Rohde & Schwarz HM8118, User Manual

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48
First-Time Operation
surement range. If the next highest measurement range is
selected for this DUT, it will display in the center of the se-
lected range. Since the measurement error is defined as
a percentage of the measurement range final value, the
measurement error in the higher range goes up nearly by
a factor of 2. Typically, the measurement error is increased
accordingly in the nearest higher measurement range. If a
component is removed from the test lead or measurement
adapter during a measuring process in the continuous
measurement mode, the automatically selected measure-
ment range and the automatically selected measurement
function can be adopted by switching to the manual mea-
surement range selection (RANGE HOLD). This allows the
measurement time during the measurement of many simi-
lar components to be reduced.
4.5.1 Example of determining the measurement
accuracy
The accuracy calculation is always based on the data sheet
table (see fig. 4.5). To calculate the corresponding measu-
rement accuracy, the following component parameters are
required (component operating point):
❙ Component impedance at corresponding measurement
frequency
❙ The measurement frequency.
As an example, a 10 pF capacitator with an impedance of
15 MΩ at 1 kHz will be measured. In this case, the top row
of the accuracy table is valid:
The accuracy decreases with the measurement voltage (test vol-
tage) because the signal / noise ratio decreases. Consequently,
this leads to additional instabilities. The accuracy decreases at
the same rate. If 0.5V is used as measurement voltage, for in-
stance, the base accuracy is one half.
Impedance: 100 MΩ
4 MΩ
20 Hz 1 kHz 10 kHz 100 kHz
0,2% + I Z I / 1,5 GΩ
The actual measured series resistance includes all series
resistances such as the component leads and the resis-
tance of series-connected foils in capacitors as well as die-
lectric losses; it is expressed by the dissipation factor DF.
The equivalent series resistance (ESR) is frequency-depen-
dent according to the formula:
ESR = Rs = D/ωCs
where ω „Omega“ = 2 π f (circular frequency) represents.
Traditionally, the inductance of coils is measured in a series
circuit; however there are cases where a parallel circuit will
yield a better representation of the component. In small
„air“ coils mostly the ohmic or copper losses are predomi-
nant , hence the series circuit is the proper representation.
The core of coils with an iron or ferrite core may contribute
most of the losses, the parallel circuit is to prefer here.
4.5 Measurement Accuracy
The measurement of impedance and phase angle is prone
to a certain amount of inaccuracy. The measurement accu-
racy of a specific test point can be calculated based on the
accuracy table in the data sheet (see fig. 4.5). Make sure
you know the impedance of the corresponding component
at the respective test point. No further information is requi-
red to calculate the accuracy. The base accuracy of 0.05%
as indicated in the data sheet pertains only to the base ac-
curacy of the R&S®HM8118 bridge. The base accuracy only
indicates the general measurement uncertainty of the in-
strument. The accuracy table describes the measurement
accuracy that additionally has be taken into account.
The highest accuracy is ensured when the DUT value (=
Device Under Test) is approximately centered in the mea-
Fig. 4.4: Example correlation Cs (or Rs) and test voltage
The resistance measurement always occurs in compliance with
the method to apply voltage (AC) and measure the resulting cur-
rent. The only difference to L or C is that the phase angle is ne-
arly 0° (real resistance). A resistance measurement with DC is
not intended.
Fig. 4.5: Table to determine the accuracy
Impedance: 100 MΩ
4 MΩ
1 MΩ
25 kΩ
100 Ω
2.5 Ω
0,01 mΩ
20 Hz 1 kHz 10 kHz 100 kHz
0.2% + I Z I / 1.5 GΩ
0.05% +
I Z I / 2 GΩ
0.1% +
I Z I / 1,5 GΩ
0.5% +
I Z I / 100 MΩ
0.2% +
I Z I / 100 MΩ
0.1% + 1 mΩ / I Z I
0.3% + 1 mΩ / I Z I
0.2% +
2 mΩ / I Z I
0.5% +
2 mΩ / I Z I
0.5% +
5 mΩ / I Z I
+
I Z I / 10 MΩ