Patent Publication Number: US-8975988-B1

Title: Impedance tuner using dielectrically filled airline

Description:
PRIORITY CLAIM 
     Not applicable 
     CROSS-REFERENCE TO RELATED ARTICLES 
     
         
         [1] Load Pull System: http://www.microwaves101.com/encyclopedia/loadpull.cfm 
         [2]“Computer Controlled Microwave Tuner—CCMT,” Product Note 41, Focus Microwaves, January 1998 
         [3] Directional Couplers: http://www.e-meca.com/rf-directional-coupler/directional-coupler-780.php 
         [4] U.S. Pat. No. 7,135,941, Triple probe automatic slide screw load pull tuner and method 
         [5] “MPT, a universal Multi-Purpose Tuner,” Product Note 79, Focus Microwaves, October 2004. 
         [6] “On wafer Load Pull Tuner Setups: A design help”, Application Note 48, Focus Microwaves, December 2001. 
         [7] U.S. Pat. No. 6,674,293, Adaptable pre-matched tuner system and method 
         [8] S-parameter Basics: http://www.microwaves101.com/encyclopedia/sparameters.cfm 
       
    
     BACKGROUND OF THE INVENTION 
     Prior Art 
     This invention relates to low noise and high power (nonlinear) testing of microwave transistors (DUT) in the frequency and time domain for Noise and_Load Pull measurements [1]. 
     Microwave tuners [2], are used to test electrical components, like transistors, in cellular telephones and other electronic products to optimize performance. A microwave tuner helps determine the best circuit environment for optimal performance based on an electrical quantity called “impedance”. Tuners can create a wide range of impedances to allow testing at different impedances. In the case of noise measurements the tuners are used to generate arbitrary source impedances and appropriate software is then used to extract the noise parameters. Impedances (Z) are related to reflection factors (Γ) through the relation: 
     Γ=(Z−Zo)/(Z+Zo), whereby Zo is the characteristic impedance of the transmission line of the test system. 
     Load pull is the method by which the load impedance presented to the DUT at a given frequency is changed systematically and the DUT performance is registered, with the objective to find an optimum depending on the overall design objectives. This may be maximum power, efficiency, linearity or else. The same is valid for the source side of the DUT. Passive (slide screw) tuners are used to emulate the various impedances presented to the DUT [2], ( FIG. 1 ). The electrical signals injected into the input of the DUT and extracted from the output can be measured using power meters directly or through sampling devices, typically signal directional couplers [3]. At high power the (nonlinear) DUT is saturating and deforming the sinusoidal input signal. As a result part of the power is contained in harmonic frequency components. The DUT performance can only be optimized when all harmonic frequency components are impedance-matched properly. This requires independent harmonic tuning, mainly at the DUT output, but often also at the DUT input. 
     A wideband slide screw tuner ( FIG. 2 ) uses a slotted airline (slabline) ( 25 ) and a mobile carriage ( 23 ) which slides along the slabline and carries a metallic probe ( 21 ,  24 ), which is insertable into the slot of the slabline. By approaching to the center conductor ( 27 ) the probe creates controllable capacitive coupling between the center conductor and the ground walls of the slabline and thus a controllable reflection factor ( FIG. 2   b ). To cover 360 degrees of reflection factor the carriage (and the probe) must travel at least one half of a wavelength along the slabline ( 22 ) ( FIG. 2   a ). 
     Harmonic impedance tuners have been introduced in 2004 ( FIGS. 3 and 4 ) [4]. They comprise a number of independent wideband probes ( 41 ,  44  and  45 ) attached to mobile carriages ( 43 ) and insertable into and movable horizontally inside the slot of a low loss transmission airline (slabline) ( 42 ). To tune independently three frequencies, harmonic or not, it has been shown experimentally, that there is need for three such probes ( 41 ,  44  and  45 ) [5]. Each probe is attached to and positioned by a precision remotely controlled gear mechanism in a carriage ( 43 ) ( FIGS. 2 ,  3 ) and must travel one half a wavelength (λ/2) along the axis of the slabline. A three-frequency harmonic tuner is therefore at least three times longer than a wideband tuner with the same lowest frequency of operation. 
     The main shortcoming of such tuners [5] is their horizontal size and weight due to the length of the slabline. Since in order to generate arbitrary reflection factors (impedances) at any frequency, each probe and associated carriage must move horizontally over at least one half of a wavelength (λ/2) at the fundamental frequency Fo ( FIG. 4 ) this means that the lowest fundamental frequency determines the length of the tuner. 
     The electrical wave length in air is λ [cm]=30/Frequency [GHz]. 
     In a practical tuner apparatus ( FIGS. 2   a ,  4 ) the size of the additional supporting items, a) the length of the mobile carriages themselves (LC) and b) the length of the side-walls (LW) of the tuner housing, add to the overall tuner length. In practical terms the minimum overall length of the slabline of a three carriage harmonic tuner, without the size of the input and output connectors, is: L=3*λ/2+3*carriage(LC)+2*side-walls(LW) ( FIG. 4 ). 
     The present invention describes a method allowing reducing the overall linear length of such a tuner, with minimal effect on its RF performance, by reducing the electrical wavelength inside the slabline; this is done by filling part of the slabline with a dielectric material with a dielectric coefficient ∈ r &gt;1. The method consists therefore in a compromise between best RF performance and smallest mechanical size and weight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and its mode of operation will be better understood from the following detailed description when read when read with the appended drawings in which: 
         FIG. 1  depicts prior art, a typical load pull test setup using impedance tuners to test RF transistors. 
         FIG. 2  depicts prior art, a) schematics of a single carriage slide screw tuner and definitions of basic elements determining its length; b) a cross section of a slabline and the tuning probe. 
         FIG. 3  depicts prior art, a photograph of an actual three carriage harmonic tuner and its actual length with a lowest frequency of operation of 0.7 GHz (700 MHz). 
         FIG. 4  depicts prior art, a schematics of an actual three carriage harmonic tuner and the definitions of all components determining the total tuner length. 
         FIG. 5  depicts a perspective view and cross section of a tuner slabline filled with liquid dielectric material and the tuning probe partly inserted. 
         FIG. 6  depicts prior art, a comparative table of various dielectric liquids, together with their dielectric constant “epsilon”, the loss tangent “delta” and the ratio “tan δ/√∈ r ”, which is a representative quantity for tuner applications. 
         FIG. 7  depicts a cross section of a dielectric liquid filled tuner slabline and initialized tuning probe. 
         FIG. 8  depicts a cross section of a dielectric liquid filled tuner slabline and tuning probe inserted to maximum reflection. 
         FIG. 9  depicts schematics of a three carriage tuner a) with air filled slabline and b) with dielectric filled slabline and the associated reduction in length. 
         FIG. 10  depicts a tuner calibration setup. 
         FIG. 11  depicts a comparison of measured slabline loss between a slabline filled with air and one filled with Mineral oil between 0 and 1000 MHz. The curves are normalized to the electrical wavelength. 
         FIG. 12  depicts a comparison of measured slabline loss between a slabline filled with air and one filled with Silicon oil between 0 and 1000 MHz. The curves are normalized to the electrical wavelength. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention discloses the concept of reducing the length of single or multi-carriage impedance tuners, by using a low loss dielectric material to fill the slabline cavity and reduce the effective wavelength of the signals transmitted through the tuner, and thus the overall length of the slabline itself. In a preferred embodiment said dielectric material shall be a fluid, wherein oil is a preferred embodiment.  FIG. 6  contains a comparative table of dielectric constants (∈ r ) and associated loss (tan δ) of various liquids. A high dielectric constant ∈ r  is obviously preferable since the effective electric wavelength is λ eff =λo/√∈ r , whereby λo is the wavelength in air (or vacuum). However, as can be seen from  FIG. 6 , liquids with high ∈ r  tend to have high losses (tan δ). In the case of tuners losses are very important, since they reduce the effective tuning range, by twice the insertion loss between the tuner test port ( FIG. 3 ) and the tuning probe and the loss between said tuning probes in case of a multi-probe tuner. An effective “figure of merit” is then the ratio between loss and dielectric constant, included in  FIG. 6  in the column (tan δ/√∈ r ). The smaller this number for comparable dielectric constants, the better the specific dielectric fluid will be suited for tuner applications. Of course ∈ r  has to be high enough to cause a significant reduction in tuner length, this reduction being approximately “1/√∈ r ” ( FIGS. 9   a ,  9   b ). 
     Considering two examples: a) a single carriage tuner starting at Fmin=200 MHz. The effective length of such an apparatus is actually 80 cm (75 cm free travel=λ/2 (200 MHz) plus 3 cm for the carriage and 2 cm for the two walls). Using a dielectric fluid with ∈ r =3, the total length is reduced to 48.5 cm. b) In the case of a three carriage (harmonic) tuner starting at Fmin=400 MHz the associated dimensions are: b1) in air: 123.5 cm, b2) with dielectric: 76 cm. The size and weight reduction of roughly 40% in both cases is considerable and leads to reducing manufacturing cost and, most importantly, mounting effort and operation stability when tests are to be carried through on wafer [6]. 
     Using dielectric fluid for filling the slabline offers a number of additional benefits: a) lubrication: the probes can slide effortlessly on the side-walls of the slabline for perfect grounding contact without any wear out; b) higher capacitance: the maximum capacitance reached between the probe approaching the center conductor is increased by the factor ∈ r  for the same gap size ( 83 ); this increases the achievable reflection factor at the probe reference plane; c) reduction of electric field: the electric field E between (grounded) probe and center conductor is reduced: the voltage V between center conductor and probe is: V=∈ r *E*S, whereby “S” is the gap between center conductor and probe ( 83 ); or E=V/(∈ r *S): i.e. the electric field across the gap is reduced by a factor 1/∈ r , which automatically reduces the risk of Corona discharge; and finally d) provides better cooling of the center conductor: filling the cavity of the slabline with a liquid provides for better heat removal (cooling) of the center conductor, which in normal, air filled slabline tuners, is thermally insulated from the environment and heats up easily at high transmitted power. 
     The effect of using dielectrically filled slablines is shown in  FIG. 9 ; each carriage only needs to travel (1/√∈ r ) far, and for low enough minimum frequencies, this corresponds to the same reduction in overall tuner length, since the width of the carriages and tuner walls are small compared with λ/2. 
     In order to be used in automatic measurements an impedance tuner has to be automated and calibrated: automation means that the carriages and probes must be attached to and driven by gear mechanisms which will be controlled by electrical motors, preferably stepper motors [2, 7] and controlled by a central or on-board processor; calibration is necessary in order to be able to extract the DUT data from the measurement setup ( FIG. 1 ). 
     A tuner calibration setup is shown in  FIG. 10 ; a control computer communicates with a pre-calibrated network analyzer (VNA) which is connected through its test ports to the tuner two-port using high quality RF cables; an appropriate algorithm determines the horizontal and vertical probe positions (in stepper motor steps) needed to create a plurality of reflection factors (impedances) covering the tuning area of interest. Typically such area is the whole Smith chart, since it is often not known ahead of time where the optimum conditions for testing a DUT are; therefore the free horizontal travel for the carriage has to be at least one half of a wavelength at the test frequency; this corresponds to a 360 degree circle on the Smith chart. The S-parameters [8] of the tuner two-port measured by the VNA for said probe positions are retrieved by the computer via digital communication (USB, GPIB or LAN) and saved in calibration files in a format which associates S-parameters with probe positions. After the calibration the data are retrieved by the measurement routines, embedded with the test fixture parameters, in which the DUT is mounted, and applied as corrections to the data measured in the test setup ( FIG. 1 ), in order to generate corrected measurement data referred to the DUT itself (phase and amplitude corrections of the reflection factor and amplitude corrections of input and output power etc. [1]). 
     This invention discloses a method for mechanically shortening single and multi-carriage tuners using a slabline filled with dielectric material; in a preferred embodiment said dielectric material is low loss silicon or mineral oil, but alternative substances are easily imaginable. Obvious alternatives of low loss high dielectric fluids shall not impede on the validity of the disclosed invention.