METRON QA-ES, User & Service Manual, Page: 32 / 55

The measuring device consists of two blocks:
•
a RMS DC converter for current measuring, and;
•
a peak detector for measuring the peak voltage in the sig-
nal.
The peak detector is divided into two actions for measuring the
positive and negative peak values.
3. Input Filtration
At the measuring system input, the incoming signal is filtered
down to a lower level through a filter based on L1, L2, C5, C6
and C7. The 3dB frequency is set to 10 MHz. The filter can be
found on diagram 1 of 3.
4. RMS DC Converter
(See QA-ES Processor Board Schematic Diagram 1) . The RMS
DC converter is based upon an analog multiplicator from analog
devices AD834, U15. U15 and U25 form a circuit that calcu-
lates the mean square of the applied signal. A filter, consisting
of C8, C9. C50, C51, R35 and R36, determines the constant
time factor for the integration, which will correspond to the am-
plitude of the outgoing signal at U25. Further calculations of
RMS values and current are carried out in the software on the
Processor Board.
Two amplification steps following the mean square circuit en-
sure optimum dynamics in the measuring system. Both steps
U13 and U14 provide 20 dB amplification. By measuring the
signal before U13, and after U13 and U14, you can determine
which signal level makes the best use of the A/D converter dy-
namics.
5. Peak Detector
(See QA-ES Processor Board Schematic Diagram 3) . The peak
detector is divided into two sections: one for detecting the posi-
tive peak voltage, and the other for detecting the negative peak
voltage. The detector is based on a transistor diode connection.
The positive peak voltage is detected by transistors Q1 and Q4,
to which are attached various components. The same goes for
the negative peak voltage with Q2 and Q5. The positive peak
voltage is built up over the C27 capacitor, whereas the negative
peak voltage is built up over the C28 capacitor. U23 drains
small amounts of current from C27, and provides the AID con-
verter with correct polarization and low impedance for the posi-
tive peak detector. U22 does the same for the negative peak de-
tector. The peak detector ran be reset by short-circuiting C27
and C28 via octol analog switch U24
(See QA-ES Processor
Board Schematic Diagram 2) .
6. A/D Converter
4