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the 100% range should be selected first,
otherwise the display will start flashing as
soon as the full-deflection value of the meas
urement range is exceeded. In case of in-
sufficient resolution of the display, the next
smaller measurement range should be
selected. The two measurement ranges
only differ with repect to the obtainable re-
solution, i.e. resolution of 0.1% in the 100%
range and of 0.01% in the 10% measure-
ment range. The difference in suppression
of the fundamental wave in the two
measurement ranges is not visible on the
display, because the inherent distortion and
noise of the HM8027 unit are less than 0.5
digit. These values are only significant
during evaluation of the signals via the
monitoroutput (see the "OUTPUT"section).
The distortion factor is directly read out on
the display in percent, requiring no further
conversion. The read-out range extends
from 99.9% to 0.1% or from 9.99% to
0.01%, respectively.
Analysis of the measurement result
When measuring mains-operated equip-
ment, the test signal is often deteriorated
by low-frequency interferences. To elimi-
nate a part of these interferences, a high-
pass filter with a cut-off frequency of 1 kHz
and a roll-off characteristic of 12dB/Oct. can
be inserted into the signal path. This will
efficiently suppress any mains-frequency
interference for distortion measurement of
frequencies above 1kHz. When measuring
any signals near the upper frequency range
limit of the HM8027 module, the frequency
response of the internal measurement
amplifier should be taken into consideration.
Its high-end cut-off frequency is around
80kHz (- 3dB). This means that the
harmonics contained in a test signal having
a fundamental frequency of 20kHz are only
evaluated up to the third order. Likewise,
the noise bandwidth is limited to 80kHz. A
further evaluation of the residual distortion
can be carried out via the OUTPUT socket
(3). This socket is used for qualitative
evaluation of distortions excluding the fun-
damental wave of the test signal, e.g. by
means of an oscilloscope. It can be deter-
mined whether the displayed distortion
Sous réserve de modifications / Reservado el derecho de modificatión
factor is caused by linear distortions or by
noise. Due to the specified sensitivity of 1
mV per indicated digit, a further evaluation
of the distortion measurement is possible
beyond the specified resolution of the
HM8027 unit. For this purpose, a 4½digit
multimeter, e.g. the HM8011-3 module, is
connected to socket (3). Now the distortion
can be read out directly from the multimeter
display in the 200 mV AC range. A read-out
of 00.75 (mV) corresponds to a distortion
factor of 0.0075%. The evaluation capability
is limited by the inherent distortion and
noise of the HM8027 module.
Operational check
Measuring equipment required
1 Low Distortion Generator HM8037
2 Generators HM8030-3 or 8037
1 Digital Multimeter HM8011-3 or any other
with 200mV AC range.
Check of input sensitivity
Setting: (2)
(7)
2k
Cal. max. 0dB released
1) Adjust output voltage of HM8037 to
300mV at 1kHz.
2) Connect the output of HM8037 to the
input (9) of HM8027.
3) With the above setting the display (1) of
the HM8027 should indicate a value
≥100.
Check of input signal attenuation
Setting: (2)
(7)
2k
Cal. max. odB released
1) Connect the output of HM8037 to the
input (9) of the HM8027.
2) Adjust readout (1) to 100 by means of
the amplitude control of the Sine Wave
Generator HM8037.
3) Check the following values:
(8)Min. Stellung, (11)=0dB, Display ≤19
(8)Max. Stellung, (11)=-10dB, Display ≤30
(8)Max. Stellung, (11)=-20dB, Display ≤10
(8)
(11)
(10)
(8)
(11)
(10)
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