WO2022033770A1

WO2022033770A1 - A high-frequency mode stirrer for reverberation chambers

Info

Publication number: WO2022033770A1
Application number: PCT/EP2021/068442
Authority: WO
Inventors: Sara Catteau; Kent Thomra; Robert Rehammar
Original assignee: Bluetest Ab
Priority date: 2020-08-14
Filing date: 2021-07-05
Publication date: 2022-02-17
Also published as: SE543894C2; SE2030254A1

Abstract

A method for calibrating a reverberation chamber (210). The method comprises arranging a high-frequency mode stirrer (100) on a spatially configurable platform (220) in the reverberation chamber (210). The high- frequency mode stirrer (100) comprises: an electrically conductive member (101) arranged to reflect electromagnetic radiation; one or more apertures (102) arranged extending through the member (101), wherein the apertures (102) are arranged to diffract and to diffuse the electromagnetic radiation; and one or more electrically conductive plates (103) arranged on the member (101), wherein the plates (103) are arranged to scatter the electromagnetic radiation. The method further comprises arranging a reference antenna (230) in the reverberation chamber (210), arranging a measurement antenna (240) in the reverberation chamber (210), and measuring losses between the reference antenna (230) and the measurement antenna (240). The method also comprises altering a spatial configuration of the platform (220) when measuring losses and calibrating the reverberation chamber (210) based on the measured losses.

Description

TITLE

A HIGH-FREQUENCY MODE STIRRER FOR REVERBERATION CHAMBERS

TECHNICAL FIELD

The present disclosure relates to test equipment for antenna systems and wireless devices in general. There are also disclosed systems and methods for measuring the performance of antenna systems and for testing wireless devices.

BACKGROUND

Reverberation chambers (RCs), also known as electromagnetic reverberation chambers (ERCs) or mode-stirred chambers (MSCs) have become effective tools for measuring over-the-air (OTA) performance of various wireless devices. RCs are mainly used for evaluating antenna system performance in radio frequency reflective environments, i.e. , when the device under test (DUT) is subjected to multipath propagation.

In an RC, the signal is injected or picked up by the DUT in a closed chamber, or cavity, comprising inwardly radio frequency reflective walls. An injected signal arrives at the DUT after multiple reflections through many different trajectories. This creates a radio frequency signal fading state at the receiver. By moving mode stirring plates and/or a turntable upon which the DUT is arranged, the geometry of the chamber changes, which in turn changes the fading state that the DUT experiences. Thus, a rich isotropic multipath (RIMP) environment is efficiently generated where a large number of fading states with different incident wave compositions can be tested in an efficient manner.

With the expansion of the available frequency range in fifth generation (5G) cellular networks to the mm-wave region (e.g. >20 GHz), there is a need to characterize devices at these frequencies as well. Accurate characterization of a DUT in an RC requires accurate calibration of the RC, which includes determining and compensating for the loss between the DUT and a measurement antenna in the RC. However, measuring the loss between the DUT and the measurement antenna becomes increasingly challenging as the frequency increases as, i.a., the loss in the cables used to connect to the antennas increases with frequency, which decreases the signal to noise ratio.

There is a need for improved RC calibration methods and mode stirrers for mm -wave characterization.

SUMMARY

It is an object of the present disclosure to provide for improved RC calibration methods and mode stirrers for mm-wave characterization.

This object is obtained by a method for calibrating a reverberation chamber. The method comprises arranging a high-frequency mode stirrer on a spatially configurable platform in the reverberation chamber. The high-frequency mode stirrer comprises: an electrically conductive member arranged to reflect electromagnetic radiation; one or more apertures arranged extending through the member, wherein the apertures are arranged to diffract and to diffuse the electromagnetic radiation; and one or more electrically conductive plates arranged on the member, wherein the plates are arranged to scatter the electromagnetic radiation. The method further comprises arranging a reference antenna in the reverberation chamber, arranging a measurement antenna in the reverberation chamber, and measuring losses between the reference antenna and the measurement antenna. The method also comprises altering a spatial configuration of the platform when measuring losses and calibrating the reverberation chamber based on the measured losses.

The high-frequency mode stirrer facilitates the distribution of high-frequency power, i.e. reducing inhomogeneity, within the RC when the reference antenna is placed off the spatially configurable platform. The stirrer is particularly effective at the range 20 GHz and above, especially compared to a prior art mode stirrer - referred to as a low-frequency mode stirrer from here on - which in general is a construction with metallic or otherwise electromagnetic reflective elements that can be moved to different orientations and locations in the RC to achieve different boundary conditions in the RC.

According to aspects, the reference antenna is arranged such that a main lobe of the reference antenna is aimed with a line of sight intersecting the high- frequency mode stirrer. This can also be expressed as having the main lobe directed towards the high-frequency mode stirrer. This way, the beam of the reference antenna is always illuminating the high-frequency mode stirrer, which guarantees good stirring at the first reflection of the beam.

According to aspects, the plates are adjustably and releasably attached to the member. This provides reconfigurability for different RC scenarios.

According to aspects, the method further comprises arranging a blocking screen to block the line of sight between the measurement antenna and the reference antenna. This reduces inhomogeneity during the calibration.

According to aspects, the losses between the reference antenna and the measurement antenna are measured with a network analyzer.

According to aspects, the method further comprises arranging one or more low-frequency mode stirrers in the reverberation chamber, and altering a spatial configuration of the one or more low-frequency mode stirrers when measuring losses. The low-frequency mode stirrers provide additional stirring and the RC can easily be reconfigured to a conventional RC by lifting out the high-frequency mode stirrer.

According to aspects, at least one of the plates is arranged perpendicular to a surface of the member at a mounting position of the at least one plate. This way, the plate can easily be mounted to the member 101 with, e.g., a nut and a bolt. Other fastening means are also possible.

According to aspects, the electrically conductive member of the high-frequency mode stirrer is formed in an arcuate shape. This provides good stirring when placed on a rotating platform and also effectively uses the limited space on the platform. According to aspects, the electrically conductive member of the high-frequency mode stirrer comprises a plurality of surfaces connected to each other to form a Z-fold configuration, wherein a fold angle is formed between two adjacent surfaces in the plurality of surfaces. The Z-fold configuration facilitates fitting a larger surface area of the electrically conductive member into a small volume in the RC. In addition, the adjustable fold angles facilitate reconfiguring the shape of the high-frequency mode stirrers. This can be helpful for fitting the high-frequency mode stirrers onto differently sized spatially configurable platforms.

According to aspects, the method further comprises adjusting any of size, shape, placement, and orientation of the one or more electrically conductive plates in dependence of the shape of the reverberation chamber and its contents and of a frequency range of the electromagnetic radiation. According to further aspects, the method comprises adjusting at least one fold angle in dependence of the shape of the reverberation chamber and its contents and of the frequency range of the electromagnetic radiation. According to other aspects, the method comprises adjusting any of size and shape of the one or more apertures in dependence of the shape of the reverberation chamber and its contents and of the frequency range of the electromagnetic radiation. Adjusting the plates, fold angles, and apertures, as well as the number of plates, apertures, and the surfaces in the Z-fold, may be done to optimize stirring according to a specific scenario.

There is also disclosed herein a method for measuring performance of a device under test (DUT) in a reverberation chamber over a frequency band. The method comprises calibrating the reverberation chamber over the frequency band using the calibration method discussed above. The reverberation chamber comprises the high-frequency mode stirrer. The method further comprises measuring performance of the DUT in the reverberation chamber over the frequency band.

According to aspects, the high-frequency mode stirrer is arranged on the spatially configurable platform in the reverberation chamber during the calibration step and during the measurement step. According to other aspects, the high-frequency mode stirrer is arranged on the spatially configurable platform in the reverberation chamber during the calibration step and is arranged outside the reverberation chamber during the measurement step.

There is also disclosed herein a calibration kit for calibrating a reverberation chamber. The kit comprises a high-frequency mode stirrer arranged on a spatially configurable platform in the reverberation chamber. The high- frequency mode stirrer comprises: an electrically conductive member arranged to reflect electromagnetic radiation; one or more apertures arranged extending through the member, wherein the apertures are arranged to diffract and to diffuse the electromagnetic radiation; and one or more electrically conductive plates arranged on the member, wherein the plates are arranged to scatter the electromagnetic radiation. The kit further comprises a control unit comprising processing circuitry and an interface. The control unit is configured to: measure losses between a reference antenna in the reverberation chamber and a measurement antenna in the reverberation chamber; altering a spatial configuration of the platform when measuring losses; and calibrate the reverberation chamber based on the measured losses.

There is also disclosed herein a computer program product comprising a computer program configured to execute a method according to the discussions above, and a computer readable storage medium on which the computer program is stored.

There is also disclosed herein a high-frequency mode stirrer for modifying electromagnetic boundary conditions in a reverberation chamber. The high- frequency mode stirrer comprises: an electrically conductive member arranged to reflect electromagnetic radiation; one or more apertures arranged extending through the member, wherein the apertures are arranged to diffract and to diffuse the electromagnetic radiation; and one or more electrically conductive plates arranged on the member, wherein the plates arranged to scatter the electromagnetic radiation, and wherein the plates are adjustably and releasably attached to the member. According to aspects, the high-frequency mode stirrer comprises a plurality of legs arranged to raise the high-frequency mode stirrer relative to a spatially configurable platform when the high-frequency mode stirrer is arranged on the platform. This way, the high-frequency mode stirrer can be placed on top of a platform without interfering with, e.g., cables and cable connectors. According to other aspects, the high-frequency mode stirrer comprises at least one handle for manually lifting the high-frequency mode stirrer. This way, the high-frequency mode stirrer can be lifted in and out of an RC with minimal effort.

The high-frequency mode stirrer may comprise fluted knobs for attaching it to a spatially configurable platform. This way, the high-frequency mode stirrer can be securely attached and detached quickly. Furthermore, the high-frequency mode stirrer may comprise hinges or the like to be foldable to some extent to facilitate moving the stirrer. The stirrer may also comprise a plurality of parts that are disassemblable.

The methods disclosed herein are associated with the same advantages as discussed above in connection to the different measurement devices. There are furthermore disclosed herein control units adapted to control some of the operations described herein.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described in more detail with reference to the appended drawings, where

Figure 1 illustrates an example high-frequency mode stirrer;

Figure 2 illustrates a reverberation chamber comprising an example high- frequency mode stirrer;

Figures 3-6 illustrate top views of example high-frequency mode stirrers;

Figure 7 is a plot showing the Rician K-value versus frequency;

Figure 8 is a plot showing chamber losses versus frequency;

Figure 9 is a flow chart illustrating methods;

Figure 10 shows an example control unit;

Figure 11 illustrates a computer program product; and

Figures 12 and 13 are flow charts illustrating methods.

DETAILED DESCRIPTION

Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

With the expansion of the available frequency range in 5G to the mm-wave region (>20 GHz), there is a need to characterize devices at these frequencies as well. The increase in the upper limit of the usable frequencies has also pushed the limit for spurious emission measurements higher, as this limit is generally twice the carrier frequency. Such measurements can be done in a reverberation chamber (RC).

An RC is essentially a cavity resonator with a high Q-factor. The spatial distribution of the electromagnetic field strengths is strongly inhomogeneous, i.e. standing waves are present. This can be reduced with one or more mode stirrers (also known as mode tuners). A mode stirrer is a construction with metallic or otherwise electromagnetic reflective elements that can be moved to different orientations and locations in the RC to achieve different boundary conditions in the RC. The lowest usable frequency (LUF) of an RC depends on the dimensions of the chamber and stirrer design. Larger chambers present a lower LUF than smaller chambers. Consequently, the volume V to some extent determines the Q-factor of the RC and has an impact on the LUF. The size and shape of a mode stirrers further affects its capability of stirring at high frequencies, i.e. to reduce inhomogeneity in the RC. To increase stirring at all frequencies, the device under test (DUT) is often placed on a spatially configurable platform, e.g. a rotating platform, such as a turntable, which is rotating during measurements.

The RC is a useful tool in identifying spurious emissions as it detects power emitted from DUTs over a wide frequency range and in any direction. To determine the spurious power level at the DUT, the loss between the DUT and the measurement antenna must be determined and compensated for. Measurements of the loss between the DUT and the measurement antenna becomes increasingly challenging as the frequency increases as the loss in the cables used to connect to the antennas increases with frequency, which decreases the signal to noise ratio. To deal with this, the shortest cables possible should be used. To facilitate the use of short cables, a reference antenna for calibration, that would often be placed where the DUT will be located, i.e. on a spatially configurable platform, can be placed off the platform and be pointed towards a mode stirring device located on the platform. Since many DllTs are large, shrinking the RC is not a viable option to increase the frequency capabilities. Therefore, the present disclosure relates to the specific design features of a high-frequency mode stirring device that facilitate the distribution of high-frequency power within the RC when the reference antenna is placed off the spatially configurable platform.

Figure 1 shows an example embodiment of the disclosed high-frequency mode stirrer. The mode stirrer consists of a folded electrically conductive member comprising a sheet mounted around the rim of a rotating platform. Perpendicular plates are added to the flat surfaces to increase the directions of energy scatter. Furthermore, apertures in the folded conductive sheet are added to increase diffraction and diffusion.

In other words, there is herein disclosed a high-frequency mode stirrer 100 for modifying electromagnetic boundary conditions in a reverberation chamber. The high-frequency mode stirrer 100 comprises an electrically conductive member 101 arranged to reflect electromagnetic radiation. In the example of Figure 1 , the member can comprise a sheet. Other shapes are, however, possible. The member comprises a good electrical conductor, e.g. copper. One or more apertures 102 are arranged extending through the member 101 . The apertures 102 are arranged to diffract and to diffuse the electromagnetic radiation. One or more electrically conductive plates 103 are arranged on the member 101. The plates 103 are arranged to scatter the electromagnetic radiation and the plates 103 are adjustably and releasably attached to the member 101 .

The disclosed high-frequency mode stirrer 100 is suitable for reducing inhomogeneity in the RC at high frequencies. Although it can stir at any frequency range, it is particularly effective at 20 GHz and above, especially compared to a prior art mode stirrer - referred to as a low-frequency mode stirrer from here on - which in general is a construction with metallic or otherwise electromagnetic reflective elements that can be moved to different orientations and locations in the RC to achieve different boundary conditions in the RC. Figures 3 - 6 show top views of high-frequency mode stirrers 100a, 100b, 100c, and 100d, which are examples of a high-frequency mode stirrer 100. As is shown in the examples of Figures 1 , 2, 5, and 6, the electrically conductive member 101 of the high-frequency mode stirrer 100 may comprise a plurality of surfaces connected to each other to form a Z-fold configuration. In this configuration, a fold angle A is formed between two adjacent surfaces in the plurality of surfaces. This fold angle may be adjustable. A Z-fold resembles the letter Z when comprising three surfaces. However, the high-frequency mode stirrer 100 may comprise any number of surfaces. Furthermore, all surfaces in the plurality do not necessarily have the same dimensions. A mode stirrer 100 comprising more than two surfaces comprises a plurality of adjustable fold angles, which may be equal or different. The Z-fold mode stirrers may be placed on a line, as is shown in Figure 5, or around the rim of a circle, as is shown in Figure 6. Other arrangements are also possible. The Z-fold configuration facilitates fitting a larger surface area of the electrically conductive member 101 into a small volume in the RC. In addition, the adjustable fold angles facilitate reconfiguring the shape of the high-frequency mode stirrers. This can be helpful for fitting the high-frequency mode stirrers onto differently sized spatially configurable platforms.

The electrically conductive member 101 of the high-frequency mode stirrer 100 may comprise an arcuate shape, as is shown in the examples of Figures 3 and 4. The arcuate shape is part of a cylinder in Figure 3 and the arcuate shape is a full cylinder in Figure 4. The cylinder may comprise end plates 107 and 108 placed at respective ends of the cylinder, thereby forming a drum-shaped member 101 . Figures 1 and 2 show other examples of end plates 107 and 108.

The one or more electrically conductive plates 103 arranged on the member 101 in the examples of Figures 1 and 2 comprise trapezoidal sheets with a mounting means for attaching them to the electrically conductive member 101 . Other shapes are also possible. The size, shape, placement, and orientation of the plates 103, as well as the number of plates, affect the effectiveness of the stirring, which can be different at different frequencies. These parameters further affect polarization balance in the RC, i.e. a ratio of orthogonal polarizations. The size, shape, placement, and orientation of the apertures 102, as well as the number of apertures, also affect the stirring and the polarization balance.

In a preferred embodiment, at least one of the plates 103 is arranged perpendicular to the surface of the member 101 at a mounting position of the at least one plate 103. This way, the plate can easily be mounted to the member 101 with, e.g., a nut and a bolt. Other fastening means are also possible.

The high-frequency mode stirrer 100 may comprise a plurality of legs 104 arranged to raise the high-frequency mode stirrer 100 relative to a spatially configurable platform 210 when the high-frequency mode stirrer 100 is arranged on the platform 210. This way, the high-frequency mode stirrer 100 can be placed on top of a platform without interfering with, e.g., cables and cable connectors.

The high-frequency mode stirrer 100 may comprise at least one handle 105 for manually lifting the high-frequency mode stirrer 100. This way, the high- frequency mode stirrer 100 can be lifted in and out of an RC with minimal effort.

The high-frequency mode stirrer 100 may comprise fluted knobs 106 for attaching it to a spatially configurable platform 210. This way, the high- frequency mode stirrer 100 can be securely attached and detached quickly. Furthermore, the high-frequency mode stirrer 100 may comprise hinges or the like to be foldable to some extent to facilitate moving the stirrer. The stirrer may also comprise a plurality of parts that are disassemblable.

It is an advantage that the high-frequency mode stirrer 100 is easily moved - an advantage made even greater with any of the optional legs, handles, and fluted knobs. This provides a flexibility when using the RC. In an example scenario, a conventional RC is used for DUT characterization (e.g. measuring spurious emission) without the disclosed high-frequency mode stirrer 100 for a lower frequency range (e.g. 100 MHz - 20 GHz). Thereafter, the high- frequency mode stirrer 100 is placed on the spatially configurable platform 210 and the DUT is characterized for an upper frequency range (e.g. 20 - 70 GHz). This way, the DUT can be characterized for a large range of frequencies with minimal reconfiguration of the RC, and therefore minimal effort and time. The high-frequency mode stirrer 100 may be used for both measurements of the DUT and for calibration of the RC, or it can be used only for calibration. In the latter case, the mode stirrer 100 is lifted into the RC before calibration at the upper frequency range and is removed afterwards.

It is naturally possible to take advantage of the benefits of the disclosed high- frequency mode stirrer 100 when it is placed on other locations than on the spatially configurable platform 210 in the RC or when it is placed in other environments.

There is also disclosed herein a reverberation chamber 210 comprising the high-frequency mode stirrer 100, as shown in the example of Figure 2. The reverberation chamber 210 may comprise any of: a spatially configurable platform 210, a reference antenna 230, a measurement antenna 240, a blocking screen 250, and one or more low-frequency mode stirrers 260. The reverberation chamber may also comprise a control unit 1060 for RC calibration, which is discussed in more detail below.

As mentioned, calibration is crucial for DUT characterization in an RC; loss between the DUT and the measurement antenna must be determined and compensated for. Therefore, there is also herein disclosed a method for calibrating a reverberation chamber 210.

The reference antenna is used for the calibration. Instead of being placed where the DUT will be located, i.e. on the spatially configurable platform, it is placed off the platform and is pointed towards the high-frequency mode stirring device located on the platform. This facilitates the use of short cables, which improves the signal to noise ratio. The moving platform and high-frequency mode stirrer distribute the energy transmitted by the reference antenna throughout the chamber, creating an isotropic environment even when the reference antenna is highly directive, which is an advantage. The chamber loss is measured between the directive reference antenna and a measurement antenna, which is optionally located behind the blocking screen, while the platform moves. The high-frequency mode stirrer 100 could be used to allow measurement of highly directive antennas or systems in an RC. These types of systems are becoming more common at higher frequencies, and the prior art RC designs may not stir the high frequency energy sufficiently without this additional type of stirrer.

In other words, there is disclosed herein a method for calibrating a reverberation chamber 210, which is illustrated in Figure 9. The method comprises arranging S1 a high-frequency mode stirrer 100 on a spatially configurable platform 220 in the reverberation chamber 210. The high- frequency mode stirrer 100 comprises: an electrically conductive member 101 arranged to reflect electromagnetic radiation; one or more apertures 102 arranged extending through the member 101 , wherein the apertures 102 are arranged to diffract and to diffuse the electromagnetic radiation; and one or more electrically conductive plates 103 arranged on the member 101 , wherein the plates 103 are arranged to scatter the electromagnetic radiation. The method further comprises arranging S2 a reference antenna 230 in the reverberation chamber 210. The method also comprises arranging S3 a measurement antenna 240 in the reverberation chamber 210 and measuring S4 losses between the reference antenna 230 and the measurement antenna 240. The method further comprises altering S5 a spatial configuration of the platform 220 when measuring S4 losses and calibrating S6 the reverberation chamber 210 based on the measured losses. Altering S5 a spatial configuration can mean to rotate the platform if the platform is a turntable. The altering occurs at the same time as when the measurements occur. In an example, the measurements comprise gathering 1000 samples over 10 seconds, wherein the platform is a turntable that is rotating during those 10 seconds.

After the calibration method has been performed. The calibrated RC can be used to characterize the DUT in various ways. For example, spurious emissions of the DUT can be measured. In that case, the DUT can be operated as “stand alone”, i.e. it is not necessary to connect a measurement instrument to it, and a spectrum analyzer is connected to the measurement antenna. Another example is to measure in-band radiated emissions.

According to aspects, the reference antenna 230 is arranged such that a main lobe of the reference antenna 230 is aimed with a line of sight intersecting the high-frequency mode stirrer 100. Since the main lobe has an angular cross section (e.g. specified by the half power beam width), “line of sight” is not necessarily a straight line from the peak power of the lobe to the center of the high-frequency mode stirrer. Furthermore, it is not necessary that the hole mode-stirrer is illuminated. As such, roll, pitch, and jaw of the reference antenna, as well as its position in the RC, may be adjusted in dependence of the shape of the reverberation chamber 210 and its contents and of the frequency range of the electromagnetic radiation. The disclosed high- frequency mode stirrer 100 together with the reference antenna 230 ensure adequate stirring. The design of the disclosed mode stirrer provides sufficient scattering, diffusing, and diffracting of the electromagnetic radiation. Since the beam of the reference antenna is always illuminating at least part of the high- frequency mode stirrer 100 (the main lobe of the reference antenna 230 is aimed with a line of sight intersecting the high-frequency mode stirrer 100), good stirring is guaranteed at the first reflection of the beam, especially since the high-frequency mode stirrer 100 is arranged on the spatially configurable platform 220, which is moving during the measurements. This mechanic can be compared to a beam that sometimes hits a wall of the chamber, which is static and provides poor stirring, which is a scenario that sometimes happen in a conventional RC with conventional low-frequency mode stirrers. Furthermore, stirring should preferably be achieved as early as possible (e.g. at the first reflection) since every reflection in the RC is lossy, which is a problem that increases with frequency. Therefore, the disclosed high- frequency mode stirrer 100 and the disclosed calibration method performs much better compared to prior art solutions.

Measuring S4 losses between the reference antenna 230 and the measurement antenna 240 can be to measure scattering parameters between the reference antenna 230 and the measurement antenna 240. In that case, the losses between the reference antenna 230 and the measurement antenna 240 may be is measured with a network analyzer. In practice, chamber losses are determined by average measurements for many different positions of the mode stirrers (for any frequency), and many positions of the spatially configurable platform. RC calibration methods are further described in Furht, Borko, and Syed A. Ahson, eds. Long Term Evolution: 3GPP LTE radio and cellular technology. Crc Press, 2016. Measuring losses can also be to take the ratio of measured power delivered to the measurement antenna over power available to the reference antenna when the reference antenna is transmitting and the measurement antenna is receiving. This may also be done vice versa since the equipment is reciprocal.

The method may further comprise arranging S7 a blocking screen 250 to block the line of sight between the measurement antenna 240 and the reference antenna 230. This reduces inhomogeneity during the calibration. The blocking screen 250 may also be arranged such that it blocks the line of sight between the measurement antenna 240 and the DUT during characterization of the DUT.

The method may further comprise arranging S8 one or more low-frequency mode stirrers 260 in the reverberation chamber 210 and altering S9 a spatial configuration of the one or more low-frequency mode stirrers 260 when measuring S4 losses. Altering a spatial configuration can, e.g., be to move a mode stirrer back and forth on a line. With this feature, the RC can easily be re-arranged to a conventional RC for lower frequencies by lifting out the high- frequency mode stirrer 100 from the RC.

According to the discussion above, the electrically conductive member 101 of the high-frequency mode stirrer 100 may be formed in an arcuate shape and/or the electrically conductive member 101 of the high-frequency mode stirrer 100 may comprise a plurality of surfaces connected to each other to form a Z-fold configuration, wherein a fold angle A is formed between two adjacent surfaces in the plurality of surfaces. According to aspects, the plates 103 are adjustably and releasably attached to the member 101. Therefore, the method may further comprise adjusting S01 any of size, shape, placement, and orientation of the one or more electrically conductive plates 103 in dependence of the shape of the reverberation chamber 210 and its contents and of a frequency range of the electromagnetic radiation. The contents include items inside the RC and their shape and electromagnet properties. The items can be the spatially configurable platform 210, the reference antenna 230, the measurement antenna 240, the blocking screen 250, one or more low-frequency mode stirrers 260, the DUT, and the control unit 1060. As previously mentioned, at least one of the plates 103 may be arranged perpendicular to a surface of the member 101 at a mounting position of the at least one plate 103. The method may also comprise adjusting S02 at least one fold angle A in dependence of the shape of the reverberation chamber 210 and its contents and of the frequency range of the electromagnetic radiation. The method may further comprise adjusting S03 any of size and shape of the one or more apertures 102 in dependence of the shape of the reverberation chamber 210 and its contents and of the frequency range of the electromagnetic radiation. The apertures can be adjusted by, e.g., covering parts of the apertures with a metal cover.

Adjusting the plates, fold angles, and apertures, as well as the number of plates, apertures, and the surfaces in the Z-fold, may be done to optimize stirring according to a specific scenario. A scenario can e.g. be a frequency range of 20 - 70 GHz in a 2 x 2 x 2 m^A3 RC comprising a turntable with a 0.5- m dimeter and 0.3-m height. Example dimensions of the apertures and plates are cross sections with dimeters in the range of 0.1 to 20 cm. An example dimension of a flat high-frequency mode stirrer is 0.5 x 0.5 m^A2. The dimensions of the mode stirrer may naturally also be adjusted for the different scenarios mentioned above. As mentioned, adjustments also affect polarization balance in the RC, i.e. a ratio of orthogonal polarizations.

There is also disclosed herein a method for measuring performance of a device under test (DUT) in a reverberation chamber 210 over a frequency band, as is illustrated in Figure 13. The method comprises calibrating SA1 the reverberation chamber 210 over the frequency band according to the calibration method discussed above, wherein the RC comprises the high- frequency mode stirrer 100 discussed above. The method further comprises measuring SA2 performance of the DUT in the reverberation chamber 210 over the frequency band. Measuring performance can be, e.g., spurious emission measurements or in-band radiated emissions measurements.

According to aspects, the high-frequency mode stirrer 100 is arranged on the spatially configurable platform 220 in the reverberation chamber 210 during the calibration SA1 step and during the measurement SA2 step. According to other aspects, the high-frequency mode stirrer 100 is arranged on the spatially configurable platform 220 in the reverberation chamber 210 during the calibration SA1 step and is arranged outside the reverberation chamber 210 during the measurement SA2 step.

One way of quantifying how well stirred the energy is in an RC is, is with the Rician K-value, which is a measure of how much energy that reaches the measurement antenna directly from the reference antenna. This energy is not stirred, so the smaller the Rician K-value is, the better. Figure 7 shows the measured Rician K-value with and without an example of the disclosed high- frequency mode stirrer in an RC. It should be noted that conventional low- frequency mode stirrers are present in both cases. The average K-value for the frequency range 24 to 67 GHz is reported for both cases in the table below. The disclosed high-frequency mode stirrer improves the K-value by almost 10 dB.

It has also been confirmed that the chamber loss measured with the mode stirrer is the same as the chamber loss measured by a reference antenna located on the rotating platform from 6 to 30 GHz. Figure 8 shows the chamber loss measured with these two methods. The peak-to-peak difference is generally below 1 dB between the three measurements.

Figure 10 schematically illustrates, in terms of a number of functional units, the components of the control unit 1060 according to an embodiment of the discussions herein. Processing circuitry 1010 is provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, etc., capable of executing software instructions stored in a computer program product, e.g. in the form of a storage medium 1030. The processing circuitry 1010 may further be provided as at least one application specific integrated circuit ASIC, or field programmable gate array FPGA.

Particularly, the processing circuitry 1010 is configured to cause the control unit 1060 to perform a set of operations, or steps, such as the methods discussed in connection to Figures 9 and 12. For example, the storage medium 1030 may store the set of operations, and the processing circuitry 1010 may be configured to retrieve the set of operations from the storage medium 1030 to cause the control unit 1060 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 1010 is thereby arranged to execute methods as herein disclosed.

The storage medium 1030 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

The control unit 1060 may further comprise an interface 1020 for communications with at least one external device, such as the reference antenna 230, the measurement antenna 240, the spatially configurable platform 220, the low-frequency mode stirrers 260, at least one DUT, and measurement equipment. As such the interface 1020 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of ports for wireline or wireless communication.

The processing circuitry 1010 controls the general operation of the control unit 1060 e.g. by sending data and control signals to the interface 1020 and the storage medium 1030, by receiving data and reports from the interface 1020, and by retrieving data and instructions from the storage medium 1030. Other components, as well as the related functionality, of the control node are omitted in order not to obscure the concepts presented herein.

A central function of the control unit 1060 is to transmit test signals via the interface 1020 to, e.g., the reference antenna 230, the measurement antenna and/or the DUT. A test signal may, e.g., comprise control signaling and data signals. The test signal may be a baseband signal, or a radio frequency signal. The control unit may also be configured to control operation of the spatially configurable platform 220 and the low-frequency mode stirrers 260, according to a pre-determined pattern of displacement, or adaptively in response to some feedback signal.

The different control programs that the control unit executes can be stored in the storage medium 1030.

In summary, there is disclosed herein a calibration kit for calibrating a reverberation chamber 210. The kit comprises a high-frequency mode stirrer 100 arranged on a spatially configurable platform 220 in the reverberation chamber 210. The high-frequency mode stirrer 100 comprises: an electrically conductive member 101 arranged to reflect electromagnetic radiation; one or more apertures 102 arranged extending through the member 101 , wherein the apertures 102 are arranged to diffract and to diffuse the electromagnetic radiation; and one or more electrically conductive plates 103 arranged on the member 101 , wherein the plates 103 are arranged to scatter the electromagnetic radiation. The calibration kit further comprises a control unit 1060 comprising processing circuitry 1010 and an interface 1020. The control unit 1060 is configured to: measure SX1 losses between a reference antenna 230 in the reverberation chamber 210 and a measurement antenna 240 in the reverberation chamber 210; altering SX2 a spatial configuration of the platform 220 when measuring SX1 losses; and calibrate SX3 the reverberation chamber 210 based on the measured losses. The steps executed by the control unit are demonstrated in Figure 12.

Figure 11 schematically illustrates a computer program product 1100, comprising a set of operations 1110 executable by the control unit 1060. The set of operations 1110 may be loaded into the storage medium 1030 in the control unit 1060. The set of operations may correspond to the methods discussed above in connection to Figures 9 and 12.

In the example of Figure 11 , the computer program product 1100 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product could also be embodied as a memory, such as a random-access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program is here schematically shown as a track on the depicted optical disk, the computer program can be stored in any way which is suitable for the computer program product.

Claims

1. A method for calibrating a reverberation chamber (210), the method comprising: arranging (S1 ) a high-frequency mode stirrer (100) on a spatially configurable platform (220) in the reverberation chamber (210), the high-frequency mode stirrer (100) comprising an electrically conductive member (101 ) arranged to reflect electromagnetic radiation, one or more apertures (102) arranged extending through the electrically conductive member (101 ), wherein the apertures (102) are arranged to diffract and to diffuse the electromagnetic radiation, and one or more electrically conductive plates (103) arranged on the electrically conductive member (101 ), wherein the electrically conductive plates (103) are arranged to scatter the electromagnetic radiation; arranging (S2) a reference antenna (230) in the reverberation chamber (210); arranging (S3) a measurement antenna (240) in the reverberation chamber (210); measuring (S4) losses between the reference antenna (230) and the measurement antenna (240); altering (S5) a spatial configuration of the spatially configurable platform (220) when measuring (S4) losses; and calibrating (S6) the reverberation chamber (210) based on the measured losses.

2. The method according to claim 1 , wherein the reference antenna (230) is arranged such that a main lobe of the reference antenna (230) is aimed with a line of sight intersecting the high-frequency mode stirrer (100).

3. The method according to any previous claim, wherein the electrically conductive plates (103) are adjustably and releasably attached to the electrically conductive member (101 ).

4. The method according to any previous claim, further comprising arranging (S7) a blocking screen (250) to block the line of sight between the measurement antenna (240) and the reference antenna (230).

5. The method according to any previous claim, wherein the losses between the reference antenna (230) and the measurement antenna (240) are measured with a network analyzer.

6. The method according to any previous claim, further comprising arranging (S8) one or more low-frequency mode stirrers (260) in the reverberation chamber (210), and altering (S9) a spatial configuration of the one or more low-frequency mode stirrers (260) when measuring (S4) losses.

7. The method according to any previous claim, wherein at least one of the electrically conductive plates (103) is arranged perpendicular to a surface of the electrically conductive member (101 ) at a mounting position of the at least one electrically conductive plate (103).

8. The method according to any previous claim, wherein the electrically conductive member (101 ) of the high-frequency mode stirrer (100) is formed in an arcuate shape.

9. The method according to any previous claim, wherein the electrically conductive member (101 ) of the high-frequency mode stirrer (100) comprises a plurality of surfaces connected to each other to form a Z-fold configuration, wherein a fold angle (A) is formed between two adjacent surfaces in the plurality of surfaces.

10. The method according to any previous claim, further comprising adjusting (S01 ) any of size, shape, placement, and orientation of the one or more electrically conductive plates (103) in dependence of the shape of the reverberation chamber (210) and its contents and of a frequency range of the electromagnetic radiation.

11 . The method according to claim 9, further comprising adjusting (S02) at least one fold angle (A) in dependence of the shape of the reverberation chamber (210) and its contents and of the frequency range of the electromagnetic radiation.

12. The method according to any previous claim, further comprising adjusting (S03) any of size and shape of the one or more apertures (102) in dependence of the shape of the reverberation chamber (210) and its contents and of the frequency range of the electromagnetic radiation.

13. A method for measuring performance of a device under test (DUT) in a reverberation chamber (210) over a frequency band, the method comprising calibrating (SA1 ) the reverberation chamber (210) comprising the high- frequency mode stirrer (100) over the frequency band using the method according to any of claims 1-12, and measuring (SA2) performance of the DUT in the reverberation chamber (210) over the frequency band.

14. The method according to claim 13, wherein the high-frequency mode stirrer (100) is arranged on the spatially configurable platform (220) in the reverberation chamber (210) during the calibration (SA1 ) step and during the measurement (SA2) step.

15. The method according to claim 13, wherein the high-frequency mode stirrer (100) is arranged on the spatially configurable platform (220) in the reverberation chamber (210) during the calibration (SA1 ) step and is arranged outside the reverberation chamber (210) during the measurement (SA2) step.

16. A calibration kit for calibrating a reverberation chamber (210) comprising: a high-frequency mode stirrer (100) arranged on a spatially configurable platform (220) in the reverberation chamber (210), the high-frequency mode stirrer (100) comprising an electrically conductive member (101 ) arranged to reflect electromagnetic radiation, one or more apertures (102) arranged extending through the electrically conductive member (101 ), wherein the apertures (102) are arranged to diffract and to diffuse the electromagnetic radiation, and one or more electrically conductive plates (103) arranged on the electrically conductive member (101 ), wherein the electrically conductive plates (103) are arranged to scatter the electromagnetic radiation, a control unit (1060) comprising processing circuitry (1010) and an interface (1020), the control unit (1060) configured to measure (SX1 ) losses between a reference antenna (230) in the reverberation chamber (210) and a measurement antenna (240) in the reverberation chamber (210), altering (SX2) a spatial configuration of the spatially configurable platform (220) when measuring (SX1 ) losses, and calibrate (SX3) the reverberation chamber (210) based on the measured losses.

17. A computer program product (1100) comprising a computer program (1110) configured to execute a method according to at least one of claims 1- 15, and a computer readable storage medium (1120) on which the computer program is stored.

18. A high-frequency mode stirrer (100) for modifying electromagnetic boundary conditions in a reverberation chamber, the high-frequency mode stirrer (100) comprising an electrically conductive member (101 ) arranged to reflect electromagnetic radiation, one or more apertures (102) arranged extending through the electrically conductive member (101 ), wherein the apertures (102) are arranged to diffract and to diffuse the electromagnetic radiation, and one or more electrically conductive plates (103) arranged on the electrically conductive member (101 ), wherein the electrically conductive plates (103) arranged to scatter the electromagnetic radiation, and wherein the electrically conductive plates (103) are adjustably and releasably attached to the electrically conductive member (101 ).

19. The high-frequency mode stirrer (100) according to claim 18, wherein at least one of the electrically conductive plates (103) is arranged perpendicular to the surface of the electrically conductive member (101 ) at a mounting position of the at least one electrically conductive plate (103).

20. The high-frequency mode stirrer (100) according to any of claims 18-19, wherein the electrically conductive member (101 ) comprises an arcuate shape.

21 . The high-frequency mode stirrer (100) according to any of claims 18-20, wherein the electrically conductive member (101 ) comprises a plurality of surfaces connected to each other to form a Z-fold configuration, wherein a fold angle (A) is formed between two adjacent surfaces in the plurality of surfaces.

22. The high-frequency mode stirrer (100) according to any of claims 18-21 , comprising a plurality of legs (104) arranged to raise the high-frequency mode stirrer (100) relative to a spatially configurable platform (220) when the high- frequency mode stirrer (100) is arranged on the spatially configurable platform (220).

23. The high-frequency mode stirrer (100) according to any of claims 18-22, comprising at least one handle (105) for manually lifting the high-frequency mode stirrer (100).

24. A reverberation chamber (210) comprising the high-frequency mode stirrer (100) according to any of claims 18-23.