Patent ID: 12259409

DETAILED DESCRIPTION OF THE INVENTION

This invention discloses a radio frequency (RF, microwave), computer-controlled harmonic impedance tuner and the associated calibration method, suitable for load pull measurements. The tuner (FIGS.5,7and8) uses a low loss transmission airline (slabline)59, which includes two parallel conducting (metallic or metallized) sidewalls80(FIG.8) forming a straight channel and a, typically but not exclusively, cylindrical center conductor51,81. The center conductor is “S” shaped and terminates at the two ends of the slabline, the input or test port52facing the DUT, and the output or idle port53facing the load. Four parallelepiped tuning probes50,54,58and55(see alsoFIG.2) are inserted in pairs opposite (diametrically) to each-other from the top and the bottom (FIG.8) into the slabline channel, are kept at fixed depth26(FIG.2) and can be moved only horizontally43,47(FIG.4, showing a two-probe case) along the slabline; each probe creates, as it moves, at its own reference plane concentric reflection factor circles92on the Smith chart (FIG.9). The total reflection factor90is created by a planetary epicycloid superposition of the two reflection factor vectors (one circle rotates around a point at the periphery of the other) and can be seen inFIG.9(see ref.4,FIG.8b). Four tuning probes create a more complicated double epicycloid trace.

The simpler concept of a two-probe slide screw tuner with vertically fixed tuning probes is explained in prior artFIG.4. The two slugs42and46move inside the channel of the slabline48along43without changing their penetration or at constant distance45from the center conductor44. The reflection factor49at the input, or test, port40is created by vector superposition of the individual reflections401and402of each probe (slug). Port41is idle and connected to auxiliary instruments. The input (test) port40is connected to the DUT. The shortcoming of this configuration is that the reflection is present at all but a number of tuner settings, that is only when the two vectors401and402arrive opposite at the test port and cancel each other; this also means that a 50 Ohm impedance can only be generated (tuned to) individually at a frequency by properly positioning the slugs. The, otherwise, always present reflection creates risk of spurious oscillations of the DUT and, either its catastrophic failure or, at least, wrong data. This invention solves this problem by bending the center conductor and allowing wideband 50 Ohm behavior.

FIGS.6A and6Bdepict the difference between the traditional tuning probe (FIG.6A) and the new tuning probe (FIG.6B) used in the new tuner type: in the case of the traditional probe (FIG.6A) the slug60has protrusions (wings)62extending beyond the center of the center conductor64of the slabline66; this is in order to capture a maximum of the stray electric field towards the bottom of the slabline and create a maximum reflection factor (at the probe reference plane) close to 1. On the other hand, the slugs61of the new tuner have their protrusions63chopped slightly above the center of the center conductor65of the slabline67, to allow crossing over the slug on the other side of the slabline channel (slugs82and83inFIG.8). This shortening of the enveloping of the center conductor, together with the larger gap between slug61and center conductor65creates smaller reflection factor of the order of 0.6 to 0.7, which is sufficient for this type of tuner, as shown inFIG.9.

The concept of the harmonic four probe tuner without vertical probe control is shown inFIG.5: the tuning probes are mounted diametrically in pairs across the channel of the slabline59, which has a length of the straight section51of the center conductor of one wavelength λ(Fmin) at the lowest frequency of operation of the tuner; the body of the slabline59itself, between the test (or input) port52and the idle (or output) port53is longer by two times the span of the bends B. Tuning probes #1,54and #3,50are mounted on one side of the slabline channel, the other two probes #2,58and #4,55are mounted on the diametrically opposite side. Probes #1 and #3 are withdrawn close to the test port, shown as54A and50A and probes #2 and #4 are withdrawn close to the idle port53, shown as58A and55A; the positions X1 to X4 of the probes #1 to #4 are defined from the test port52reference line, as shown clearly inFIG.5; it is obvious that the carriages holding probes #1 and #4 are positioned in withdrawal state next to the edges of the slabline using proximity limit switches501and502mounted on the slabline end walls and probes #3 and #2 using proximity limit switches503and504mounted on the adjacent carriages holding probes #1 and #4. It is also obvious that X1 cannot be larger than X3 and X2 larger than X4. Otherwise, all opposite probes (#1 with #2 and #4, and #2 with #1 and #3 etc.) can cross-over and this must be considered in the calibration algorithm. In any case the shape of the bends of the center conductor relative to the width of the probes when withdrawn is chosen such that the capacitive coupling between the probes and the center conductor is negligible to avoid a residual reflection; on the other hand, the bends56and57do not change the characteristic impedance of the slabline, as long as the center conductor keeps its diameter and remains centered between the sidewalls of the slabline.

The four-probe tuner with neutral initialization state is shown inFIG.7: it includes an input (test) port76and an output (idle) port77; the horizontal control of the carriages70,71,77and701(FIG.7) is best accomplished using linear electric actuators (see ref. 6) or stepper motors72,73,79and702and associated horizontal ACME rods703,704,74,75. Since the tuner does not have adjustable vertical axes, not being able to withdraw the tuning probes from the slabline, to create a reflection-free neutral state, means that another way is needed: this is made possible by bending56,57the center conductor close to the coaxial connectors of the tuner ports into a stretched “S” shape. This then allows to move the tuning probes to the neutral positions50A,55A and54A,58A and55A, where the capacitive coupling with the center conductor51vanishes and so does the residual reflection factor. The center conductor is bent in the plan of the channel of the slabline staying always at the same distance from the sidewalls80(FIG.8); since the characteristic impedance, even at differential ΔX level, is defined by the ratio of the channel width and the center conductor diameter, as long as the bending shape is smooth enough and the electro-magnetic field is not overly deviating from the predominant coaxial TEM propagation mode, the transmission and residual reflection perturbation is negligible (item501inFIG.10) and, at least, below the threshold needed for a tuner slabline (a tuner works by creating reflection, perfect 50 Ohm matching is not a strict requirement).

The center conductor78is bent (twisted) at both ends56,57terminating with coaxial connectors. The tuning probes are attached to carriages70,77,71and701, which slide or roll84on the edges801of the sidewalls80(FIG.8) along the slabline. In this embodiment the carriages are threaded and driven by ACME screws74,704,75and703controlled by remotely controlled stepper motors79,73,702, and72correspondingly. In real operation, since the horizontal coordinates X1 to X4 are measured from the input (test) port76reference plan, then withdrawal of probes #2 and #4 occurs at bend57and of probes #1 and #3 at bend56. The span B of the bends (FIG.5) depends on the mechanical layout and is such that, when the probe is in the extreme outside position, the coupling with the center conductor is insignificant; typically, the required gap between the bottom of the tuning probes and the center conductor is no more than 2 times the center conductor diameter. The withdrawal positions are detected using limit switches.

As already mentioned, the interaction of four tuning probes creates multiple reflections and seems chaotic, thus impossible, and not intuitive to visualize graphically. InFIG.9, though, there is an attempt to visualize the reflection factor generation when two out of four probes interact in the active straight section of the “S” shaped center conductor. Assuming probe #1 is at position X1, closer to the test port than probe #2 and generates a (fixed) vector92; probe #2 creates a superimposed vector93which rotates around the tip of vector92, following the trajectory94, creates a total vector90. Both vectors rotate as the probes move as shown by the dotted circles95and96. If only probe #2 moves, then the total vector90follows trace96; when both probes move together at the same pace, then the total vector90follows trace91and if they both move at different paces it follows a Lissajous contour. In the case of four independently moving probes the traces are simply the superposition of such epicycloid traces. What seems too complex to visualize, is though handlebar using numerical data processing, as shown in ref. 8.

Cascading two two-probe tuners (FIG.5) creates enough combined tuner settings to generate solutions for two-frequency or harmonic tuning; this means the four-probe tuner ofFIG.5can synthesize random impedances independently at two harmonic frequencies; a condition is that all tuning probes create enough reflection factor at both harmonic frequencies. Other than harmonic tuners with vertical probe control, where two probes are enough to control two frequencies, in our case four probes are required; logically it comes down to degrees of freedom versus targets: two impedances include four values Real and Imaginary or amplitude and phase times two. The difference with prior art two-probe harmonic tuners (see ref. 4) is that here the probes are not always cascaded in the same order; the calibration must account for that as is going to be disclosed further down.

The tuner can be calibrated and operated to create user-defined impedances. We distinguish two special states: the initialization state (FIG.5B), used in calibration, and the withdrawn state (FIG.5A) used in operation. In the withdrawn state the tuner presents to the DUT a wideband 50 Ohm load, that minimizes the risk of spurious DUT oscillations. However, free control of all probes along the entire length of the slabline requires probe #3 not to conflict with probe #1 and probe #2 not to conflict with probe #4. Therefore probes #3 and #2 are initialized by moving them along the slabline one wavelength A out of the way of probes #1 and #4 correspondingly, which in both cases amounts to X2=B (for probe #2) and X3=λ+B for probe #3; this is the initialization state.

The tuner calibration process uses a setup as inFIG.11and is depicted, step by step, in the flowchart ofFIG.12: each tuning probe shall be able to move over one half of a wavelength at the fundamental operation frequency, which means that two cascaded probes need a total travel of λ, divided in several, typically, 100 incremental steps and, whereas this corresponds to 3.6° per step transmission phase at Fo, at the harmonic frequency 2Fo this corresponds to 7.2° per step. This coarser resolution is, though, countered by the movement of the probe over 720° (since λ(Fo)/2=λ(2Fo)); In the calibration setup the tuner116is connected using RF cables115with a Vector Network Analyzer (VNA)110pre-calibrated at the frequencies of interest. The linear actuators1101,1102,114and117(see ref. 6) are controlled by the control computer112using a digital cable118via the electronic on board control module113to move the carriages, attached to the actuators, which slide along the axis of the slabline119, in order to control the reflection factor S11, while reading the four scattering parameters (s-parameters) from the VNA110using standard, digital communication cable111and a prior art communication protocol. Computer112includes a temporary RAM and a permanent (hard-disc) electronic memory for saving large amount of data containing probe positions and the associated s-parameters in calibration data files for later use. The PC controller112also includes and runs appropriate tuner and VNA control and communication software as well as appropriate numerical data processing (s-parameter matrix conversion to T (cascade) format, matrix inversion and multiplication and cascading) routines (see ref. 9).

The fast de-embedding calibration procedure comprises the following steps (FIG.12): After the tuner is connected to the VNA, pre-calibrated at a selected frequency Fo and the harmonic frequency 2Fo inside the operation range of all probes of the tuner, the tuning probes are initialized as follows: probe #1 is moved to position54A, probe #3 is moved to position X3=B+λ(Fo), probe #4 is moved to position55A equal to X4=λ+B and probe #2 is moved to position X2=B; S-parameters of the tuner are measured and saved in a init matrix [S0]. Then the third tuning probe #3 is moved along the slabline to a multitude M of approximately 100 positions X3 between the initialization position (X3.0=B+λ(Fo) and X3=B at the selected frequency Fo and s-parameters are measured at Fo and 2Fo and saved, together with the position X3 in a first data file S3. Subsequently the tuning probe #3 is moved to position X3.0=λ+B and tuning probe #2 is moved from the init position54A in a multitude of M positions X2 and at every step s-parameters at Fo and 2Fo are measured and saved in a second data file S2 together with the position X2.

The same procedure is repeated with probes #2 and #4 after probes #1 and #3 are initialized to positions X1=B and X3=λ+B; first probe #4 is moved to X4=λ+B and probe #2 is moved from X2=B to X2=λ+B in M=100 steps, s-parameters are measured and saved in a file S2; then probe #2 is moved to X2=B and probe #4 is moved from X4=2+B to X4=B in M=100 steps and s-parameters are measured and saved in file S4. At this point we have created four data files S1 to S4 containing sets of s-parameters associated with probe coordinates X1 to X4, then we have the following possible probe position arrangements: 1) X1<X2 and X2<X3 and X4<X3; 2) X1<X2 and X2<X3 and X3<X4; 3) X1<X2 and X3<X2 and X3<X4; 4) X2<X1 and X1<X4 and X4<X3; 5) X2<X1 and X3<X2 and X1<X4; 6) X2<X1 and X1<X4 and X3<X4. Assuming s-parameters in file Si are declared [Si] with the associated cascade ABCD format [Ti], then, in a de-embedding procedure, matrices are de-embedded and cascaded as follows: The first matrix [T1] or [T2] remains intact, if the associated probe is closest to the input port (X1<X2 or X2<X1); the remaining matrices [Ti] (including [T2] or [T1]) are multiplied with the inverse matrix [T0]−1in the order of the signal flow: i.e., if Xi<Xj then multiply [Ti]*[Tj] in this order, because the instant position of the probes defines the order of the cascaded twoports and yields the tuner calibration matrix as follows: [TC]=[Ti]*[T0]−1*([Tj]*[Tk]*[Tm]), wherein i, j, k, m are values between 1 and 4 per the previous rules. Or if Xi<Xj<Xm<Xk multiply [Ti]*[Tj]*[Tm]*[Tk], etc., If Xi=Xj, i.e., if the tuning probes face each-other across the channel, the multiplication order is irrelevant,

This procedure creates the actual tuner calibration file containing s-parameters Sij(X1,X2,X3,X4,Fo) and Sij(X1,X2,X3,X4,2Fo) for a total of (in case of M=100 steps) 2*1004=2*108sets of four s-parameters per 4 bytes pro value or ˜6.4 GB of data, to be processed in RAM. A direct calibration is impossible. The de-embedding calibration only requires measurement of 400 points at 2 frequencies for a total of approximately 7-8 minutes plus probe movement or a total of less than 15 minutes. Using the calibration data for harmonic impedance synthesis (tuning) purpose requires algorithms based on numerical data crunching equivalent to the algorithms used in ref. 8.

Obvious alternatives to the disclosed concept of a low-profile load pull tuner with four independent fixed-insertion, diametrically mounted and sharing the same slabline, tuning probes with neutral state capability but without adjustable vertical axis, using a double-bent center conductor, shall not impede on to the validity of the present invention.