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12
1.3.3.
Analog
section
(extended measurements).
1.3.3.
1.
Frequency measurements
The
unknown
frequency source
to
be
measured
is
applied to the
V
and
0
input
terminals. After suitable
attenuation
as
necessary
in
the
a.c.
voltage
attenuator
and impedance
conversion
(to
match
the
low-impe-
dance comparator)
the
signal
is
fed to
one
input of the
comparator A502.
The
trigger
level
input
selected
by
the front-panel
thumbwheel
control
is
applied
via
an
impedance
convertor
to
the other input of the comparator.
The
trigger
level
is
set
to
a
value
lower than the amplitude of the
signal
to
be measured. During counting,
each time the amplitude of the
signal
from
the input exceeds the
selected
trigger
level,
the
compartor
gives
an output
pulse to the counter.
The
internal
counter
of
the
IIP
is
also
used
as
an overflow counter
for
frequency measurements.
Triggering
is
also possible
on
the negative-going
pulses
of
a signal
by
using
the
+/—
switch to
reverse
the
polarity
of the
trigger
level.
+
ST3365
811127
Fig.
9.
Block diagram-frequency
measurements
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