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A.7
Reverberation Test Results Assessment
A.7.1
Introduction
The results of the reverberation chamber measurements performed by QinetiQ are presented
in this section of the report. No limit line has been applied to these emissions graphs. When
making comparisons it must be considered that the directivity of the devices was not known
when calculating the equivalent radiated field. As discussed previously the directivity was
therefore assumed to be unity within the equation E3, Appendix E of EN 61000-4-21. Note
the directivity D factor is a linear factor to be multiplied within the equation. Unless the
device tested was purely isotopic, the estimated E-fields from the reverberation chamber
method may be ‘artificially’ lower than the ‘worst case’ value measured within the FAR.
There are a number of other factors that could impact the comparison between measured
emissions, some of these can also exist within conventional techniques, for example, turntable
rotation speed, cable layout, antenna scan rate, equipment cycle/dwell time. These factors can
and will impact the measurement of EUT’s performed within various test facilities,
understanding these factors and controlling them will reduce measurement differences.
A.7.2
Results
The following table summarises the tests completed and the related figure numbers where
graphical results are presented. Problems were in encountered in testing the two of the device
that required a radio link to operate, this was discussed previously in Section A.5.2. The two
devices that experienced these problems were the Philips multi media system and the DECT
phones. The radio link issue is related to the continuously perturbed test environment, leading
to the wanted radio link signal being disrupted, thus leading to loss of communications link.
One possible solution for this problem would be to mode tune (only) these devices and
establish a good link at every discrete paddle position. An alternative possibility would be to
terminate the radio link into a matched load and use exercise software routines to fully
exercise the devices. This method may also be required in conventional emissions
measurements, if the radio link signal causes saturation or overload of the receiver /
preamplifiers.
Table 29:
Reverberation chamber test results
Figure
EUT
Mode
RBW
72
Noise floor
Mode stirred
1 MHz, 300 kHz and 100 kHz
73
Laptop PC
Mode stirred
1 MHz, 300 kHz and 100 kHz
74
XBox
Mode stirred
1 MHz, 300 kHz and 100 kHz
75
DVD and PC
Mode stirred
1 MHz, 300 kHz and 100 kHz
76
Desktop PC
Mode stirred
1 MHz, 300 kHz and 100 kHz
77
Laptop PC
Mode stirred and mode tuned
300 kHz
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Figure
EUT
Mode
RBW
78
XBox
Mode stirred and mode tuned
1 MHz
Figure 72 presents the results of the noise floor of the instrumentation used in the
measurements in the reverberation chamber indicating the achievable sensitivity in
comparison with the applicable CISPR limits for a peak detector.
Estimated maximum (free space) E-field generated by EUT at 3 metres
Noise Floor
120
100 KHz Bandwidth
300 kHz BANDWIDTH
1 MHz BANDWIDTH
Limit line
110
100
90
Efiel
d
(dB
/uV
/m)
80
70
60
50
40
30
20
10
0
-10
1.000 GHz
2.000 GHz
3.000 GHz
4.000 GHz
5.000 GHz
6.000 GHz
Frequency
Figure 72: Reverberation Chamber Noise Floor
It can be seen from the graphs that the noise floor is between 40 and 60 dB below the peak
limit line, dependent upon the measurement RBW.
A.7.2.1
Laptop
The radiated emissions from the Laptop computer are presented in Figure 73.
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Estimated maximum (free space) E-field generated by EUT at 3 metres
HP Compaq Laptop (Mode stirred)
100
100 kHz BANDWIDTH
300 kHz BANDWIDTH
1.0 MHz BANDWIDTH
90
80
E-field (dB/uV/m)
70
60
50
40
30
20
10
0
1.000 GHz
2.000 GHz
3.000 GHz
4.000 GHz
5.000 GHz
6.000 GHz
Frequency
Figure 73: Laptop – Mode Stirred
The mode stirred data for the Laptop measured is shown in Figure 73 above, the main CPU
clock frequency and second harmonic are clearly measured. The CPU fundamental frequency
of 2.6GHz, is over 20dB below the peak limit defined.
A.7.2.2
X Box
The 2.4 GHz ISM frequency dominates this measurement band, as expected. Due to the high
ISM signal at 2.4 GHz investigations were performed to ensure the preamplifier and receiver
input were not being overloaded during these measurements.
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Estimated maximum (free space) E-field generated by EUT at 3 metres
XBOX 360 Game system (Mode stirred)
120
100 kHz BANDWIDTH
300 kHz BANDWIDTH
1 MHz BANDWIDTH
110
100
90
80
Efie 70
ld
(d 60
B/
uV
/m 50
)
40
30
20
10
0
1.000 GHz
2.000 GHz
3.000 GHz
4.000 GHz
5.000 GHz
6.000 GHz
Frequency
Figure 74: XBox radiated emissions
The harmonic frequency at 4.8 GHz has been captured and shown to be some 50 dB lower
than the ISM fundamental. The processor, which is believed to be at 3.2 GHz has been
captured and is some 35dB below the defined peak limit. There also appears to be related
clock/bus signals and harmonics that are significantly below the defined peak limit.
A.7.2.3
DVD/TV
The radiated emissions from the DVD/TV system are presented in Figure 75.
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Estimated Maximum (free space) E-field generated by EUT at 3 metres
LCD Television and DVD (Mode stirred)
120
100 kHz BANDWIDTH
300 kHz BANDWIDTH
1 MHz BANDWIDTH
110
100
90
80
Efie
ld
(d
Bu
V/
m)
70
60
50
40
30
20
10
0
1.000 GHz
2.000 GHz
3.000 GHz
4.000 GHz
5.000 GHz
6.000 GHz
Frequency
Figure 75: TV/DVD Mode Stirred
It is considered that some of the emissions frequencies generated by the TV/DVD
combinations are ‘sporadic’ in nature. It is possible that these non-continuous signals are not
fully captured during both the reverberation chamber and conventional chamber
measurements. This would require a further detailed investigation to perform an evaluation of
dwell time for signals appearing to be ‘sporadic’ in nature.
A.7.2.4
Desktop
The radiated emissions from the Desktop PC are presented in Figure 76.
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Estimated maximum (free space) E-field generated by EUT at 3 metres
Desktop computer (Mode stirred)
120
100 KHz Bandwidth
300 kHz BANDWIDTH
1 MHz BANDWIDTH
110
100
90
Efiel
d
(d
B/
uV/
m)
80
70
60
50
40
30
20
10
0
1.000 GHz
2.000 GHz
3.000 GHz
4.000 GHz
5.000 GHz
6.000 GHz
Frequency
Figure 76: Desktop – Mode Stirred
It is considered that in this case some of the emissions frequencies generated by the Desktop
computer are ‘sporadic’ in nature. It is possible that these non continuous signals are not fully
captured during both the reverberation chamber and conventional chamber measurements.
This would require a further detailed investigation to perform an evaluation of dwell time for
signals appearing to be ‘sporadic’ in nature. This could explain the variance in some of the
signal amplitudes, when comparing the three traces for 100 kHz, 300 kHz and 1 MHz
bandwidths. It is considered these variances are not related to the measurement bandwidth,
however, possibly related to variations in the cycling and exercising of the EUT.
A.7.3
Comparison of Mode-stirred and Mode-tuned
A.7.3.1
Laptop PC
The emissions signal at 2.4 GHz, which is an ISM frequency probably radiating from the
Laptop’s Wireless LAN card, was only apparent during the mode tuned measurement (as
shown in Figure 77). This is due to again this signal being non-continuous or ‘sporadic’ in
nature.
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Estimated maximum (free space) E-field generated by EUT at 3
t Compaq Laptop (300 kHz RBW) - (Mode stirred Vs Mode
HP
t
d)
120
Mode tune
Mode
110
100
90
80
Efie
ld
(d
Bu
V/
m)
70
60
50
40
30
20
10
0
1.000
GH
2.000
GH
3.000
GH
4.000
Frequency GH
5.000
GH
6.000
GH
Figure 77: Laptop – mode stirred vs mode tuned
During mode tuned testing the paddle wheel is stationary for a defined dwell period, thus the
emissions signal from the wireless LAN was observed, however, this signal was only present
at one of the discrete paddle positions. Additional tests were carried out which prove that it
was possible to detect these emissions measurements during mode stirred, if the paddle wheel
was left to rotate for many rotations (i.e 10’s rotations). This is considered to be a dwell time
versus paddle speed issue. Again due to the non-continuous nature of these emissions, it is
considered that the conventional chamber measurements would suffer the same issues, related
to turntable rotation speed and receive antenna height scan rate. The apparent increase in
emissions between 3 GHz and 6 GHz is also related to dwell time, as the noise floor of the
analyzer will increase for long sample times, as the analyser is selected to ‘maximum hold’.
A.7.3.2
X-Box 360
The mode stirred and mode tuned data show very good correlation over the frequency range
1-6 GHz. There are very small differences between the methods at both 2.4 GHz and
2.5 GHz, for the wireless LAN and wireless control related signals. It is considered these
very small differences (3-4 dB) are related to possible dwell time versus paddle speed issues,
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allowing the paddle to rotate for further rotations or slowing the paddle significantly could
resolve these small differences. The largest difference between the mode stirred and mode
tuned emissions results is at a frequency of 2.7 GHz. In this case the mode tuned data is
showing to be approximately 10 dB lower than the corresponding mode stirred data. It is
possible this is due to under sampling for the mode tuned method, increasing the number of
discrete paddle positions thus increasing the sample rate and permitting the measurement to
be resolved fully at this frequency.
Estimated maximum (free space) E-field generated by EUT at 3 metres
XBOX 360 Game system (Mode stirred Vs Mode tuned)
120
Mode
Mode
110
100
90
80
Efie
ld
(d
Bu
V/
m)
70
60
50
40
30
20
10
0
1.000
G
2.000
G
3.000
G
4.000
Frequenc G
5.000
G
6.000
G
Figure 78: XBox 360 – mode stirred vs mode tuned
It is also highly likely that many of the dwell time/rotation rate issues are also apparent within
the anechoic chamber method. As with the reverberation chambers paddle rotation speed,
equally the rotation rate of the turntable and height scan of the receive antenna are of equal
importance and can impact the emissions amplitudes measured.
A.7.4
DECT Phone and Multimedia System
There were problems encountered in testing this product due to the loss of link in the
reverberation chamber, due to the high reflection within the chamber, this issue has been
discussed previously within this report.
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A.8
Measurement Times
A.8.1
FAR Measurement Time for the Basic CISPR Test
This section lists the actual test time taken for each product under test. The test time
calculated is based on various parameters, i.e. set-up and configuration of the equipment
under test, sweep time of the spectrum analyser for each antenna polarisation and rotation of
the turntable.
Table 30 shows the test time for one product (Desktop PC) using peak detector, one
bandwidth (RBW = 1 MHz) and horizontal and vertical antenna polarisations.
Table 30:
Testing time for basic CISPR test
Function
Actual time
(Hours)
Estimated time
(Hours)
EUT set-up and configuration
0.50
1
EUT Operation
0.50
1
Test equipment preparation and set-up
1
1
Testing Vertical scans (X axis)
4
4
Testing Horizontal scans (X axis)
4
4
Total
10
11
Depending on the complexity of the EUT and the physical dimensions, some EUTs may
require more time for setting up and operation in order to achieve normal operating mode or
worst-case scenario.
A.8.2
Time for Additional Testing
The additional scans can determine the emission levels when the EUT is placed in the Y and
Z planes and rotated 360 degrees, this requires additional testing time but is very useful for
compliance issues, it gives an early indication of the EUT emission profile where some EUTs
might have higher emission levels from the Y and Z planes.
Table 31 gives the estimated testing time for the additional tests (1-6 GHz for Y and Z axis)
and polar scans in (X, Y, Z) for the highest three frequencies.
Table 31:
Testing time – additional tests
Function
Time (Hours)
Vertical scans (1-6 GHz) for Y and Z axis
8
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Horizontal scans (1-6 GHz) for Y and Z axis
8
Polar graphs (X, Y Z) planes for 3 frequencies
6
For the products listed in Section A.3, the report shows that there was no increase in emission
levels when the EUT was placed on its side in the Y and Z planes and rotated 360 degrees to
those measured in the X plane.
A.8.3
Reducing Measurement Times
In the development of the CISPR standards, there has been consideration of possible methods
for reducing measurement times, whilst not reducing the probability of measuring the
maximum emissions from strongly directive narrow beam emissions.
In order to reduce unnecessary testing at frequencies above 1 GHz of equipments that are
unlikely to radiate profusely across the spectrum to 18 GHz, a conditional requirement for
testing has been proposed in the standards committees. The conditional testing procedure of
CIS/I/151 is defined as:
•
If the highest internal source of the EUT is less than 108 MHz, test to 1 GHz
•
If the highest internal source of the EUT is between 108 and 500 MHz, test to 2 GHz
•
If the highest internal source of the EUT is above 500 MHz, test to five times the
highest frequency or 6 GHz.
Although these proposals are likely to be included in the final version of the harmonised
product standards it has not been considered in this programme of work. All products were
tested up to 6 GHz, irrespective of the internal clock frequencies and data processing speeds.
The general conclusions of the sections above are that there is unlikely to be a significant
increase in testing time and most testing at frequencies above 1 GHz, whether internal to the
manufacturer or contracted out to a UKAS accredited EMC test laboratory, will be able to be
completed at a reasonably economical price and within reasonable timescale.
A.9
Comparison of FAR and Reverberation Measurements
Figure 79 shows the comparison between the radiated emissions from the Desktop PC
measured in the reverberation chamber compared with those measured on the same item of
equipment in the Fully Anechoic Room.
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20
15
10
dB
5
0
-5
-10
-15
-20
1
1.5
2
2.5
Frequency, GHz
Figure 79: Desktop PC reverberation chamber radiated emission results relative to FAR
results
On average the reverb chamber gave an average reduction in field of 7 dB but there was clear
and apparently uniform frequency dependence. This could lead to a factor, which could be
included in EN 61000-4-21 to obtain representative fields when using the reverberation
chamber, but further work is required to determine the value and precision of the gain figure
to be applied.
The same outline analysis was performed for the DVD/TV system that was measured using
both facilities. The results are presented in Figure 80.
20
15
10
dB
5
0
-5
-10
-15
-20
1
1.2
1.4
1.6
1.8
2
2.2
2.4
Frequency, GHz
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Figure 80: DVD/TV reverberation chamber radiated emission results relative to FAR
results
The mean value is again in the order of about 10 dB with a +/-5 dB variation about the mean.
Both of these results imply that the directivity factor of 1.7 would probably need to be
adjusted in order to realise a more representative reverberation chamber result closer in value
to those determined in the FAR for these types of product. It is encouraging that the
differences do not vary significantly with frequency since this could be an expected effect
where the EUT are omni-directional radiators at low frequencies but have higher gain lobes at
higher frequencies.
A.10
Impact on Industry
A.10.1
Product Manufacturers
For product manufacturers having to meet the new CISPR limits for Information Technology
and Multi-media Equipment, the test methods and limits to be applied would appear to be
reasonably acceptable. The test method proposed by CISPR is effectively an extension to
1-6 GHz of existing field strength measurement methods below 1 GHz but with the reduction
of ground reflection and should present no major technical problem for most EMC
laboratories.
For the existing EMC laboratories the development of a semi-anechoic room facility extended
to cover the frequency range 1-6 GHz would mainly comprise the introduction of floor
absorber as is currently used for immunity testing. This is usually carbon loaded
polyurethane material in a pyramidal format, most of the existing material should be capable
of operating effectively up to 6 GHz if not significantly higher in frequency. More material is
probably required than is possessed at present in some laboratories but the major costs will be
cost is likely to be associated with the initial calibration which may involve about two weeks’
work to complete. The calibration in later years should only take two or three days.
Once established as a calibrated facility, the time taken for emission measurements in the
frequency range 1-6 GHz will be similar to that taken for the frequency range 30-1000 MHz.
From the results of the measurements made on the six products examined in the series of
measurements undertaken in this project, there were few frequencies that need to be further
checked out in detail unlike the process that is common at frequencies below 1 GHz where a
second assessment using the quasi-peak detector follows to initial scan using the peak
detector, but measurements are often required at a large number of frequencies, taking several
hours.
There should be reasonable acceptability of the current test method and the initial limits.
Although consideration will need to be given to the product design to ensure compliance at
frequencies above 1 GHz, designing for EMC at the early stages of development usually deals
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with design cost issues. The desktop computer examined in this work had a metal case where
the shielding effectiveness in the range 1-3 GHz was in the order of 20 dB and this change to
the design had not apparently affected the product price, as it was still highly competitive.
Overall the costs to manufacturing industry of designing and approving to the new CISPR
limits at frequencies above 1 GHz should be financially acceptable.
A.10.2
Radio service operators
The compliance of all products in large-scale production in the commercial market should
result in the provision of adequate protection to radio services. The CISPR limits have been
developed scientifically and are very similar to the FCC limits in the USA that appear to have
achieved adequate acceptance. The new tests and limits will be adopted by all major
manufacturers probably before the revised version of the standard is approved by the
European Commission and consequently any short-term compliance matters will be
addressed. When the harmonised EN standard is finally published compliance with the limits
will be a legal requirement.
The avoidance of any interference issues will depend on whether the test methods adequately
address interference to digitally modulated radio services. Peak and average detectors are
currently called up but CISPR is already considering more appropriate detectors specifically
devised for digital services. The key factor is more likely to relate to the setting of the limits.
Whilst they have been set using standard procedures there is a dependence on probability
factors that may vary between product types and their location. It is possible in any standard
that compliance with the limit does not guarantee that there will be no interference.
Compliance with European standards is acknowledged as providing only a “comfort of
conformity” i.e. a finite probability with a small possibility of non-compliance. Obviously if
a transistor radio is placed directly on a compliant microwave oven there could be some
interference experienced as a 3 metre separation distance is considered applicable, the same
could apply at higher frequencies for individuals attempting to receive on a GSM 1800 mobile
phone close to a machine in a computer room where the wanted signal may be weakened by
building shielding. The checking of the applicability of the limits should involve a scientific
review but normally complaint statistics are minimal so the emission limits, which are
initially based on compromise, need to be monitored carefully if full assurance in the standard
is to be achieved.
If there considerations of permission for some products having low power radio frequency
sources to enter the market without a requirement for strict compliance with similar limits the
controllers of the spectrum will need to be aware of the possible conditions. For a peak
radiated emission limit of 70 dB(μV/m), i.e. 3.16 mV/m, the maximum power radiated from a
product having a source at the frequency of interest would have to be restricted to 3 μW in
order to comply with the limit. For relaxation of any regulations relating to short-range
devices, typically a maximum of 10 mW is quoted, there would have to be some control on
the radiated emission levels at the frequencies of interest, just as emissions from ITE would
have to be controlled.
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A.11
Summary of Measurements
Measurements at frequencies greater than 1 GHz have been made on six products of the type
that are generally available to the commercial market and are purchased in large numbers.
The aim was to determine whether these products are likely to cause interference to radio
services operating in the 1 to 6 GHz band. The test facilities adopted were the Fully Anechoic
Room (FAR) and Reverberation Chamber so that measurements could be made according to
the latest approved CISPR modification to the Information Technology Equipment (ITE)
standard CISPR 22 and the methods of IEC 61000-4-21 for emission measurements.
With regard to the CISPR measurements a fully anechoic room previously used for the
approvals of GSM mobile phones was set up using the test method and procedure, and
calibration techniques set out in the new CISPR standards for emission measurements at
frequencies above 1 GHz. The FAR facilities can be calibrated easily to achieve the CISPR
requirements for emissions in the frequency range 1 to 6 GHz. The CISPR calibration was
performed over the period of one week using the VSWR method. The pre-calibrated
reverberation chamber is calibrated only in terms of a good response in the range 1 to 6 GHz,
and working volume field uniformity, based on BS EN 61000-4-21 requirements. Other than
insertion loss measurement requirements, there are no additional calibration requirements for
performing radiated emissions within a reverberation chamber.
The FAR measurements required a 30 dB pre-amplifier to achieve sufficient sensitivity from
equipment with the CISPR average limits. The reverberation chamber provides high level of
sensitivity because of the reflections but difficult when testing products having radio links.
With regard to the measured radiated field strengths from the products tested, in the fully
anechoic room, except for the X-Box and products under test with approved licensed radio
frequency for transmitters, all products complied with the limit lines with reasonable margins.
The X-Box radiated is Wi-Fi enabled and transmitted at about 2.6 GHz.
The measurements in the fully anechoic room were made with both peak and average detector
as required by the CISPR standard for emissions at frequencies above 1 GHz. For a
resolution bandwidth of 1 MHz the average levels at any frequency were lower than the peak
levels but the average limits are also lower. Generally the margins of compliance with the
peak limits were larger than those for the average limits by a few dBs but in some cases the
margins were virtually identical. This indicates that the selection of the peak and average
limits values is reasonable in relative terms when measured using the 1 MHz bandwidth.
The measurements were initially made with the equipment under test in a normal
configuration, i.e. the base of the equipment located on a horizontal support, this being the X
plane. Measurements were made of the detailed field strength distribution in the horizontal
plane by rotating the equipment under test through 360 degrees about a vertical axis.
Significant variations were observed indication the importance of equipment rotation; failure
to rotate could result in lower values than the maximum. However when the equipment was
placed on its side (Y plane and Z plane) and also rotated through 360 degrees there were no
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records of any increase of levels above those measured initially in the X plane indicating that
the CISPR method provides a reasonable worst case. This is fortunate because if this was not
the case there would be a requirement for vertical scans which would make the test method
more complex and costly.
The polar plot distributions showed that for some individual equipment the horizontal polar
distribution was dipole like or almost omni-directional at frequencies close to 1 GHz but there
were more high gain lobes at higher frequencies. With systems comprising more than one
item, i.e. equipment plus TV receiver, the high frequency polar distributions were more
uniform and were probably the result of random reinforcement of the radiation from a number
of sources. Again this rein forces the point that the planned CISPR method is adequate and
does not need to be any more complex than the current version.
A.12
FAR Calibration Data
The calibration of the ERA 10 x 5 x 5 metre fully anechoic room was performed over the
frequency range 1-6 GHz using the VSWR method of CISPR/A/500/CD. The calibration was
performed in two frequency ranges, 1-3 GHz and 3-6 GHz in both horizontal and vertical
polarisation.
The transmit antenna was placed at the locations shown in Figure 81 and the receive antenna
was placed at a distance of 3 metres from the transmit antenna with the main lobe of the
receive antenna directed towards the transmit antenna. The receive antenna was located at the
same height of the transmit antenna, Figure 81 also shows the seven transmit antenna
locations.
3 metres
from each
point
Transmitter locations
Receive antenna.
Main lobe directed
towards a single
calibration location
Figure 81: The seven transmitter locations of the calibration
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A.12.1
A.2 Calibration results
The calibration data is presented in Figures 82 to 89 for the four conditions. Figure 82 shows
the responses received by the receive antenna for all seven transmit locations in the frequency
range 1 - 3 GHz for horizontally polarised antennas. Figure 83 presents the data normalised
to the minimum signal level received at any location. The range of normalised data is less
than a VSWR of 3 dB and the ERA FAR is acceptable in so far that the variations are less
than 3 dB.
The only marginal compliance was for vertical polarisation at frequencies in the range
1 - 1.1 GHz where some VSWR levels exceeded 3 dB but by values less than 0.5 dB. These
errors were considered acceptable in comparison with the overall measurement uncertainty
for the measurements.
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1 - 3GHz Cal Horizontal
0
-10
-20
-30
Point 1
Point 2
Point 3
Point 4
Point 5
Point 6
Point 7
dBm
-40
-50
-60
-70
-80
-90
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Frequency GHz
Figure 82: Calibrations 1-3 GHz - Horizontal
1 - 3 GHz Calibration Horizontal
3.5
3
2.5
2
dB
VSWR
3dB limit
1.5
1
0.5
0
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Frequency GHz
Figure 83: Calibrations 1-3 GHz – Horizontal - VSWR
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3-6GHz Calibration Horizontal
0
-10
-20
Point 2
Point 3
Point 4
Point 5
Point 6
Point 7
-30
-40
-50
-60
3
3.5
4
4.5
5
5.5
6
Frequency GHz
Figure 84: Calibration 3-6 GHz – Horizontal
3 - 6GHz Calibration VSWR
3.5
3
2.5
2
dB
dBm
Point 1
1.5
1
0.5
0
3
3.5
4
4.5
5
5.5
6
Frequency GHz
Figure 85: Calibration 3-6 GHz – Horizontal - VSWR
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1 - 3 GHz Cal Vertical
0
-10
-20
Point 1
-30
dBm
Point 2
Point 3
-40
Point 4
Point 5
Point 6
-50
Point 7
-60
-70
-80
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Frequency GHz
Figure 86: Calibration 1-3 GHz - Vertical
1 - 3 GHz Calibration VSWR Vertical
4
3.5
3
2.5
2
1.5
1
0.5
0
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Figure 87: Calibration 1-3 GHz – Vertical - VSWR
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3 - 6GHz Calibration Vertical
0
-10
Point 1
Point 2
Point 3
Point 4
-30
Point 5
Point 6
Point 7
-40
-50
-60
3
3.5
4
4.5
5
5.5
6
Frequency GHz
Figure 88: Calibration 3-6 GHz - Vertical
3 - 6 GHz VSWR Vertical
3.5
3
2.5
2
dB
dBm
-20
1.5
1
0.5
0
3
3.5
4
4.5
5
5.5
6
Frequency GHz
Figure 89: Calibration 3-6 GHz – Vertical - VSWR
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A.13
Reverberation Chamber Emissions Test Methods
As part of the project to investigate interference and EMC limits above 1 GHz, the EMC
Group, QinetiQ, Farnborough were tasked with investigating the reverberation chamber
techniques for measuring radiated emissions above 1GHz. This section of the report discusses
the reverberation chamber measurement procedures, practical measurements and also
highlights the advantages/disadvantages of employing reverberation chamber methods for
performing radiated emissions.
This section discusses methods of directly comparing the data from the reverberation chamber
method to conventional methods, employing semi anechoic, full anechoic or Open Area Test
Sites (OATS). In addition this section also discusses the differences in the test methods,
assumptions made as regards the equipments directivity and includes detailed procedures for
performing radiated emissions testing within the reverberation chamber.
The reverberation chamber radiated emissions measurements are performed in basic
accordance with BS EN 61000-4-21:2002. This standard was written as an alternative to BS
EN 61000-4-3 to permit radiated immunity testing to be performed using the mode tuned
chamber methods. Included within BS EN 61000-4-21:2002, is Annex E, related to the
measurement of radiated emissions within a reverberation chamber and includes techniques
for comparing data to radiated emissions performed within conventional semi anechoic
chambers or OATS.
Currently radiated emissions measurements of commercial equipment for the European
market, from personal electronic devices (PED’s) to Information Technology Equipment
(ITE) are performed from 30 MHz to 1 GHz, to satisfy the requirements of BS EN
55022:1998 / CISPR22. Currently these test standards are being revised and measurement
requirements/limits are being extended to cover an agreed upper frequency of 6 GHz. The
use of the reverberation chamber technique for performing radiated emissions is being
considered, as an alternative method.
Note: the US FCC standards do have existing methods and limits for performing radiated
emissions above 1 GHz, depending upon the device under tests highest clock frequency.
Typically a computer system (unintentional radiator) will be tested to the 5th harmonic of the
highest clock frequency.
QinetiQ, Farnborough have been involved in the research and development of reverberation
chambers for the past 12 years. Primarily the development of reverberation chambers has
been concentrated on radiated susceptibility / immunity and the measurement of shielding
effectiveness. Alternative reverberation chamber techniques have been developed and are
accepted for both commercial and military equipment radiated susceptibility testing and as
discussed as an alternative to conventional radiated immunity techniques within BS EN
61000-4-21. Reverberation chambers are currently being employed as alternative methods
from as low as 80 MHz to typically 18 GHz.
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Over the past 8 to 10 years the EMC Group, QinetiQ, Farnborough, have been actively
involved in the development of the reverberation chamber (mode stirred and mode tuned) for
radiated emissions from as low as 80 MHz to 1 GHz and now being extended to cover the
above 1 GHz frequency region. The commercial equipment European standard BS EN
61000-4-21: 2003 was developed to provide an alternative to conventional anechoic chamber
testing for both radiated immunity from 80 MHz to 2 GHz and for radiated emissions from as
low as 80 / 100 MHz to 1 GHz. Techniques based on EN 61000-4-21 are currently being
developed and adopted to cover radiated emissions measurements above 1GHz within this
measurement programme.
A.13.1
Anechoic chamber versus reverberation chambers techniques
Many measurements have previously been performed over the frequency range 30 MHz to
1000 MHz to compare conventional anechoic chamber radiated emissions to the reverberation
chamber techniques. Investigations have been performed in the US and in the UK to provide
a comparison of radiated emissions for various electronic equipments and simulated
equipments. An initial investigation was performed by the European Civil Aircraft EMC
Research Club to compare both conventional anechoic chamber emissions to three different
reverberation chamber test facilities. Measurements were performed for comparison purposes
employing Emissions Reference Sources (ERS’s) and simulated equipment. Only limited
comparisons have been performed between various methods above 1 GHz. This phase of the
investigation addresses the mode stirred and mode tuned methods above 1 GHz, as part of the
Ofcom permitted interference and EMC limits above 1 GHz project.
Currently defence standards, commercial aircraft standards and US ITE standards include
requirements to perform radiated emissions above 1 GHz. For ITE equipment US FCC
requirements (CFR47, part 15) address radiated emissions above 1GHz. In addition specific
standards such as Bellcore GR1089-CORE (now Telcordia), have radiated emissions
requirements up to 10GHz, specifically related to Network Telecommunications Equipment
to
meet
US
telecommunications
requirements.
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Table 32 shows some of the current commercial standards including radiated emissions above
1 GHz, including measurement detectors, bandwidths and measurement distances.
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Table 32:
Current radiated emissions standards above 1GHz
Standard
Upper
Frequency
Distance
Detector
Bandwidth
Limits
BS
EN55022:1998
Present – 1GHz
10/3m
below
1GHz
Q Peak –
below 1GHz
120kHz
below 1GHz
CLASS A
Peak/Average
above 1GHz
1MHz
above 1GHz
Draft (revision)
– 6GHz
3m above
1GHz
(Industrial limit)
CLASS B
(Domestic limit)
Bellcore
GR1089 CORE
10GHz
3m
Peak/Average
1MHz
As FCC
requirement to
10GHz
CFR47, Part15,
Part 3
5th clock
harmonic
(unintentional)
3m
Peak/Average
1MHz
CLASS A and B
limits – both
peak and average
limits apply
10th clock
harmonic
(intentional)
A.13.2
Reverberation Chamber radiated emissions
The reverberation chamber technique lends itself very well to performing thorough radiated
emissions measurements, due to the isotropic nature of the test environment, resulting in an
extremely repeatable test method. This test environment removes the requirement to use
turntables and antenna height scanning, thereby simplifying the overall test set-up. In
addition this reduces equipment set-up time and the need to carefully route and manage
equipment cabling etc.
For immunity testing there has been much investigation and discussion over the use of
continuous mode stirred techniques compared to mode tuned methods. The discussions have
been related mainly to the response or cycle time of the equipment under test compared to the
rate at which the tuner or paddle is continuously rotated. The same arguments may apply to
the testing of equipment radiated emissions within the reverberation chamber. For fast
responding equipment, continuous mode stirred operation may well be adequate, however,
slow responding/slow cycle time equipment may require further investigation using mode
tuned test methods. Within this measurement programme investigations have been performed
mainly using the continuous mode stirred method, however, two devices were also measured
using the mode tuned method for comparison to both mode stirred and conventional
techniques. The issues of equipment dwell time/cycle time will and can impact the methods
used for conventional radiated emissions tests, related to turntable rotation speed and height
scanning rates. This investigation was only applied in detail to two devices and as such a full
investigation of this phenomena has not been possible.
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Prior to measuring the electrical/electronic samples selected for this programme, initial
investigations and discussions were carried out to set various test parameters, as detailed
below:
•
Paddle or tuner speed (mode stirred).
•
Paddle or tuner step size (mode tuned).
•
Measurement bandwidths.
•
Frequency range.
•
Detector functions.
•
Sweep rates.
The above parameters that would impact both the reverberation chamber and anechoic
chamber measurements were applied by both measurement parties set such that measurements
were performed in a controlled and consistent manner. It was agreed early in the programme
to employ the mode stirred chamber technique for all devices selected and perform a limited
set of mode tuned tests for comparison purposes of two devices.
The mode tuned method permits the tuner or paddle wheel to be stepped in discrete angles,
allowing an emissions sweep to be performed at each discrete angle of the paddle. In terms of
the equipment under test, by making multiple sweeps at each paddle position, this increases
the probability that the maximum emissions would be captured over multiple paddle or tuner
positions. It is considered for a majority of ITE type equipment that the response/cycle time
would be adequately fast to allow mode stirred methods to adequately capture the maximum
or ‘worst case’ emissions.
Reverberation chambers have been employed in the past for performing ‘worst case’ radiated
emissions testing and for emissions comparisons. Computer manufacturers have employed
the mode stirred technique for performing very repeatable batch to batch variation tests. In
addition the reverberation chamber has been used successfully by computer manufacturers to
perform comparative radiated emissions testing i.e. for investigating variations in CPU
samples. Although both the aspect and polarisation angle information is lost, the excellent
test repeatability (in the region of +/- 2dB), provides confidence in any modifications
performed to the device under test or when measuring various samples or investigating batch
variations.
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Below are a list of perceived advantages and disadvantages of the reverberation chamber
method compared to conventional anechoic chamber or OATS emissions measurement
techniques:
Advantages:
•
The Reverberation Chamber provides a thorough measure of radiated emissions in
terms of total radiated power.
•
For mode stirred operation for fast operating equipment – provides a fast
measurement method.
•
Measurement of maximum radiated emissions from multiple aspect angles and
multiple polarisations of the EUT.
•
Excellent test to test repeatability – good and accurate comparison technique.
•
Cost effective to set-up – chamber requires no anechoic material.
•
Set-up time is short – as cable positions and location of equipment within the
chamber are insignificant, providing the EUT and cables ≥ λ/4 away from the
chamber boundaries and paddle wheel.
•
Lower measurement noise floor, for same analyser/set-up used – this is related to both
the chambers ‘Q’ factor and the antennas characteristics within the chamber.
Disadvantages:
•
There is no aspect angle and polarisation information – related to identifying specific
equipment emission issues in terms of absolute aspects and polarisations.
•
Measure of total radiated power not directional maximum field strength from EUT –
comparisons/emissions limits require investigation
•
For EUTs with slow cycle/response time – use of mode tuned methods increases test
time for stepped paddle positions, however, this would also impact the
turntable/height scan rates within an anechoic chamber
•
Possible lack of acceptance by some measurement communities for performing
alternative reverberation chamber techniques
•
Limited lower frequency is related to chamber dimensions – generally not an issue for
above 1 GHz (Note: typical shielded chamber can be used from as low as 80 /
100 MHz)
•
Possible issues related to comparing to conventional techniques for ‘high’ directivity
devices - devices. (see Note 1)
Note 1: From previous work it is considered that a majority of commercial
electrical/electronic equipment would have low values of directivity. The se are considered to
be in the region of a maximum of approximately 2-3 dB. The information from this
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investigation will yield results related to equipment directivity based on the data from the
conventional techniques.
A.13.3
Reverberation Chamber measurement parameters
Prior to performing the EUT radiated emissions testing within the reverberation chamber,
several calibration parameters were measured to prove the chamber meets the uniformity
requirements, establish the loading factor or loss factor for the EUT installed within the
chamber and determine the insertion loss of the chamber. The required measurement
parameters are described below.
A.13.3.1 Volume Uniformity
As opposed to the Normalised Site Attenuation (NSA) measured within an anechoic chamber
or OATS, the uniformity within a defined working volume for the reverberation chamber
must be measured and proved to be within required standard deviation (SD) limits. As the
present European commercial radiated emissions techniques do not cover frequencies above
1 GHz, the chamber uniformity techniques and allowable standard deviations (SD’s) will be
adopted, as used in susceptibility/immunity methods above 1GHz. This is based on the
required volume uniformity, as detailed within DO160E, Defence Standard 59-41:2003 and
BS EN 61000-4-21:2002. The volume calibration is based on an eight point working volume
uniformity test from the lowest useable frequency (LUF) to 10 x the LUF, of the chamber.
Above this frequency the volume uniformity measurements are performed for three locations
within the working volume. Providing the volume uniformity lies within the required
standard deviation, the chamber is deemed to have met the requirements. Based on the 1 GHz
requirements of EN 61000-4-21, and the general standard deviations in immunity standards
above 1 GHz, the allowable standard deviation for a 3 point calibration within the working
volume is 3 dB. At each of the 3 locations, field measurements are performed at 3 axis
(X,Y,Z) at each position. Note: all 3 axis and the resultant of X,Y,Z axes must be within the
allowable field uniformity standard deviation. The reverberation chambers employed in this
measurement programme have been calibrated in terms of their uniformity and have been
shown to meet the Standard Deviation requirements of the commercial test standards. Figure
90 below shows a diagram of a typical measurement set-up within a reverberation chamber,
as detailed within BS EN 61000-4-21.
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Paddle wheel or tuner
Stepper
motor
EUT
Rx antenna
Calibrated
Working volume
Ante Chamber
Support
Non conductive
equipment
EUTsurface
support
λ/4 at LUF from any chamber
Penetration panel
Signal/control connections
Figure 90: Typical reverberation chamber measurement set-up
A.13.3.2 Chamber Insertion Loss
A parameter that is critical to the technique is the measured insertion loss factor of the
reverberation chamber. This is a measure of the amount of radiated power that is lost to
boundary losses, leakage and absorption within the chamber. This parameter is measured
within an empty chamber, and measured within the working volume of the chamber. This
measure provides a factor that shall be applied to the antenna base power, to ascertain the
EUT’s total radiated power, i.e. corrected for all losses within the chamber. By correcting for
this measured loss factor, the total or maximum radiated power within the measured
bandwidth can be presented for the EUT.
A typical empty chamber insertion loss measurement from 100 MHz to 18 GHz is as shown
in Figure 91. This is a typical empty chamber insertion loss from for the 7 x 5 x 3m chamber
installed at QinetiQ, Farnborough. This reverberation chamber was employed for radiated
emissions measurements performed during this measurement programme.
The range of insertion values for the frequency range 1-6 GHz are 13-26 dB.
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Mode stirred Chamber G (Small MSC) - Insertion Loss measurement - Mode stirred
5
0
-5
Gain (dB)
-10
-15
-20
-25
-30
-35
-40
1E+8
1E+9
1E+10
1E+11
Frequency (Hz)
IL1073.MA
IL1071.MA
IL1067.MA
Figure 91: Typical empty chamber insertion loss for the 7x5x3m reverberation chamber
A.13.3.3 Determining total radiated emissions power
Within an anechoic chamber or OATS the measured radiated emissions parameter is in terms
of E-field (dBμV/m) with limits based on defined EUT to antenna distances, measurement
bandwidths and receiver detectors. Conversely, within the reverberation chamber the
parameter measured is the antenna base voltage (dBμV). From the antenna base voltage
measurement and correcting for the chamber insertion loss, it is possible to determine the
amount of total radiated power (Pradiated ) radiated by the EUT into the chamber, for at least one
complete rotation of the paddle wheel or tuner, during this investigation the paddle
wheel/tuner was rotated for a minimum of 5 complete rotations per measurement band.
The total radiated power into the chamber from the EUT can be determined using either
maximum received power or using mode tuned methods it is possible to measure the average
radiated power from the EUT. This is discussed later within the results section of the report.
The equation (from EN61000-4-12:2002 Annex E) for calculating the maximum radiated
power from the EUT is as shown below in [1].
Pradiated = PMaxRec x NTx /(CLF x IL)
[Equ 1]
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Where,
Pradiated is the radiated power from the EUT (within measurement bandwidth)
CLF is the chamber loading factor
IL is the chamber insertion loss
PMaxRec is the maximum power received (within the measurement bandwidth) over the
number of tuner steps
NTx antenna efficiency factor for the Tx antenna used in calibrating the chamber and is
assumed to be 0.75 for a log periodic type antenna and 0.90 for a horn type antenna
A.13.3.4 Estimation of maximum (free space) E-field generated from EUT
As the parameter measured within the reverberation chamber is the maximum or average
received power from the EUT, determining the effective E-field based on this measurement
can only be an estimate, based on the effective radiated power (ERP). The field strength in
terms of V/m can be estimated using the following equation [2].
[2] ERadiated = √(DxPRadiated x 377/4πR2 )
[Equ 2]
Where,
ERadiated is the estimated field strength generated by the EUT in V/m
PRadiated is the radiated power from equation [1] in W
R the required distance from the EUT in metres – shall be sufficient to ensure far field
conditions exist
D is the equivalent directivity of the EUT
A.13.4
Radiated emissions set-up within reverberation chamber
Providing the reverberation chamber meets the uniformity requirements as detailed in Section
A.13.3.1, and the insertion loss factors have been measured, the EUT radiated emissions can
be performed over the required frequency range, in this case 1-6GHz. As detailed within the
set-up requirements of BS EN61000-4-21, the EUT should be set-up on a non-conductive
support, this includes mains and network cabling.
The equipment under test set-up and position should be representative of its installation, if
required a conductive ground plane can be used, providing the EUT and ground plane is
within the bounds of the calibrated working volume. The additional requirement for testing
equipment within the reverberation chamber is that the EUT shall be at least λ/4 from the
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chamber boundaries, at the lowest measured frequency. Floor standing EUTs shall be
supported 10cm above the floor by low loss/permittivity dielectric supports.
The receive antenna employed shall be set-up such that it is not directly illuminated by the
EUT. The receive antenna shall be directed either into the corner of the chamber or at the
paddle wheel / tuner. Figure 92 shows a typical set-up within a reverberation chamber of a
computer server and associated network cables. In this example the computer server system
is set-up on a non conductive/low loss support. At 1GHz and above, the EUT shall be at least
7.5cm from all boundaries and the paddle wheel/tuner.
In this example, the network cabling is routed to a patch panel and decoupled outside the
chamber. The non conductive supports used allow the system to be set-up at a height of
greater than 0.8m above the chamber boundary.
Figure 92: Computer server set-up within reverberation chamber for radiated emissions
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