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CHAPTER
1_
1 1
Ai^
JLEJJIILLPJJ
INTRODUCTION
1
.1
The 9902 and 9906
are
compact
universal
counter
timers,
powered
from
a.c.
line supplies,
and
with a
comprehensive range
of facilities.
The
two
instruments
differ
in
that the
9902
has a
frequency autoranging
facility
and
the
9906
provides an
extended (VHF) frequency
range, with prescaling
and reduced
sensitivity
above
150
MHz.
In
all
other respects the instruments are operationally
identical
.
OPERATING
FACILITIES
1
.2
(a)
Frequency measurement;-
'A'
Channel,
AC
mode;
9902;
10
Hz
-
50
MHz
9906;
10
Hz
-
30
MHz
(HF
range)
30
MHz
-
200
MHz
(VHF
range)
'A'
Channel,
DC
mode
(both models);
d.c.
-
20
MHz.
'B'
Channel,
AC
mode
9902;
100
Hz
-
10
MHz
9906;
10
Hz
-
10
MHz
'B'
Channel,
DC
mode
(both models);
DC
-
10
MHz
On
the
9902
an
AUTO
facility
automatically
selects the
optimum
resolution
up
to
a
one second
gate time.
(b)
Period
Measurement;
1
to
10^
periods
in
decade
steps,
a.c.
or
d.c. coupled.
(c)
Ratio
Measurement;
Refer
to
Chapter
4.
(d)
Time
Interval, single
or
double
line,
with
manual
or
electrical
start/stop
and
trigger
slope selection.
Fixed
start
delay
is
included
to
minimise
the effects of
contact
bounce
in
the
customer's
equipment
when
using contacts.
9902
&
9906
Vol
.
1
1-1
(e)
Totalize
will
provide
the
total
of
a succession
of
events, using
either
manual
or electronic start/stop signals.
NOTE;
The
front
panel symbol /1\ advises
the user
to
consult the
handbook
for
further details.
CHECK
FACILITIES
1
MHz
Readout
1
.3
When
CHECK
mode
is
selected the
1
MHz
internal
reference
signal
is
fed
through the instrument
to
provide an operational
check
on
Frequency
and
Period
modes.
Segment
Check
1
.4
With
CHECK
selected
the
instrument
will
display
'all
eights'
when
the
RESET
button
is
pressed, thus
checking
all
segments
in
the display I.e.d.'s
DELAY
CIRCUIT
1
.5
To
minimise contact
bounce
effects
on external
timing, the inputs
to
the
'A'
and
'B'
amplifiers are
fitted
with
identical
delay
circuits,
which
are
,
selected simultaneously by a
front
panel switch.
The
circuits
give a 'hold-off
to
the input signal of
approximately
10 ms,
and
are
thus
effective
in
suppressing
contact
bounce
effects
up
to
50 Hz.
The delay
is
operative on
'A'
and
'B'
channels (d.c.
coupled)
and
on
'B'
channel (a.c. coupled).
The
delay
capacitors are
mounted
on
pins
to facilitate
removal,
or to
permit a
change
of
value
to
obtain a smaller delay.
For
advice
on
external
contact
closure
refer to
para.
2.10.
AC
(LINE)
POWER
SUPPLY
1
.6
The
instrument operates from
a.c.
supplies
between 94 and 265
volts,
45-450 Hz.
Tappings
and
a
link
on
the internal line transformer
provide
for
a
choice
of
six
voltage ranges.
A
rear
panel switch must be
set to
select either
the
upper
or
lower
half of the
chosen range.
Refer
to
Chapter
2
for
selection
instructions.
A
label
fixed
to
the
rear
panel should indicate
the selected
voltage
range.
Spare
labels are
supplied with
the instrument
for
voltage ranges other than
212
-251V.
9902
&
9906
Vol
.
1
1-2
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