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Heathkit IO-4550 - Page 19

Heathkit IO-4550
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GagumAcrriTe?
The
vertical
input
circuit
basically
consists
of
an
attenuator,
an
input
follower,
and
a
switched-gain
amplifier.
These
circuits
function
together,
through
the
VOLTS/CM
switch,
to
provide
the
total
desired
attenuation
or
gain.
The
attenuator
obtains
its
four
attenuation
factors
(1,
10,
100,
and
1000)
from
four
divider
networks
(resistors
R101-1
thru
R106-1,
and
capacitors
C101-1,
C103-1,
C104-1,
C106-1,
C107-1,
and C109-1).
At
DC
and
low
AC
frequencies,
the
resistive
dividers
reduce
the
input
signal
level;
while
at
higher
frequencies,
attenuation
is
determined
by
_
the
resistor-capacitor
(RC)
networks.
Cops
in
the
Atfenuator
netunel
Trimmer
capacitors
C101-1,
C104-1,
and
C107-1
are
used
to
adjust
the
capacitor
division
ratio
to
match
the
resistor
ratio.
Trimmer
capacitors
C102-1,
C105-1,
C108-1,
and
C111-1
are
adjusted
during
calibration
to
make
the
input
capacitance
of
the
Oscilloscope
equal
on
all
positions
of
the
VOLTS/CM
switch.
This
is
essential
when
an
attenuation
probe
(usually
X10)
is
used.
i
tupet
Follawe.
|
The
input
follower
circuit
consists
of
a
FET
(field-effect-transistor)
source
follower,
DC
current
source,
and
an
impedance
translator.
The
attenuated
input
signal
is
coupled
through
resistors
R108-1
and
R109-1,
and
capacitor
C112-1
to
the
gate
of
FET
source
follower,
Q101-1.
Capacitor
C112-1
forms
a
high
frequency
path
around
R109-1
for
improved
frequency
response.
Input
protection
is
provided
by
two
FET’s
(D101-1
and
D102-1)
wired
as
reverse
biased
diodes.
They
are
connected
to
the
plus
(+),
and
minus
(—)
15-volt
supplies.
Thus,
if
the
input
signal,
after
the
input
attenuator,
exceeds
15
volts
the
FET’s
become
forward
biased
and
clamp
the
signal
to
within
a
diode
drop
of
15
volts.
This
prevents
damage
to
Q101-1
if
the
VOLTS/CM
switch
is
in
a
low
range,
and
a
high
potential
is
applied
to
the
input.
'
Transistor
Q101-1
provides
the
high
input
impedance
necessary
to
prevent
attenuator
loading
and
a
low
output
impedance
to
drive
emitter
follower
transistors
Q103-1
and
Q104-1.
To
compensate
for
the
DC
voltage
present
at
the
source
of
Q101-1
when
no
signal
is
applied,
FET
Q102-1
forms
a
DC
current
source.
DC
BAL
control
R85
is
adjusted
so
that
the
current
supplied
is
sufficient
to
produce
a
zero
output
at
the
source
of
Q101-1
for
a
zero
input
at
the
gate
of
Q101-1.
The
circuit
formed
by
diodes
0103-1
and
D104-1,
and
transistors
Q103-1
and
Q104-1
acts
as
an
impedance
translator,
It
reduces
the
output
impedance
of
the
input
follower
to
approximately
50
ohms.
The
output
of
the
input
follower
is
coupled
to
the
switched-gain
amplifier.
Page
17
This
switched-gain
amplifier
is
formed
by
transistors
Q105-1
and
Q106-1
to
provide
a
double-ended
output
from
a
single-ended
input
signal.
A
relatively
constant
current
is
supplied
through
resistor
R119-1
to
the
amplifier,
so
that
an
increase
in
current
through
Q105-1
will
cause
a
corresponding
decrease
in
current
through
Q106-1.
Thus,
as
Q105-1
amplifies
the
input
signal,
Q106-1
produces
an
equal
but
opposite
signal.
This
creates
a
push-pull
effect
on
the
signal,
which
is
amplified
in
the
following
stages
to
drive
the
vertical
deflection
plates
of
the
CRT.
Front
panel
VARIABLE
control
R128-1
adjusts
the
gain
of
the
amplifier
when
it is
turned
from
its
detented
CAL
(fully
clockwise)
Position.
Two
switch-selected
RC
networks
reduce
the
gain
of
this
switched-gain
amplifier
from
8
to
4
and
1.6.
Table
I
shows
how
the
VOLTS/CM
switch
selects
the
various
attenuation
factors
and
gains
of
the
switched-gain
amplifier
to
provide
the
desired
total
gain.
STEP
BALANCE
control
R124-1
adjusts
the
collector
currents
of
Q105-1
and
Q106-1
so
that
the
CRT
trace
does
not
shift
when
the
gain
(VOLTS/CM
switch)
is
switched.
TABLE
I
VOLTS/CM]
ATTENUATION
|
AMPLIFIER]
TOTAL
FACTOR
GAIN
FACTOR
-
=
4
8
6
4
8
1.6
4
8
6
4
8
To
illustrate
how
the
attenuator
and
switched-gain
amplifier
work
together
for
the
proper
gain,
assume
the
VOLTS/CM
switch
is
in
the
10mV
position
and
a
10mV
signal
is
applied
to
the
input.
Since
the
total
gain
factor
is
8,
the
input
signal
is
amplified
by
a
factor
of
eight
before
it
is
coupled
to
follower
Q107-1/0108-1.
An
BOmV
signal
at
the
follower
i
se
a_
1
cm
deflection
in
ow
assume
the
VOLTS/CM
switch
is
in
the
500mV
position
and
a
500mV
signal
is
applied
to
the
input.
The
total
gain
factor
is
now
.16.
Multiplying
the
input
signal
by
the
total
gain
factor
results
in
an
80mV
signal
to
the
follower
(S00mV
x
.16
=
80mV),
again
causing
a
1
cm
deflection
on
the
CRT
screen.

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