Transcript EMC testing the easy way

RF Immunity testing…basic
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.
SAC – Semi-anechoic chamber
RF 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
Stirred mode chamber
RF 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
TEM cell
Stripline Cell and Transverse
Electromagnetic Mode Cell
To RF power
amplifier
•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
EUT
Terminating
network
GTEM cell
Gigahertz TEM cell
Avoids limitations of TEM cell by taking the coaxial 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.
Septum plate
EUT
RF absorbing
end face
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
Transmitting
antenna
EUT
Partially lined with RF Absorber on all internal faces
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
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
Power vs Stress level for Lc600 cell - 125MHz
60
50
Field Strength
40
30
20
10
0
0
10
20
30
40
50
Power in (W)
60
70
80
90
100
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.
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
Key features
• Full height door provides great accessibility to EUT volume.
Full height access
EUT Volume
(Polypropylene
Lining)
Control and
Monitoring
compartment
Access door
Key features
Balun for Balancing and
Impedance matching
Impedance
matching
terminations
Dual septum
EUT Volume
(Polypropylene
Lining)
Control and
Monitoring
compartment
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.
Dual septum
EUT Volume
(Polypropylene
Lining)
Control and
Monitoring
compartment
Impedance
matching
terminations
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.
Dual septum
EUT Volume
(Polypropylene
Lining)
Control and
Monitoring
compartment
Impedance
matching
terminations
Key features
End detail showing absorber and access door.
RF absorbing end face (Ferrite
tiles), prevents internal
reflections
Balun for Balancing and
Impedance matching
Dual septum
Impedance
matching
terminations
EUT Volume
(Polypropylene
Lining)
Control and
Monitoring
compartment
Access door
Key features
Arrangement of I/O filtered feeds. Feeds can be installed to suit individual customer
requirements
Dual septum
RF absorbing end face (Ferrite
tiles), prevents internal
reflections
Cabling
to EUT
Balun for Balancing and
Impedance matching
Access
door
EUT Volume
(Polypropylene
Lining)
Filter
section
Control and
Monitoring
compartment
Cabling to
Ancillary
equipment
Filter section
Inner connection compartment
External connection panel
EUT access
The 3GHz system
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
Cell calibration
A VERY IMPORTANT TOPIC
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.
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….
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.
Individual
plotsare
for each
locationwe
in EUT
volume (Cell020)
•If all these
results
plotted…
obtain….
70
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
Output (dB)
60
50
40
30
20
10
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0
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200
300
400
500
600
Frequency (MHz)
700
800
900 1000
ERS calibration data
ERS cal data
80
70
60
50
40
30
20
0
100
200
300
400
500
600
700
800
900
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
1000
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
Comparison of actual and highest/lowest A.F.
values (Cell 020)
Antenna Factor (dB)
50
40
30
20
10
0
-10
-20
0
100
200
300
400
500
600
Frequency (MHz)
700
800
900
1000
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.
20
15
10
5
W ors t c as e A F dev (L)
0
W ors t c as e A F dev (H )
-5
-10
-15
83 0
63 0
43 0
23 0
30
-20
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.
Percent scan readings
Error distribution (% of frequencies)
60.00
50.00
40.00
B,D,H,J
30.00
C, E, F, K
20.00
D, F, I, M
10.00
0.00
-10.00
-5.00
0.00
Deviation (dB)
5.00
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.
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.
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