Abstract:
A non-directional coupler including a semiconductor junction in series with a capacitor, the semiconductor junction being formed so that the threshold frequency short of which it behaves as a rectifier is smaller than the coupler&#39;s operating frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/949,941, filed Sep. 24, 2004 entitled INTEGRATED COUPLER, which application is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of couplers having the function of sampling part of the power present on a main line towards a secondary line for control and feedback. Couplers are generally used in closed-loop gain-control systems to provide a real measurement of the power. 
   2. Discussion of the Related Art 
     FIG. 1  very schematically shows an example of a conventional circuit using a coupler. This example relates to the control of a power amplifier  1  (PA) for amplifying a useful signal UTI for a transmit antenna  2 . In this type of application, the transmit power is controlled with a power reference PL. A coupler  3  is interposed between the output of amplifier  1  and antenna  2  to extract data proportional to the power actually transmitted. This data is exploited by a detector  4  (DET) providing a measured value MES to a comparator  5  with required power PL. Comparator  5  provides a control signal REG to amplifier  1 . 
   Two large coupler categories are essentially known. A first category relates to so-called distributed couplers, which are formed from coupled transmission lines. A second category relates to couplers with local components, based on capacitors and inductances. 
   Distributed couplers are directional, that is, they detect the direction of the measured power and are sensitive to dimensional variations of the lines. Such couplers are bulky due to the size of the lines to be formed, especially for radio frequency applications (from several hundreds of MHz to a few GHz). 
   Couplers with local components are non-directional. They have the advantage of having a large passband and of being more compact. 
   As illustrated in  FIG. 1 , a coupler is defined by four ports or terminals IN, DIR, CPLD, and ISO. Terminals IN and DIR are on the main line while terminals CPLD and ISO define the coupled secondary line. In  FIG. 1 , terminal IN is on the side of power amplifier  1  while terminal DIR is on the side of antenna  2 . Terminal CPLD is the terminal on which is sampled the information proportional to the power in the main line. In a non-directional coupler, to which the present invention applies, terminals IN and DIR are one and the same and terminal ISO generally does not exist. 
   The main parameters of a non-directional coupler are: 
   the coupling factor (generally on the order of from 10 to 30 dB) which corresponds to the path loss between ports IN and CPLD (the other port being loaded with a standardized impedance, generally 50 ohms); and 
   the insertion loss in the desired passband which corresponds to the path loss between ports IN and DIR (the other port being loaded with a standardized impedance, generally 50 ohms) and which is desired to be as small as possible (smaller than 1 dB and preferably on the order of 0.5 dB) to minimize the attenuation of the wanted signal due to the presence of the coupler. 
     FIG. 2  shows the electric diagram of a conventional non-directional coupler with local components. Such a coupler is essentially formed of the association of two cells  31  and  32  respectively forming high-pass and low-pass filters. Cell  31  comprises a capacitor C 31  having a first electrode connected to transmit line  12  (confounded terminals IN and DIR) and having a second electrode connected, by an inductance L 31 , to ground. The second electrode of capacitor C 31  also constitutes an input terminal of cell  32  formed of an inductance L 32  connecting this second electrode to terminal CPLD, terminal CPLD being further grounded by a capacitor C 32 . 
   A disadvantage of passive couplers with local components such as that illustrated in  FIG. 2  is linked to the dispersions (on the order of 20%) of the inductive and capacitive components upon manufacturing thereof. Such dispersions are reflected on the coupler parameters, which are given for an operating frequency band. 
   Theoretically, it is also possible to form high-pass and low-pass filters based on resistive and capacitive elements to form a coupler. However, the required number of stages (filter order) results, in practice, in a large size filter. Further, the dispersion problem is also present for resistors. 
   Above all, such structures are, in practice, not integrable in high-frequency applications (over one hundred MHz) more specifically aimed at by the present invention, due to the small required values, especially for capacitors (less than one picofarad). 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a novel integrable coupler architecture. 
   The present invention more specifically aims at providing a non-directional coupler, the parameters of which are free of the dispersion problems of conventional couplers with local components. 
   The present invention also aims at enabling easy and accurate setting of the values of the coupler components. 
   To achieve these and other objects, the present invention provides a non-directional coupler comprising a semiconductor junction in series with a capacitor, the semiconductor junction being formed so that the threshold frequency short of which it behaves as a rectifier is smaller than the coupler&#39;s operating frequency. 
   According to an embodiment of the present invention, said semiconductor junction is formed in an epitaxial layer, the thickness of which conditions the threshold frequency from which the junction no longer has a rectifying function. 
   According to an embodiment of the present invention, said capacitor has a value greater than 10 picofarads. 
   According to an embodiment of the present invention, the semiconductor junction is sized to exhibit, at the coupler&#39;s operating frequency, a series capacitance on the order of a few hundreds of femtofarads and a series resistance on the order of a few tens of ohms. 
   The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2 , previously described, are intended to show the state of the art and the problem to solve; 
       FIG. 3  shows the diagram of an embodiment of a coupler according to the present invention; 
       FIG. 4  shows a coupler according to the present invention connected to a detector of a control loop of the type illustrated in  FIG. 1 ; 
       FIG. 5  shows in a very simplified cross-section view, an example of the forming of a coupler in a silicon wafer according to the present invention. 
   

   DETAILED DESCRIPTION 
   The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those components which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the exploitation that is made of the measurements performed by a coupler according to the present invention has not been described in detail, the present invention applying whatever the type of measurements performed and whatever the transmit line on which the coupler is connected. 
   A feature of the present invention is to form an integrated coupler in the form of a semiconductor junction (PN) in series with a capacitor. 
     FIG. 3  shows an embodiment of a coupler  3  according to the present invention. 
   A PN junction  35  is connected by a first terminal (indifferently P or N) to transmit line  12  (confounded terminals IN and DIR) while its other terminal is connected to a first electrode of a capacitor  36  having its other electrode defining terminal CPLD of the coupler. 
   According to the present invention, PN junction  35  is used, not as a rectifying element but, at the frequencies desired for the coupler operation, to form a capacitor  351  in series with a resistor  352 , both of very small value. “Very small value” means a capacitance  351  of less than one picofarad and a resistance  352  of less than 100 ohms. The PN junction is thus formed to avoid rectifying the signal at relatively high operating frequencies (greater than some hundred MHz) chosen for the coupler. According to a preferred example, it is formed with an intrinsic area (PIN diode), for example, in an epitaxial layer. 
   Capacitor  36  has the function of blocking the D.C. component. Its value is sufficient to be neglected in the series association with capacitor  351 . Preferably, a value greater than 10 picofarads fulfils these requirements. The function of capacitor  36  will be better understood in relation with the description of  FIG. 4  integrating the coupler in its application. 
     FIG. 4  shows a coupler  3  according to the present invention shown in the form of a diode  35  in series with a capacitor  36 , and having its terminal CPLD connected to a detection circuit  4  providing a measurement signal MES for a comparator. Circuit  4  is a conventional circuit, for example, of the type illustrated in  FIG. 1 . It comprises a rectifying element  41  (for example, a diode) having its anode connected to terminal CPLD and having its cathode connected, via a capacitor  44 , to ground. The cathode of element  41  is further connected, by a resistor  42 , to terminal MES which is grounded by a capacitor  43 . Capacitors  43  and  44  form with resistor  42  a low-pass filter reducing the ripple of the D.C. signal sampled on terminal MES. 
   Detector  4  further comprises a temperature-compensated bias circuit setting a level Vdc on the anode of diode  41 . This circuit is formed of two resistors  45  and  46  in series between a terminal  47  of application of voltage Vdc and the anode of diode  41 . The midpoint of this series connection is grounded by a diode  48  in series with a resistor  49 . Such an assembly enables obtaining an exploitable level even for low powers of signals carried on main line  12  (smaller than 0 dBm). Without this biasing, diode  41  would be blocked for such levels. However, the presence of this bias voltage requires using capacitor  36  to avoid a continuous biasing of PN junction  35  of coupler  3 , which would null out the desired operation. 
     FIG. 5  illustrates an example of integration in a silicon substrate  7  of a PN junction  35  of a coupler according to the present invention. To obtain the desired absence of a rectifying effect, an epitaxial region  71  is provided between a P+ doped region  72  and N+ doped substrate  7 . This is an example, and the doping types may be inverted. A first (anode) contact  73  is taken on region  72  and a second (cathode) contact  74  is taken on the N+ region that is, on substrate  7 . 
   Other junction configurations may be envisaged provided to respect the absence of a rectifying effect at the desired operating frequencies. For example, the PN junction may be formed from a diode-assembled NPN-type bipolar transistor (connected base and collector). 
   The threshold frequency fs from which the PN junction no longer rectifies the signal is a function of the carrier transit time (designated as tt). This frequency is proportional to the inverse of the transit time. 
   If the main line signal has a frequency greater than threshold frequency fs, the voltage switches from a negative value to a positive value and conversely, with a periodicity smaller than the transit time. The forward incursion is too steep to cause a current and the carrier is carried off by the negative halfwave before recombining, and thus before generating a rectified current. Under such circumstances, the PN junction is considered as a capacitor in series with a resistor. 
   As a first approximation, it can be considered that the transit time essentially is a function of the epitaxial layer thickness and of the carrier diffusion coefficient. More specifically, time tt is proportional to W 2 /D, where W represents the thickness of the epitaxial layer and D represents the carrier diffusion coefficient. 
   For a diffusion coefficient of carriers D on the order of 13 cm 2 /s, which is a usual value in present technologies, frequency fs approximately is 1300/W 2  (fs in MHz and W in μm). Generally, in light dopings used to form the diodes, diffusion coefficient D of the carriers can be considered as being constant. Accordingly, the smaller the thickness of the epitaxial layer, the higher the frequency from which the PN junction does not have a rectifying behavior. 
   It should be noted that the doping level of the regions has but little influence on the threshold frequency of the PN junction. 
   A specific example of application of the present invention relates to couplers used in the field of mobile telephony (GSM and DCS). The value of capacitor  351  is on the order of a few hundreds of femtofarads. This value resulting from the diode forming can be adjusted by setting, according to the desired response and taking into account possible stray capacitances, especially the epitaxy doping, the active surface, and the epitaxy thickness in case of a full depletion. Such a value is compatible with frequencies on the order of one GHz. 
   Similarly, resistive component  352  is on the order of a few tens of ohms, which is compatible with the forming of a PN junction and, again, adjustable according to the desired characteristics, by setting the interval between the P+ and N+ regions. 
   It should be noted that, in case of an integration with the detection circuit, the anode of the diode thus formed directly forms the terminal on which line  12  is connected, that is, the antenna and the output of the power amplifier. 
   An advantage of a PN junction to form the coupler is that, in the form of an active coupler, its parameters are controllable even for small capacitance and resistance values, with a much smaller dispersion (linked to the semiconductor technology). The “active” coupler thus becomes integrable. It can then be integrated on a same chip as that of the detection circuit ( 4 ,  FIG. 4 ). 
   The values to be given to resistance  352  and capacitance  351  by the configuration of junction  35  are determined by usual modeling and simulation tools according to the desired and/or acceptable coupling factor and insertion loss at the selected operating frequencies. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the practical forming of a diode fulfilling the constraints given by the present invention to form a coupler is within the abilities of those skilled in the art based on the functional and sizing indications given hereabove. Further, the present invention applies for a lateral diode as well as for a diode made in a vertical technology and whatever the type of formed diode (PN diode, PIN diode, etc.), provided that it is sufficiently slow with respect to the desired operating frequencies. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.