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1.4.
OPERATING
PRINCIPLE
1.4.1.
Description of the
block diagram,Fig.
30
The
16
MHz
crystal
clock
generates the
basic
frequency
for
all
signals,
so the
count
pulses for
the
analog
to
digital
converter
ADC.
The
frequency
divider
generates the
8
MHz
clock pulse
for
the microprocessor
and
the
1
kHz
test
fre-
quency
in
3 reference phases,
namely
0°,
90° and 180°.
In
the Phase
selector
the
CPU
selects
the appropriate reference phase 0°,
90°
or
180°
for
the
phase
sensitive
rectifier
and
the
ADC.
The
band-pass
filter
1
converts
the
TTL
signal
into a
1
kHz
sine
wave
signal.
The
test
voltage amplifier amplifies
.
the
1
kHz
sine
wave
signal
to
a
2 Veff
open
circuit
voltage
at
the
component
under
test
(CUT)
connection.
In
the 'Cs
biased'
mode
2
Vdc
are
added
to the
1
kHz
signal.
The
isolating
buffer senses the voltage
at
the
CUT
The
inverting
amplifier
feeds
a
compensating
current
via
capacitor
C
(90° phase
shift)
into
the current
to voltage converter
input
for equalizing the stray capacitances.
The
amplitude
of the
compensating
current
is
set
by Co
TRIM.
The
current
to
voltage converter converts the current
through
the
CUT
into
a
proportional
voltage.
The
conversion
factor
can be
switched
by
a
factor
of
10.
For
current
or
voltage
measurement
the
input
of the
subsequent
differential
amplifier
is
switched
over
by
the
voltaqe/current
(
V/l) selector controlled
by
the
CPU.
In
the
programmable
amplifier
gain factors x0.1,
xl
or
xIO
are selected
by
the
CPU
depending
on
the
impedance
of
the
CUT.
For
the reference
measurement
the
input
is
short-circuited.
The
1
kHz
band-pass
filter
2 suppresses
hum
interference
and
reduces the
harmonic
components
of the
1
kHz
measurement
signal..
The
level
detector
compares
the
output
voltage
of band
filter
2
with
a preset reference value.
If
this
value
is
exceeded,
the
CPU
switches the
programmable
amplifier to a lower gain
factor.
The
phase
sensitive rectifier
generates
dc
voltages
which
are
proportional
to that
component
of the
measuring
voltage being in-phase with the reference
voltage.
The
analog to
dig
ital
converter
ADC
converts the
output
signal
of the
rectifier
into
a
binary
number
which
can be processed
by
the
CPU.
.&•
The
central
proce
ssing
unit
CPU
with
the inherent
microprocessor
controls
and monitores
the measu-
rement
process,
computates and
stores
the
measurement
values
and
transfers the
result
to the
display.
The
LCD
control
transforms
the
serial
data
transmitted
by
the
CPU
into
parallel
data
and
controls
the
liquid -crystal
display
which
operates
in
duplex
mode.
In
the
LED
control
the parameter
key
actuations
are verified
and
processed.
The
selected
parameter
is
indicated
by
a
LED.
Simultaneously
the
information
is
BCD-coded
and
sent to
the
CPU, whereby
the
most
significant bit directly
switches
on
the
2
Vdc
voltage,
when
the
parameter Cs
(2
V
Bias)
is
set.
The
power
supply generates the
required
stabilized
dc
voltages
+15
V,
-15
V
and +5
V
for
the
circuitries.
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