Philips PM 6303 - 1.4. OPERATING PRINCIPLE

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 processing 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.