RF Immunity testing

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RF Immunity testingbasic options

• Fundamental requirement is to create strong EM
  fields in an enclosed area.
• The options are
• Large SAC or FAC.
• Stripline (TEM cell).
• GTEM cell.
• Other proprietary types.

Note that the above cells or chambers are equally
effective at measuring radiated emissions.
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SAC Semi-anechoic chamberRF immunity power
requirements
VERY inefficient design, most power is absorbed
by the absorber. Needs large power amps to create
required field strengths
RF power amplifier
Signal generator
EUT
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Stirred mode chamberRF immunity power
requirements
Unlined screened room, fitted with irregular
paddle wheels designed to change room
resonances as they rotate. Only effective above
300MHz, but capable of creating high field
strengths for moderate power input.
RF power amplifier
Signal generator
EUT
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TEM cell

• Simple and efficient, needs low power for
  required field strengths.
• Can only be used in a screened room
• Limited to small sizes
• Limited upper frequency
• 15 x 15 x 5 cell 500MHz
• 30 x 30 x 10 cell 200MHz

Stripline Cell and Transverse Electromagnetic
Mode Cell
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GTEM cell
Gigahertz TEM cell
Avoids limitations of TEM cell by taking the
co-axial input feed and gradually expanding the
section. Absorber on end wall prevents
reflections. Co-axial core feeds septum plate
which is offset to increase EUT volume. Very
efficient. Needs low power input for required
field strength. No power is absorbed until
wavefront passes EUT and reaches end face.
RF absorbing end face
Septum plate
EUT
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ARCell (Modified MAC cell)

• Evolutionary design Creates partial FAC inside
  cell
• Field uniformity achieved by using absorber to
  limit reflections.
• Generally limited accuracy.
• Absorber reduces RF input efficiency, more power
  required to generate necessary field strength.

RF absorbing end face
EUT
Transmitting antenna
Partially lined with RF Absorber on all internal
faces
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LaplaCell

• Evolutionary design performance to match the
  GTEM
• Field uniformity achieved with unique dual
  septum design
• Balanced septum configuration offers maximum
  EUT volume for given chamber size.
• RF Efficiency high, same as GTEM.

EUT Volume (Polypropylene Lining)
Access door
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RF power amplifier requirements
Plot shows characteristics at 125MHz. This is a
low point on the efficiency curve, therefore this
plot represents the worst case situation. Other
frequencies require less power for a given field
strength
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Ancillaries.. For emissions and immunity up to
20V/m

• Compact
• Very easy to use
• All pre-calibrated with the cell

• All USB interface
• Just plug-and-play

3GHz Emissions analyser 3GHz Synthesiser
1GHz power amp. LETIS (auto switch unit) 3GHz
power amp.
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Key features

• Best EUT size vs overall size characteristics.
• Delivered fully ready for use.
• Pre-calibrated for both emissions and immunity.
  No on-site assembly or commissioning required

RF absorbing end face (Ferrite tiles)
EUT Volume (Polypropylene Lining)
Control and Monitoring compartment
Access door
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Key features

• Full height door provides great accessibility to
  EUT volume.

Full height access
EUT Volume (Polypropylene Lining)
Control and Monitoring compartment
Access door
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Key features
Balun for Balancing and Impedance matching
Impedance matching terminations
Dual septum
EUT Volume (Polypropylene Lining)
Control and Monitoring compartment
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Key features
Note that the balun both balances the septums and
acts to convert the 50ohm Input impedance to over
200 ohms characteristic impedance inside the
cell. This provides a far better match to the
free space impedance of 377 ohm than obtained by
the 50 ohm characteristic impedance of the GTEM
cell.
Impedance matching terminations
Dual septum
EUT Volume (Polypropylene Lining)
Control and Monitoring compartment
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Key features
This matters because the ideal test cell would
emulate the real world as closely as possible,
so the impedance inside the cell should match
free space impedance as closely as possible.
This match is far better achieved with the
LaplaCell design. It means that the relationship
between magnetic field and electric field in the
EM emissions is more accurately determined.
Impedance matching terminations
Dual septum
EUT Volume (Polypropylene Lining)
Control and Monitoring compartment
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Key features
End detail showing absorber and access door.
RF absorbing end face (Ferrite tiles), prevents
internal reflections
Impedance matching terminations
Dual septum
Balun for Balancing and Impedance matching
EUT Volume (Polypropylene Lining)
Control and Monitoring compartment
Access door
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Key features
Arrangement of I/O filtered feeds. Feeds can be
installed to suit individual customer requirements
RF absorbing end face (Ferrite tiles), prevents
internal reflections
Cabling to EUT
Dual septum
Balun for Balancing and Impedance matching
Access door
EUT Volume (Polypropylene Lining)
Filter section
Control and Monitoring compartment
Cabling to Ancillary equipment
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Filter section Inner connection
compartment External connection panel
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EUT access
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The 3GHz system
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Key features
Sketch to show how the LaplaCell design gives a
significantly more uniform field than
conventional GTEM cells. The views below show the
obvious fact that the field in the GTEM is more
dense at the top than at the bottom of the EUT
volume, therefore resulting in a less uniform
field
EUT volume
LaplaCell fields
GTEM fields
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Cell calibration
A VERY IMPORTANT TOPIC
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Cell calibration
A VERY IMPORTANT TOPIC

• LaplaCells are calibrated for emissions and
  immunity to 3GHz
• They are calibrated by measurement (not by any
  theoretical calculation)
• They have a volumetric calibration measured over
  27 points in the EUT volume.
• They are calibrated for every 4MHz step from
  30MHz upwards (Actually every 2MHz for emissions
  from 30MHz 1000MHz)
• This calibration process is extremely detailed
  and is accomplished using computer processing and
  takes 2 days. No other cell undergoes anything
  like this.
• The result is a calibration characteristic that
  is both highly detailed and dependable.

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Cell calibration
A VERY IMPORTANT TOPIC

• For emissions calibration, an ERS (Emissions
  Reference Source), is placed inside the cell in
  locations to cover the whole EUT volume.
• At each location a scan is measured.
• If all these results are plotted we obtain.

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Cell calibration
A VERY IMPORTANT TOPIC

• For emissions calibration, an ERS (Emissions
  Reference Source), is placed inside the cell in
  locations to cover the whole EUT volume.
• At each location a scan is measured.
• If all these results are plotted we obtain.

Note how most results form a tight group. At
higher frequencies, odd results (corresponding to
an odd location in the cell), drop out. This
effect is acknowledged in IEC61000-4-20. At each
frequency, the lowest 25 of values are discarded
and the rest are averaged. This result is then
compared with the known ERS result on a perfect
3m site
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ERS calibration data
This plot is what we would obtain if the cell
acted as a perfect OATS. The Antenna Factor for
the cell is therefore the difference between this
and the actual results obtained in the cell
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This plot shows the calculated correction data
using the averaged values (thick black line). The
Red line is using only the highest values (hence
lowest correction). Note how close this Is to
the black line denoting good consistency). The
blue line is worst case, using the lowest values
(hence highest correction). In practice These
values are never used due to multiple scanning
techniques
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This shows worst case deviations from true
readings calculated from previous plots. Note
that we can read up to 4dB high, or 10dB low if
we happen to be in just the wrong location at
each frequency. In order to avoid these drop
out points, standard practice in cells is to
scan 3 or 4 times with the EUT/cabling in
different positions for each scan, and max hold
the results.
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This shows the statistical error distribution of
3 different groups of scans, each with 4
locations selected at random. Note how these
distributions compare well with accredited test
lab performance. Also note that these results all
correspond to near point sources. Larger EUTs and
cables will degrade the accuracy significantly.
This affects all cells, and to a lesser extent,
chambers and OATS too.
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System operation
Immunity. The software is pre-loaded with the
cell calibration files and the field strength
feedback signal is automatically fed back from
the cell via the cell control cable. The
Synthesiser is the core of the system. It
interfaces the system to the PC, generates the RF
signal and controls the signal level as required
by monitoring the feedback signal from the
cell. Once the Synthesiser is connected to the PC
and all other connections to the power amps,
LETIS and cell are made. Basic operation is very
simple. Frequency start, frequency end, stress
level (V/m), frequency step () and dwell time
are entered in appropriate fields and click
RUN. If the alternative strategy of setting the
field with an empty cell first is required, just
click PRE SCAN. Subsequent clicking of RUN will
automatically use the pre-scanned
settings. During any scan, the actual field
levels are plotted in real time, as is the target
field level. Modulation can be selected from a
list (off, 1KHz am or pulsed modulation 200, 20
2Hz). An EUT prompt signal is available which can
pulse each time a dwell period starts. An EUT
monitor input is available, the status of which
is plotted on screen. All results and setup data
can be saved to file using standard Windows pull
down menus and the file format is such that Excel
(and other applications) will read the files
directly.
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System operation
Emissions As with the immunity system, the
emissions software is pre-loaded with the antenna
factor for the cell. Obviously, because the cell
is fully screened, there is no ambient noise to
contend with. These factors make emission
measurements extremely quick and
straightforward. The software is also pre-loaded
with the common emission limits for FCC, EU and
Aus/NZ standards, so a direct assessment of
compliance can be made. The PC software always
defaults to the standard analyser settings such
as scan rate, RBW (resolution bandwidth) and
detector characteristics. So there is little room
for errors! In accordance with standard practice
for cells, the software can run in Max Hold mode
whilst several scans of the EUT are made in
various positions/orientations. Because the cell
is calibrated with a traceable source, the
measurements are directly related to OATS
results. Measurement uncertainty is dependant on
the type/size of EUT and the presence of cabling.
For compact EUTs without cables, a measurement
uncertainty of 6dB (same as normally offered by
accredited test labs) is recommended. For less
benign EUTs, the uncertainty should be increased.
The increase depends on the above factors, and
will be subject to the rigour with which the test
is performed.
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Background

• Laplace Instruments Ltd have been a significant
  supplier to the EMC test industry for the past 14
  years.
• The first cells were delivered in year 2000.
• These LaplaCells were originally developed for
  test lab use with inputs from NPL (London),
  Manchester University and PERA (Production
  Engineering Research Association).
• They were the first cells to be compliant with
  IEC61000-4-20 (due to the collaboration with NPL,
  who originated the calibration techniques that
  were adopted for the standard).
• There are now 46 Lc300 cells and 27 Lc600 cells
  in service with customers worldwide.
• Customers include
• Rolls Royce (Aero Engines), UK
• Siemens Instrumentation, UK
• ABB Motor Drives, Finland.
• Samsung, USA
• Thales Navigation, France
• Dundee University (Space satellite systems), UK
• MTL (Instrinsically safe instrumentation), India
• Sauter (Industrial instrumentation), Switzerland
• Unitech (Test Lab), Indonesia