Section
3— DD
501
THEORY OF OPERATION
INTRODUCTION
This
section
of
the
manuai
describes the
circuitry
used
in
the
DD
501
Digitai
Delay.
The
description
begins
with a
discussion
of
the instrument, using the block
diagram
shown
in
Section
7.
Next,
each major
circuit
is
described, using the
biock
diagram
to
show
the relationship
between
stages
in
each major
circuit.
Detaiied
schematics
of
each
circuit
are
located
in
the
Diagrams
section
at
the
back
of this
manual;
refer to
these
schematics throughout
the foliowing
circuit
description
for
specific electricai
values
and
relationships.
BLOCK DIAGRAM
DESCRIPTION
Trigger
Circuit
The
function
of
each
biock
in
the Events
trigger
circuit
is
identicai to
the function
of the Start trigger
circuit,
therefore,
only the
Start
portion
of
the
T
rigger
circuit
wiil
be
discussed.
A
Start trigger signai
is
connected
from an
externai
source
to
the
Start
INPUT
connector
J120.
The
Start
Trigger
Preamp
presents a
high-impedance
input
and
iow-impedance
output
to
the input
trigger signai.
The
iow-impedance
output
trigger
from
the
Start
Trigger
Preamp
drives
the
Start
Trigger
Levei
Com-
parator.
The
front-panel
LEVEL
control
in
the
comparator
circuit
selects
a
dc
reference
point
on
the
trigger
waveform.
The
dc
reference voltage selected
triggers
the
Start
Schmitt Trigger
circuit.
The
front-panel
SLOPE
switch
selects
the
positive-
or negative-going
slope
on
the
Schmitt square-wave
trigger for
the
dc
reference
point.
Counter
Circuit
The
first
trigger
pulse
to
reach the
Start
T
rigger
enables
the Least
Significant
Digit
Counter
circuit.
This allows the
counter
to
start
counting the events pulses from
the
Events Delay
circuit.
The
EVENTS DELAY
COUNT
switch
setting deter-
mines
the
number
of
events pulses
to
be counted.
Assume
an
EVENTS DELAY
COUNT
switch
setting of
00010
in
the
following
discussion.
The
switch
setting
of
00010
programs
the
counters
to
99989.
After the
first
events
pulse the four
most
significant dig
its
are stored
in
the
Most
Significant Digits Latch.
The
Most
Significant Digits
Counter
Reset generates a 50
nanosecond
pulse
and
resets
the
Most
Significant Digits
Counters.
After
10
events pulses
have been counted,
the
counters
will
be
set
at
99999.
The
eleventh events pulse
transfers
one
count
through
the
Final
Count
Detector
to
the
Dly'd
T
rig
Output
Amplifier
and
resets
the Least
Significant
Digit
Counter.
The
Final
Count
Detector
resets
the
Most
Significant
Digits
Latch
and
the
Start
Trigger Gate.
The
counters
and
latches are
now
reset to
99989, ready
to
begin another
count.
The
Manual
Reset
circuitry
allows the operator
to clear
and
reset
all
counters
and
latches'
with
a
front-panel
switch during a
count
cycle.
Power
Supply
and
Triggering
indicator
The
Start
Trigger
Lamp
Multi
is
a
50
millisecond
multivibrator
which performs two
functions.
It
allows the
START
TRIG'D
indicator
to
remain
on
long
enough
for
viewing during high-frequency
start
pulses
and
holds the
START
TRIG'D
indicator
on whenever
the
Start
Trigger
Gate
is
open.
The
Events Delay
circuit
enables
the
Events Trigger
Lamp
Multi (50
millisecond
multivibrator) to drive
the
EVENTS
TRIG'D
indicator.
The
count output
pulse
from
the
Final
Count
Detector
is
amplified
in
the Dly'D
Trig'd
Output
Amplifier.
The
Dly'd
Trig'd
Lamp
Multi
is
a
50
millisecond multivibrator
that
enables
the
Dly'd Trig'd
Lamp
Driver to
remain
visible
during high-frequency pulse output
signals.
The
+5
Volt
Regulator supplies
power
for
all
integrated
circuits
and
the
POWER
indicator.
The -15
Volt
Regulator supplies
power
for
all
other
circuit
functions.
REV.
A.
NOV
1975
3-1
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