Tektronix DC 501 Instruction Manual page 11

Theory
of
Operation—
DC
501
fig. 2-2.
Time
Base generator
norrrtal
gating
mode
ladder diagram.
U2228,
causirtg
its
Q
output
to
go LO,
closing the
GATE.
Simultaneously, the
5
output goes HI,
removing
current
from
the
GATE
irtdicator
LED.
When
the
GATE
output goes
LO. the negative transition
toggles
U220A,
switching
Q
LO
and
U
HI.
Now
NANO
gate
U230D
has
two
HI
inputs, placing a
LO
at
the input of
OR
gate
U230B
and
activating the
LATCH
control signal (HI
state).
One
microsecond
later
(T^), a negative edge
from
the
1-MHz
clock toggles
U222A,
switching
its
outputs
arxf
placing a
LO
on
the input of
NANO
gate
U230D.
U230D
reverts to
its
original
condition, terminating the
LATCH
signal.
The
display time begins
when
the
GATE
signal ertds
(Tg),
When 0230
turns off,
C2^
begins to charge through
R232 R235
toward the Vcc supply. R235.
DISPLAY
TIME,
provides
an
adjustable time constant to vary the
display
time
from
about
0.1
second
to
about 10
seconds.
When
the
DISPLAY TIME
control
is
fully
clockwise
(HOLD
detent
position),
S235
opens,
and C235
stops
charging.
When
S235
is
closed
and
C235
charges
sufficiently to bring
(3238
to
its
firing potential (T,), the
display
time
ertds
aixl
the
rtext
GATEopening
sequence
begins.
Manual
Gate.
The
manual
mode
of operation
is
selected
by
placing the
MEASUREMENT
INTERVAL
switch
in
the
MANUAL
position.
The
switch closure
to
ground (cam 5 of
the switch) places a
LO
on
the
set
inputs of
U220B
and
U222A,
and
a
LO
on
the
clear
input of
U220A.
This
forces
the
Q
outputs of
U222A
and
U220B
HI,
and
the
Q
output
of
U220A
LO. With both
inputs of
U230D
held HI,
the
LATCH
output
is
held HI. allowing the counter
to
update
the display continuously.
The
GATE
is
opened
when
the
front-panel
START
button
is
pushed
in,
opening
S210 and
applying a HI
to the clear input of
U222B. As
before, the
GATE
open
condition
is
HI
at
theQ
output of
U222B.The
GATE
is
then closed
when S210
is
set
to
STOP
(button
out).
To
reset
the counters
in
the manual mode,
the
RESET
butt
on
mus
t
be pushed
to activate
the
CLEAR, CLEAS,
and
reset
control
signals.
Automatic Gate
(For Instruments
Having Option
2).
The
automatic
gate
mode
is
selected
by
placing the
MEASURE-
MENT
INTERVAL
switch
in
the
AUTO
position.
The
output
of the
automatic time base
circuit
is
connected
to
the gate generator
via
contact
1
of the witch. Contact 2
opens
to
enable the readout
scaling circuit.
The
automatic
gating cycle
begins with the
CLEAR
pulse,
which
occurs
when 0238
reaches
its
firing
potential, as discussed for the
24