Rohde & Schwarz HM8118 User manual

HM8118: Measurement of capacitors and
coils
From the expert’s desk
Products:
| HM8118
1. Introduction:
The HM8118 is an LCR Bridge with a basic accuracy of 0,05%. The
test frequency can be set from 20Hz up to 200kHz.
This Application Sheet helps to perform first measurements on capaci-
tors and coils without having a doubt concerning the results.
It does not claim to be complete. All information has been compiled
with care. However errors can’t be ruled out.
2. General information about the measurement principle
The HM8118 is not a typical Wien, Maxwell or Thomson bridge based
instrument. It measures the impedance Z and the phase angle Φ with
an AC stimulus signal. In combination with other known parameters
(e.g. frequency) the desired characteristics can be calculated.
When performing measurements always remember, that e.g. the ca-
pacity varies with certain parameters, specially over the frequency. Al-
so aging, temperature, the level of the measurement signal and addi-
tional bias are dependency factors.
The equivalent circuit of each DUT always contains inductive, real and
capacitive parts. E.g. a capacitor will have inductive and real parasit-
ics.
This means, that ideal Φ (90°=pure inductive, 0°= pure ohmic, -90°
=pure capacitive) will never be reached.
It is also important, that due to the opositional parasitic parts each
measurement contains a certain inaccuracy being a systematic meas-
urement error.
Extraction from the user manual: Phase angle and integral parts of L, C, R
As an example a bigger coil always contains a parasitic ohmic
and capacitive part in addition to the inductive essential part.
The inductive part will prevail, while the capacitance can not
directly be measured with the HM8118. However this coil will
have a self resonance showing the capacitive part to be pre-
sent.
3. Test assembly
It is important to allways keep in mind, that the test assembly
may have appreciable influence to your measurement. When
talking about capacitors in the range of picofarads, the position
of the clamps has definitely influence to the measurement.
You can easily give it a try by performing the following steps:
- use the basic setting (1kHz / Single Mode)
- shorten the clamps and perform a short calibration
- lay down the clamps with at least 20cm difference and per-
form an open calibration
- change to C-D Mode
- Now hold the clamps, that the tips are just not touching and
check the measurement result.
I can achieve more than 3pF with this method
So allways choose the best test adaptor for your application.
The HZ181 is a great tool for leaded (through hole) compo-
nents avoiding such distance issues:
4. Alignment of the test structure
We recommend to align the instrument on all available fre-
quencies (20Hz –200kHz in 69 Steps) when measuring un-
know devices. After choosing the right test assembly and have
the instrument warmed up for at least 30 minutes, please per-
form the following steps:
- Perform a Factory Preset to start with a dedicated setup:
- Press [RECALL] [9]
- Prepare the instrument to calibrate on all frequencies:
- Press [SELECT] [3]
- Change to (Mode) using the rotary knob
Technical Information
P. Jahns - 03.2020
5TO

- Press the rotary knob to select this menu point
- Change from “SGL” to “ALL” and press the knob again
- Leave the menu using [ESC]
- Perform a complete alignment:
- Shorten the clamps and press [SHORT]
- Now the short calibration will run for about two minutes.
- Open the clamps (If using e.g. kelvin clamps take care to
have about the same position as it will be for the meas-
urements to come ) and press [OPEN]
- Also this procedure will take up to two minutes
5. Measurement of an unknown coil
Preliminary remarks: Coils containing ferromagnetic core ma-
terials may not give exact results as the core material is speci-
fied for a certain frequency range. This has to be checked be-
fore.
- Switch on the measurement voltage / current monitor:
- Press [SELECT] [2]
- Change to (Vm/Im) using the rotary knob
- Change the value to “ON” and press the knob again
- Leave the menu using [ESC]
- Connect the unknown coil to the test structure
- Change to Z / Phi Measurement by pressing [Z - Φ]
- The phase angle should already be positive
- Now search for the best acquisition frequency
- Press [FREQ]
- Use the rotating knob to change the frequency as long
as Φ is closest to 90°
- Press [L-R]
- Now the nominal inductance is displayed in combination
with the serial resistance
- Check the voltage not to drop down too much (down to
35% of the nominal voltage is fine –in this case the nomi-
nal voltage is 1V; see section “LEV” on the left upper dis-
play)
-
PHI and Z over frequency for a standard coil with 132µH / enam. copper wire
5. Measurement of an unknown capacitor
Preliminary remarks: Gold caps can not be measured with the
HM8118 as they are too inert for that kind of measurement.
- Switch on the measurement voltage / current:
- Press [SELECT] [2]
- Change to (Vm/Im) using the rotary knob
- Change the value to “ON” and press the knob again
- Leave the menu using [ESC]
- Connect the unknown capacitor to the test structure
- Change to Z / Phi Measurement by pressing [Z - Φ]
- The phase angle should already be negative
- Now search for the best acquisition frequency
- Press [FREQ]
- Use the rotating knob to change the frequency as long
as Φ is closest to -90°
- Press [C-D]
- Now the nominal capacitance is displayed in combination
with the serial resistance
- Check the voltage not to drop down too much (down to 35%
of the nominal voltage is fine –in this case the nominal volt-
age is 1V; see section “LEV” on the left upper display)
6. Additional Information
If the voltage drops down to less than 35% of the nominal
frequency, switch on the constant voltage mode. The cali-
bration done before ist still valid when changing to this
mode.
In the constant voltage mode the impedance of the HM8118
is fixed to 25 Ω, meaning that the output voltage is held con-
stantly to the preset level for devices with an impedance of
more than 25 Ω.
This can improve the results mostly for lower inductance
values. It is important to remark, that the accuracy is re-
duced by the factor of 2 when working in this mode.
It is also possible to acquire the DUT’s behavior using the in-
tended working frequency without tuning. If this is pur posed,
the calibration can also only done for this very frequency us-
ing “SGL” Mode (see “calibration, calibrate all frequencies)
References:
- R&S HM8118 User Manual
- R&S FAQ "How to startup your HM8118 using a specific calibration”
- R&S FAQ “HM8118 different measurement result of the same coil”
- R&S FAQ “HM8118 tu use external BIAS”
Rohde & Schwarz GmbH & Co. KG
Mühldorfstraße 15 | D - 81671 München
Phone + 49 89 4129 - 0 | Fax + 49 89 4129 –13777
www.rohde-schwarz.com