Filter
Top Products
Theory ol Operation
-
494A/494Ap
Sewice,
Vot. 1
time. When a
filter is
selected,
the series
diodes
are
biased on and
the
shunt
diodes
are
biased off.
The
diode conditions
are
opposite
for
the
filters
that
are
not
selected.
Since
the switching
operation
is
th€ same
for
all ftlters,
the following
description
for
the 100
kHz
filter
selection applies
to all filters.
When
the 100 kHz
filter
is sel€cted,
line
2 from
U4035 will be low.
This
tums
on
switching
transistors
Q3015 and
Q3055.
With
input
switch
03015
turned on,
the current
path
is
through
R4012,
R4010, L4010,
CR3013,
L3015,
R3015,
and
e3015.
This current
is
determined
the
resistors
in
this
series
circuit.
The vol-
tage drop
across
the resistors
is
enough
to
turn
the
seraes
diode
on and
reverse
bias
shunt
diode
CR3012.
The same case
exists
for
the lilter
output
switch.
03055.
SEries resistors
establish
the current
to for-
ward
bias CR3063
and
reverse
bias
CR3062.
Therefore
the
signal
from
the
VR
Input
circuit,
via
jumper
B,
is
applied
through
the sel€cted
tilter
to
the
10 dB
Gain Steps circuit
via
jumper
K. Nominal
loss
through
the
lilter
circuit
is
approximately
6
dB, with
slight variations among
the filt€rs.
The
1st
Filter
Select
output
level
is nominally
-25
dBm.
Any difference
in
gain
between the filters
is compensated
for
later
in
the
2nd Filter
Select circuits.
In
the
non-selected
filter
sections,
the input and
out-
put
switch
transistors
are
tumed
off
by
the
high
outputs
from
decimal decoder
U4035. The
collectors
are
pulled
toward
-15V
through
the
resistors
that forward
bias
th€
shunt diodes in
the
input and
output.
Since one
filter is
always selected,
the voltage
drop across
the
common
input and
output
resistors
back biases
the
series
diodes.
A filter
is
not
used
in
the 3 MHz
section
because
the
wide
bandwidth filtering
takes
ptace
in
the 110 MHz
filters
between the
znd and
3rd
ConvertErs. Instead
of
a filter, a
6 dB
attenuator
is contained
in
the
3 MHz
selection circuit. This
attenuator
helps
match the lEvels
of
the
various
bandwidths
by
simulating
the insertion
loss of
the
other sections.
The
1
MHz filter
section
consists
of
a
pi
attenuator
and an
LC band-pass
filter.
The
attenuator
adjusts
to
match
the
level of
the
3 MHz
section.
Gain
is
adjusted
for
both bandwidths in
the 2nd
Filter
Select
circuit.
The
100
kHz
filter
is
a
doubte-tuned
LC
circuit
designed for a
good
time-domain
response shape.
The
filter
is
tun€d
with
composite
variable
capacitors
con-
sisting of small
air variables paralleled
with switched
fixed
capacitors.
A
third variable
capacitor
may
be
adjusted
to
establish
the
desired
bandwidth. For
Option
07
instruments,
a similar
300 kHz filter
replac€s
the
100 kHz filter.
The 10 kHz filter uses a
pair
of
two-pole monolithic
crystal
filters
that
ar€
interconnected
by variable
shunt
capacitor C2030. Input and outprrt
impedances
are
matched
with
broadband transformers
T2025
and
T3040.
A
3 dB
pi
attenuator
is included
at
the
filter
input to help
match
the loss
of
the other sections.
The 1
kHz
resolution
filter consists of a single
two-
pole
monolithic
crystal filter,
matched
to
the
50
ohm
impedance with
broadband transformers
T2035
and
T2040. A
2
dB attenuator is also included at
the
filter
input
to
h€lp match
the
loss of
the oth€r filters.
The loHzfiAA Hz filter,
A69,
is contained in
a
separate assembly with switching
done on this
board.
One set
of switching
transistors
enables
thE
filter
path
when either
the
100 Hz
or
10 Hz
bandwidth
is
selected.
Another
switch
selects between
the two bandwidths.
Decimal
decoder
U4035
selects
100 Hz
bandwidth by
pulling
output
5
(pin
6)
low, and selects
10 Hz
bandwidth
with output 6
(pin
4.
The filter
path
is
selected
when
either
output 5
or
6
are
low.
Diode
pair
CR1030 turns on
01025
at the input
to
forurrard
bias
diode CR1011.
Diode
pair
CR4055
tums
on
Q4050
at the
output
to fonilard
bias
diode CR4061.
Diodes
pairs
in
CR1012
and
CR1020
provide
limiter
and
clamp
action
at
the
filter input
to
remove
RF
excursions
caused
by the
dc switching.
The filter
has
a
bandwidth
of
100 Hz when
its input
port
is low
(-15V)
and 10Hz when high
(+15\4.
Transistor
Q1027
does
the
switching. When
100H2 is
selected. output
5
(pin
6)
of
decoder U4035
is low and
output
6
is high. This turns transistor Q1025
on, for-
ward
biasing
CR1011
and reverse
biasing CR101O.
This applies the
lF signal to
the
filter input.
This
also
biases 01027
off,
placing
-15
V
at the
filter
input
to
select
the 100
Hz bandwidth.
(04040
also switches the
output,
but
is not
effective in
this
instrument
since
A69
only requires
switching at
the
input).
When output 6
of
U4035
is low
and output 5
is high,
Q1027
and Q4040
are
on in addition to
Q1025
and
04050.
This selects the
filter
path
and applies
*15
V
to
the
input and
output
ports,
switching
the
tilter
to the
10 Hz mode.
100 Hz
and
10 Hz Bandpass Filter
(Diagram
19A)
The 100
Hz
and
10
Hz bandwidths
are
provided
by a
dual-bandwidth
filtering assembly
(A69).
The
signal
is
converted from
10 MHz
down
to the
250 kHz
center fre-
quency
of
the
filter. Filtering at 250
kHz
makes
the
bandwidth a
much higher
percentage
of
the
filter
center
frequency
than
if filtered
at
10
MHz. The
ltltered
signal
is
then
converted back
up
to
10 MHz.
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7-24