Hameg HM8118, Руководство пользователя

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41
Subject to change without notice
F i r s t - t i m e o p e r a t i o n
4.6 Display of measurement results
Three different representations of measurement results are
available on the LCD display of the HM8118 LCR bridge:
– MEASUREMENT VALUE,
– absolute DEVIATION ∆ ABS or
– relative DEVIATION ∆ % (in percent).
By pressing the SELECT button
3
with the menu SETUP you can
switch the display of the measurement values between DEV_M
(for the display of the “Main Measurement Value“) and DEV_S
(for the „Second Measurement Value“). The LCD display will
show the main and second measurement values, decimal point
and units. The resolution of the main value display (L,C,R,G,Z
or Y) is 1 to 3 digits preceding and 3 to 5 digits following the
decimal point.
The resolution of the second value (D, Q, R, B, X or
Θ
) is 1 to 3
digits preceding and 3 to 5 digits following the decimal point.
If any value is beyond the measurement range selected OVER-
RANGE will be displayed.
∆ % (#, %)
The „#“ sign preceding a value and the „%“ sign following it
designate a display of the relative deviation ∆ % (in percent)
of the measured value of L,C,R,G,Z or Y or D,Q,R,B,X or
Θ
with respect to a stored measurement value (reference
value).
∆ ABS (#)
The „#“ sign preceding a value designates the display of the
absolute deviation ∆ ABS of a measured value with respect
to a stored measurement value (reference value), similar
to ∆ %. The deviation is displayed with the appropriate unit
(Ohms, Henries etc.).
Nominal value (REF_M, REF_S)
This function allows you to store nominal values for the
measurement values which serve as the references when
any of the functions „∆ %“ or „∆ ABS“ is selected. In order
to store a reference value, press the SELECT button 3 ,
use the SETUP menu and choose REF_M for the main
value reference resp. REF_S for the second value reference.
The proper units are automatically chosen according to the
measurement functions selected for the main (H, F, Ω, S)
and second (Ω, S, degrees) measurement values. A nominal
value may be entered with up to 5 digits following the deci-
mal point. Alternatively, by pressing the TRIG button
27
a
measurement occurs and the resulting measurement value
is taken as the reference value.
4.7 Choosing of the measurement range
The measurement range can be selected manually or au-
tomatically. In some cases it is advisable to choose manual
operation, because autoranging always requires a full cycle
until the proper range is found. Manual range selection is
especially helpful when similar components are to be inserted
and removed into/from a test fixture. The LCR bridge HM8118
will automatically switch to the highest range, any time a com-
ponent is inserted it will switch to the appropriate range and
back to the highest range when the component is removed. If
autoranging is disabled, and if the impedance of a component
is
>
100 times larger than the nominal value of the range, the
bridge will display OVERRANGE. If this happens, an appropriate
range must be selected.
If a component is measured in a non-optimum
range, the accuracy of the measurement will be
impaired!
The deterioration of the measurement accuracy outside the
optimum range is calculated as follows (please refer to exa-
mple on page 42):
The best accuracy is achieved, if the measuring value of the
DUT (= device under test) is located in the middle of the selected
measurement range. If the next higher range for the DUT is
selected, this value will appear in its center. As the measuring
error is given in percent of full range the permissible error in
this new range will be appr. twice that in the former, proper
range. Typically, the measurement error increases with ranges
higher than the proper range.
If a component is removed or inserted from resp. into the test
fixture or connected/disconnected to/from the test cables
during a measurement cycle, the result will be wrong, also, in
auto mode, the instrument will switch to the highest range und
back to the proper one after the next component was inserted.
If similar components are to be measured in turn, it is advisa-
ble to hold the appropriate range and the function by pressing
AUTO/HOLD/RANGE 17 .
4.7.1 Automatic range selection
If autoranging is chosen the HM8118 bridge will automatically
select the optimum range for precise measurements of the DUT.
If the result becomes < 22.5 % of a range the next lower range
will be switched in, if it exceeds > 90 % of a range the next hig -
her range will be switched in. An integrated hysteresis (about
10 %) prevents channel hopping if a value is close to the value
where range switching occurs. The following table shows the
levels at which ranges will be changed (the constant voltage
CST V must be turned off):
Measurement Impedance
range of the DUT
1 to 2 Z > 3.00 Ω
2 to 3 Z > 100.00 Ω
3 to 4 Z > 1.60 kΩ
4 to 5 Z > 25.00 kΩ
5 bis 6 Z > 1.00 MΩ
2 to 1 Z < 2.70 Ω
3 to 2 Z < 90.00 Ω
4 to 3 Z < 1.44 kΩ
5 to 4 Z < 22.50 kΩ
6 to 5 Z < 900.00 kΩ
When measuring certain inductors in AUTO MODE
it may occur that the HM8118 continuously changes
the range. The source impedance depends on the
range used, hence it may happen that the measure-
ment results differ more than the hysteresis of 10 %
when the range is changed, causing this effect. It is
then advisable to switch to manual range selection.
4.7.2 Manual range selection
The HM8118 bridge has 6 measurement ranges (1 to 6). It is
possible to preselect them manually or automatically. The fol-
lowing table specifies the source resistance and the impedance
of the connected component for each range. Please note, that the
specified ranges are related to impedance and not resistance. Be
aware, that capacitors or inductances are frequency dependent
components.