Hameg Instruments HM8018 Handbuch Seite 15

the accuracy of L and C measurements is not very
important. However, when series and parallel
components are determined, test errors of more
than 100% may occur without the HZ18 cable.
Furthermore the Kelvin probe permits to connect
components which are difficult to access.
DC bias
The test voltage of the HM8018 unit is sinusoidal
with frequencies between 160Hz and 16kHz. The
amplitude is about 1V
nnected without biasing, they are negatively poled
at every second halfwave. This may lead to irre-
versible damage, particularly when tantalum
capacitors are used. For this reason a 2V DC
voltage can be applied to the test object. This will
prevent any negative polarization. However,
correct polarity should be observed. when
connecting the capacitor. The positive pole of the
DC voltage is applied to the red banana socket.
Dissipation factor and Q-factor
Any passive electronic part is subject to losses
when operated. In case of capacitors, the losses
depend on the type of dielectric material, capa-
citance temperature and operating frequency.
In the equivalent circuit diagram, the losses are
shown as a first approximation by an equivalent
shunt resistor of the capacitor. This resistance
can be determined as a parallel conductance
with the HM8018 unit by using the „G" test
function. Beside the dissipation of the core
material used, the quality of an inductance coil
is essentially dependent on the ohmic resistance
of the winding. In the equivalent circuit diagram,
these losses are characterized by a resistor con-
nected in series with the induction coil. Its resis-
tance is determined with the HM8018 by using
the „Rs" test mode. When measuring Rs or G.
it is possible to use a smaller measuring range.
However, the measuring value of imaginary parts
may not be exceeded by a factor of 2.5 (e.g:
determination of Rs or G is allowed in the 2000pF
range for capacitors up to 5nF).
The formlulas required to evaluate quality and loss
factor are as follows:
tan δ = G/ω · Cx
tan δ = Rs/ω · L
where:
D = tan δ = dissipation factor(no dimension)
Q = loss factor(no dimension)
G = parallel conductance in S(1 S = 1/Ω)
Rs = series resistance in Ω
C. L = measured value in F or H
ω= 2 x π x f i n rad/sec (f =test frequency)
16
. When capacitors are co-
ss
Q = I/tan δ
Q = I/tan δ
The test principle of the HM8018 module permits
to determine the equivalent components up to
phase angles 45" (tan δ ≤ 1)with a deviation of
<1%. This exceptionally good result is achieved
due to a special test procedure which has not been
used in sets of this price category up to now.
However, when measuring iron core coils, no
reliable result can be achieved in many cases in
spite of the used test procedure. This is largely
due to the saturation behaviour of the core
material used. For this reason, the result should
be checked with an approximate calculation, when
determining the inductance values of iron core
transformers and coils.
Alignment procedure HM8018
Measuring equipment required:
Oscilloscope 20MHz (e.g. HM303)
Frequency counter le.g. HM8021)
Digital Multimeter DMM le.g. HM8011)
Load 100kΩ 0,1%,
Load 1kΩ 0,1%
Capacitor 470pF PP oder Polysulfon
Capacitor with known value between 10...16nF
0.1%
Presettings:
To start the calibration, first adjust the R-trimmer
„Probe comp." and „Display Zero" at the front
panel to center position. Disconnect all cable of
the frontpanel HM8018.
1. Test of oscillator frequency:
Connect frequency counter ( TI measuring) to
testpoint „Synchro". Set LC-Meter to range
2µF, „G". Adjust period to 6280µs ± 25µs with
R111.
2. Adjustment of measuring frequency:
Set LC-Meter to the ranges described below.
Adjust period time as following:
2µF VR103 (3) 6283µs ± 1µs
200nF VR102 (2) 628,3 µs ± 0,1µs
200pF VR101(1) 62,83 µs ± 0,1µs
3. Adjust offset (PD):
Connect oscilloscope (AC-coupling, max.
sensitivity) to TP5 and set LC-Meter to range
200nF, „G". Adjust amplitude to minimum with
VR106 (6).
4. Adjust offset (Uref):
Connect DMM (V
200 mH. Adjust VR109 (9) to 0 ± 1 mV.
5. Symmetry:
Set LC-Meter to 200nF, „G". Connect
resistance 100kΩ to the banana jacks and the
oscilloscope (AC, 10mV/Div., 0,1ms/Div.) to
TP7. Adjust VR108 (8) for same amplitude.
Änderungen vorbehalten / Subject to change without notice
) toTPG. Set LC-Meter to
DC