Operating Principle; Description Of The Block Diagram - Philips PM 6303 Service Manual

2.1.
2-1
OPERATING PRINCIPLE
DESCRIPTION OF THE BLOCK DIAGRAM, Fig. 3
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 i 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 voitage 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 voltage/current (V/l) selector controlled by the CPU.
In the programmable amplifier gain factors x0.1, x1 or x10 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 digital 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 contrds
the liquid-crystal display which operates in duplex mode.
In the LED control the parameter key actuations are verified and processed. The selected parametr
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.