Rohde & Schwarz HM8118, User Manual

Page: 53 / 76

53
Instrument Functions
7.1.2 Voltage LEV
The LCR bridge R&S®HM8118 generates a sinusoidal mea-
surement AC voltage between 50 mV
eff
and 1.5 V
eff
with a
resolution of 10 mV
eff
. You can set the measurement AC
voltage either in the SETUP menu via LEV or via LEVEL
9
key by means of the knob
6
or the arrow keys
7
. You can select the decimal point to be changed via ar-
row keys. Using the SETUP menu additionally provides
you with the option to select the measurement AC voltage
by means of the numeric keypad. The amplitude accuracy
is ±5 %. This voltage is applied to the component through
a source resistance. Depending on the impedance of the
connected component, the source resistance may auto-
matically be selected in accordance with the following ta-
ble. The source resistance is dependent on the selected
measurement range.
Component Impedance Source Resistance
10.0 µΩ to 3.0 Ω 25.0 Ω
3.0 Ω to 100.0 Ω 25.0 Ω
100.0 Ω to 1.6 kΩ 400.0 Ω
1.6 kΩ to 25.0 kΩ 6.4 kΩ
25.0 kΩ to 2.0 MΩ 100.0 kΩ
2.0 MΩ to 100.0 MΩ 100.0 kΩ
7.1.3 Preload/ Bias Current BIAS
To permit a forecast on how a component will behave in
the circuit at a later point, you can preset a DC BIAS which
corresponds to the subsequent supply voltage (current).
The BIAS function offers the option to overlap a DC with
the AC measurement range voltage. Components such as
electrolytic or tantalum capacitors require a positive
preload for an accurate measurement. An internal preload
of 0 to +5 V
DC
with a resolution of 10 mV or an external
preload of 0 up to +40 V
DC
/ 0.5A through an external
power supply (instrument back panel) allow reality-orien-
ted measurements (function C-R / C-D). Additionally, the
The constant voltage (CST V function) must be switched on for
measurements with bias current or external preload.
Fig. 7.2: Constant voltage CST_V activated
Is is necessary to unload coils before removing them, i.e.after
switching off the bias current, it is required to wait for the coils
to discharge before the component is disconnected from the
measuring instrument. During the discharge, "Please wait...“ is
shown in the LCD display. The bias current (BIAS) is only availa-
ble for the inductance measurement.
internal preload helps measurements on semiconductor
components.
For measurements of inductances, (function L-R / L-Q),
only an internal bias current is available which can be set
from 0 to +200 mA (DC) with a resolution of 1mA. An ex-
ternal bias current is not possible in this case.
Use the BIAS
10
key to select the value for the preload or
the bias current. Press the BIAS key again after entering
the value to complete the process. You can use the knob
6
and the arrow keys
7
(decimal point) to se-
lect the amount of the preload / bias current. You can acti-
vate the internal preload or bias current (BIAS) by pressing
the BIAS /
26
key. If the preload or bias current is ac-
tivated, the BIAS / key will be illuminated. By pressing
the BIAS / key again, the preload / bias current will be
deactivated and the key will no longer be illuminated.
Example for internal BIAS preload:
In this example, a 1000µF (20V) electrolytic capacitator
was measured with a measurement voltage of 5kHz. The
C-R mode is activated as function and the BIAS
10
key is
used via knob
6
or arrow keys
7
(decimal point)
to select the value for the internal preload. The BIAS /
26
is used to activate the internal BIAS preload.
Example for external BIAS preload:
The error message "DCR too high“ indicates that the resistance
of the connected DUT is too high for the selected bias current. In
this case, the bias current cannot be activated.
Unipolar capacitators must be connected with the correct pola-
rity, i.e. the positive capacitator pole must be connected to the
left contact and the negative pole to the right contact. The pre-
load (BIAS) is only available for the capacity measurement.
Fig. 7.3: Internal BIAS preload
Fig. 7.4: Connectors for external BIAS preload