Theory Of Operation; Block Diagram Description; Trigger Circuit - Tektronix DD 501 Instruction Manual

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