Abstract:
High reliability impedance tuners used in high power measurements suffer fast heating and consequently thermal expansion of the central conductor, which has a very small mass and is thermally isolated from the tuner housing. This leads to false measurements or catastrophic tuner failure (short) of either the DUT or the tuner. A method is introduced for allowing the center conductor to expand linearly without deforming and risking loss of accuracy first and catastrophic failure later. Practical tests have shown significant thermal behavior improvement.

Description:
PRIORITY CLAIM 
     This application claims priority on provisional application 62/311,130 named “Temperature compensated High Power Tuners”, filed on Mar. 21, 2016. 
    
    
     CROSS-REFERENCE TO RELATED ARTICLES 
     
         
         1. “Load Pull for Power Devices” [online], [retrieved on Mar. 14, 2017], &lt;URL: https://www.microwaves101.com/encyclopedias/load-pull-for-power-devices&gt;. 
         2. “Computer Controlled Microwave Tuner—CCMT”, Product Note 41, Focus Microwaves, January 1998. 
         3. “Standing wave ratio, VSWR”, [online], [retrieved on Mar. 2, 2017], &lt;URL: https://en.wikipedia.org/wiki/Standing_wave_ratio&gt;. 
         4. Tsironis, U.S. Pat. No. 6,674,293, “Adaptable pre-matched tuner system and method” 
         5. Euler formula, “Buckling” [online], [retrieved on Mar. 11, 2016], &lt;URL:https://en.wikipedia.org/wiki/Buckling&gt;. 
       
    
     BACKGROUND OF THE INVENTION AND PRIOR ART 
     This invention relates to remotely controlled electro-mechanical RF impedance tuners used in load and source pull testing of high power RF transistors and amplifiers. 
     Modern design of high power RF amplifiers 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 for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models only. 
     A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” ( FIG. 1 ). Load pull is a measurement technique employing microwave impedance tuners  2 ,  4  and other microwave test equipment, such as signal sources  1 , test fixtures and DUT  3  and power meters  5 , the whole controlled by a computer  6 ; said computer controlling and communicating with said tuners  2 ,  3  and other equipment  1 ,  3  and  5  using digital cables  7 ,  8  and  9 . The tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (see ref. 1); tuners allow determining the optimum impedance conditions for designing amplifiers and other microwave components for specific performance targets, such as gain, efficiency, inter-modulation etc.; this document refers hence to “tuners” as being “impedance tuners”, in order to distinct from “tuned receivers (radios)”, commonly referred to as “tuners” because of the included tuning circuits (see ref. 2). 
     Impedance tuners comprise, in general, a slotted transmission airline (slabline)  23 ,  24  and adjustable reflection probes  22 ,  FIG. 2 ; the probe  22  is attached to a precision vertical axis  21  which is mounted inside a mobile carriage  28 ; the axis  21  can move the probe  22  vertically  216  towards the center conductor  23  and the carriage  28  can move the probe  22  horizontally  217  parallel to the center conductor  23  of the slabline  24 . The vertical movement  216  changes the amplitude of the reflection factor seen at the tuner test port  25  whereas the horizontal movement  217  changes the phase. This way the whole impedance plan (Smith chart) is covered allowing a quasi-infinity number of impedances from Zmin to Zmax to be synthesized at any given frequency within the “tuning range” of the tuner. Typical values of state of the art tuners are |Zmin|≈2Ω and |Zmax|≈1250Ω; this corresponds to Voltage Standing Wave Ratio (VSWR) of 25:1 ( FIG. 6 ). The relation between reflection factor and impedance is given by GAMMA=|GAMMA|*exp(jΦ), (Z−Zo)/(Z+Zo) {eq.1}, wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50Ω (see ref. 3). The equivalent to GAMMA is the Voltage Standing Wave Ratio: VSWR=(1+|GAMMA|)/(1−|GAMMA|) {eq.2}. 
     Metallic probes  22 ,  30  or “slugs” are typically made in a cubical form  41  with a concave bottom  35  which allows to capture, when approaching the center conductor  32  (see  FIG. 11  in ref. 4), the electric field, which is concentrated in the area  36  between the center conductor  32  and the ground planes of the slotted airline (slabline)  31 ,  FIG. 3 . This “field capturing” allows creating controllable high capacitance and reflection factors. The critical part is the required high proximity of the probe to the center conductor and the high accuracy of the vertical probe movement ( FIG. 6 ), whereby changes in the vertical probe position  62  of a few micrometers affects the reflection factor (and the VSWR) by a large amount. 
     When the center conductor  53  of the slabline heats up, the solution disclosed here prevents it from bending  57  and eventually “buckling”. This is obtainable if the center conductor is allowed to expand axially  105 ,  106 , which it cannot do, if not modified, as shown in  FIG. 5 , because its expansion is limited by the connectors  518 ,  519 . The prior art configuration of  FIG. 9 , whereby a protrusion  91  machined out of the main segment  94  of the center conductor slides into a hole  92  drilled into the fixed portion  93 , does allow the center conductor to expand, but the required gap  96  must be at least twice as large as in the case of the present invention which is depicted in  FIG. 10 , whereby the gap is split in two, approximately equal, parts  104  and  109 . Also, in the configuration of  FIG. 9  the unmoving segment  93  of the center conductor is a short piece of center conductor, anchored permanently on the connector plate  95 , and much shorter than the moving part  94  of the center conductor. This means not only that the thermal expansion of the free part  94  is larger (typically at least twice as long as in the case of  FIG. 10 , since its length is larger, thermal expansion of a rod is proportional to its length), but also that, because of its larger length, the slenderness factor “Length/Diameter” is larger. In the case of  FIG. 10 , if the joining tube  102  is placed in the middle of the center conductor, i.e. each free-standing segment is one half as long as the tuner (Length/2), then the slenderness factor will also be half, and, following Euler&#39;s formula (see ref. 5), the required force for the center conductor to bend or “buckle” will be four times larger. Therefore the risk of “buckling” under axial force will be four times smaller. 
     When microwave power is injected into the tuner, some of it is absorbed by the center conductor  53  of the slabline. This leads to a rise of its temperature and associated thermal expansion mostly along its axis. Since the center conductor  53  has only limited range for expansion, because it butts at the connectors  518  and  519 , this leads to bending and eventually “buckling”  57  in  FIG. 5A ; “Buckling” of the center conductor happens in different ways, depending on its pre-forming, which cannot be “perfectly” straight, its “slenderness” factor Length/Diameter and the anchoring method at the connectors. Whereas in an ideal, low power and room temperature, situation FIG.  5 B 1 , the center conductor is positioned exactly in the center of the slabline channel, when heated and it bends (deflects) it may either deflect sideways  515  or downwards  514  or both; of course, it may also deflect upwards (not shown), in which case we may have a premature electrical short circuit. In either case the effect is “at best” loss of accuracy or “at worst” an electrical short and damage of the tuner and/or the DUT. 
     In case a short happens with either temporary or permanent damage of the tuner or the DUT, at least the operator will be alerted and can take measures to correct the situation. But if it does not come to a short (case of FIG.  5 B 3 ), then the result will “only” be false measurement. This is because, as shown in  FIG. 6 , a relative movement between tuner probe and center conductor will change the calibrated VSWR; in other words, the data retrieved from the measurement instruments ( FIG. 1 ) will be recorded at the wrong VSWR values. There will be no warning, just wrong data; it comes even worse: would the operator have doubts about the tuner accuracy, and would he disconnect the tuner from the test setup in order to re-calibrate or verify the calibration on a vector network analyzer (VNA), he, probably, will find that the tuner is accurate. This is because, during the dismantling of the setup the center conductor will cool down and recover its initial (calibrated) position (transition from states FIG.  5 B 2  or FIG.  5 B 3  back to original state FIG.  5 B 1 ); this can be a substantial systemic problem for high power testing using such tuners. 
     This invention discloses solutions allowing avoidance of such catastrophic and systemic test problems. 
    
    
     
       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 typical automated transistor load pull test system. 
         FIG. 2  depicts Prior Art: a front view of an automated slide screw impedance tuner using a single vertical axis and RF probe (slug). 
         FIG. 3  depicts Prior Art: cross section of RF probe inside a slotted airline (slabline) approaching the center conductor. 
         FIG. 4  depicts Prior Art: a perspective view of relevant dimensions and parameters of the operation of a vertically adjustable RF probe (slug). 
         FIGS. 5A ,  5 B 1 ,  5 B 2  and  5 B 3  depict prior art:  FIG. 5A  depicts the center conductor deformation due to heating and expansion and FIGS.  5 B 1  to  5 B 3  depict the possible deflection of the center conductor related to tuning probe: FIG.  5 B 1  corresponds to room temperature; FIG.  5 B 2  corresponds to sidewise deflection and short circuit and FIG.  5 B 3  to deflection away from the center conductor and false measurements. 
         FIG. 6  depicts prior art: the measured dependence of tuner VSWR from the proximity (distance) of the tuning probe to the center conductor. 
         FIG. 7  depicts the frequency response of a tuner probe positioned close to the center conductor in an ideal case  71  and in the case where the center conductor has moved  72  because of heating. 
         FIG. 8  depicts the test setup used to calibrate an automatic impedance tuner. 
         FIG. 9  depicts prior art: the method used to allow thermal expansion of center conductor in tuners (sliding contact). 
         FIG. 10  depicts a segmented center conductor using an expansion ring, allowing the segments to expand linearly under influence of dissipated power and heat. 
         FIG. 11  depicts a front view of two segments of thermally expanding center conductor, joined by a conductive ring and associated dimensions. 
         FIG. 12  depicts actual tube and rod diameters used to estimate inductive versus capacitive disturbance of the characteristic impedance of the segmented and temperature compensated center conductor. 
         FIG. 13  depicts detail of reliable RF contact between the expansion tube and interface rod protrusion. 
         FIG. 14  depicts the contact tube sliding over massive protrusions inserted and attached permanently into the adjacent center conductor segments; the tube may have the same or different diameter as the center conductor, depending on the application. 
         FIG. 15  depicts the 3D view of the contact tube sliding over the protrusions made as massive pin insertions into the center conductor segments. 
         FIG. 16  depicts an embodiment whereby it is the inserting protrusion which is slotted and not the expansion tube. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A prior art solution allowing for the thermal expansion of a metallic rod, used in mechanical engineering, involves a sliding contact ( 92  in  FIG. 9 ); sliding contacts, in electrical engineering in general and microwave engineering can be unreliable and bear the risk of being worn out after a certain period of cycles, which in this case are heat/cool cycles. Also, in the area where one segment of the center conductor of a transmission line enters the other, the required jump in diameter  91  causes a spurious impedance jump. In this particular case the spurious impedance jump is inductive, since the exposed portion of the sliding contact  91  represents a series inductance and creates a higher characteristic impedance (Z 1 ) than the slabline itself (Zo); it is therefore important (a) to reduce this gap  91  and associated impedance jumps as much as possible, especially at high frequencies, where the length of the gap  96  may be critical, relative to the signal wavelength; and (b) to introduce a compensation by creating a parallel capacitive load. This second point is possible by making the diameter of the tube  93  larger than the diameter of the center conductor  94 . This has not been disclosed in the prior art ( FIG. 9 ). 
     In order to account both (c) for the thermal expansion of the center conductor segments  105  plus  101  and  106  plus  103  and (d) for the characteristic impedance discontinuity, caused by the gaps  109 , the diameter of the tube  102  is made larger than the diameter of the center conductor  105 ; an appropriate method for compensation the characteristic impedance discontinuity is to select the diameters of the adjacent center conductor segments, such as to cause the same decrease in Zo by the tube  114  as the increase to Zo created by the protrusions  1105  and  1106 ; as an example, if the diameter of the center conductor is 3 mm and the slabline channel 5,425 mm is wide, a Zo=Z(Do)=50Ω is created. A protrusion of a diameter of 2.5 mm creates a characteristic impedance of Z1=Z(D1)=60.9Ω; if the tube has a wall thickness of 0.5 mm, corresponding to an outside diameter of 3.5 mm, this creates a characteristic impedance of Z2=Z(D2)=40.8Ω; all together, both the arithmetic and geometric averages of those characteristic impedances amount to either 50.85Ω or 49.85Ω correspondingly, i.e. close to compensation. As a general rule one should aim to Z(D1)−Z(Do)≈Z(Do)−Z(D2), whereby Do is the diameter of the center conductor, D1 is the diameter of the protrusion and D2 is the diameter of the sliding tube. 
     Details of this configuration are shown in  FIG. 12 : the center conductor (typically a gold-plated steel rod)  120 ,  125  is cut short and two interface rods  128  and  129  are attached to center conductor ends tightly (typically using screw inserts) (see also items  1001  and  1002  in  FIG. 10 ). The inserts  128  and  129  have the same diameter as the center conductor and thus create the same characteristic impedance Zo: Z(Do)=Zo (typically 50Ω). In order to allow the tube  126  to slide over the inserts and make RF contact the ends of the inserts  121 ,  124  must be reduced by carving out protrusions that will enter into the cavity  116  of the tube  114 . The characteristic impedance of the protrusions is higher than Zo; we assume the difference to be ΔZ (in Ω), Z1=Z(D1)=Zo+ΔZ; To compensate for this disturbance the diameter D2 of the tube  126 ,  127  is made larger than Do leading to Z2=Z(D2)=Zo−ΔZ. In terms of symmetry the inductive behavior of protrusion  121  is compensated by the capacitive behavior of the left half of the tube  127  and the inductive behavior of protrusion  124  is compensated by the capacitive behavior of the right tube half  126 . The symmetry plan  123  is arbitrary and can as well be the geometrical center of the tube. In any case this compensation holds only for a limited temperature range, because, as the center conductor heats and expands the exposed length of the protrusions shrinks and so does their inductive effect, whereby the capacitive effect of the higher diameter of the tube (D2&gt;Do) does not really change. This compensation is though, better than no compensation at all. 
     A further problem of the prior art structure ( FIG. 9 ) is reliability; if the thermal cycle is repeated several times, the silver or gold surface plating of the protrusion and/or the inner wall of the tube are going to wear out and the electrical conductivity is going to change (increase sharply). This depends on the thickness of the plating layer and the tightness of the sliding contact; of course if the whole body of the center conductor would be made of massive silver, brass or copper, excluding massive gold for cost reasons, the wearing out problem would disappear. However, high electrical conductivity is associated with lower mechanical sturdiness, i.e. such long center conductors would be prone to “sag” over time under its own weight. The material of choice for long center conductors is steel, which, because of its inherent high electrical resistance, has to be gold or silver plated. In this case the reliability problem of surface wear-out re-surfaces. The solution introduced here is to insert two interface rod segments on each side of the sliding tube, on which′ protrusions the tube slides, made of homogenous massive high electrical conductivity material, such as brass, bronze, silver or copper, whereby there is no surface plating to be worn-out. This is shown in  FIGS. 10 and 11 . 
     The center conductor  105  and  106  is cut short by the sum of the lengths of the inserts  101  and  103 , the tube  102  and the exposed sections of the protrusions  109  and  104 . The tube  102  has several slots  100  cut parallel to its axis allowing a pre-bend inward to establish secure sliding contact on the protrusions on both sides. RF contacts are extremely critical; they must always be well defined and mechanically repeatable. This is ensured here using the pre-bending of the ends of the tube  102 , which allows the RF contact to take place at the inside surface of the pre-loaded edge of the tube cylinder (see self-explanatory  FIG. 13 ): The protrusion enters the cavity of the tube; the tube has axial slots  100 , shown in  FIG. 10 ; these slots allow to pre-bend the edge of the tube so it creates sliding RF contact with the protrusion body. This type of contact is reliable because of the pre-loading force and defines well the contact plan, at the edge of the tube. Since both, the tube and the protrusion are made of homogenous material, and are not surface-plated, rubbing of the tube edges on the protrusion body will, in fact, remove any residuals and keep the RF contact polished and highly conductive over time. On the side of the center conductor, the inserts  101  and  102  must make perfect surface contact with the center conductor segments at plans  1001  and  1002  to avoid spurious reflections. This is best ensured when both parts are tightened using centered screws  1102 . The inserts and the center conductor expand and contract, under heating cycles, as a single unit. The expansion must be absorbed internally, since the limiting RF connectors  107  and  108  are considered non-yielding obstacles. 
     The thermal expansion of the center conductor can be calculated using the known formula ΔL=L*α*ΔΘ, whereby L is the length of the center conductor, ΔL the expansion, α is the linear thermal expansion coefficient of the center conductor material (in this case steel α≈45 ppm/° C.) and ΔΘ is the temperature increase in degrees. For instance, a typical steel center conductor of L=1000 mm would expand by approximately 1.2 mm if heated from 20° C. to 100° C., values which are typical for high power tuner operation. The larger gap required, in the prior art configuration, when the sliding contact is placed close to the connector ( FIG. 9 ) is causing a larger parasitic inductance and inferior RF performance. When the sliding contact (connecting tube  102 ) is placed in the middle of the center conductor, meaning that the gap allowed is half as wide, each parasitic inductance, due to the protrusion  104 , is half as large. The connecting tube  102  shall be manufactured from well conducting and elastic material, preferably spring-Bronze or Beryllium Copper (BeCu), because of its good electrical conductivity and strong spring-metal behavior, which allow long lasting and reliable DC and RF sliding contact between the tube&#39;s  102  inner walls and the protrusions  104 , which move into and out of the tube; the protrusions  104 ,  109  are concentric with and machined (carved out) from the body of the interface rods  101 ,  103 . The gaps are dimensioned to allow for maximum thermal expansion  105 ,  106 , caused by the DC and RF power, the tuner is expected to handle. Slots  100  cut into the mantle of the connecting tube  102  provide for and enhance the spring mechanism for the sliding contact. As shown in  FIG. 10 , each segment  101 ,  103  will expand  105 ,  106  only towards the connecting tube  102 , since the connectors  107  and  108  are unmovable obstacles (borders). 
     The exact dimensioning of the connecting tube, the gaps and the protrusions is shown in  FIG. 11 : Each segment can expand a certain amount  113  and  117  before hitting the connecting tube walls  114 . The gap  115 , left between the tips of the protrusions  116 , shall be larger than the sum of the two gaps  113  and  117  between the cores  118 ,  112  of the center conductor and the edges of the tube  114 . 
     An alternative, simplified embodiment, fulfilling the same requirements as hitherto disclosed in this invention is shown in  FIG. 14 . In this case the interface rods  112  and  118  are replaced by center pins  141  and  142 , which are made of massive non-plated highly conductive metal, and are inserted and permanently attached to the center conductor segments  140  and  143 , while sliding on the central tube  144  (and making good RF contact, as shown in  FIG. 13 ) to allow for thermal expansion and contraction of the center conductor. 
       FIG. 16  depicts an embodiment whereby it is the inserting protrusion which is slotted making contact from inside and not the expansion tube. This configuration has the reliability advantage that the spread of the protrusion is going to maintain its shape and therefore the sliding RF contact, even if slightly compressed, which is not the case with the embodiment of  FIGS. 10, 11, 13 and 15 . 
     The tuner is calibrated using a network analyzer  80 , VNA and a control computer  82  in room temperature; the tuner is connected using RF cables  85  to the VNA ports and using digital cables  88  to the computer  82 ; the computer controls also the VNA using different digital cables and communication protocol  81 ; the probes  84  are positioned horizontally and vertically inside the slabline  89  in order to generate desired reflection factors within the tuning range of said tuner ( FIG. 8 ); the collected s-parameter data from the VNA are saved in tuner calibration files. This generic tuner calibration method is used abundantly in the specific art and is hereby applied to the high reliability high power tuner as well with a higher probability of eliminating shorts and systemic measurement errors at high power operation. 
     Obvious alternative embodiments to the herein disclosed method of controlling the temperature and thermal expansion of the center conductor of slide screw impedance tuners are imaginable and possible but shall not impede on to the validity of the basic idea of the present invention.