Frequency Measurements - Philips PM2521 Service Manual

39
1.4.3.
Analog
section
(extended
measurements)
1.4.3.
1.
Frequency measurements (Hz)
The Hzl function switch connects the input
signal
to
be measured
to the voltage attenuator.
After attenuation
it is
applied
via
function switch contact
Hz2
to the
impedance
convertor.
The low impedance output
is
applied
to the
+
input of the
comparator A502.
When
positive triggering
is
selected,
the
+10V
is
applied
via
R502
to
one end
of the
TRIGGER LEVEL
potentiometer
R502 and
the resulting
+
signal
selected
is
fed
via
the
+ input of the buffer amplifier
A501
to
impedance
convertor
V302, which
gives a
+
input to pin
3
of the
comparator A502.
When
the
signal
pulse
peaks
cross
the
trigger level
threshold,
the
comparator produces output pulses
via
the
grounded
base transistor
V501
.
On
comparator output
pulses,
V502
conducts
and
the resistor
R512
results in
a
drop
in
output
of
50mV
approx. This
is
done to prevent
unwanted
oscillation.
When
the negative triggering
is
selected,
control relay
ReM
causes
V507
to
conduct
(logic
1
)
which means
that
—10V
is
applied via
R505
to the
end of the
TRIGGER LEVEL
control.
Although the
+10V
is still
applied
via
R504,
the smaller
value of
R505 makes the input
negative.
Diode
V551
also
conducts to give
a
negative
polarity signal to invert
the
comparator output
in
the
digital
section.
In
the negative trigger
mode,
V502
is
already conducting, so
V503
and
V504
act as a level
shifter
to
compen-
sate for the difference
in
levels
between
logic
0 and
logic
1
.
TRIGGER LEVEL
Fig.
28.
Frequency measurements