Patent Publication Number: US-2021194190-A1

Title: Electrical Equipment Adapted to Detect the Presence of an External Antenna

Description:
The invention relates to the field of electrical equipment including a radio module capable of operating with an internal antenna or an external antenna if such an external antenna is connected to the electrical equipment. 
     BACKGROUND OF THE INVENTION 
     Certain kinds of electrical equipment, and in particular certain smart meters and certain gateways, include a radio module capable of operating either with an internal antenna incorporated in the electrical equipment, or else with an external antenna added to the electrical equipment and connected to an external connector of the electrical equipment that is provided for this purpose. 
     By way of example, the radio module may be a cellular radio module, and the external connector may be a coaxial connector. 
     Using the external antenna serves to improve the reception and transmission of data by the electrical equipment. Thus, and by way of example, when an electricity meter is to be installed in a location having poor cellular network coverage, e.g. in a cellar, an external antenna is connected to the electricity meter in order to enable it to transmit uplink data and to receive downlink data more effectively. 
     It is therefore appropriate to detect whether an external antenna is connected to the external connector of the electrical equipment so as to be able to connect the radio module to the external antenna when it is present. 
     In the prior art, the following solutions are known for detecting the presence of an external antenna connected to an external connector of coaxial connector type. 
     A “mechanical” solution can be seen in  FIG. 1 . A piece of electrical equipment  1  has a cellular radio module  2 , an internal antenna  3 , and a coaxial connector (e.g. a subminiature version A (SMA) connector) that incorporates a switch  5 . It is the coaxial connector  4  that acts mechanically to detect the connection of an external antenna  6  or of a cable to which the external antenna  6  is connected. 
     In an “optical” solution, an optical link arranged downstream from the coaxial connector detects the obstruction caused by inserting the external antenna or a cable to which the external antenna is connected. 
     In an “electrical” solution, a direct current (DC) signal is interrupted by the presence of an external antenna fitted with a DC resistor. 
     In a “radio transmission” solution, transmission by the radio module is activated, and reflection of the transmitted signal is measured. 
     In a “radio reception” solution, a comparison is made between the powers of signals received on the two channels of the radio module, i.e. the channel including the internal antenna and the channel including the coaxial connector. 
     Those detection techniques raise the following problems. 
     The “mechanical” solution requires special coaxial connectors, which are bulky, expensive (about three times the price of a conventional coaxial connector), and difficult to incorporate on a printed circuit. The main functional drawback of that solution lies in the appearance of mechanical chatter in the event of the external antenna or the cable connected to the coaxial connector being loose. Furthermore, when in the presence of a coaxial cable, it is not possible to detect whether an antenna is present at the end of the coaxial cable. In spite of the above-mentioned lack of robustness, that method remains the simplest, and it is in very widespread use when the radio module is of the cellular type. 
     The “optical” solution is not robust because elements can obstruct the optical link (e.g. dust). 
     As mentioned above, the “electrical” solution requires the use of external antennas that are special in that they are equipped with a DC resistor, thereby greatly limiting the external antennas that can be selected. 
     The “radio transmission” solution constitutes the technique that is the most reliable at present. Nevertheless, that solution is not applicable with a cellular radio module. Specifically, transmitting a continuous wave signal is permitted only in a “test” mode, which cannot be set up in the field. Waiting for a signal with standard signaling to be transmitted (i.e. a signal complying with the third generation partnership project (3GPP) protocol and with radio standards) can take a long time (up to 1 hour if it is necessary to scan several technologies such as the second, third, and fourth generation (2G, 3G, and 4G) technologies, for example). 
     The “radio reception” solution is not very reliable since it depends greatly on external conditions (network quality, transient noise, immediate environment of the electrical equipment, etc.). 
     OBJECT OF THE INVENTION 
     An object of the invention is to provide a solution making it possible to detect that an external antenna is connected to electrical equipment as described above, said solution not presenting the above-mentioned drawbacks. 
     SUMMARY OF THE INVENTION 
     In order to achieve this object, there is provided electrical equipment comprising:
         an internal antenna;   an external connector to which an antenna external to the electrical equipment can be connected;   a first radio module;   a second radio module;   a radiofrequency (RF) link enabling the second radio module to be connected to the external connector;   a detector device arranged, when a test signal is transmitted over the external connector via the RF link, to produce a detection signal representative of whether or not the external antenna is connected to the external connector;   control means arranged to control the second radio module so that it generates and transmits the test signal via the RF link, to acquire the detection signal, and depending on the detection signal, to connect the first radio module to the external connector if the external antenna is connected to the external connector, or else to connect the first radio module to the internal antenna if the external antenna is not connected to the external connector.       

     Thus, in the electrical equipment of the invention, an additional RF link is added serving to connect the second radio module to the external connector, and advantage is taken of the presence of the second radio module to detect whether an external antenna, for connecting to the first radio module, is or is not connected to the external connector. 
     The solution of the invention is very advantageous. 
     Specifically, the solution of the invention is performed using a conventional external connector and therefore does not present the problems associated with the special connector of the above-described “mechanical” solution. Furthermore, the detection performed by the test signal and the detection signal serves, when a cable is connected to the external connector, to detect whether an external antenna is or is not connected to the other end of the cable. 
     The solution of the invention is robust and cannot be disturbed by dust. 
     The solution of the invention can be performed regardless of the type of external antenna that is used. 
     When the first radio module is a cellular radio module and when the second radio module is a radio module of the industrial, scientific, and medical (ISM) type, the solution of the invention does not present the difficulties involved in performing the “radio transmission” solution. Specifically, it is possible to transmit a test signal from the ISM radio module at any time, naturally providing that ISM standards are complied with. 
     Finally, the solution of the invention is performed entirely within the electrical equipment and it is not disturbed by conditions outside it. 
     There is also provided electrical equipment as described above, wherein the RF link is a conducted link. 
     There is also provided electrical equipment as described above, wherein the RF link is a radiated link, the detector device including a link antenna connected by the RF link to a communication antenna of the second radio module. 
     There is also provided electrical equipment as described above, including a main RF transmission line comprising a main RF track connected to the external connector, the detector device comprising a detector RF transmission line comprising a detector RF track coupled to the main RF track, and detector components connected to the detector RF track. 
     There is also provided electrical equipment as described above, wherein the detector components comprise first detector components connected to a first end of the detector RF track and arranged to produce a first voltage representative of a forward power resulting directly from transmission of the test signal, and second detector components connected to a second end of the detector RF track and arranged to produce a second voltage representative of a reflected power resulting from reflection of the test signal, the detection signal being obtained from the first voltage and from the second voltage. 
     There is also provided electrical equipment as described above, wherein the first and second detector components comprise respective first and second voltage boost circuits followed by respective first and second peak detector diodes. 
     There is also provided electrical equipment as described above, wherein the main RF transmission line is a wide band transmission line while the detector RF transmission line is a selective transmission line tuned to a test frequency of the test signal. 
     There is also provided electrical equipment as described above and including a switch device, the control means being arranged to control the switch device so as to connect or disconnect the second radio module selectively to or from the external connector, and so as to connect the first radio module selectively to the internal antenna or to the external connector. 
     There is also provided electrical equipment as described above, wherein the switch device comprises a first double-throw switch and a second double-throw switch, the first double-throw switch having a first input connected to an output of the first radio module and a second input connected to an output of the second radio module via the RF link, and the second double-throw switch having an input connected to an output of the first double-throw switch, a first output connected to the internal antenna, and a second output connected to the external connector. 
     There is also provided electrical equipment as described above, wherein a test frequency of the test signal is included in a frequency band in which the first radio module operates. 
     There is also provided electrical equipment as described above, wherein the test signal is encoded so as to avoid an interfering signal at the test frequency disturbing the detector device. 
     There is also provided electrical equipment as described above, wherein the first radio module is a cellular radio module and wherein the second radio module is an ISM radio module. 
     There is also provided electrical equipment as described above, the electrical equipment being a meter. 
     There is also provided electrical equipment as described above, the electrical equipment being a gateway. 
     There is also provided a method of detecting and connecting an external antenna, the method being performed in electrical equipment as described above and comprising the steps of:
         controlling the second radio module so that it generates and transmits the test signal over the external connector via the RF link;   acquiring the detection signal;   deducing from the detection signal whether or not the external antenna is connected to the external connector; and   if the external antenna is connected to the external connector, connecting the first radio module to the connector; or else   connecting the first radio module to the internal antenna.       

     There is also provided a computer program including instructions that enable the above-described electrical equipment to execute the steps of the above-described method of detecting and connecting an external antenna. 
     There is also provided a computer readable storage medium, having stored thereon the computer program as described above. 
     The invention can be better understood in the light of the following description of particular, nonlimiting embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the accompanying drawings, in which: 
         FIG. 1  shows a prior art “mechanical” solution for detecting the presence of an external antenna; 
         FIG. 2  shows electrical equipment in a first embodiment of the invention; 
         FIG. 3  also shows electrical equipment in the first embodiment of the invention; 
         FIG. 4  shows a detector device in simplified manner; 
         FIG. 5  shows the detector device more accurately; 
         FIG. 6  is a perspective view of a portion of an electrical circuit card including the detector device and a coaxial connector; 
         FIG. 7  shows steps of a detection and connection method; 
         FIG. 8  is a graph plotting a forward power curve, a reflected power curve, and a curve of power measured on a main RF track, the curves being obtained while an external antenna is connected; 
         FIG. 9  is a graph similar to the graph of  FIG. 8 , the curves being obtained while the external antenna is not connected; 
         FIG. 10  comprises graphs, each comprising a first voltage curve and a second voltage curve, the curves being obtained by simulation with different standing wave ratios (SWRs) and with different impedances; 
         FIG. 11  is a table of values used for obtaining the curves of  FIG. 10 ; 
         FIG. 12  shows electrical equipment in a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 2 to 6 , in this example electrical equipment in a first embodiment of the invention is an electricity meter  10  comprising a housing incorporating a first radio module  11  and a second radio module  12 . 
     In this example, the term “radio module” is used to mean a module arranged to perform communication (transmission and/or reception) by radio. 
     The first radio module  11  is a cellular radio module capable of communicating by using some or all of the following standards: 2G, 3G, 4G, Cat-M, NB-IoT, etc. 
     The second radio module  12  is an ISM radio module. In this example, the second radio module  12  operates at an ISM frequency of 868.3 megahertz (MHz). 
     The meter  10  has an internal antenna  13  situated inside the housing, and an external connector, specifically a coaxial connector  14 , that enables an external antenna to be connected to the meter  10 . 
     It should be observed that the external antenna may be connected directly to the coaxial connector  14 , or else it may be connected via a cable that then has a first end to which the external antenna is connected and a second end that is connected to the coaxial connector  14 . 
     The meter  10  includes a first main RF transmission line  16  that serves to connect the first radio module  11  to the internal antenna  13 , and a second main RF transmission line  17  that serves to connect the second radio module  12  to the coaxial connector  14 . 
     The second main RF transmission line  17  can be seen more clearly in  FIGS. 4 to 6 . It can be seen that the second main RF transmission line  17  has a main RF track  18 . The main RF track  18  is a copper track formed on a face of a portion of a circuit card. The remainder of the face of the portion of the circuit card is covered for the most part by a copper surface  19  that forms a ground plane, such that the main RF track  18  extends in said ground plane  19  while being insulated therefrom by narrow strips of substrate that are not covered in copper. 
     The meter  10  also includes an RF link  20  that enables an output S 1  of the second radio module  12  to be connected to the coaxial connector  14 . In this example, the RF link  20  is a conducted link that comprises an RF track or an RF cable. 
     The meter  10  also has a switch device  21  that comprises a first double-throw switch  22  and a second double-throw switch  23 . The first double-throw switch  22  has a first input E 1  connected to an output S 2  of the first radio module  11  and a second input E 2  connected to the output S 1  of the second radio module  12  via the RF link  20 , and an output S 3 . The second double-throw switch  23  has an input E 3  connected to the output S 3  of the first double-throw switch  22 , a first output S 4  connected to the internal antenna  13  via the first main RF transmission line  16 , and a second output S 5  connected to the coaxial connector  14  via the second main RF transmission line  17 . 
     The meter  10  further includes control means that in this example comprise a control component  25  adapted to execute instructions of a program for performing the steps of the method described below for detecting and connecting an external antenna. By way of example, the control component  25  is a microcontroller, a processor, or indeed a programmable logic circuit such as a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). 
     The control component  25  is connected to the first double-throw switch  22  and to the second double-throw switch  23  and it is arranged to control them, i.e. to connect the first input E 1  or the second input E 2  of the first double-throw switch  22  selectively to the output S 3  of the first double-throw switch  22 , and to connect the input E 3  of the second double-throw switch  23  to the first output S 4  or to the second output S 5  of the second double-throw switch  23 . 
     The meter  10  also has a detector device  26  that can be seen more clearly in  FIGS. 4 to 6 . The detector device  26  comprises a coupler-detector circuit including a detector RF transmission line  27  comprising a detector RF track  28  coupled to the main RF track  18 , and detector components connected to the detector RF track  28 . 
     The detector components comprise first detector components connected to a first end of the detector RF track  28 , and second detector components connected to a second end of the detector RF track  28 . 
     The first detector components comprise a first voltage boost circuit  29  followed by a first peak detector diode  30  and a resistor-capacitor (RC) network  37 . The second detector components comprise a second voltage boost circuit  31  followed by a second peak detector diode  32  and an RC network  41 . 
     The first voltage boost circuit  29  comprises a first capacitor  35  connected to the first end of the detector RF track  28  and a first inductor-capacitor (LC) circuit  36  to which the first peak detector diode  30  is connected. Likewise, the second voltage boost circuit  31  comprises a second capacitor  39  connected to the second end of the detector RF track  28  and a second LC circuit  40  to which the second peak detector diode  32  is connected. 
     Because of the presence of the first detector components and of the second detector components, the detector RF transmission line  27  is a selective transmission line tuned to the above-mentioned ISM frequency (868.3 MHz). The coupler-rectifier is thus likewise tuned to the ISM frequency. 
     In contrast, the second main RF transmission line  17  is a wide band transmission line. 
     The method performed in the meter  10  for detecting and connecting an external antenna is described below in detail. The sequence of the main steps of the method can be seen in  FIG. 7 . 
     By default, the first double-throw switch  22  and the second double-throw switch  23  are in a configuration such that the output S 1  of the second radio module  12  is connected to the coaxial connector  14  (via the RF link  20  and the second main RF transmission line  17 ; step E 1 ). The second input E 2  of the first double-throw switch  22  is thus connected to the output S 3  of the first double-throw switch  22  and the second output S 5  of the second double-throw switch  23  is connected to the input E 3  of the second double-throw switch  23 , and thus to the second input E 2  of the first double-throw switch  22 . 
     The control component  25  then controls the second radio module  12  so that it generates and transmits a test signal St over the coaxial connector  14  via the RF link  20  (step E 2 ). 
     The test frequency of the test signal St is the ISM frequency of 868.3 MHz. It should be observed that it is preferable for the test frequency of the test signal St to be included in the frequency band in which the first radio module  11  operates, as in this example. 
     The detector device  26  then produces a detection signal representative of whether or not the external antenna is connected to the coaxial connector  14  (step E 3 ). The detection signal is acquired by the control component  25 . 
     In this example, the detection signal is obtained from a first voltage V 1  produced across the terminals of the RC network  37  and from a second voltage V 2  produced across the terminals of the RC network  41 . Specifically, in this example, the detection signal is equal to: V 2 −V 1 . 
     The control component  25  acquires, digitizes, and analyzes the first voltage V 1  and the second voltage V 2 . 
     The first voltage V 1  is representative of a forward power, obtained from the first end of the detector RF track  28 , and resulting from the forward transmission of the test signal St. 
     The second voltage V 2  is representative of a reflected power, obtained from the second end of the detector RF track  28 , and resulting from the test signal St being reflected, as a function of the configuration, either from the coaxial connector  14  on its own, or else from the coaxial connector  14  and the external antenna (and also the cable, if any, connected to the coaxial connector  14  and to the external antenna). 
     In  FIG. 8 , it can be seen that, when the external antenna, which forms a tuned load, is connected, the forward power Pf at the test frequency is much greater than the reflected power Pr. In comparison, in  FIG. 9  it can be seen that, when the external antenna is not connected, the forward power Pf and the reflected power Pr are very close to each other. In the graphs of  FIGS. 8 and 9 , the curve P 1  corresponds to the power detected on the main RF track  18 . 
     The difference between the second voltage V 2  and the first voltage V 1  thus forms a detection signal that is representative of whether or not the external antenna is connected to the coaxial connector  14 . 
     The control component  25  compares the detection signal, i.e. the difference between the second voltage V 2  and the first voltage V 1 , with a predetermined detection threshold Vth. 
     If the following applies: 
         V 2− V 1&lt; Vth  
 
     then the control component  25  detects that the external antenna is not connected. 
     In contrast, if the following applies: 
         V 2− V 1≥ Vth  
 
     then the control component  25  detects that the external antenna is connected (step E 4 ). 
     If the control component  25  detects that the external antenna is not connected, then the control component  25  controls the first double-throw switch  22  and the second double-throw switch  23  so that the output S 2  of the first radio module  11  is connected to the internal antenna  13 . 
     If the control component  25  detects that the external antenna is connected, then the control component  25  controls the first double-throw switch  22  and the second double-throw switch  23  so that the output S 2  of the first radio module  11  is connected to the coaxial connector  14  and thus to the external antenna (step E 5 ). 
     It should be observed that the value of the predetermined detection threshold Vth is determined from measurements taken in a plurality of configurations, each corresponding to a possible termination for the coaxial connector  14 . 
     In a first configuration, this gives: 
     SWR=1 
     that corresponds to the standing wave ratio for a perfectly matched external antenna (50 ohm (Q) load). 
     In a second configuration, this gives: 
     SWR=2 
     that corresponds to the SWR of a well-matched external antenna (90% of the signal is passed from the coaxial connector  14  to the external antenna). 
     In a third configuration, this gives: 
     SWR=3 
     that corresponds to an external antenna of poorer quality (this situation is possible in use, since most multi-band external antennas have quality of this order). 
     In a fourth configuration, this gives: 
     SWR is infinite
 
that corresponds to an open circuit, and thus to the absence of an external antenna. It should be observed that this applies also when a cable has its second end connected to the coaxial connector  14 , but has no external antenna connected to its first end.
 
     The predetermined detection threshold is optimized as a function of the type of load, in such a manner that even an ordinary external antenna (presenting an SWR of 3) can be detected easily. 
     Advantageously, the test signal St is encoded by simple coding, e.g. of on-off keying (OOK) type. 
     This avoids an interfering signal at the test frequency disturbing the detector device  26 , and in particular this makes it impossible to take a decision on the basis of the received interfering signal. 
     This makes discrimination between the two states even more robust. 
     There follows a description of the results of simulations performed on the detector device  26  for different SWR values and using different impedance values. 
     The graph G 1  corresponds to the SWR being equal to 1 and the impedance at the second end (connected to the coaxial connector  14 ) of the main RF track  18  is equal to 50Ω. The graph G 2  corresponds to the SWR being equal to 1 and the impedance at the first end (connected to the switch device  21 ) of the main RF track  18  is equal to 50Ω. 
     On the graph G 1 , the curve for the first voltage V 1  is obtained from the values in column C 1  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 2  in the table of  FIG. 11 . 
     On the graph G 2 , the curve for the first voltage V 1  is obtained from the values in column C 3  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 4  in the table of  FIG. 11 . 
     Column C 0  contains the values (in decibels (dB)) of the power detected on the second main RF transmission line  17 . These values are plotted along the abscissa axis in the various graphs. 
     The graph G 3  corresponds to the SWR being equal to 2 and the impedance at the second end of the main RF track  18  being equal to 25Ω. The graph G 4  corresponds to the SWR being equal to 2 and the impedance at the second end of the main RF track  18  being equal to 100Ω. 
     On the graph G 3 , the curve for the first voltage V 1  is obtained from the values in column C 5  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 6  in the table of  FIG. 11 . 
     On the graph G 4 , the curve for the first voltage V 1  is obtained from the values in column C 7  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 8  in the table of  FIG. 11 . 
     The graph G 5  corresponds to the SWR being equal to 3 and the impedance at the second end of the main RF track  18  being equal to 16.5Ω. The graph G 6  corresponds to the SWR being equal to 3 and the impedance at the second end of the main RF track  18  being equal to 150Ω. 
     On the graph G 5 , the curve for the first voltage V 1  is obtained from the values in column C 9  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 10  in the table of  FIG. 11 . 
     On the graph G 6 , the curve for the first voltage V 1  is obtained from the values in column C 11  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 12  in the table of  FIG. 11 . 
     The graph G 7  corresponds to the SWR being infinite and the impedance at the second end of the main RF track  18  being equal to 0Ω. The graph G 8  corresponds to the SWR being infinite and the impedance at the second end of the main RF track  18  being infinite. 
     On the graph G 7 , the curve for the first voltage V 1  is obtained from the values in column C 13  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 14  in the table of  FIG. 11 . 
     On the graph G 8 , the curve for the first voltage V 1  is obtained from the values in column C 15  in the table in  FIG. 11 . The curve for the second voltage V 2  is obtained from the values in column C 16  in the table of  FIG. 11 . 
     It can be seen that the detectable difference between the first voltage V 1  and the second voltage V 2  is at least 6 dB (for a mediocre external antenna) and is 9 dB for a well-matched external antenna. This difference is much greater than the situation where the external antenna is absent, which leaves a comfortable margin for defining a predetermined detection threshold Vth that is robust. Detecting the presence or the absence of an external antenna is thus both robust and reliable. 
     With reference to  FIG. 12 , electrical equipment in a second embodiment is once again an electricity meter  50 . 
     The electricity meter  50  has a first radio module  51  (which is cellular), a second radio module  52  (which is ISM), an internal antenna  53 , and a coaxial connector  54 . 
     In this embodiment, the RF link enabling the second radio module  52  to be connected to the coaxial connector  54  is a radiated link. The detector device  55  has a link antenna  56  connected by the RF link to a communication antenna  57  of the second radio module  52 . The communication antenna  57  is tuned to the test frequency, which is the ISM frequency of the second radio module  52 . 
     Naturally, the invention is not limited to the embodiments described, but covers any variant coming within the ambit of the invention as defined by the claims. 
     The electrical equipment in which the invention is performed need not necessarily be an electricity meter, but could be any other type of meter, and could even be any electrical equipment other than a meter, e.g. a gateway. 
     In the description above, it is stated that the control component controls the second radio module so that it generates and transmits the test signal via the RF link, acquires the detection signal, and depending on the detection signal, controls the switch device. Naturally, these operations could be performed by a plurality of distinct components. 
     The first radio module need not necessarily be a cellular radio module, and the second radio module need not necessarily be an ISM module.