Abstract:
A power detector for measuring the average power, without constant voltage, of modulated or unmodulated high frequency or microwave signals is described. The power detector includes a signal line connected to a high frequency input. A detection line is capacitively and/or inductively coupled to the signal line and, seen in the longitudinal direction, is connected to the signal line. The signal is tapped from the detection line at two or more detection positions which are staggered in the longitudinal direction.

Description:
CONTINUING DATA 
   This application is a 371 of PCT/EP03/00857 filed Jan. 28, 2003. 
   FIELD OF THE INVENTION 
   The invention relates to a power detector for measuring the average power of modulated or un-modulated high-frequency and/or microwave signals. 
   BACKGROUND OF THE INVENTION 
   Power detectors are already well known in an extremely diverse range of embodiments. The most frequently used realisations contain either a thermal sensor, which generates an electrical measurement parameter proportional to the absorbed electrical power, or they contain a diode regulator in a one-way or two-way circuit, which supplies an electrical output parameter equivalent to the voltage at the terminal resistance, from which the power to be measured can then be determined. 
   In particular, the demands of the communications standards and the second and third generations of mobile telephones have led to the development of a new group of power detectors with a substantially greater dynamic range for modulated signals. 
   A power detector of this kind is known, for example, from the previously unpublished DE 100 39 665 of the applicant. In the case of the power detector described in this application, a stripline without electrical isolation is connected directly to the input terminal. The stripline terminates with a power distributor. A detector diode, which detects the positive and the negative half-wave of the measured signal in each case is disposed at the input and output ends of the stripline. The detector diodes can be connected to a difference amplifier in order to register the measured signal. As described in the application, a spatial isolation of the detector diodes has the advantage that the arrangement is less sensitive to reflections. The difference between the output voltages at the two detector diodes is therefore less dependent on the adaptation of the power distributor and the parasitic diode capacities. Other measurement branches with series connected attenuation stages are connected to the power distributor, so that a relatively large dynamic range is achieved in combination. 
   However, if the measured signal is associated with a direct-voltage component, a considerable measurement error occurs because of the direct-current-resistance of the stripline connecting the detector diodes and the associated drop in voltage. 
   Hitherto, it has been conventional to suppress the direct-voltage components of the measured signal with an isolating capacitor arranged between the high-frequency terminal and the detector diodes. An isolating capacitor of this kind is used, for example, in U.S. Pat. No. 4,943,764. Furthermore, with the solution proposed in this document, several diode pairs isolated from one another by power distributors are used to increase the dynamic range. However, the detector diodes pick up the two half-waves at the same measurement point. Accordingly, the relative insensitivity to reflections, which results from a spatial isolation of the detector diodes, as suggested in DE 100 39 665, is not available in this context. 
   The use of an isolating capacitor to suppress the direct-voltage component of the measured signal has several disadvantages. Firstly, the capacity of the isolating capacitor must be relatively large, in order to achieve a low lower-threshold frequency thereby allowing a broadband realisation of the power detector. This leads to a mechanically relatively large structure. Secondly, the isolating capacitor must be integrated in a low-reflection high-frequency line for microwave applications. This leads to additional interference reflections and therefore also to greater measurement errors. 
   SUMMARY OF THE INVENTION 
   The present invention addresses a need for providing a power detector, which is relatively insensitive to a direct-voltage component and at the same time insensitive to reflections, for example, on a terminal resistance or a power distributor. 
   The invention is based on the knowledge that a direct voltage de-coupling can be achieved by placing the pickups for the detector elements, for example, the detector diodes, not directly on the signal line, but on a detection line, which is coupled to the signal line in a capacitive and/or inductive manner. In theory, the detection line could be completely electrically isolated from the signal line. However, it is advantageous to connect the detection line to the signal line at only one contact position, and in this manner to provide a defined reference potential. Under no circumstances, however, should more than one contact position be provided along the longitudinal direction of the signal line, so that no direct current flows in the detection line, and an optionally present drop in voltage at the direct-current-resistance of the signal line does not lead to a drop in voltage in the detection line. The voltage pickups at the detection positions of the detection line are therefore at the same direct voltage potential and a direct-voltage component of the measured signal does not lead to a measurement error. 
   The contact position between the signal line and the detection line can be disposed at any desired position, for example, in one of the two terminal regions of the lines. 
   The detection line can be attached to the signal line using thin-layer, multi-layer technology with the interposition of a thin insulating layer. In principle, the signal line and the detector line could also be designed as coupled lines alongside one another in a coplanar manner. 
   It is advantageous if the line impedance of the detection line is relatively low and is in the same order of magnitude as the direct-current-resistance of the signal line. Even if the non-terminated detection line is resonant for individual measurement frequencies, strongly disturbing resonances do not occur because of the low circuit quality as a result of the high attenuation. 
   The further development according to the invention is suitable for two-way rectifiers each with only one detector diode for both half-waves and also for one-way rectifiers with only one detector diode, wherein the reference potential for the difference amplifier can be picked up at a pickup position on the detection line, which is offset relative to the pickup position of the detector diode. 
   Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Several exemplary embodiments of the invention will be described in greater detail below with reference to the drawings. The drawings are as follows: 
       FIG. 1  shows a schematic diagram of the solution for the arrangement of detector diodes suggested in the previously unpublished application DE 100 39 665; 
       FIG. 2  shows a schematic diagram of a first exemplary embodiment of the invention; 
       FIG. 3  shows a schematic diagram of a second exemplary embodiment of the invention; 
       FIG. 4  shows a possible realisation of the first exemplary embodiment of the invention as presented in  FIG. 2  using coplanar technology; 
       FIG. 5  shows a schematic diagram of a third exemplary embodiment of the invention; 
       FIG. 6  shows a schematic diagram of a fourth exemplary embodiment of the invention and 
       FIG. 7  shows a schematic diagram of a fifth exemplary embodiment of the invention with several measurement branches to increase the dynamic range. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a schematic diagram of a solution with an offset arrangement of the detector diodes of a power detector  1 , as proposed in the previously unpublished application DE 100 39 665 of the applicant. 
   A high-frequency input  2  is connected to a signal line  3 , which has a direct-current-(DC)-resistance R B  and terminates with a terminal-resistance  4 , which has the resistance value R 0 . Furthermore, a first detector diode  5 , which is connected by its anode to the signal line  3 , and a second detector diode  6 , which is connected by its cathode to the signal line  3 , are provided. At each connection of the detector diodes  5  and  6  disposed opposite to the signal line  3 , a charging capacitor  7  and/or  8  and a supply line  9  and/or  10  for the difference amplifier are provided. The first detector diode  5  supplies an output voltage corresponding to the measured signal of positive polarity relative to earth, while the second detector diode supplies an output voltage corresponding to the measured signal of negative polarity relative to earth. The difference amplifier  11  suppresses any direct voltage components superimposed on the measured signal and therefore on both output voltages, so that, at its output  12 , a measured signal is available, which is proportional to the power of the measured signal, provided the detector diodes  5  and  6  are adjusted in their quadratic range. 
   The resistance value R 0  of the terminal resistance  4  is, in general, adapted to the line impedance Z 0  of the signal line  3 . In the context of DE 100 39 665, this terminal resistance is formed by a power distributor, which allows a branching into further measurement branches, which have been omitted in the present context for the sake of simplicity. 
   In practice, a completely reflection-free termination of the signal line  3  cannot be achieved. Slight reflections may occur even at the detection positions  13  and  14  of the detector diodes  5  and  6 . The effect of these reflections can be reduced by means of an offset in the longitudinal direction of the detection positions  13  and  14 , as suggested for the first time in the previously unpublished DE 100 39 665 A1 of the applicant. It can be assumed that the distance between the detection positions  13  and  14  for the maximum frequency of the measured signal to be measured is approximately λ/4, wherein λ signifies the wavelength. If reflections occur, for example, at the terminal resistance  4 , then the phase difference between the incident and the reflected wave at the detection position  13  is displaced by approximately 180° relative to the detection position  14 . 
   This means that incident and reflected waves are superimposed, for example, at the detection position  13 , in a destructive manner, while they are superimposed at the detection position  14  in a constructive manner. The positive measurement voltage at the detector diode  5  is accordingly somewhat reduced, while the negative measurement voltage at the detector diode  6  is somewhat increased in magnitude. By addition of the values of the two measured voltages in the difference amplifier  11 , these effects are compensated relative to one another, so that, at least for the maximum measured frequency, a lower sensitivity to reflections is provided. For lower measurement frequencies, the compensation is no longer complete; however, with declining frequency, the reflection factor declines anyway, and accordingly, the need for compensation exists primarily at the upper end of the measurement frequency band. 
   However, the offset arrangement of the detection positions  13  and  14  on the signal line  3  has the disadvantage that, because of the voltage drop at the direct-current-resistance R B  of the signal line  3 , the detection positions  13  and  14  are not at exactly the same potential, if the measured signal at the high-frequency input  3  has a direct-voltage component. The detector diodes  5  and  6  typically have a rectifier sensitivity s=800 μV/μW, that is to say, with a supplied high-frequency power of 1 μW, there is a rectified voltage of 800 μV, wherein a linear relationship between the high-frequency power to be measured and the detected measured voltage can be assumed for small high-frequency powers (quadratic characteristic range). The direct-current-resistance R B  between the two detector diodes  5  and  6  is typically R B =0.5Ω. The terminal resistance R 0  is adapted to the impedance of the signal line  3  and is typically R 0 =50Ω. 
   The lower measurement threshold of detector diodes of this kind is approximately 0.1 nW. Now, if a measured signal with a direct voltage component of U DC =1 mV is provided at a high-frequency input  2 , then there will be a difference voltage of 10 μV across the detector diodes  5 ,  6  at the difference amplifier  11  because of the voltage distribution between the terminal resistance R 0  and the direct-current-resistance R B . This corresponds to an erroneously detected high-frequency power of 10 nW; that is to say, the drop in direct voltage at the resistance R B  is erroneously detected as a high-frequency power of 10 nW at the input, which exceeds the possible lower measurement threshold of 0.1 nW by a factor of 100. 
   As already explained, the insertion of a isolating capacitor between the high-frequency input  2  and the signal line  3  would also be disadvantageous. This would have to be of a relatively large dimension in order to achieve a low lower-threshold frequency for a broadband design of the power detector. On the other hand, the isolating capacitor would have to be integrated into the signal line, which would lead to additional interfering reflections and therefore greater measurement errors. 
     FIG. 2  shows a diagram of a first exemplary-embodiment of a power detector  1  according to the invention. Identical or corresponding components are indicated in all drawings with the same reference numbers, to avoid repetition of description. 
   According to the invention, it is proposed that the detection positions for the detector diodes  5  and  6  and/or other detector elements should not be provided directly on the signal line  3  but rather on a detection line  20 . The detection line  20  in the exemplary embodiment presented in  FIG. 2  is connected to the signal line  3  by a contact position  21  arranged at an end region facing towards a high-frequency input  2 . At all other positions, the detection line  20  is electrically isolated from the signal line  3 . It is important that only one contact position  21  is present along the longitudinal direction of the signal line  3 . Because the input resistances of the difference amplifier  11  are of very high-resistance, no significant current flows in the detection line  20 , and the entire detection line  20  is at a uniform direct voltage potential. Accordingly, there is no voltage drop between the detection positions  13  and  14 . The connection of the detection line  20  to the signal line  3  at exactly one contact position, seen in the longitudinal direction, is advantageous, because as a result, the detection line  20  is placed at a defined reference potential. It is also important that the detection line  20  is coupled to the signal line  3  so firmly in a capacitive and/or inductive manner, that, with reference to the longitudinal coordinate L, a voltage distribution of a high-frequency signal present on the detection line  20  is similar to that on the signal line  3 . 
     FIG. 3  shows-a second exemplary embodiment of the invention. By contrast with the exemplary embodiment shown in  FIG. 2 , the contact position  21  in this exemplary embodiment is not disposed at the input end of the detection line  20  facing towards the high-frequency input, but at the output end of the detection line  20  facing towards the terminal resistance  4 .  FIGS. 2 and 3  present only two examples of the arrangement of the contact position  21 . The contact position  21  may be located at any desired position in the longitudinal extension of the detector line  20 . 
     FIG. 4  shows a possible realisation of the exemplary embodiment presented in  FIG. 2  using thin-layer multi-layer technology. The signal line  3 , which, in the exemplary embodiment is realised as a coplanar stripline, is disposed on a substrate  30  made from a dielectric. The signal line  3  is therefore a thin stripline  31 , generally made from a metal, which in each case is isolated by a distance  32  and/or  33  from a metallic earth surface  34  and  35  respectively, each carrying an earth potential. A thin insulation layer  36  made from a suitable dielectric, for example, silicon nitride or silicon oxide, is disposed on the stripline  31 , which forms the signal line  3 . A further stripline  37 , which forms the detection line  20 , is disposed above the insulation layer  36 . The detection line  20  is only connected to the signal line  3  at the contact position  21 , which is formed by a metallic coating  38 . At all other positions along a longitudinal direction L, the detection line  20  is electrically isolated from the signal line  3  and only coupled in a capacitive manner via the thin dielectric insulation layer  36 . 
   The detection diodes  5  and  6  are shown only schematically in  FIG. 4 . Furthermore, the charging capacitors  7  and  8 , which are designed as thin-layer capacitors, are also shown. The base electrode of the thin-layer capacitors is formed from a part of the earth surfaces  35  and/or  34 , above each of which a thin insulation layer  39  and/or  40 , preferably made from the same material as the insulation layer  36 , is disposed in the place of the charging capacitors  7  and/or  8 . A metallic layer  41  and/or  42  is disposed above the insulation layer  39  and/or  40  respectively. Since the local form of the insulation layers  39 ,  40  above the first metallic coating and a second metallic coating arranged above that, is necessary anyway for the manufacture of the thin-layer condensers  7  and/or  8 , manufacturing the detection line  20  isolated from the signal line  3  by the insulation layer  36  does not represent an additional expenditure and can be realised using the same technology with a variation of the manufacturing masks. 
   While the exemplary embodiments according to the invention shown in  FIGS. 2 to 4  are presented for a two-way rectifier,  FIGS. 5 and 6  show two exemplary embodiments of a one-way rectifier with only one detector diode  5 , which is arranged at the detection position  13 . The other detection position  14  is connected via a resistance  50  in each case to the other input of the difference amplifier  11 . The exemplary embodiments shown in  FIGS. 5 and 6  differ again in that the contact position  21  in the exemplary embodiments shown in  FIG. 5 , is disposed at the end facing towards a high-frequency input  2 , and in the exemplary embodiment shown in  FIG. 6 , is disposed at the end of the detection line  20  facing towards the terminal resistance. With a one-way rectifier, it is also important that the reference potential tapped via the resistance  50  and supplied to the difference amplifier  11  is at the same direct-voltage potential as the pickup of the detector diode. 
     FIG. 7  shows a further exemplary embodiment of the invention. In the exemplary embodiment shown in  FIG. 7 , three measuring branches A, B and C are provided in order to increase the dynamic range, as suggested in principle in the previously unpublished application DE 100 39 665 of the applicant. Each measuring branch A in the exemplary embodiment comprises two detector diodes  5   A ,  6   A ,  5   B ,  6   B ,  5   C  and  6   C  respectively, which tap the signal, as described with reference to  FIG. 2 , at the detection positions  13   A ,  14   A ,  13   B ,  14   B ,  13   C  and  14   C  respectively of the detection line  20   A ,  20   B  and  20   C . As described with reference to  FIG. 2 , the detection lines  20   A ,  20   B  and  20   C  are coupled in a capacitive and/or inductive manner to the signal lines  3   A ,  3   B  and  3   C  respectively and are connected electrically to the signal lines only at one contact position  21   A ,  21   B  and  21   C  respectively. The measuring points  51   A ,  52   A ,  51   B ,  52   B ,  51   C  and  52   C  are each connected to a difference amplifier, which is not illustrated in  FIG. 7 . 
   The signal line  3   A  of the first measurement branch A is connected to the high-frequency input  2  without the intermediate connection of a isolating capacitor and, at the output end, is connected to a power distributor  53 , which evenly distributes the power to the second measuring branch B and the third measuring branch C. A first attenuation element  54   B , which attenuates the output power of the power distributor  53  by a defined attenuation factor, is disposed between the input of the signal line  3   B  of the second measuring branch B and one of the outputs of the power distributor  53 . The output of the signal-line  3   B  terminates with a terminal resistance  4   B . 
   The input of the signal line  3   C  of the third measuring branch C in the exemplary embodiment is connected via two series arranged attenuation elements  54   C,1  and  54   C,2 , to the other output of the power distributor  53 . The attenuation elements  54   C,1  and  54   C,2  are preferably somewhat spatially isolated by a line  55  to avoid any direct crosstalk from the input of the attenuation element  54   C,1  to the output of the attenuation element  54   C,2 . The signal line  3   C  terminates with a terminal resistance  4   C . 
   The measured signal is accordingly supplied to the signal lines  3   A  and  3   B  and  3   C  of the different measurement branches A, B and C with a different attenuation, so that the measuring branch A is suitable for measurement in the lower power range, the measuring branch B is suitable for measurement in the medium power range and measuring branch C is suitable for measuring in the upper power range. 
   The invention is not restricted to the exemplary embodiments presented. For example, the signal line  3  and the detection line  20  should be designed as inductively coupled microstriplines adjacent to one another with only one metallic coating on the upper side of the substrate  30 , wherein the substrate has only one continuous metallic earth on the underside, as is conventional with microstriplines. 
   While the present invention has been described in connection with a number of embodiments and implementations, the present invention is not so limited but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.