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Tektronix 502A

Tektronix 502A
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SECTION
3
CIRCUIT
DESCRIPTION
Introduction
The
Type
502A
is
a
dual-beam,
high-gain,
low
frequency
oscilloscope
employing
a
T5021
type
dual-gun
cathode-ray
tube.
The
instrument
has
identical
Vertical
Deflection
Am
plifiers,
one
for
the
UPPER
BEAM
and
one
for
the
LOWER
BEAM.
Simultaneous
horizontal
deflection
of
both
beams
is
provided
by
a
single
Time-Base
Generator
and
Horizontal
Sweep
Amplifier
circuit.
The
Type
502A
circuitry
is
arranged
so
that
the
instru
ment
can
be
used
in
any
of
several
configurations.
It
may
be
used
as
a
conventional
single-beam
oscilloscope
by
apply
ing
an
input
signal
to
either
of
the
Vertical
Deflection
Amplifiers.
It
may
be
used
to
examine
two
waveforms
simultaneously
by
applying
input
signals
to
both
Vertical
Amplifiers.
Both
deflection
amplifiers
may
be
used
in
a
differential
mode,
to
examine
the
difference
between,
or
the
algebraic
sum
of,
two
signals.
The
UPPER
BEAM
Deflection
Amplifier
can
be
connected
to
the
horizontal
deflection
plates,
so
that
the
instrument
may
be
employed
as
a
single
beam
X-Y
oscilloscope.
And,
by
means
of
the
EXTERNAL
horizontal-input
connector,
the
instrument
may
be
used
as
a
dual-beam
X-Y
oscilloscope,
with
both
traces
plotted
on
the
same
X
scale.
The
UPPER
BEAM
Vertical
and
the
LOWER
BEAM
Vertical
Deflection
Amplifiers
are
identical,
so
the
description
that
follows
applies
to
both.
The
push-pull
Vertical
Amplifier
consists
of
three
stages
of
amplification,
the
Input
Amplifier,
the
Second
Amplifier
and
the
Output
Amplifier.
A
cathode-follower
stage
drives
the
Output
Amplifier.
The
overall
gain
of
the
Amplifier
is
controlled
by
three
feedback
networks,
two
providing
negative
feedback
and
the
third
positive
feedback
(see
Fig.
3-1).
The
Input
Amplifier
is
a
cathode-coupled
paraphase
amplifier
(it
may
also
be
operated
differentially)
whose
gain
is
controlled
by
negative
feedback
from
the
cathodes
of
the
Driver
C.F.
stage
and
the
Output
Amplifier
stage.
The
Second
Amplifier
has
a
positive
feedback
network
that
extends
from
the
plate
circuit
on
one
side
to
the
grid
circuit
on
the
other;
this
configuration
makes
this
stage
an
almost
"infinite-gain
”
amplifier.
The
result
of
both
feedback
net
works
is
an
amplifier
having
a
sensitivity
of
100
microvolts
per
centimeter.
The
Input
Circuit
The
Input
Selector
switch
SW403
determines
the
mode
of
operation
for
the
Amplifier.
When
in
any
of
the
three
posi
tions
marked
AC
the
signal
is
ac-coupled
through
C400
(for
Input
A)
and/or
C401
(for
Input
B).
When
in
any
of
the
three
positions
marked
DC
the
input
capacitor
(C400
and
C401)
is
shorted
and
the
signal
is
dc-coupled
to
the
Input
stages.
The
sensitivity
of
the
Vertical
Amplifier,
as
mentioned
previously,
is
100
microvolts
per
centimeter.
However,
by
means
of
attenuation
and
degeneration
networks,
the
vertical
deflection
factor
can
be
increased
to
20
volts
per
centimeter.
It
is
possible,
through
the
use
of
the
VARIABLE
SENSITIVITY
Control,
to
introduce
enough
cathode
degeneration
into
the
cathodes
of
the
Output
Stage
to
increase
the
deflection
factor
to
approximately
50
volts
per
centimeter.
Either
of
two
attenuation
networks
can
be
connected
in
series
with
the
Input
connectors
of
the
Vertical
Amplifiers.
One
attenuates
the
signal
by
a
factor
of
10,
the
other
by
100.
For
d
c
and
low
frequency
signals,
these
networks
are
resistance
dividers,
and
the
degree
of
attenuation
is
pro
portional
to
the
ratio
of
the
resistance
values.
The
reason
for
this
is
that
the
impedance
of
the
capacitors,
in
this
range
of
frequencies,
is
so
high
that
their
effect
in
the
circuit
is
negligible.
For
higher-frequency
signals,
how
ever,
the
impedance
of
the
capacitors
is
less
and
their
effect
in
the
circuit
is
more
pronounced.
Near
the
upper
frequency
range
of
the
Amplifier
the
impedance
of
the
capa
citors
becomes
so
low,
compared
to
the
resistance
of
the
circuit,
that
the
attenuators
become
capacitance
dividers.
For
these
frequencies
the
degree
of
attenuation
is
inversely
proportional
to
the
capacitance
ratio.
In
addition
to
providing
the
proper
degree
of
attenuation,
the
resistance
values
of
the
attenuators
are
chosen
so
as
to
provide
the
same
input
resistance,
regardless
of
the
setting
of
the
SENSITIVITY
switch.
For
example,
in
the
"straight
through
”
positions
of
the
SENSITIVITY
switch
(.1
mVOLTS
PER
CM
to
.2
VOLTS
PER
CM),
the
1
-megohm
grid
resistors
.
.
.
R410
for
Input
A
and
R440
for
Input
B
.
.
.
constitute
the
input
resistance
of
the
Vertical
Amplifiers.
In
the
range
from
.5
VOLTS
PER
CM
to
2
VOLTS
PER
CM,
the
X10
Attenuator
is
connected
into
the
input
circuit.
The
resistor
in
the
low
end
of
the
divider
.
.
.
R406E
for
Input
A
and
R407E
for
Input
B
.
.
.
shunts
the
grid
resistor
to
create
an
equivalent
resistance
of
100
K
ohms.
This
100
K
equiv
alent
resistance
is
then
in
series
with
the
resistor
in
the
high
side
of
the
divider
(900
K
ohms)
to
produce
a
total
input
resistance
of
1
megohm.
The
X100
attenuator
works
in
the
same
manner.
The
10.1
K
resistor
at
the
lower
end
of
the
divider
shunts
the
1
meg
grid
resistor
to
form
an
equivalent
resistance
of
10
K
ohms.
This
equivalent
resistance
is
then
is
series
with
the
990
K
resistor
in
the
upper
side
of
the
divider
to
create
a
total
input
resistance
of
1
megohm.
The
capacitance
values
in
the
attenuators
are
also
selected
to
provide
a
constant
input
capacitance
...
47
pf
..
.
regardless
of
the
setting
of
the
SENSITIVITY
control.
In
the
"straight
through"
positions
of
the
switch,
the
total
input
capacitance
is
equal
to
the
capacitance
of
C410
3-1

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