Filter
Top Products
Theory
ot
Operation
-
494A/4g4Ap
Servlce,
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Figure
7-1.
Ctpsr
section of
a four-cavlty
filtee
Selection
between
the
two 2nd
lF signals
also
takes
place
within
the 829
MHz
converter
system.
A
diode
switching
network
connects
the
active
j10
MHz
2nd
lF
signal
to the output
to drive
the
grd
Converter.
2472
MHz
2ND
CONVERTER
(Diagram
12)
The
2072
MHz
2nd
Converter
converts
the
2O72MHz
signal
output
from the
1st
Convener
to
110 MHz
for
eventual
application
to the
grd
Converter.
The assembly
consists
of a
four-cavity
filter
connected
to
a
narrow
band
mixer
through an
external
cable,
a
110 MHz low-pass
filter,
and
a
mixer-biasing
circuit.
Four-Cavity
Filter
The four-cavity
filter (FL14)
is
a
low-loss
narrow-
band fifter
that only
pass€s
lhe 2072
MHz
IF
signal
to
the mixer.
Any
other
frequencies
are
reflected
back
to
the
lst
Converter
and
terminated.
In
addition.
the
filter
prevents
the converter
LO and
mixer
products
from
entering
the 1st
Converter.
This filter
has a 1
dB bandwidth of 15 MHz and
an
insertion
loss
of 1.2
dB.
Each end resonator
ls capacity
coupled
to external
circuits through a coupling
hat
plugged
into
a
3 millimeter
connector.
Intercavity
cou-
pling
is
provided
by
coupling loops
that
protrude
from
the machined filter
top.
The
resonant
frequency of each
cavity is
determined
primarily
by the depth of a
gap
in
the
underside
of
the
filter top,
and is
fine
tuned
with a
tuning screw on
the side of
each cavity.
All of
the tight
machining
tolerances are confined
to the top. Thus,
the
main
cavity milling
need not be
a
high
precision part.
When
properly
tuned, using
a
network analyzer,
the
filter return
loss is
greater
than
25
dB from either end
(in
a
50 ohms
system).
Figure
7-1 shows
a cross sec-
tional view of
the
filter, and Figure 7-2
shows
the
equivalent
electrical
circuit.
Mixer
Circuit
The
mixer circuit consists of a single-balanced,
two-diode mixer, a
bias
circuit for
th€
mixer, a delay
line, and
a 110 MHz low-pass filter.
COUPLING
HAT
COUPLING LOOP FREOUENCY DETERMINING
GAP
FILTE
J
R
TOP
cAvrrYPosr+
l+
FtRsr
cAVrry
INTER-cAVttt *ott"raoND
cAvrry
2727-707A
7-8