Tektronix DC 501 Instruction Manual page 6

Operating
Instructions—
DC
501
reading will
be displayed.
To
change the display time,
which
is
continuously variable
from about
0.1
second
to
10
seconds, or to
hold
a
display indefinitely, turn the
DISPLAY TIME
control.
Totalizing
(Counting
Number
of Events).
The
DC
501
will
display the
accumulated number
of pulses applied
to
the External or
Internal
input
circuit
up
to
9,999,999
pulses.
Input
signal rate
should not exceed 100
MHz.
Before applying the
signal,
set
the
MEASUREMENT
INTERVAL
switch to
MANUAL.
Apply
the
signal
and
push
the
START
button.
The
GATE
indicator
will light
and
the progressing
count
will
be
displayed. Adjust
the
ATTEN
and
TRIGGER LEVEL
controls
as
necessary to obtain
a
steady count.
To
stop the counting,
release
the
START
button.
The
GATE
light will
go out and the displayed
count
will
be
held.
The
displayed count
will
continue by
pressing the
START
button
again.
The
counter can be
reset
to zero
at
any
time
by
pushing the
RESET
button.
Signal
Connection
Coaxial
cables
and probes
offer
very convenient means
of connecting the
signals
to the front-panel
INPUT BNC
connector. These devices
are shielded to prevent
pickup
of
electrostatic interference.
A
10X
attenuation
probe not
only reduces the
size
of the input
signal,
but
is
also
presents
a
high input
impedance
to
allow the
circuit
under
test to
perform
very close to
normal
operating conditions.
Input
and output
data access to the
DC
501
is
made
via
the
plug-in
connector
contacts
at
the
rear
of the main
circuit
board.
Fig. 1-1 identifies the contacts and
their
associated
input/output assignments.
An
optional multi-pin
connector,
to
which
these data
can be hard-wired
to
provide
external access,
is
available to
install
on
the
rear
panel of the
power
module.
Input Attenuation and
Trigger Level
Adjustment
Signals to
be counted
may
have
a
wide
variety of shapes
and amplitudes,
many
of
which
are
unsuitable to drive the
counting
circuits.
Because of
this,
the
signal
is
first
passed
through an
attenuator, then
applied to a signal-shaping
circuit
which
converts
it
to rectangular pulses of
uniform
amplitude. This
circuit
includes
a reference
level adjustable
between +
and
—
2
volts to
which
the
incoming
signal
is
compared,
allowing the 300-millivolt
sensitivity
window
of
the
signal-shaping
circuit
to be
adjusted to
a
convenient
amplitude on
the
incoming waveform
(see Fig.
1-2).
Obtaining
a
steady,
reliable reading
is
dependent upon
the
proper
selection of input attenuation
and proper
adjust-
ment
of the
TRIGGER LEVEL
control.
Generally, the best point
on
a
waveform
for triggering
the
counter
is
where
the slope
is
steep and therefore usually
free 0 ^ no'se
O"
a s'ne-'vave signal, fo''
examp'e,
the
steepest slope
occurs
at
the
zero-crossing point. Noise
pulses or other
signal
components
of sufficient
amplitude
to
produce unwanted
trigger
pulses
will
cause an erratic or
incorrect
count.
Fig.
1-2
shows
the
TRIGGER LEVEL
control adjusted to avoid
error. In critical
measurement
applications,
monitor
the incoming
signal
with
a test
oscilloscope.
Measurement
Interval
and
Display
Time
Controls
The
MEASUREMENT
INTERVAL
switch
selects
the
time
interval (also called
gate time) during
which
the
DC
501
counts.
The
internal time-base circuit
derives gate
times
from
an accurate
1-MHz
reference
signal
to
make
frequency measurements. These
gate
times
are
0,01
s,
0.1
s,
1
s,
or 10
s.
The
measurement
interval selected
determines
the
measurement range and
resolution. Also,
the displayed
decimal point
is
positioned correctly
and
the correct
measurement
units
(MHz
or
kHz)
are indicated for the
corresponding
switch position.
Contact
Assignment
28B
Second
decimal point (D2) output.
27A
Internal
scan clock disable
input.
27B
MHz
light
output.
26A
Reset input/output.
25A
TS 0
(Time Slot Zero) output.
25B External scan clock
input.
24B
Internal
scan clock
(2
kHz)
output.
23B Overflow
output.
22B
MSD
(most
significant
digit)
20B
8
20A
4
BCD
output,
serial
by
digit.
21B
2
19A
1
19B Data good output.
17A
Signal
input ground.
16A
Signal input.
Fig.
1 1
.
Input/Output assignments
of
p!ug-in
connector
contacts.
1-2