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Tektronix DC 502 User Manual

Tektronix DC 502
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Section
2
—
DC
502
SECTION
2
THEORY
OF
OPERATION
INPUT
CIRCUITS
Direct
Input
Attenuators.
Signals
to
be
counted
are
applied
via
front-panel
DIRECT
INPUT
connector
J100
to
the
attenu

ators.
The
attenuators
are
frequency-compensated
voltage
dividers
consisting
of
resistors
R102-R107
and
capacitors
C102-C107.
Switches
S100A
and
S100B
allow
front-panel
selection
of
X1,
X5,
X10,
or
X50
attenuation
of
the
input
signal.
C1
10
provides
AC
coupling.
FET
source
follower
Q115
and
emitter
follower
Q122
present
a
high
impedance
to
the
input
signal.
The
diodes
in
the
base
circuit
of
E.
F.
Q128
form
a
series-limiter
and
clamping
network,
which
reduces
the
input
signal
to
limits
suitable
for
driving
the
shaping
circuits.
The
clamping
diodes
limit
the
voltage
at
the
emitter
of
Q128
to
a
dynamic
range
of
about
1.2
volts.
Signal-Shaping.
U150B,
an
OR
gate
integrated
circuit
with
push-pull
outputs,
is
connected
as
a
Schmitt
trigger.
It
shapes
the
input
signal
into
a
square
wave.
Its
“
hysteresis
window
“
is
a
width
of
about
200
mV.
The
output
changes
states
when
the
signal
voltage
passes
through
the
upper
threshold,
then
reverts
to
its
original
state
when
the
signal
voltage
passes
through
the
lower
threshold.
For
this
reason,
an
input
signal
smaller
in
amplitude
than
the
width
of
the
hysteresis
window
cannot
activate
the
counting
circuits.
The
quiescent
level
at
the
input
of
U150B
can
be
adjusted
to
overcome
some
of
the
triggering
difficulties
arising
from
various
input-signal
shapes
and
frequencies.
Integrated-circuit
operational
amplifier
U135
and
its
associated
discrete
components
are
connected
as
a
voltage
follower.
TRIGGER
LEVEL
potentiometer
R135
selects
a
voltage
between
ground
and
about
—
2
volts
and
applies
it
to
pin
3
of
U135.
This
level
is
then
established
at
pin
2,
and
hence,
the
input
of
U150B,
through
the
action
of
the
operational
amplifier.
The
output
of
U150B
is
applied
to
U150A,
whose
push-pull
outputs
drive
Q160
and
Q162,
which
are
connec

ted
as
a
differential
pair.
This
circuit
provides
a
level
shift
to
TTL
level,
and
further
shapes
the
signal
to
be
counted.
A
waveform
with
fast
rising
and
falling
edges
is
produced
at
the
collector
of
Q160.
CR165
limits
the
amplitude
of
the
count
signal
to
5
volts,
clamping
the
negative-going
portion
of
the
signal
to
ground.
The
signal
is
then
passed
through
emitter
follower
Q170
to
U160B,
where
it
receives
a
final
phase
inversion
(to
correspond
with
the
input
signal)
and
becomes
the
decade
input.
4-10
Pre-Scale
Input
Circuit
50
MHz
to
550
MHz
signals
applied
to
J180
are
AC-coupled
into
a
50
environment.
A
quiescent
10
mA
current,
set
by
R185
and
R187,
keeps
CR185,
186,
187
and
188
in
the
diode
clamping
bridge
turned
on
until
the
input
signal
amplitude
reaches
about
±0.4
V.
While
the
bridge
is
turned
on,
the
signal
source
sees
about
20
equivalent
resistance
of
the
bridge
in
series
with
R189,
24,
and
the
emitter
resistance
of
Q190,
about
5.
As
the
signal
amplitude
exceeds
±0.4
V,
one
pair
of
diodes
(CR185,
CR188)
begins
to
turn
off
during
positive-going
excursions,
while
the
other
pair
(CR186,
CR187)
turns
off
during
negative-going
excursions.
As
these
diode
pairs
alternately
turn
off
with
signal
amplitude
changes,
a
matched
pair
of
diodes
(CR181A,
CR181B)
on
the
input
side
of
the
coupling
capacitor,
C182,
alternately
turn
on
to
maintain
the
50
input
impedance.
C181
prevents
CR181A
&
CR181B
from
being
biased
on
or
off
by
DC
levels
which
may
be
part
of
the
input
signal.
The
clamping
action
of
the
bridge
diodes
limits
the
changing
DC
level
at
the
bridge
output
to
a
maximum
0.8
V
peak-to-peak.
This
signal
is
then
coupled
by
C188
to
Q190
via
the
high-frequency
peaking
network,
C189-R189.
C192
couples
the
amplified
signal
to
pin
10,
the
high
impedance
input
of
the
4-10
counter,
U190.
C192
is
mounted
on
the
circuit
board
with
special
lead
dress
to
aid
high
frequency
response
at
minimum
signal
amplitudes.
Bead
L192
is
mounted
on
one
lead
of
C192
to
suppress
high
frequency
oscillations.
R191
establishes
a
bias
at
pin
10
of
U190.
C191,
193,
and
194
decouple
pins
14,
13,
and
12,
respectively,
right
at
the
terminal
connections
to
the
circuit
board.
L199
and
C199
suppress
power
supply
noise
and
decouple
the
Vcc
input,
pin
14.
Pin
4
of
U190
produces
one
output
cycle,
a
level
change
of
about
0.5
V,
for
every
ten
input
events
to
pin
10.
The
base
of
Q195
swings
approximately
between
3.4
V
and
3.9
V.
The
emitter-follower
action
of
Q195
provides
a
healthy
current
drive
to
the
base
of
Q198,
which
would
saturate
in
no-signal
conditions
except
for
CR196.
The
collector
of
Q198
can,
therefore,
respond
immediately
to
signal
changes,
producing
a
TTL-compatible
output
to
drive
pin
9
of
U160B
in
the
gating
logic
to
the
Decade
Counting
Units.
2-1

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Tektronix DC 502 Specifications

General IconGeneral
BrandTektronix
ModelDC 502
CategoryCash Counter
LanguageEnglish