Philips pm2521 - 2. TECHNICAL DATA; 2.1.1. Direct voltage measurements

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