cl
01
4 at
the
base of e1
01
7,
Cl 0.t
g
at the collector
of
Q1017, L1021
and
Cl023
at
the
base
of
Q'1025, and
Cl013
at
the
collector
of
Q1017
decou-
ple
the
signal
path
from
the
bias network.
The
3 dB
pad
(R1026,
R1027,
R102B,
and
R1029)
helps maintain a
wide-band
50O interface
between
the
second amplifier
stage
and
the
829 MHz Bandpass
Filter
on
the
829
MHz
2nd
Converter
board.
Test
point
J1029 at
the output
port
of
the 3
dB
pac,
is
used
for checking
amplifier
performance
and
to aid
in
adjustrnent
of
the
929 MHz
band-pass
lilter
on
the
829 MHz 2nd
Converter
board
(A2347).
7-14
+12
V
2?27-1A6A
Figure 7-5. Equivalent dc circult ot an 829 MHz
amplifiee
829 MHz
2nd
Converter.
Down-conversion lrom
829 MHz to
110 MHz
lF occurs on
the 829 MHz 2nd
Converter board
(A23A7).
The
board
contains
a
829
MHz
Band-Pass
Filter, a 1.3 GHz Low-Pass Filter,
a
3 dB
pad,
a 450
MHz High-Pass Filter, a single-
balanced mixer,
and
a
300
MHz Low-Pass
Filter.
The 829 MHz
Band-Pass Filter blocks unwanted
inputs,
primarily
the
609
MHz image
signal. lt consists
of
four,
quarter-wave,
coaxial type resonators, mounted
on
the 829 MHz 2nd
Converter
board. The
end
resona-
tors
are
tapped
near
their
grounded
end
to
facilitate
input and output coupling
of the
filter. lnter-resonator
coupling
is
provided
by
printed
"through-the-board"
capacitors
that
connect b€tween
the resonators
at
their
high
impedance end.
A
bendable tab
is located at the
high impedance end
of
each resonator
for fine
adjust-
ment of resonant frequency.
The bendable tab
acts
as
a small,
variable
capacitor
to
ground,
making fine
adjustments of resonant
frequency
possible.
When
properly
tuned, the
filter
presents
at least
12
dB
input
return loss and
about
2
dB
insertion loss at
829 MHz.
The
1.3
GHz Low-Pass
Filter
blocks
high trequency
signals
that would
otherwase
be
admitted
at
the
re'
entrant frequencies
of the band
pass
in excess
of
2
GHz.
The function
of
the
1.3 GHz Low-Pass
Filter
is
shared by the 1.2
GHz
Low-Pass
Filter on
the
829
MHz
Diplexer board.
o
o
o
o
O
a
o
a
o
a
o
a
o
o
o
o
o
o
o
o
a
o
o
o
a
o
o
o
t
o
o
o
O
o
O
o
o
o
o
a
a
o
o
o