Filter
Top Products
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- -.
The.assembly
consists
of
four
subassemblies:
1st
Mix-er (A69A't),
Bandpass
Fitter
(A69A2),
2nd
Mixer
(A69A9),
and
Local
Osciilator
(A69i4).
These
subass€mblies
are
on
individual
ciriuit
boards
con-
tained in
shielded
compartments
within
a
metal
cas€
mounted
above
th€ crt.
lst
Mlrer.
The
lst
Mixer (A6gA1)
is
the
input
mixer.
lt
converts
the
l0
MHz
lF
signal
down
to
250 kHz.
An
input
filter
reduces
the signil
skirts
before
going
to
the
mixer.
The
buffered
9.ZS
i4Hz
Local
Oscil-
lator
(LO)
mixes
with
lhe
19
MHz
signat,
producing
the
250
kHz
mixer
output
that
drives
ttre
-bandpass
filter.
-
The
input
signal
consists
of
the
10
MHz
lF
signal
with
a
dc control
signal
also
riding
on
the
line.
Ttre
Oc
control
signal
selects
the
10
Hz or
100
Hz lilter.
The
10
MHz
signal
couples
through
a
capacitor
and
transformer
to
feed
the
input
filter.
The
dc control
sig-
nal feeds
through
a resistor
and
capacitor
to isolate
ttie
10Ir4Hz
signal
and provide
a clean
dc signal
to
the filter
switches
on
the
Bandpass
Fitter
board
(469A2).
Input
filter
Y2025
is
a
monolithic
second-order
cry-
stal
fflt€r
with
a i
kHz
bandwidth.
The
tilter
limits
tarje
signals
outside
the
bandpass
before
they
enter
the
mixer.
This
reduces
th€
intermodulation
Oistortion
(lMD)
in
the
circuit.
The
l0
MHz
signal
driv€s
the fitter
at about
-20
dBm
signat
levet.
The filter
drives
high
level
mixer
USO20,
built
from
a
monolithic
ring
of
MOSFET
switches.
Differentiat
pair
05020
and
Q5025
buffers
the
g.7S
MHz
LO
signal
and
drives
the
mixer
at
the
LO
input.
The
buffer
provides
opposite
polarity
high
amplitude
square
waves
to
drive
the mixer
differential
LO
inputs.
The
square
wave
helps
provide
low
mixer
lMD.
A
potentiomet€r
adjusts
rnixer
bias
to help
provide
low
IMD
and
low
mixer
insertion
loss.
The
adjustment
is
made
for
best conversion
gain.
The main
power
supplies
also
enter
this
board.
The
117V
power
suppty
is re-regutated
to +1S
V
to avoid
loading
the existing
+.1S
V suppty.
The
_1
5 V
suppty
acts
as
the reference
supply
for
th€
+1S
V
regutaior
which
consists
of
US010B,
e5010,
and
eS015.
hesis-
tors
R5125
and
R5'l27
act
as
a
voltage
divider
to
set
the output
voltage.
Bandpass
Fitter.
The
250
kHz
signat
produced
by
the
previous
mixer
is
filtered
and
-ampiified
in
the
Bandpass
Filter
board (A69A2).
The
filter
consists
of
thr€e
nearly
identical
stages.
Bandwidth
is
changed
from
10
Hz
to
100
Hz
with
transistor
switches
in
each
ijaS.".
The
control
signal
for
this
switching
comes
from
the lst
Mixer
board
(A69Al).
Theory
of
Operalion
-
4g4A/lg4Ap
Service,
Vol.
1
The
filtEr
sections
consist
of crystals
in
series
with
the signal path.
Each
crystal
is
driven
from
a
low
impedance
source.
Resistive
loading
then
sets
the
bandwidth
of each
section.
Ttre
switihing
transistors
connect
smaller
resistors
in
parallel
with
the filter
load
resistors
to
reduce
the bandwidth.
Each
crystal
is embedded
in
a
balanced
network.
The
balancE
adjustment
compensates
for
the
effects
of
crystal
parallel
capacitance.
This
improves
fitter
stop-
band attenuation
and
shape
symmetry.
each
stage
alio
contains
a variable
capacitor
in
series
with
the Lrystal
to
provide
a
fine
frequency
adjustment
with
a
tuning
range
of
about
15
Hz tor
each
stage.
_ lach
amplifier
consists
of
a feedback
circuit
using
a
JFET and
a PNP,
providing
voltage gain
of
about
trio.
The
stage
is completed
by driving
an
emitter
follower
to
provide
the low
output
impedance
neEded
to drive
the
next
crystal
or
the
output
mixer
(2nct
Mixer).
The
vatues
of
the f€edback
resistor
and
the
input
resistors
deter_
mine
the
stage
gain.
The
250 kHz
signal
from
the
mixer
drives
a smatl
attenuator
and
the
transformer
Tgoi
6.
The attenuator
terminates
the
mixer and
transformers.
The
balanced
output
from
T3016
contains
crystal
y2o2o
and
the Fre_
guency
adjust
capacitors
on
one
side
and
the Balance
adjustment
capacitor
on
the
other
side.
Load
resistor
R4163
terminates
the crystal
network
When
10Hz
bandwidth
is
selected,
Q4159
turns
on,
ptacing
R9162
in
parallel
with
load
resistor
R4l69.
This
raises
the e
and
reduces
the
bandwidth.
lt also
increase
the
inser-
tion
loss,
so
Q21 59 also
switches
on,
placing
R3162
in
parallel
with
R4027
to
increase
the
gain
and overcome
the added
insertion
loss.
Series
network
R2145 and
C2150
provide
positive
feedback
for
the amplifier.
This looks
inductive
and
so
compensates
for
the
capacitance
an
the impedance
seen
at
the amplifier
input.
Capacitor
Cg14g,
across
feedback
resistor
R3160,
rolls
ofr
the
amplifier
gain
above
250
kHz
to
prevent
10 Mhz
feedthrough.
Emitter
follower
Q4145
provides
a low
impedance
drive
for
the next
filter stage.
The
second
stag€
filter
uses
Q4140 to
invert
the sig-
nal
for
Balance
adjustment
C4045.
Other
than that,
the
operation
of
this stage
is
identical
to the
first.
The
third
stage
is
identical
to the second.
2nd
Mker.
The 2nd
Mixer (A694A)
converts
the
filtered
signal
trom
250 kHz
back
up
to 10 MHz. The
mixer
in
this
circuit
is a
MOSFET
ring
(U5022)
like
that
used
in
the 1st
Mixer
stage.
Less
LO
voltage
is needed
because
IMD requirements
are
less
stringent
in
this
stage.
A
potentiometer
adjusts
mixer
gate
bias.
The
adjustment
is
made
for
best conversion gain.
7-25