Abstract:
A harmonic rejection tuner, used for reflecting RF power at the harmonic frequencies in a load pull measurement setup, uses adjustable capacitive grounding of half wavelength long resonator stubs in order to vary the electrical length and thus the resonant frequency of the resonators. Since the resonators themselves are by nature narrowband, this frequency adjustability allows for a higher frequency bandwidth and better coverage of operational frequency range of the test setup. Since load pull measurements are carried out mostly at fixed frequencies, adjustment of the capacitive shorts can be either manual or by remote control. Capacitive shorting the resonator stubs allows DC bias to be applicable to the device under test through the tuner.

Description:
CROSS-REFERENCE TO RELATED ARTICLES 
     
         
         
           
             [1] U.S. Pat. No. 6,297,649, “Harmonic rejection load tuner”, Christos Tsironis 
             [2] U.S. Pat. No. 7,042,233, “Harmonic rejection tuner”, Philippe Boulerne 
             [3] “Computer Controlled Microwave Tuner System—CCMT”, Product Note  41 , Focus Microwaves 
           
         
       
    
     PRIORITY CLAIM 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates to load pull testing of microwave power transistors, which use automatic microwave tuners in order to synthesize reflection factors (or impedances) at the input and output of the transistors at various harmonic frequencies. 
     Modern design of high power microwave amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device&#39;s (microwave transistor&#39;s) characteristics. In such circuits, it is insufficient and inaccurate for the transistors operating at high power in their highly non-linear regions and close to saturation, to be described using analytical or numerical models only. Instead the devices must be characterized using specialized test setups under the actual operating conditions. 
     A popular method for testing and characterizing such microwave components (transistors) for high power operation is “load pull” and “source pull”. A typical load pull setup is shown in  FIG. 1 . It includes a signal source  1 , a microwave source tuner  2 , a test fixture  3  in which a device under test (DUT) is mounted, an output tuner  4  and a power meter  5 . The tuners  2 ,  4  and the test equipment  1 ,  5  are remotely controlled  6 ,  7 ,  8  by a system computer  9  running appropriate software for tuner control and instrument communication (GPIB) for data collection and processing. 
     The microwave tuners  2 ,  4  in the setup of  FIG. 1  synthesize the microwave impedances presented to the DUT  3  for which the test takes place. The end-result of a load pull testing session is the dependence of DUT parameters, like Gain, Power and Efficiency etc. on load and source impedance, created by the load and source tuners. 
     Electro-mechanical tuners [3] have several advantages compared to electronic and active tuners, namely long-term stability, higher handling of microwave power, easier operation and lower cost. Electro-mechanical tuners are made using the slide-screw concept, of which a cross section is shown in  FIG. 2 , which employs adjustable mechanical obstacles (probes or “slugs”)  10  inserted into the transmission media of the tuners, which is a slotted coaxial or parallel plate airline (slabline)  11 ,  FIG. 2 , at an adjustable distance from the central conductor  12 ; this insertion  13  of the slugs creates capacitive coupling between probe and central conductor and allows to reflect part of the power coming out of the DUT output port back into the DUT and to create, this way, a controllable reflection factor (or impedance) that is presented to the DUT. 
     When the DUT&#39;s (transistors) operate at high power, close to power saturation, they become strongly non-linear and deform the sinusoidal input signal injected by the signal source and therefore, following Fourier&#39;s law, they generate power at harmonic frequencies. If the injected signal is at a fundamental frequency fo then harmonic power is generated at 2fo, 3fo, 4fo etc. ( FIG. 1 ). This harmonic power is normally lost when fed into a wideband RF load  5  and reduces the operational efficiency of the transistor  3 . A test method, which allows the harmonic power to be recovered and re-injected back into the DUT, is called “harmonic load pull”. Harmonic load pull uses harmonic tuners in order to manipulate the impedances at the harmonic frequencies. A typical harmonic load pull setup is shown in  FIG. 3 . 
     DESCRIPTION OF PRIOR ART 
     The harmonic load pull setup of  FIG. 3 , uses a signal source  14 , an input bias tee  15  for applying control voltage to the DUT  20 , a wideband input tuner  16 , which uses a capacitively coupled probe  17 , and an input harmonic rejection tuner  18  [1], which uses two sets of open λ/4 resonator stubs  19 , before the DUT  20 . The combination of wideband tuner  16  and harmonic tuner  18  allows controlling the reflection factor (or impedance)  21 , seen by the DUT  20  at the fundamental frequency fo and the harmonic frequencies 2fo and 3fo. 
     The same is achieved in the setup of  FIG. 3  in the section between DUT  20  and load  22 : A harmonic rejection tuner  23  using λ/4 open stubs  24  is inserted between DUT  20  and wideband load tuner  25 , which also uses capacitively coupled probes,  25   a , and to an output bias tee  26 , which serves to apply output supply voltage to the DUT  20 . On both sides of the DUT  20 , towards the signal source  14  and the load  22 , designated here as “source” and “load” side of the DUT correspondingly, the reflection factors  21  and  21   a  are maximized, because the insertion loss of any section between DUT  20  and tuners  18 ,  23  is minimized. 
     Harmonic rejection tuners, as used in the setup of  FIG. 3  are described in [1],  FIG. 5 . They use a slotted airline (slabline) and a set of open stubs with a length of λ/4 at the harmonic frequency, which slide on top of the central conductor of the slabline. The open stubs create serial resonance, which corresponds to an RF short circuit at the contact point with the central conductor  43  at the harmonic frequency, thus reflecting back harmonic power to the transistor. By moving the open stubs along the slotted airline the phase of the microwave reflection factor can be freely controlled. This, in general, creates test conditions satisfying most measurement requirements for harmonic load pull [1]. 
     Another possible configuration of such “harmonic rejection” tuners uses galvanically shorted stubs [2],  FIG. 6 , instead of open stubs [1]. In the case of shorted stubs the length of each stub must be λ/2 at the harmonic frequency. The galvanic short is then transformed by the λ/2 length of the stub into another RF short at the reference plane of the central conductor  75   a    FIG. 11 . The effective length and therefore the resonant frequency of such resonators may be modified by moving the galvanic short closer or further away from the central conductor using perfect sliding metallic contacts [2]. 
     Harmonic rejection tuners using galvanized adjustable shorted stubs [2] have a number of practical disadvantages: 
     A first disadvantage is that harmonic rejection tuners using shorted stubs [2] cannot be used in a setup as shown in  FIG. 3 ; this is because the DC shorts, as described in [2] will not allow the DUT  20  to be biased. Therefore an alternated test setup, as shown in  FIG. 4  must be used. 
     In this setup ( FIG. 4 ) the signal source  27  injects a signal into the DUT  34  through a wideband tuner  28  using capacitive probes (slugs)  29  followed by the harmonic rejection tuner  30  using galvanized shorted λ/2 stub resonators  31  and a bias tee  32 ; the reflection factor  33  seen by the DUT  34  is lower than in the setup of  FIG. 3 , because of the insertion loss  35  of the bias tee  32 . The same is valid at the output of the DUT  34 , where an output bias tee  36  must be inserted between the DUT  34  and the load harmonic rejection tuner  37 , the wideband load tuner  39  and the microwave load  40 ; the reason for this is, that the output harmonic rejection tuner  37  uses shorted stub resonators  38 , which do not allow DC bias to reach the DUT  34 . For the same reasons as before the reflection factor  39  and therefore the tuning range, seen by the DUT  34 , is going to be reduced, compared with the setup in  FIG. 3 . 
     Another disadvantage of harmonic rejection tuners using shorted stubs [2] is that the galvanic short circuits at the end of the stubs will create short circuits not only at the harmonic frequencies, which are in general quite high, in the GHz range, but also at lower frequencies down to DC, as seen by the transistor DUT  34  and this, because the gain of the transistor DUT&#39;s is always much higher at lower frequencies, always creates favorable conditions for spurious low frequency oscillations of the whole test system, in which case testing becomes impossible. 
     To avoid the problems associated with galvanic shorts [2] the solution proposed hereby is a harmonic rejection tuner apparatus where only RF shorts, that is short circuits effective only above a certain frequency, are used; this is possible through capacitive coupling between the resonator stubs and the ground plane surrounding them; these capacitive shorts can also be moved up and down along the stub to adjust the effective length and the resonance frequency, while at the same time there is no galvanic contact between the parallel stub itself and the ground to prevent the DUT to be biased and there is very low reflection at low frequencies, thus reducing the risk for spurious oscillations. 
     What is also proposed is an arrangement for resonance frequency adjustment of open λ/4 stubs as described in [1]; this is made using movable dielectric rods (tubes), which envelop the open stub and add adjustable electrical length by creating an adjustable capacitive load. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention and its mode of operation will be more clearly understood from the following detailed description when read with the appended drawings in which: 
         FIG. 1  depicts prior art, a load pull test set-up using passive electro-mechanical tuners. 
         FIG. 2  depicts Prior Art; Cross section of slide screw Load Pull Tuner 
         FIG. 3 : depicts Prior Art; Harmonic Load Pull setup using harmonic rejection tuners based on open stub λ/4 resonators 
         FIG. 4 : depicts Harmonic Load Pull setup using harmonic rejection tuners with λ/2 shorted stub resonators 
         FIG. 5 : depicts Prior Art; Harmonic rejection tuner using λ/4 open stub resonators 
         FIG. 6 : depicts Prior Art; Shorted stub including another resonator adapted to compensate for the effect of moveable shorted stub 
         FIG. 7 : depicts Harmonic rejection tuner using a pair of open-stub resonators with adjustable capacitive loading for modifying the resonance frequency. 
         FIG. 8 : Detail view of free standing capacitive load of λ/4 open stub harmonic resonator 
         FIG. 9 : Harmonic rejection tuner using a pair of capacitive loaded stub resonators 
         FIG. 10 : Detail view of close fitting capacitive load λ/4 open stub harmonic resonator 
         FIG. 11 : Harmonic rejection tuner using a pair of capacitive adjustable shorted λ/2 stub resonators 
         FIG. 12 : Detail view of adjustable capacitive shorts applied on λ/2 resonator stubs 
         FIG. 13 : Semi-automated harmonic rejection tuner, in which the resonance frequency of open λ/4 resonators is adjusted by computer control 
         FIG. 14 : Semi-automated harmonic rejection tuner, in which the resonance frequency of capacitive shorted λ/2 resonators is adjusted by computer control 
         FIG. 15 : Harmonic rejection tuner using λ/4 open stub resonators with fully automated resonance frequency adjustment 
         FIG. 16 : Harmonic rejection tuner using capacitive shorted λ/2 stub resonators with fully automated resonance frequency adjustment 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention describes harmonic rejection tuner configurations, which allow adjustability of the frequency of operation, while at the same time being free of practical problems and limitations of previously proposed alternatives. 
     The proposed harmonic rejection tuner is similar to prior art [1] as depicted in  FIG. 5  comprising a transmission media in form of a slotted coaxial or parallel plate airline (slabline) with an input (test) and an output (idle) port and one set of two harmonic resonators per harmonic frequency to be rejected (reflected back to the DUT), said test port being connected to the device under test (DUT) and said idle port being connected to the RF source or load (FIGS.  3 , 4 ). 
     The harmonic resonators are made of a coaxial structure comprising an external ground conductor  41  and a metallic stub  42  serving as central conductor of the resonator, which has sliding galvanic contact on its bottom end  43  with the central conductor  44  of the airline  45  and is either open [1] or shorted [2] on the top end  46 ; said central conductor stub of the resonator  42  being centered and secured against the metallic ground walls of said resonator by means of low loss and low dielectric constant dielectric washers  47 . 
     In a first configuration ( FIG. 7 ) the λ/4 open resonator stubs of the tuner disclosed in [1], the top end  46  of each resonator stub  42  is embedded in a dielectric cylinder  48 , which creates a capacitive load to the resonator stub  42 , as it is moved up and down  49  in such a way as to increase or decrease the depth at which the open end of the stub is inserted inside the dielectric material  49 . 
     A detail view of the free-standing capacitive load of λ/4 open stub resonators is shown in  FIG. 8 . The open stub is secured against the ground walls by a low dielectric washer  58 . The dielectric cylinder  54  slides vertically  56  and closely fitting on the open stub  55 , but free standing inside the cylindrical resonator ground walls  57 . This creates a relatively small capacitive load, but ensures continuous operation, because there are no critical sliding contacts with the ground walls  57 . 
     In a second configuration of the open stub λ/4 harmonic resonators ( FIG. 9 ) the dielectric load consists of dielectric cylinders  51  which envelop the open stub closely  50  and can be moved up and down  52  in such a way as to increase or decrease the capacitive load and therefore the effective electrical length of the resonators and their resonance frequency. Compared with the configuration in  FIGS. 7 and 8 , where the dielectric cylinder  48  is free standing, in the configuration of  FIG. 9  the dielectric cylinder makes sliding contact with the ground plane  53  and this increases the capacitive load and therefore the range of frequency tuning. A disadvantage of this contacting dielectric load  51  is that, would the sliding contact not be perfect, the capacitive load may change abruptly and cause a jump in resonance frequency. 
     A detail view of the closely fitting capacitive load of λ/4 open stub resonators is shown in  FIG. 10 . The open stub is secured against the ground walls by a low loss dielectric washer  59 . The dielectric cylinder  60  slides vertically  61  and closely fitting on the open stub  62  as well as on the inside  63  of the cylindrical ground walls  64 . This creates a higher capacitive load than in the configuration of  FIG. 8  at the risk of discontinuity of frequency tuning as described before. 
     In a third configuration ( FIG. 11 ) the galvanically shorted λ/2 resonator stubs used in the tuner disclosed in [2] ( FIG. 6 ), the galvanic shorts between the resonator stubs ( 11 - 15  and  12 - 16  in  FIG. 6  [2]) and the ground plane are replaced by capacitive shorts  65  ( FIG. 11 ) between the resonator stubs  67  and a metallic cylinder  66 ; details are shown in  FIG. 12 . The resonator stubs  67  themselves are centered inside the cylindrical metallic ground plane  73  by means of low loss dielectric washers  74  and make sliding metallic contact with the central conductor  75  of the slabline  76 . 
       FIG. 12  shows a detail of the capacitive shorts applied on λ/2 resonator stubs  68 ; said capacitive shorts are made using a metallic ground rod  69 , which slides on the inner side of the cylindrical ground walls  70  and makes perfect galvanic ground contact, whereas on the inside diameter  71  it is isolated from the λ/2 resonator stub  68  by a thin dielectric film  72  such as Capton or Mylar, which are flexible dielectric materials of high dielectric constant and high durability, able to withstand for long periods of time friction caused by the two metals, the resonator stubs  68  and the surrounding metallic cylinders  69  sliding one inside the other. The thickness of such dielectric films is on the order of 0.002″ to 0.004″ with dielectric constants between 4 and 10, thus creating sufficient capacitive coupling between the resonator stubs  68  and the ground  69  for effective short circuiting the resonator stubs above a few hundred MHz. 
     Capacitively shorted resonant stubs  67  do not create a DC ground short circuit and allow therefore harmonic rejection tuners to be used in setups as shown in  FIG. 3 , where the bias tee can be inserted before the source tuner and after the load tuner, thus not reducing the tuning range due to their insertion loss. 
     The resonators in all hitherto tuner configurations,  FIGS. 13-15 , are mounted into appropriate holdings (carriages) which are driven by lead screws and computer controlled stepper motors and slide on top of the slabline in such a manner as to make perfect galvanic contact between the bottom of the resonators  77  and the top  78  of the slabline  81  and the bottom tip of the resonator stubs  79  with the top of the central conductor  80  of the slabline  81 . 
       FIGS. 7 ,  9  and  11  show harmonic tuners with manually adjustable resonance frequency of the harmonic resonators. In a first semi-automated configuration the depth of the dielectric loads of the λ/4 open stub resonators ( FIG. 13 ) are remotely controlled by stepper motors  81  and lead screws  82  are attached to the dielectric cylinders  84  and can move them deeper over the resonator stubs  85  to modify the capacitive load and thus the electric length and resonance frequency of the resonators. 
     A similar semi-automated setup is used in the case of shorted λ/2 stub resonators ( FIG. 14 ); in this case, same as in the apparatus in  FIG. 13 , computer controlled stepper motors  86  and lead screws  87  hold the sliding shorts  88  capacitively coupled  89  to the resonator stubs  90 , which slide on the central conductor  91  of the slabline  92 . The resonator stubs  90  are centered and secured inside the ground walls  94  of the resonators using low loss dielectric washers  93 . 
     In previously presented harmonic rejection tuners, [1][2], there is a solid adjustable link between resonators of the same harmonic frequency, as shown in  FIG. 5  (prior art,  FIGS. 11A and 11B  of [1]). One of the carriages is then linked to a remote control mechanism consisting of a lead screw, gear, stepper motor and computerized control, which allows positioning the set of both resonators at will along the slotted airline. In this case the horizontal distance between resonators must be re-adjusted manually for each new resonance frequency. 
     In a fully automated configuration ( FIG. 15 ) each carriage  95 ,  96  carrying a harmonic resonator  97 ,  98  is linked to a remote moving mechanism, consisting of lead screws  99 ,  100 , and stepper motors  101 ,  102 , linked to a computerized control and can be moved at will along the said airline  103 , making sure that the sliding contacts, both of the central conductor of the resonator with the central conductor of the airline  104 ,  105  and of the ground conductor of the resonator with the ground conductor of the airline  106 ,  107  are perfect and continuous during the movement. 
     The equivalent setup for harmonic rejection tuners using capacitively shorted λ/2 resonators [2] is shown in  FIG. 16 : each carriage  108 ,  109  carrying a harmonic resonator  110 ,  111  is linked to a remote moving mechanism, consisting of lead screws  112 ,  113  and stepper motors  114 ,  115 , linked to a computerized control and can be moved at will along the said airline  116 , making sure that the sliding contacts, both of the central conductor of the resonator with the central conductor of the airline  117 ,  118  and of the ground conductor of the resonator with the ground conductor of the airline  119 ,  120  are perfect and continuous during the movement. 
     Whereas the configuration proposed in [1],  FIG. 5  lacks the capacity to adjust the resonance frequency of the open stubs, the configuration proposed in [2],  FIG. 6  uses galvanic shorted stubs  15 ,  16  in [2], which do not allow DC power to traverse the tuner and reach the DUT, and is prone to low frequency spurious oscillations of the device under test (DUT, transistor). 
     The tuner proposed here eliminates all these shortcomings by using variable capacitive loads of the open stubs in [1] and capacitively coupled shorts of the shorted stubs in [2]. 
     As can be seen in  FIGS. 7 to 10  the open stubs  46  of the parallel open stub resonators are surrounded by a tube made of dielectric material  48 . This dielectric cylinder may or may not touch the ground plane  41  surrounding the central stub of the resonator  42 . If it does make sliding contact with the ground plane and the stub itself ( FIG. 10 ), the capacitive load increases and therefore the adjustable frequency range increases as well; however sliding contacts are more difficult to manufacture and, if sliding is not continuous and an air gap between dielectric and ground plane or stub is created during movement, this will cause a sudden drop in capacitive load and a jump in frequency. Non contacting dielectric loads ( FIG. 8 ) offer less capacitive load, but are less critical in adjustment and offer continuous frequency adjustability. 
     A capacitive load at the end of an open stub increases the effective electrical length of the stub itself. This decreases the frequency at which the stub is λ/4 long and therefore its series resonance frequency. This phenomenon is used to adjust the resonance frequency over several hundred MHz in the 2-3 GHz frequency range. This increases the effective frequency bandwidth of the open stub resonators from around 5% without adjustable capacitive load to over 30%. 
     Adjustment of the position of the dielectric cylinder  54 ,  60  with respect to the open stub  55 ,  62  can be made either manually, using a simple or micrometric screw, or remotely, using a stepper motor and gear  81 ,  82 . In general manual adjustment is simpler, but it requires on-line observation on a network analyzer in order to adjust the resonance frequency. Another possibility is to use a micrometric screw and pre-calibrate the positions versus resonance frequency. Each time the electrical length of the open or shorted resonant stub changes, so does its resonance frequency and the distance between resonator assemblies must be adjusted as well. This is taken care either by an adjustable spacer  49   a  or by automatic gears  99 ,  100 ,  FIG. 15 . 
     An automatic adjustment of the position of the dielectric cylinder by means of a gear, lead screw and stepper motor can be fully automated and pre-calibrated. This may facilitate the operation, but, in this particular case, where the resonators are used for load pull measurements, which are typically single frequency operations, this feature is of lesser importance. However this version of the apparatus ( FIGS. 15 ,  16 ) is also possible and may be used. 
     In an automatic configuration, where the position of the dielectric cylinder and resonance frequency are remotely controlled, the horizontal distance between resonators  119 ,  120  must also be automated and pre-calibrated. Otherwise automatic adjustability of the resonance frequency alone is not enough to maintain a fully automatic operation. 
     A variable short applied on a stub adjusts the effective electrical length of the stub [1]. This changes the frequency at which the stub is λ/2 long and therefore its series resonance frequency. This phenomenon is used to adjust the resonance frequency over several octaves in the GHz frequency range. This, theoretically, increases the effective frequency bandwidth of the open stub resonators from around 5% to whatever hardware limitations exist, depending on loss of the resonators, quality of sliding contacts and general dimensions of the airline. 
     However this, theoretically unlimited, adjustment range will be limited by technical difficulties in manufacturing the resonators and the sliding contacts. 
     Adjustment of the position of the sliding short  71 ,  89  can be made either manually, using a screw or remotely, using a stepper motor and gear  86 ,  87 . In general manual adjustment is simpler, but it requires on-line observation on a network analyzer in order to adjust the resonance frequency. Another possibility is to use a micrometric screw and pre-calibrate the positions versus resonance frequency. 
     An automatic adjustment of the position of the sliding short by means of a gear, lead screw and stepper motor  86 ,  87  can be fully automated and pre-calibrated. This may facilitate the operation, but, in this particular case, where the resonators are used for load pull measurements, which are typically single frequency operations, this feature is of lesser importance. However this version of the apparatus is possible and may be used. 
     In an automatic configuration, where the position of the sliding short and resonance frequency are remotely controlled, the horizontal distance  119 ,  120  between resonators must also be automated and pre-calibrated. Otherwise automatic adjustability of the resonance frequency alone is not enough to maintain a fully automatic operation, since the electrical distance between resonators affects sensibly the overall reflection behavior of the tuner and creates several frequency jumps and spurious resonances if this distance is not adjusted very carefully. 
     In conclusion, several practical configurations of harmonic rejection tuners are set forth, allowing full manual and automatic frequency adjustability, both using λ/4 open resonators using capacitive loads and capacitively shorted λ/2 resonators as well as mandatory adjustability of the physical distance between harmonic resonators.