Microwave vector modulator and device for matching a microwave load
BACKGROUND OF THE INVENTION 
     The invention relates to a microwave vector modulator provided with a transmission line, to an input of which microwave radiation can be applied, and adjustable reflection means for adjustably reflecting at least part of the microwave radiation applied to the input. 
     The invention also relates to a device for the matching of a microwave load comprising a microwave source, a microwave load, a transmission line through which microwave radiation generated by the microwave source is applied to the microwave load, a vector modulator incorporated in the transmission line, with which an adjustable part of the microwave radiation generated by the microwave source can be reflected, and a quadrature detector to which at least part of the microwave radiation reflected by the microwave load and vector modulator is applied and to which also part of the microwave radiation generated by the microwave source is applied, whereby the quadrature detector controls the vector modulator so that a closed loop is obtained with which the microwave radiation reflected by the microwave load and vector modulator is cancelled out. 
     A vector modulator and its application in a device for cancelling reflections at a microwave load are known from the contribution of P. D. L. Beasley et al. at the International Radar Conference 1990, Washington. Mr. Beasley describes a CW radar installation which employs a single antenna for simultaneous transmitting and receiving. This necessitates the use of a vector modulator which in a closed loop compensates for reflections of microwave radiation at the antenna. It should be noted that actually it is not matching of a microwave load that is at issue here, but the prevention of reflections from penetrating into the receiver branch. 
     This solution permits a low-power vector modulator to be employed, whose internal losses can practically be ignored. 
     SUMMARY OF THE INVENTION 
     The invention described relates to a low-loss vector modulator which can handle large amounts of energy. It is therefore characterised in that the adjustable reflection means are provided with semiconducting surfaces incorporated in the transmission line and light-emitting means for the adjustable generation of a quantity of light with which the semiconducting surfaces are illuminated so as to obtain an adjustable reflection. 
     It is moreover characterised in that the vector modulator is provided with semiconducting surfaces incorporated in the transmission line and light-emitting means for the adjustable generation of a quantity of light with which the semiconducting surfaces are illuminated so as to obtain an adjustable reflection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Below, the invention is explained in detail with reference to the following figures: 
     FIG. 1 is a schematic diagram of the matching of a microwave load in which a vector modulator according to the invention is applied; 
     FIG. 2 is a cross-section of the vector modulator; 
     FIG. 3 is a cross-section of an alternative embodiment of the vector modulator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, marked by reference number 1, a microwave source is represented which through a transmission line 2 applies microwave radiation ot a microwave load 3. Incorporated in the transmission line is a vector modulator 4, a first coupler 5 which diverts part of the radiation generated by the microwave source and applies it as a reference signal to a quadrature detector 6, and a second coupler 7 which diverts at least part of the microwave energy reflected by the microwave load 3 and the vector modulator 4, and applies it to the quadrature detector 6. The inphase output signal I and the quadrature output signal Q of the quadrature detector 6 fully characterise the combined reflection of the microwave load 3 and the vector modulator 4. The elements mentioned so far, such as the microwave source 1, the transmission line 2, the microwave load 3, the coupler 5, the quadrature detector 6, and the coupler 7, are all known within the field of microwave technology. 
     The vector modulator 4 has two control inputs, an in-phase control input I and a quadrature input Q, which are connected with the in-phase output I and the quadrature output Q of the quadrature detector 6 via, respectively the amplifiers 8a, 8b and the low-pass filters 9a, 9b. In a line 10, through which the reference signal is applied to the quadrature detector 6, a phase shifter 11 is incorporated, with which the relative phase of the reference signal can be adjusted such that the just described configuration forms a closed loop, with which the combined reflection of the vector modulator 4 and the microwave load 3 is adjusted to zero. To achieve a specific adjustment behaviour, the loop gain and the loop bandwidth can be entirely selected in conformity with established theory in the field of control engineering, given a required step response and an acceptable residual reflection. 
     In FIG. 2 an embodiment of the vector modulator 4 is represented. The vector modulator 4 is accommodated in a transmission line 12 having connecting flange 13 as its input and connection flange 14 as its output. The transmission line 12 has been divided into two compartments 15 and 16 by a partition 17 which is tapered in a manner well-known in the field such that at the partition substantially no reflections arise. The adjustable reflection occurs at four semiconducting surfaces 18, 19, 20, 21. The semiconducting surfaces 18 and 19 are fixed to a spacer element 22 of, for example, plastic foam such that their distance apart is approximately ##EQU1## where λ is the wavelength of the microwave radiation in the transmission line; they are accommodated in the compartment 16. The semiconducting surfaces 20 and 21 are fixed to a spacer element 23 of, for example, plastic foam such that their distance apart is approximately ##EQU2## they are accommodated in the compartment 15. The spacer elements 22 and 23 are shifted, relative to the propagation direction of the microwave radiation by a distance of ##EQU3## 
     The combination of two semiconducting surfaces fixed to a spacer element will produce substantially no reflection because the reflections of the individual semiconducting surfaces cancel each other out owing to the carefully chosen distance apart. Consequently the entire modulator will produce virtually no reflection. 
     The situation becomes different if one of the semiconducting surfaces is illuminated by photons capable of creating free charges in the semiconducting material. Significant reflection will occur as ##EQU4## where σ is the conductivity of the semiconducting material caused by the photoelectric effect, c the speed of light, ε the dielectric constant of the semiconducting material, and λ the wavelength of the microwave radiation. 
     To utilize this physical effect, light-emitting diodes 24, 25, 26, 27 are provided to illuminate, respectively, the semiconducting surfaces 18, 19, 20 and 21 through the holes 28, 29, 30, 31 in the transmission line. These light-emitting diodes may be replaced by semiconductor lasers. The holes may be provided with light conductors. 
     When we apply a current I d  to the light-emitting diode 24, the semiconducting surface 18 is illuminated via the hole 28. Semiconducting surface 18 then gives off a reflection, the relative phase of which we situate in the complex plane along the positive x axis. The magnitude of the reflection as a function of the current I d  depends on several parameters: the type of the light-emitting diode, the carrier lifetime of the silicon, the surface recombination rate of the silicon. It is certain that for the reflection R the following applies: 
     
         R=f(I.sub.d) with f rising monotonously. 
    
     The selection of the parameters will be discussed later. 
     In the manner outlined above we can cause, by applying a current I d  to the light-emitting diode 25, a reflection at semiconducting surface 19; because of the path length difference of ##EQU5## the reflection is situated in the complex plane along the negative real axis. Likewise, a current I d  through light-emitting diode 26 will cause a reflection which is situated in the complex plane along the positive imaginary axis. A current I d  through light-emitting diode 27 will cause a reflection which is situated in the complex plane along the negative imaginary axis. 
     To realise a reflection of specified phase and amplitude, we resolve the reflection into a real part and an imaginary part. The real part is effected by a current I d  through light-emitting diode 24 or light-emitting diode 25, depending on the specified reflection being situated in the first or fourth quadrant of the complex plane, or, in the second or third quadrant. 
     The imaginary part is effected by a current I d  through light-emitting diode 26 or light-emitting diode 27, depending on the specified reflection being situated in the first or second quadrant of the complex plane, or, in the third or fourth quadrant. 
     Because always one at most of the two light-emitting diodes 24, 25 draws current, it is possible to connect them in antiparallel. Then, a positive control current causes a reflection along the positive real axis, and a negative control current causes a reflection along the negative real axis. Likewise, connecting the light-emitting diodes 26, 27 in antiparallel achieves a control input which in the case of a positive control current effects a reflection along the positive imaginary axis and in the case of a negative control current effects a reflection along the negative imaginary axis. In this way, control of the vector modulator by the amplifiers 8a, 8b is a simple matter. The amplifiers need to be dimensioned such that they control the light-emitting diodes 24, 25, 26, 27 through current, control through voltage adversely influencing the operation of the closed loop because of the threshold voltages of the light-emitting diodes 24, 25, 26, 27. 
     In FIG. 3 an alternative embodiment of the vector modulator 4 is represented. The vector modulator 4 is accommodated in a transmission line 12 having connecting flange 13 as its input and connecting flange 14 as its output. The adjustable reflection occurs at two semiconducting surfaces 32, 33 each of a thickness such that the reflections of microwave radiation of the front and the rear exhibit a phase difference of 180°. For semiconducting material having a relative dielectric constant ε r  this means that the thickness is substantially ##EQU6## 
     The semiconducting surfaces 32, 33 are shifted, along the propagation direction of the microwave radiation in the transmission line, over a distance of substantially ##EQU7## relative to each other. 
     Entirely analogously to the previously described embodiment, the alternative embodiment is provided with light-emitting diodes 24, 25, 26, 27 and holes 28, 29, 30, 31 in the transmission line which, together with the front and rear of the semiconducting surfaces 32, 33, can realise any required reflection. 
     When the semiconducting surfaces 32, 33 are not illuminated they will, because of their usually high dielectric constant ε r , cause a considerable reflection. To prevent this, they are embedded in λ/4 adapters 34, 35, according to a well-known technique in the field of microwaves, involving the compensation of a disturbance in a transmission line by an intentionally introduced second disturbance, which causes a reflection of equal amplitude but with a 180° phase difference. 
     Among the parameters to be selected, an important one is the carrier lifetime of the semiconducting material used. A short carrier lifetime enables a vector modulator to be realised which has a fast step response. Two drawbacks are attached to this. A short carrier lifetime requires a larger control power for the light-emitting diodes 24, 25, 26, 27. Moreover it increases the risk that noise and interferences, penetrating into the control lines of the light-emitting diodes 24, 25, 26, 27, occasion a noise modulation of the microwave radiation which is applied to the microwave load 3. Particularly if the microwave load 3 is formed by an antenna for a CW radar installation, noise modulation becomes a serious problem. 
     A very economical embodiment of the vector modulator is obtained when the semiconducting material is chosen such that the closed loop bandwidth is determined by the carrier lifetime of the semiconducting material. This obviates the need for the low-pass filters 9a, 9b, and makes the vector modulator substantially unsusceptible to high-frequency noise and interferences in the control lines. 
     If a semiconducting material is chosen which has a long carrier lifetime, account has to be taken with an additional recombination mechanism, the surface recombination. This effect can be minimized through the etching of the semiconducting surfaces, which is an established technique in the field of semiconductor physics. This technique may be combined with the application of an anti-reflection coating on the semiconducting surfaces, without which 40% of the incident light is reflected.