Filter
Top Products
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_
The
3
dB
pad
helps
ensure
a
consistent
5Oo
inter-
face
for
the
829
MHz
Band-pass
Filter.
.
T!"
450
MHz
High-pass
Filter
btocks
the
tower
lF
signals generated
within
the
mixer.
.
The
mixer
generates
several
intermodulation
pro-
ducts
of
the
829MHz
RF
and
71g
MHz
LO
signats.
The
mixer
diodes
are
transformer
driven
Oy
a
targe
ampli-
tucre
{+12dBm)
719
MHz
signar
trom
t'he
rocit
osciita-
tor.
.This
large
signal
driveJ
the
diodei
in
and
out
of
conduction,
switchinglhe.lower
amplitude
g29
MHz
sig_
nal
on
and
off
at
a
71
g
MHz
rate,
to
generate
the
mixer
producJ::
Onty
the
difference
trequeicy
of
110
MHz
is
passed
through
the
A00
MHz
Low-pasj
filter.
The
sum
product
of
1548
MHz,
is
reflected
back
to
th€
mixer
by
the
829
MHz
Band-pass
Filter,
in-phase
with
LO
har-
monics,
to
increase
the energy
of
ine
110
MHz
signal.
A..printed
de-lay
line,
Uetween
itre
S2g
MHz
Band_pass
Filter
and
1.3
GHz
Low-pass
Filters,
"ontrot"
the
phase
delay,
The
net
rssult
of
this
,'image
enhancem€nt,,
is
low
conversion
loss
and
good
intJr-modulation
distor-
tion
performance.
The
O
dB
pad
reduces
the
image
enhancement
effect
and permits
the
use
of
non-critical
line
lengths
and
filter
characteristics.
Overall
conversion
loss,
from
829
MHz
to 1.10
MHz,
is
about
g.5
dB,
includ-
ing
2
dB from
the
929
MHz
Band-pass
Fitter
and
g
dB
from
the attenuator.
The
300
MHz
Low-pass
Filter
btocks
LO,
RF,
and
higher
frequency
products.
110
MHz
lF
Select
The
110
MHz
lF
Select
circuit
{1?3161
selects
the
110
MHz
tF
signat
from
either
the
829
MHz
2nd
Converter
or
the
2OlZ-UAz2nd
Converter
for
transmission
to
the
110
MHz
tF
Amptifier.
The
110
MHz
tF
signat
from
the
929
MHz
2nd
Con_
y919r-boa(
is
apptied
direcfly
to
thE select
circuit.
The
1'f 0
MHz
lF
signal
from
the
iaZZu{z
Convert€r
board
is
applied
to
the
select
circuit
via
p233
and
a controiled
amplifier
(Ot012/Oi011).
Switching
between
the
two
lgft:_
is
done
by
CR2011,
CRtOt2,
CR2013,
and
cR1015.
Diode
CR2011
is
turned
on
when
the
tF
SELECT
line
to the
110MHz
lF
SElect
is
low.
Thls
steers
the
110
MHz
lF
signal,
from
the
g29
MHz
2nd
Converter
1119:-to
the_output port.
At
the same
time
CR2012,
CR2013,
and
CR't01S
turn
on
and
el011
turn
off
to iso-
lalg_the
output
port
from
any
spurious
signals
from
the
2072
MHz
2nd
Converter.
When
the lF
SELECT
tine
goes
high,
etOtl
turns
on
and
CR2012,
CR201g,
and
CBl01S
tirn
otr
to ailow
the
110
MHz
lF
signal,
from
the
2O72MHz
2nd
Converter,
to be apptied
to
the
output port.
Series
diode
CR2011
also
turns
off
to
prevent
signal
loss
into
th€ inactive
Theory
of
Operation
-
4g4[l4g4Ap
SeMce,
Vot.
1
829
MHz
2nd
Converter.
Because
the 7l
g
MHz
LO
is
also
turned
off
by
the state
of
the
lF
sELECT
line,
isola_
tion
for
the
829
MHz
2nd
Converter
is
nol
critical
wlien
the
converter
is inactive.
The
switch
and
amplifier
logic
is summarized
in
Table
Z-2.
Tabte
Z-2
SWITCH
AND
AMPLIFIER
SELECTION
lF
Select
Llne
Serles
Swltch
Shunt
Ampllfier
110
MHz
lF
Source
High
Low
On
otr
otr
On
829 MHz
2nd
Conv.
2O72MHz
2nd
Conv.
The
diodes
are
used
as
the
basic
switch
el€ments.
They present
only
a
few
ohms
of
series
resistance
to
RF
signals
when fonrvard
biased,
with
current
of
several
milliamps.
When
reverse
biased,
the
diodEs
present
€s_sentially
an
open
circuit.
The
control
signal
from
0201
5
is
connected
in a
series
path
through
the four
diodes (CR2011,
CR2012,
CR201g,
anO
CAtOtSl
anC
inductors
L2011,
L2019,
and
L2019.
Thus,
only
a
small
current
is
required
to fonarard
bias all
four
diodes.
This
bias
current
is
also
used
to turn off
el01 1.
Diodes
CR2012
and
CR2013
are
incorporat€d
into
a
pi+!pj_
matching
network,
consisting
of
L2011,
L2O1O,
and
C2012.
Therefore
both switches
shunt
the signal
at
moderately
high
impedance points.
In addition,
when
the
switch
diodes are
turned
on,
parallel
resonance
between
L2011
and
CzAl2
presents
virtually
an
open
circuit
to
signals passed
by switch
CR2O11.
Switch
diode
CR2013 is
tocated
at
th€ high
impedance
node
created
by
the
series
resonance
of
L2019
and
C2017.
Diode
CRl015
direcily
shunts
the output
from
el01
l.
Transistor
Q101
1
operates
as a
common-ernitter
ampfifier
for
the 110
MHz
tF
signat
from
the 20Z2MHz
2nd
Converter.
lts
gain
and
impedance
match
are
con-
trolled
by feedback
resistors
R101i
and
Rl012.
Resis-
tors R1013
and
R1018
attenuate
the
output
by approxi-
mately
6
dB
for
enhanced
control
of
impedance
match
and
stability
characteristics.
Transistor
Q1012
maintains
a constant
dc
current
through
01011.
Dc
Coilector current
for
01011 is
set at
approximately
15
mA.
Collector
current
from
e1011
develops
a voltage
across
Rl017.
Transistor
el012
then compares
this voltage
with
the
fixed
voltage
of
the
voltage
divider, Rl015
and
R1016.
Any variation
in
the
collector
current
of
Ql
01
1 is
sensed
by
e101
2,
and
ofiset
by a resultant
change
in
the
base curent
of
Q1011
When
th€ control
current
from
e2015
(through
the
switching
diodes)
develops
a
voltage
across
Rl017
that
exceeds
the control
limits
of
01 01 2,
it effectively
removes
the base-bias
from
e101 i
and
turns el 01 I
7-15