A microwave radio transceiver test arrangement TECHNICAL FIELD

The present disclosure relates to a microwave radio transceiver test arrangement adapted for testing a microwave radio transceiver.

BACKGROUND

In many fields of wireless communication, such as microwave communication, as well as for applications associated with radars and other sensors using microwave technology, waveguides are used for transporting wireless signals, due to the low losses incurred in a waveguide.

Many radio systems and radio units comprise in-built functions for self-testing and diagnostics. Normally, such tests are made by diverting a relatively small amount of signal power from the transmitter to the receiver in a test loop and evaluate the received signal qualify. If the test is successful, it is quite likely that the whole transmit and receive chains in the radio unit are fully working. This controlled diverting or decoupling can for example be generated by means of switches on a radio unit printed circuit board (PCB), by means of directional couplers on the PCB, in filter units etc. In this context, it is important to obtain enough isolation between the transmitter and the receiver in the radio unit when the test loop is not in use.

When a test loop is implemented, there is always also a path for leakage of transmitter noise and transmitter signal that could enter the receiver and impact receiver performance, such as for example bit error rate (BER) thresholds etc. A possible solution to this problem is to transmit signal power on frequencies that normally are not used in the radio unit, and some of this signal power may radiate out from the antenna on a level that exceeds current regulated levels. However, there is always a risk that transmitter noise will disturb the receiver, and in a worst case the transmit signal will find a path to the receiver and more or less jam the receiver.

Due to the above problem, there is often a limitation regarding which power ranges that can be used in a test loop, which limits the settings that could be used on the transmitter in the test mode. Therefore, there is a risk that the test is incomplete and potential failing components will not be discovered.

There is thus a need for a microwave radio transceiver test arrangement with an enhanced test loop that does not suffer from the above limitations.

SUMMARY It is an object of the present disclosure to provide an enhanced microwave radio transceiver test arrangement.

Said object is obtained by means of a microwave radio transceiver test arrangement adapted for testing a microwave radio transceiver and comprising a control unit adapted to control a signal generator and an amplifier device that is adapted to amplify a generated signal. The test arrangement further comprises an externally arranged detachable RF (radio frequency) loop device that comprises a loop input port, a loop output port and an RF loop. The RF loop device is adapted to transfer an amplified generated signal from a transmitting radio port to a receiving radio port. The test arrangement further comprises an RF power detecting device adapted to detect power received at a receiving radio port.

In this way, the microwave radio transceiver can be tested using any suitable frequency and/or power ranges, and without introducing any leakage paths during normal use. Since the test settings that can be used in the test mode are not limited as for previous test solutions, the test will be complete and potential failing components will be discovered. A true wide band RF loop function according to the present disclosure can be used for failure analysis and calibration in field. Thereto, the risk for unwanted signal into the air or between transmitter and receiver is eliminated during normal operation.

Microwave radio transceiver calibration is made possible in field, or at least close to the site, and will enable reconfiguration of the microwave radio transceiver from one frequency to another frequency without shipping the hardware back to the factory. This in-field configuration possibility will help to minimize shipping as the supplier or customer can have one main product that can be easily reconfigured to the version that is finally needed. If the network requires alterations, the hardware could be reused again, only needing re-configuration. Microwave radio transceiver calibration can be with a minimum of expensive test equipment, possibly even without needing any supplementary test equipment.

According to some aspects, the RF loop device is adapted to confer a predefined attenuation to a signal that is transmitted via the RF loop device.

In this way, the characteristics of the RF loop can be clearly defined.

According to some aspects, the RF loop device comprises an RF power coupler that is adapted to couple a predetermined fraction of a transferred signal to a coupled transition port. In this way, the microwave radio transceiver can be calibrated quite easy in field or at least on a location near the customer. The power detector could either be inbuilt in the radio or be in an external test instrument.

According to some aspects, the RF loop device is constituted by a coaxial cable that is adapted to be connected between a first coaxial connector that constitutes the transmitting radio port and a second coaxial connector that constitutes the receiving radio port. The coaxial cable comprises a first cable coaxial connector that constitutes the loop input port and a second cable coaxial connector that constitutes the loop output port.

In this way, an uncomplicated and inexpensive RF loop device is provided.

According to some aspects, the RF loop device is constituted by a waveguide device that comprises an RF loop waveguide section that constituted the RF loop, a first transition port that constitutes the loop input port and a second transition port that constitutes the loop output port. The RF loop waveguide section is adapted to be connected between a first radio port that constitutes the transmitting port and a second radio port that constitutes the receiving radio port via the first transition port and the second transition port.

This enable for a reliable and possibly controllable RF loop device.

According to some aspects, the waveguide device comprises a diplexer filter, a diplexer input transition port, a diplexer output transition port and a diplexer filter port. In this way, the waveguide device is adapted for testing the microwave radio transceiver when the RF loop waveguide section is connected to the microwave radio transceiver. In this way, a combined waveguide device is provided where the RF loop device is easily available when the diplexer filter is mounted.

According to some aspects, the RF loop device and the diplexer filter are adapted to be connected to the microwave radio transceiver simultaneously.

In this way, the testing can be performed without switching any components.

According to some aspects, the microwave radio transceiver comprises a transmitting test radio port adapted to transmit a generated signal, and a receiving radio test port adapted to receive and detect a signal that is transferred from the transmitting test radio port. According to some aspects, the microwave radio transceiver comprises a coupled receiving test radio port that is adapted to receive and detect a predetermined fraction of a signal that is transferred from the transmitting test radio port.

In this way, the microwave radio transceiver can be calibrated quite easy in field or at least on a location near the customer. The power detector could either be inbuilt in the radio or be in an external test instrument.

According to some aspects, each one of the radio ports comprised in the microwave radio transceiver comprises a corresponding radio cavity, where each radio cavity comprises a probe of a fixed length, a bottom and a top. The probe is connected to a radio part and extends within the radio cavity, via an inner insulating part in the bottom towards the top. Each one of the transition ports that is comprised in the waveguide device comprises a corresponding transition cavity that is adapted to be inserted into a corresponding radio cavity. Each transition cavity comprises a first end that is adapted to face the bottom, and a bottom wall with an outer insulating part, through which outer insulating part a corresponding probe is adapted to protrude a protrusion distance within the transition cavity when mounted. The protrusion distance is dependent on a thickness of the bottom wall.

In this way, the same type of radio trasnsceiver can be adapted to handle different frequency bands, where different separately available waveguide adapters are sued to adapt the radio trasnsceiver to a desired frequency band.

This object is also obtained by means of waveguide devices and methods that are associated with the above advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 schematically shows a microwave radio transceiver with a microwave radio transceiver test arrangement;

Figure 2 schematically shows a device, such as a waveguide device, according to a first example;

Figure 3 schematically shows a device, such as a waveguide device, according to a second and third example; Figure 4 schematically shows a device, such as a waveguide device, according to a fourth example;

Figure 5 schematically shows a side view of a microwave radio transceiver according to the first example;

Figure 6 schematically shows a side view of a microwave radio transceiver according to the first example with a waveguide device mounted.

Figure 7 shows a schematic section side view of Figure 6;

Figure 8 shows an enlarged cut-open view of a radio cavity and a transition cavity mounted to each other;

Figure 9 schematically shows a side view of a microwave radio transceiver and a waveguide device according to the second example;

Figure 10 schematically shows a side view of a microwave radio transceiver and a waveguide device according to the third example;

Figure 11 schematically shows a side view of a microwave radio transceiver and a waveguide device according to the fourth example;

Figure 12 schematically shows a side view of a microwave radio transceiver and a coaxial cable; and

Figure 13 shows a flowchart for methods according to the present disclosure.

DETAILED DESCRIPTION

Aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings. The different devices, systems, computer programs 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 reference to Figure 1, there is schematically shown a microwave radio transceiver test arrangement 100 adapted for testing a microwave radio transceiver 110 and comprising a control unit 101 adapted to control a signal generator 102 and an amplifier device 103 that is adapted to amplify a generated signal. According to the present disclosure, the test arrangement 100 further comprises an externally arranged detachable RF (radio frequency) loop device 104 that comprises a loop input port 111, a loop output port 112 and an RF loop 115. The RF loop device 104 is adapted to transfer an amplified generated signal from a transmitting radio port 105 to a receiving radio port 106. The test arrangement 100 further comprises an RF power detecting device 107 adapted to detect power received at the receiving radio port 106.

The RF loop device 104 can thus easily be connected to the radio ports 105, 106, and a radio test mode be started. During the radio test mode, the RF power detecting device 107 detects power received at the receiving radio port 106 for a certain power transmitted at the transmitting radio port 105. Based on the detected result, it can be determined whether the microwave radio transceiver 110 is functioning within a predetermined specification or not.

By means of the present disclosure, a discrete RF loop is provided that does not require any switches and reduces the risk for transmit leakages into a receiver part.

According to some aspects, the RF loop device 104 is adapted to confer a predefined attenuation to a signal that is transmitted via the RF loop device 104. The exact attenuation could be stored into the system.

A user or service crew can thus, during installation or in fail a mode analysis, easily install the RF loop device 104 and initiate the radio test mode where tests and calibration can be done using a plurality of power levels and frequencies could be swept, according to some aspect power levels and frequencies exceeding and/or falling below the ones used during normal running. Radio calibration is possible in field, or at least close to the site, and will enable reconfiguration of the radio from one frequency to another frequency without shipping the hardware back to the factory. This in-field configuration possibility will help to minimize shipping as the supplier or customer can have one main product that can be easily reconfigured to the version that is finally needed. If the network requires alterations, the hardware could be reused again, only needing re configuration. Radio calibration can be with a minimum of expensive test equipment, possibly even without needing any supplementary test equipment. This is enabled by the RF loop device 104 that works as a true wide band RF loop function that could be used for failure analysis and calibration in field. Thereto, the risk for unwanted signal into the air or between transmitter and receiver is eliminated during normal operation.

According to some aspects, with reference to Figure 12, there is a microwave radio transceiver test arrangement 100”” where the RF loop device 104 is constituted by a coaxial cable 23 that is adapted to be connected between a first coaxial connector 24 that constitutes the transmitting radio port 105 and a second coaxial connector 25 that constitutes the receiving radio port 106. The coaxial cable 23 comprises a first cable coaxial connector 26 that constitutes the loop input port 111 and a second cable coaxial connector 27 that constitutes the loop output port 112.

According to some aspects, with reference to Figure 2, Figure 5 and Figure 6, there is a microwave radio transceiver test arrangement 100 where the RF loop device 104 is constituted by a by a waveguide device 108 that comprises an RF loop waveguide section 22 that constitutes the RF loop 115, a first transition port 21a that constitutes the loop input port 111 and a second transition port 21b that constitutes the loop output port 112. The RF loop waveguide section 22 is adapted to be connected between a first radio port 20a that constitutes the transmitting port 105 and a second radio port 20b that constitutes the receiving radio port 106 via the first transition port 21a and the second transition port 21b.

According to some aspects, with reference to Figure 3, and Figure 9, there is a microwave radio transceiver test arrangement 100’ where the waveguide device 108’ comprises a diplexer filter 109, a diplexer input transition port 21c, a diplexer output transition port 21d and a diplexer filter port 117. The diplexer filter port 117 can for example be in the form of a diplexer antenna port that is adapted to be connected to an antenna device. The waveguide device 108’ is adapted for testing the microwave radio transceiver 14’ when the RF loop waveguide section 22 is connected to the microwave radio transceiver 14’. The diplexer filter port 117 can for example, as shown in Figure 9, be in the form of a slot or waveguide port that is adapted to couple microwave signals to and from a further microwave component, such as for example an antenna device. The waveguide device 108’ is here a combined device that both comprises a diplexer filter 109 and an RF loop waveguide section 22.

In this example, depending on how the waveguide device 108’ is turned, either the diplexer filter 109 or the RF loop waveguide section 22 is connected to the first radio port 20a and the second radio port 20b since, on the one hand, the first transition port 21a and the second transition port 21b, and on the other hand, the diplexer input transition port 21c and the diplexer output transition port 21d are positioned on opposite sides. If the control unit 101 determines that the RF loop waveguide section 22 is connected to the microwave radio transceiver 14’, the control unit 101 can initiate a test mode automatically.

According to some aspects, with reference to Figure 10, there is a microwave radio transceiver test arrangement 100” where the microwave radio transceiver 14”comprises a separate transmitting test radio port 20c adapted to transmit a generated signal, and a separate receiving test radio port 20d adapted to receive and detect a signal that is transferred from the transmitting test radio port 20c.

Here, there is a waveguide device 108” that comprises the previously described diplexer filter 109 and RF loop waveguide section 22, and since the first transition port 21a, the second transition port 21b, the diplexer input transition port 21c and the diplexer output transition port 21d are positioned on the same side, these ports can be connected to the radio ports 20a, 20b, 20c, 20d of the microwave radio transceiver 14” simultaneously, such that the RF loop device 104 and the diplexer filter 109 can be connected to the microwave radio transceiver 14” simultaneously. The microwave radio transceiver 14” comprises active RF loop functions with switches or couplers that are adapted to switch between normal operation and a test function such that normal operation or testing can be performed without having to move the waveguide device 108”.. The RF power detecting device 107 is then adapted to detect power received at the receiving test radio port 20d.

According to some aspects, with reference to Figure 4 and Figure 11, there is a microwave radio transceiver test arrangement 100” where the microwave radio transceiver 14’” comprises a coupled receiving test radio port 20e that is adapted to receive and detect a predetermined fraction of a signal that is transferred from the transmitting test radio port 20c. For this purpose, there is a waveguide device 108’” that comprises the previously described diplexer filter 109 and RF loop waveguide section 22 as well as the first transition port 21a, the second transition port 21b, the diplexer input transition port 21c and the diplexer output transition port 21 d. The waveguide device 108’” further comprises an RF power coupler, or divider, 116 that is adapted to couple a predetermined fraction of a transferred signal to a coupled transition port 21e. This coupled signal is transferred to the coupled receiving test radio port 20e, and the RF power detecting device 107 is then adapted to detect power received at the coupled receiving test radio port 20e. A waveguide device with an RF power coupler/divider 116 that is adapted to couple a predetermined fraction of a transferred signal to a coupled transition port 21e without the diplexer filter is of course conceivable. It is also conceivable that any suitable type of RF loop device 104 can comprise an RF power coupler/divider 116 that is adapted to couple a predetermined fraction of a transferred signal to a coupled transition port 21e, not only having to be realized in waveguide technology. As in the previous example, the microwave radio transceiver 14’” comprises active RF loop functions with switches or couplers that are adapted to switch between normal operation and a test function such that normal operation or testing can be performed without having to move the waveguide device 108’”

For the waveguide device examples above, any type of suitable ports is conceivable, for example ports comprising a traditional coax/waveguide transition. In the following, special type of port configuration will be described.

With reference to Figure 5-8, each one of the radio ports 20a, 20b comprised in the microwave radio transceiver 14 comprises a corresponding radio cavity 2a, 2b, where each radio cavity 2a, 2b comprises a probe 3a, 3b of a fixed length, a bottom 5 and a top 6. The probe 3a, 3b is connected to a radio part 4 and extends within the radio cavity 2a, 2b, via an inner insulating part 7a, 7b in the bottom 5 towards the top 6. According to some aspects, the inner insulating part 7 is formed in a plastic material such as for example polytetrafluoroethylene (PTFE).

Each one of the transition ports 21a, 21b that is comprised in the waveguide device 108 comprises a corresponding transition cavity 9a, 9b that is adapted to be inserted into a corresponding radio cavity 2a, 2b. Each transition cavity 9a, 9b comprises a first end 15 that is adapted to face the bottom 5, and a bottom wall 12a, 12b with an outer insulating part 13a, 13b. When mounted, a corresponding probe 3a, 3b is adapted to protrude a protrusion distance D within the transition cavity 9a, 9b, through the outer insulating part 13a, 13b. The protrusion distance D is dependent on a thickness T of the bottom wall 12a, 12b.

Figure 5 shows the radio transceiver 14 with the radio ports 20 a, 20b, Figure 6 shows a waveguide device 108 mounted to the radio transceiver 14, Figure 7 shows a section of Figure 6 and Figure 8 shows an enlarged cut-open side view of a transition cavity 9a mounted to a radio cavity 2a.

In this way, by choosing a waveguide device 108 with an adapted protrusion distance D, it is possible to acquire a tuning to a desired frequency band for the RF loop waveguide section 22. This means that one and the same microwave radio transceiver 14 can be used to be connected to different waveguide devices, where the microwave radio transceiver 14 comprises first radio port 20a and a second radio port 20b according to the above. There can be a number of waveguide devices 108 that together provide functionality for the frequency bands the radio part 4 is capable of handling. Therefore, one standard microwave radio transceiver 14 can be made for all frequency bands the radio part 4 is capable of handling, having identical radio cavities 2a, 2b. With reference also to Figure 9-11, this is applicable for all waveguide devices 108, 108’, 108”, 108’” disclosed, where all the transition ports 21a, 21b, 21c, 21d, 21e can be formed in this manner, having corresponding transition cavities 9a, 9b, 9c, 9d, 9e that are adapted to be inserted into corresponding radio cavities 2a, 2b, 2c, 2d, 2e.

With reference to Figure 13, the present disclosure also relates to a method for testing a microwave radio transceiver 110, where the method comprises providing SI 00 an externally arranged detachable RF (radio frequency) loop device 104 used for transferring an amplified generated signal from a transmitting radio port 105 to a receiving radio port 106, and providing S200 an RF power detecting device 107 used for detecting power received at a receiving radio port 106, 20b, 20d, 20e.

According to some aspects, the RF loop device 104 is used for conferring a predefined attenuation to a signal that is transmitted via the RF loop device 104.

According to some aspects, the method comprises providing an RF power coupler/divider 116 that is used for coupling a predetermined fraction of a transferred signal to a coupled transition port 21e.

The present disclosure is not limited to the above, but may vary freely within the scope of the dependent claims. For example, a radio cavity and a corresponding transition cavity can have any suitable shape such as circular, oval, octagonal etc.

In this context, a port is any type of RF interface part that is connectable to a corresponding RF interface part.

In the examples related to a waveguide in general, as schematically indicated in for example Figure 2-4, any suitable type of RF technology can be applied for realizing any one of the RF loop device 104, the diplexer filter 109 and the RF power coupler/divider 116, for example any one of the mentioned coax cable, a microstrip line connection, a stripline connection, a coplanar transmission line connection and a coupled coaxial resonator connection is conceivable.

Generally, the present disclosure relates to a microwave radio transceiver test arrangement 100, 100’, 100”, 100’”, 100”” adapted for testing a microwave radio transceiver 110 and comprising a control unit 101 adapted to control a signal generator 102 and an amplifier device 103 that is adapted to amplify a generated signal. The test arrangement 100, 100’, 100”, 100’”, 100”” further comprises an externally arranged detachable RF (radio frequency) loop device 104 that comprises a loop input port 111, a loop output port 112 and an RF loop 115, where the RF loop device 104 is adapted to transfer an amplified generated signal from a transmitting radio port 105, 20a to a receiving radio port 106, 20b. The test arrangement 100, 100’, 100”, 100’”, 100”” further comprises an RF power detecting device 107 adapted to detect power received at a receiving radio port 106, 20b, 20d, 20e.

According to some aspects, the RF loop device 104 is adapted to confer a predefined attenuation to a signal that is transmitted via the RF loop device 104.

According to some aspects, the RF loop device 104 comprises an RF power coupler 116 that is adapted to couple a predetermined fraction of a transferred signal to a coupled transition port 21e.

According to some aspects, the RF loop device 104 is constituted by a coaxial cable 23 that is adapted to be connected between a first coaxial connector 24 that constitutes the transmitting radio port 105 and a second coaxial connector 25 that constitutes the receiving radio port 106, where the coaxial cable 23 comprises a first cable coaxial connector 26 that constitutes the loop input port 111 and a second cable coaxial connector 27 that constitutes the loop output port 112.

According to some aspects, the RF loop device 104 is constituted by a waveguide device 108, 108’, 108”, 108’” that comprises an RF loop waveguide section 22 that constituted the RF loop 115, a first transition port 21a that constitutes the loop input port 111 and a second transition port 21b that constitutes the loop output port 112, where the RF loop waveguide section 22 is adapted to be connected between a first radio port 20a that constitutes the transmitting port 105 and a second radio port 20b that constitutes the receiving radio port 106 via the first transition port 21a and the second transition port 21b

According to some aspects, the waveguide device 108’, 108”, 108’” comprises a diplexer filter 109, a diplexer input transition port 21c, a diplexer output transition port 21d and a diplexer filter port 117, such that the waveguide device 108’, 108”, 108’” is adapted for testing the microwave radio transceiver 14’, 14”, 14’” when the RF loop waveguide section 22 is connected to the microwave radio transceiver 14’, 14”, 14’”.

According to some aspects, the RF loop device 104 and the diplexer filter 109 are adapted to be connected to the microwave radio transceiver 14”, 14’” simultaneously.

According to some aspects, the microwave radio transceiver 14”, 14’” comprises a transmitting test radio port 20c adapted to transmit a generated signal, and a receiving radio test port 20d adapted to receive and detect a signal that is transferred from the transmitting test radio port 20c. According to some aspects, the microwave radio transceiver 14’” comprises a coupled receiving test radio port 20e that is adapted to receive and detect a predetermined fraction of a signal that is transferred from the transmitting test radio port 20c.

According to some aspects, each one of the radio ports 20a, 20b, 20c, 20d, 20e comprised in the microwave radio transceiver 14, 14’, 14”, 14’” comprises a corresponding radio cavity 2a, 2b, 2c, 2d, 2e, where each radio cavity 2a, 2b2c, 2d, 2e comprises a probe 3 a, 3b of a fixed length, a bottom 5 and a top 6, where the probe 3a, 3b is connected to a radio part 4 and extends within the radio cavity 2a, 2b, 2c, 2d, 2e, via an inner insulating part 7a, 7b in the bottom 5 towards the top 6, and where each one of the transition ports 21a, 21b, 21c, 21d, 21e that is comprised in the waveguide device 108, 108’, 108”, 108’” comprises a corresponding transition cavity 9a, 9b that is adapted to be inserted into a corresponding radio cavity 2a, 2b, each transition cavity 9a, 9b comprising a first end 15 that is adapted to face the bottom 5, and a bottom wall 12a, 12b with an outer insulating part 13a, 13b, through which outer insulating part 13a, 13b a corresponding probe 3a, 3b is adapted to protrude a protrusion distance D within the transition cavity 9a, 9b when mounted, where the protrusion distance D is dependent on a thickness T of the bottom wall 12a, 12b.

Generally, the present disclosure also relates to a waveguide device 108, 108’, 108”, 108’” comprising an RF (radio frequency) loop waveguide section 22, a first transition port 21a and a second transition port 21b. The RF loop waveguide section 22 is adapted to be connected between a first radio port 20a and a second radio port 20b via the first transition port 21a and the second transition port 21b where the RF loop waveguide section 22 is adapted to transfer an amplified generated signal from the first radio port 20a to the second radio port 20b.

According to some aspects, the RF loop waveguide section 22 is adapted to confer a predefined attenuation to a signal that is transmitted via the RF loop waveguide section 22.

According to some aspects, the RF loop waveguide section 22 comprises an RF power coupler/divider 116 that is adapted to couple a predetermined fraction of a transferred signal to a coupled transition port 21e.

According to some aspects, the waveguide device 108’, 108”, 108’” comprises a diplexer filter 109 comprising a diplexer input transition port 21c, a diplexer output transition port 21d and a diplexer antenna port 117, such that the waveguide device 108’, 108”, 108’” is adapted fortesting a microwave radio transceiver 14’, 14”, 14’” when the RF loop waveguide section 22 is connected to the microwave radio transceiver 14’, 14”, 14’”. According to some aspects, the RF loop device 104 and the diplexer filter 109 are adapted to be connected to the microwave radio transceiver 14’, 14”, 14’” simultaneously. According to some aspects, each one of the transition ports 21a, 21b, 21c, 21 d, 21e that is comprised in the waveguide device 108, 108’, 108”, 108’” comprises a corresponding transition cavity 9a, 9b that is adapted to be inserted into a corresponding radio cavity 2a, 2b having a corresponding bottom 5, top 6, and probe 3a, 3b of a fixed length that extends within the radio cavity 2a, 2b via an inner insulating part 7; 7a, 7b in the bottom 5 towards the top 6 and is adapted to protrude a protrusion distance D within the corresponding transition cavity 9a, 9b via the outer insulating part 13a, 13b when mounted, where the protrusion distance D is dependent on the thickness T of the bottom wall 12a, 12b.