Philips pm2521 - Page 30

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4. PRINCIPLE OF OPERATION
The PM 2521 is a digital multimeter by which it is also possible to measure: time, frequency and dB.
As control-unit in the PM 2521 the 8035 microcomputer is used. The PM 2521 can be subdivided into three
main part, viz; Analog section. Control section and Display section.
Figure 10 page 134 shows the parts by which the sections are built-up.
A more detailed blockdiagram is given in Fig. 11 page 134, which shows the basic principles of signal measure¬
ments
4.1. ANALOG SECTION
4.1.1. Measuring voltages, currents, resistances and diodes
In the analog section, the voltages (V-r-r-r, V~), currents (A-^-r-r, A~), resistances (12) or diodes ( ->F ) input
signals are converted into a voltage that is suitable for the analog-to-digital convertor (ADC).
The analog voltage is supplied to the ADC and measured.
4 '
4.1.2. Measuring time^and frequency
Time (s) and frequency (Hz) input signals are first attenuated by the voltage attenuator and then supplied to
a comparator. The switching level of the comparator can be influenced with the TRIGGER LEVEL potentio¬
meter at the front of the PM 2521. Influencing the comparator input level makes selective measuring of pulses
possible. From the comparator the time signals &re directly supplied to the microcomputer, where the time is
software counted. The frequency signals are supplied to a counter.
4.1.3. Measuring temperatures
Temperature measurements are made with the optional Pt-100 probe PM 9249. The Pt-100 probe is connected
to a Thomson bridge, to which the current source is connected. The voltage from the Thomson bridge (4-wire
resistance measurements) is directly supplied to the ADC and measured.
4.1.4. Decibel measurements
v
In function dB the measured a.c. voltage is software converted into a dB value. The dB function is combined
with the V~ function.
4.1.5. Measurements in relative reference mode
In the relative reference mode the measured value is captured by the microcomputer. Each subsequent positive
or negative deviation from the relative reference level will be displayed. The relative reference mode is only valid
in the functions V^-r, dB, 12, - ► (- and °C.
f
4.1.6. Control of the analog section
The analog section is controlled by the microcomputer. The setting of the directions for the measuring functions
is determined by the function selector.
The setting of the ranges, in one function is determined by the ranging switches AUTO/MAN*, UP and DOWN.
The range information switches the attenuation of the input signal, the sensitivity of the RMS convertor and the
sensitivity of the current source. The function and range information is supplied to the analog section via reed
relay control.
4.1.7. ADC
The analog-to-digital conversion used in the PM 2521 is in accordance with the Delta modulation principle.
The ADC system has been built into two LSI circuits. The analog signal is converted into a data signal.The
data cycle of the data signal is proportional to the height of the analog input signal of the ADC.
The ADC system is also equipped with an automatic zero point correction.