Philips PM6302 Instruction Manual page 32

32
Points O and O° do not coincide with point P, the diagonal voltages Ug and Ug' are finite. By alternate use of
712 and 713, point O can be made to coincide with point P.
The r.m.s. supply voltage is reduced from 700 mV to 70 mV by means of the measuring-range selector.
Input DC-BEAS can be used to superimpose the supply voltage with an external d.c, voltage, The d.c. voltage
is supplied to the sample via the output winding of 751 and the measuring-range resistor in operation.
Connection conditions:
— maximum d.c. voltage 60 V, included the eventual common mode voltage. Pay attention to the maximum
test voltage and the polarity of the sample.
— specific d.c. internal resistance of source > 50 2/1 V d.c.; smallest value 30 kQ for 60 V.
—- maximum a,c. internal resistance (output impedance) of the d.c, voltage source < 10 Q.
— if possible, supply the (battery) d.c. voltage symmetrically and floating.
Set up the symmetry parts (e.g. 18 kQ resistors conform to Fig. 9) and the by-pass capacitor very close to
sockets DC-BIAS (Fig. 7}.
Electronically stabilized and other mains parts can not be used,
Arrangement in searching mode for C measurement, see 3.1.4.
3.7.1.2 L measurement
The Maxwell-Wien bridge (Fig. 12) comprise the same measuring-range resistors 701-711 as for C measurement.
Reference reactance 515, with coarse and fine attenuation adjustment by means of 712/A and —/B, forms
in series with adjusting part 713 + 636 the second arm. However, compared to the C measurement, the parts
are interchanged. As a result of this, the voltage divider has the same frequency ratio as that of the sample.
After adjustment of the bridge, the time constants Lx/Rm and (R713 + Rg36) C515 are identical; the diagonal
voltage amounts to 0 V, provided the phase has been adjusted.
The phase adjustment can be performed by means of adjustable attenuation of reference reactance 515,
Two ranges are selectable by means of push-button Q4 801/E. Q = 1/D = 1/tand can be compensated from
<1 to 200. Values 1-14 can be read directly from the scale of 712/..; the value indicated when push-button
Qis not depressed must be inverted.
Fig. 13. illustrates an adjusted Maxwell-Wien bridge.
The losses of sample Ly are supposed attenuation resistor Ry in series with the ideal inductance (fig. 13a),
The reference reactance 515 can be attenuated in a variable way by means of resistor 712/B, which is
connected in parallel with 515 for depressed push-button Q. This applies for values from 1 to 14 (fig. 13¢),
tn thé other case, resistor 712/A is in series with the reference reactance 515. The quality range Q covers 7
to > 100, In this case, the result depends on the frequency of the supply veltage (see afso figs. 13b and d).
For the three {ower ranges the supply voltage is reduced from 700 mV to 70 mV by means of range switch
822/.. but the frequency remains the same.
Arrangement in searching mode for { measurement, see 3.1.4.
3.1.2. Zero indicator (fig. 25)
The zero indicator comprises a d.c.-a.c. converter with stages 301 and 302, impedance converter 303 and 304,
searching stage 305 as well as a control part. Furthermore, the zero indicator includes a voltage-feedback
operational amplifier 306 with bridge rectifier 401-404 and measuring instrument 821,
31.2.1, D.c.-a.c. converter
As the diagonal of al! bridge arrangements is earthed at one side, the input is asymmetrical. For R measurement
transistors 301 and 302 are opened and blocked by means of overloaded amplifier 307/308 in feedback loop.
When 301 is blocked for instance, conductive transistor 302, the operating point of which is related to
transistor 303, takes over the function of transistor 301 and forms a conductive connection to earth via 100 k&.
The diagonal voltage of the not-earthed bridge arm, point P, reaches impedance converter 303 during its "on"
phase, also via 100 kQ and 301.
Voltage feedback of impedance converter 303/304 is performed by resistors 603 and 604 via 304 resulting in
an amplification of 2. For the three first measuring ranges, which are operated with a 10 times reduced supply
voitage, the amplification of the impedance converter is magnified 10 times as a result of 603 by-passing 505.
This procedure does not apply for the R-searching mode.
Searching stage 305 between the output of the impedance converter and the input of the operational
amplifier suppresses shortly the signal for approximately 10 % of the period time at every changeover.
When using an a.c, voltage supply, 305 is open (Fig. 14}.
The control part delivers two antivalent search signais with a period time of 20 ms and a pulse signal with a
frequency of 100 Hz.