Abstract
A new two-probe waveguide slide screw load-pull tuner of which the probes share the same waveguide section; they are inserted diametrically at fixed depth into facing each other slots on opposite broad walls of the waveguide. The tuner does not have cumbersome adjustable vertical axes controlling the penetration of the probes and its low profile is optimized for on-wafer operations. The carriages holding the probes are moved along the waveguide using electric stepper motors or linear actuators.
Claims
1. An automated two-probe waveguide slide screw tuner comprising: a waveguide transmission line, having two broad walls, a top broad wall, and a bottom broad wall, and two narrow side-walls, an input port and an output port, two slots parallel to a longitudinal axis of the waveguide transmission line placed facing each-other on the top and the bottom broad walls, two remotely controlled mobile carriages C #1 and C #2, mounted opposite to each-other on the top and bottom broad walls, sliding along the waveguide transmission line and holding associated tuning probes P #1 and P #2, which are inserted in the slots sharing the waveguide transmission line; wherein the slots are positioned offset of a center line of the waveguide transmission line by at least a thickness of the tuning probes, and wherein the tuning probes are inserted diametrical into the slots at a fixed penetration into the waveguide transmission line, and wherein carriage C #1 moves probe P #1 to a position X1, and carriage C #2 moves probe P #2 to a position X2.
2. The slide screw tuner of claim 1, wherein the slots are at least one half of a wavelength (λ/2) long at a minimum operation frequency (Fmin).
3. The slide screw tuner of claim 1, wherein the carriages are controlled by remotely controlled stepper motors and appropriate gear.
4. The slide screw tuner of claim 1, wherein the tuning probes are metallic or at least partly metallized rods.
5. The slide screw tuner of claim 1, wherein the tuning probes are metallic or at least partly metallized blocks.
6. The slide screw tuner of claim 1, wherein the tuning probes traverse the slots in a contactless manner.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
(1) The invention and its mode of operation will be better understood from the following detailed description when read with the appended drawings, in which:
(2) FIG. 1 depicts prior art: a typical generic automated load pull test system.
(3) FIGS. 2A through 2B depict prior art: a single probe waveguide impedance tuner; FIG. 2A depicts a front view of the entire tuner; FIG. 2B depicts a cross section of the tuning probe (typically a conductive rod) entering the waveguide slot.
(4) FIG. 3 depicts partly prior art: a Smith chart and two possible trajectories of impedance synthesis (tuning) to reach a target impedance starting from the origin of 50Ω. Reaching target-1 uses the prior art single-probe technique with horizontal and vertical control; reaching target-2 uses the new two-probe technique with horizontal only and without vertical control.
(5) FIG. 4 depicts a cross-section through a waveguide with two conductive tuning rods (probes) entering from opposite sides into slightly offset slots.
(6) FIG. 5 depicts a front view of the waveguide tuner with two crossing over tuning probes.
(7) FIGS. 6A through 6C depict the signal flow graph (SFG) of the two-probe impedance tuner: FIG. 6A depicts the reflection area of a tuning probe; FIG. 6B depicts the overall signal flow graph; FIG. 6C identifies the signal transmission and nodes of the tuner depicted in the signal flow graph.
(8) FIG. 7 depicts signal flow and multiple reflections created by the two probes of a two-probe impedance tuner with shared waveguide. The probes can swap positions with regard to the test port 70.
(9) FIG. 8 depicts the Smith chart coverage mechanism using two-probe impedance tuner with fixed probe insertion depth.
(10) FIG. 9 depicts prior art: vertical axis of waveguide slide screw tuner.
DETAILED DESCRIPTION OF THE INVENTION
(11) This invention discloses a high frequency (RF, microwave, millimeter wave), computer-controlled impedance tuner, suitable for load pull measurements. The tuner (FIGS. 4 and 5) uses a low loss waveguide transmission airline 40, which includes two broad top walls, two narrow sidewalls and two slots 46 cut into the broad walls, one on the top and one on the bottom. The slots run parallel to the waveguide longitudinal axis and are positioned opposite to each other and slightly offset from the symmetry center line of the waveguide. The offset eccentricity is selected to allow two tuning probes (typically metallic or metallized rods 42) to cross over (pass next to each other) without touching. This structure is chosen for economy of space, because it uses slots of a total length of one half of a wavelength 56 plus the thickness of one tuning rod. An alternative configuration, where the tuning rods would not cross over, would, in principle, also work, but the slot (and the tuner) would have to be twice as long. The horizontal control of the carriages 52 and 53 can be made using stepper motors 54 and ACME screws 51, but can also be accomplished using linear electric actuators (see ref. 6), instead of motors 54 and ACME screws. The actuators have a motorized body, and their rotor axis is the extended horizontal ACME screw 51.
(12) FIG. 5 shows a front view of the waveguide tuner created by the cross section 41, 44 of FIG. 4: the slots 46 can be seen and their effective length as well 56; the mobile carriages 52 and 53 are controlled by the ACME screw 51 along the waveguide via the stepper motors 54; the reference position of carriage 53 is X1 and of carriage 52 is X2 relative to a common arbitrary zero position. The electric distance of the first tuning rod 55 from the test port 50 is L1 and the electric distance between the two tuning rods is L2; the lower slot 46 is hidden in FIG. 5 as it is evident from the cross section 44 in FIG. 4.
(13) The signal propagation trajectories are shown in FIG. 6B supported by FIGS. 6A and 6C; the total reflection factor 70 is the sum of all internal reflections in the waveguide and a function of the positions X1 and X2 of the two probes: S11(X1,X2) and is the vector-sum of the reflection factors 71 and 75 of both probes S11(X1) and S11(X2): S11(X1,X2)=S11(X1)+S11(X2), all referenced at the test port 70. The tuning probe 76 closest to the test port generates the primary reflection 71 i.e., S11(X1). Since the reflection of the first probe 76 is selected not to be total S11(X1)≈0.5-0.7, there is a signal portion 74 traversing towards the secondary probe 77; this signal portion is then reflected back 72 towards the primary probe 76; again, a portion 75 of this reflected back signal traverses the primary probe towards the test port and adds to the total reflection 70. The signal 72 reflected at the secondary probe is also reflected back 73 at the primary probe, and so on . . . creating the phenomenon of a multiple reflection. In mechanical terms this appears like a turbulence. This back and forth bouncing of signal is well described using signal flow graphs (see ref. 5). Instead, since the tuner is terminated 701 with a matched load any escaping signal 78 is not reflected back 79 and therefore ignored.
(14) FIG. 6B shows the signal flow graph describing the tuner, which is also shown schematically in FIG. 6C: Each node represents a port and each branch the complex signal flow factor S11 or S21 between nodes; each tuning probe (FIG. 6A) is described by a two-port having a reflection factor R (S11=S22) and a transmission factor T (S21=S12): probe 1 has R1 and T1, probe 2 has R2 and T2. The injected signal <a> returns as <b> after following several loops: between each probe there is a delay exp(−jβL); loop 1 represents the primary reflection, loop 2 the secondary reflection and loop 3 the multiple reflections; it must be remarked that loop 3 is revolving (repeating) adding each time a decreasing amount to the total reflection 70; loop 3 resumes in an infinite, but rapidly converging, series of reflected signal power vectors generating the overall reflection vector 70. The transmission factors T1 and T2 represent the portion of signal traversing the tuning probe, whereas the reflection factors R1 and R2 represent the reflected portion. Transmission lines are described by simple phase delays β1=2πL1/λ or β2=2πL2/λ. Assuming the probes represent capacitors C1 and C2 then we can approximate: R1=−jωC1Zo/(2+jωC1Zo); R2=−j ωC2Zo/(2+jωC1Zo); T1=2/(jωC1Zo) and T2=2/(2+jωC2Zo). The section after probe 2 is 50Ω matched, therefore signal is not returning and the associated R2 and T2 are ignored. By generating the signal loops 1, 2, 3, . . . which contribute to the overall reflected signal <b> we obtain for the reflection factor at the test port:
S11=<b>/<a>≈{e.sup.−j2βL1−R1−e.sup.−j2β(L1+L2)≠R2.Math.T1.sup.2−e.sup.−j2β(L1+2 L2).Math.T1.sup.2R2.sup.2.Math.R1+ . . . } [eq. 1]
the series of loops is truncated, since the third loop (LOOP 3) is repeated n times with n=1, 2, 3, 4 . . . towards infinite, but its contribution to the total reflection factor at the test port is rapidly decreasing, since all s-parameters R (reflection) and T (transmission) and their products appearing at higher power T1.sup.4n, R1.sup.2n, R2.sup.4n etc. are smaller than 1, forming this way a converging infinite series with finite sum. The transmission between the test port 70, probe 1, 73 and probe 2, 77 is described by lossless transmission lines having a phase −2 πL/λ, where λ is the wavelength λ=300 mm/Frequency (GHz).
(15) FIG. 8 depicts schematically the overall reflection factor synthesis mechanism: each probe creates at its own reference plane Γ1 or Γ2 (FIG. 6C) concentric reflection factor circles represented at test port reference plane as trace 100 on the Smith chart (FIG. 8). The total reflection factor trace 101 is created by a planetary epicycloid superposition of the two reflection factor vectors around the center 102 (one circle rotates around a point at the periphery of the other). When the probes cross over the circles swap. The total reflection factor vector 105 is created by the vector sum of vector 103 (from the first probe) and the vector 104 (from the second vector) as described in equation 1.
(16) Obvious alternative embodiments of fixed penetration tuning probes, diametrically inserted into and sharing the same slabline of waveguide slide screw impedance tuners and associated calibration and tuning methods shall not impede on the core idea of the present invention.
