Impedance tuners with rotating multi-section probes

A new slide-screw impedance tuner structure uses multi-segment, disc-shaped, rotating metallic probes. This ensures probes covering multiple frequency bands to be served using a single vertical control mechanism, instead of multiple axes, motors, motor control and double or triple carriages. Additional benefits are high resolution in the area where the gap between center conductor and probe is small (high GAMMA), smooth increase of proximity between probe and center conductor (basic anti-corona discharge form) and the possibility to compensate for the negative phase slope at higher GAMMA, native to traditional slide screw tuners.

CROSS-REFERENCE TO RELATED ARTICLES

BACKGROUND OF THE INVENTION

This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners. Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor'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”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment. The microwave 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); this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits (see ref 2).

Impedance tuners in particular “slide screw tuners” consist, in general, of a transmission line and a number of adjustable parallel tuning elements (22,30),FIGS. 2 to 5, which, when approaching the center conductor (32) of the airline (31) and moved along the axis of the airline (45) create a variable reactance, allowing thus the synthesis of various impedances (or reflection factors) covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA=|GAMMA|*exp(jΦ)=(Z−Zo)/(Z+Zo), wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50 Ohm (see ref. 3). The equivalent is the Voltage Standing Wave Ratio: VSWR=(1+|GAMMA|)/(1−|GAMMA|).

Up to now such tuning elements are made as metallic probes (slugs) and have had a cubical form (30,41) with a concave bottom (35) which allows to capture (when closing in to the center conductor) the electric field (99) inFIG. 9, which is concentrated in the area (36) between the center conductor (32,98) and the ground planes of the airline (31,97) (FIGS. 3 and 9). This field capturing allows creating high and controllable reflection factors. The disadvantage of this technique is the requirement of very high precision and resolution vertical movement mechanisms (33,21,53), which is controlled by a precision stepping motor (20). Because most of the field capturing effect occurs when the probe is very close to the center conductor, a high resolution vertical movement mechanism is needed. This, on the other hand, slows down the tuning procedure, since, when the probe is away from the center conductor, the tuning effect is much less effective, but the vertical moving speed is the same. In praxis, it takes typically 5 seconds to tune from |GAMMA|=0.05 to |GAMMA|=0.1 (VSWR from 1.1:1 to 1.22:1) and only 1 second to tune from |GAMMA|=0.9 to |GAMMA|=0.95 (VSWR from 19:1 to 39:1). Tuning speed is proportional to the change in VSWR divided by the change in vertical position (Y): SPEED=(Δ.VSWR)/(Δ.Y), whereas the vertical position is directly proportional to the motor stepping speed.

The probes used in slide screw tuners cover only a certain frequency range each. The lower frequency limit (61) is due to limited capacitance, the high end (62) due to resonances and propagation cut-off modes. In general the “longer” the probe, the lower the frequency (FIGS. 5 and 6). Hereby “length” of a probe is its size in direction of the axis of the airline. In order to cover a wide frequency range multiple probes are used (52,55inFIG. 5); each probe is attached to a precision vertical axis (53) in a mobile carriage (56) and is controlled by a vertical stepper motor (54). In case of even larger bandwidth three parallel probes are used (see ref. 6, FIG. 4a) which requires three axes, triple carriages and three vertical motors.

This invention discloses multiple frequency band probes using a single carriage, without need for a vertical axis and using a single vertical motor.

DETAILED DESCRIPTION OF THE INVENTION

This invention describes a radio frequency, computer controlled electro-mechanical impedance tuner apparatus, which uses multi-section metallic disc-formed RF reflection probes, instead of block-formed probes used hitherto in such devices. The probes are shown inFIGS. 7(b),8,9,12and15to20in various configurations and views. Further differences to prior art tuners are the missing vertical axis and associated gear. Beyond that the impedance tuners operate in a similar way and provide equivalent RF performance as prior art tuners; i.e. a horizontal carriage displacement changes the phase and approaching the probe to the center conductor changes the amplitude of the reflection factor (see ref. 2).

The fundamental difference between prior art and new probes is shown inFIG. 7:FIG. 7(a) shows a prior art RF probe (slug) setup with two slugs (71,75); each slug has a different horizontal length in order to cover a different frequency band. Slug (71) covers higher frequencies and slug (75) covers lower frequencies: inFIG. 6, slug (71) generates trace (62) and slug (75) trace (61). Each slug can be inserted into the slot of the airline (74) and be positioned (72) in various distances from the center conductor (76); the capacitive coupling between each slug (71,75) and the center conductor (76), shown also inFIGS. 3,4and9, creates a controllable reflection factor GAMMA. As is shown inFIG. 4, the distance between slug and center conductor must be very small (of the order of 50 micrometers) in order to create sufficient VSWR. This is the case also in the new probes (discs) ofFIGS. 7(b),8,9,12and15to20.

The operation of the disc probes is as follows (FIG. 7(b): A disc-shaped metallic RF probe (78), best shown inFIG. 19, has a cross section as shown inFIG. 9(93); the disc is rotating around the axis (92,77) which is perpendicular to the axis of the airline slot (713) and the center conductor (712) of the airline. One or more moon-shaped segments (79,714) have been removed from the original disc. This means that, by rotating the disc around the axis (77) one can reach a state where the coupling between center conductor and disc-probe varies between maximum (FIG. 16)and minimum (17). When the disc rotates the protrusion of the probe channel over-hang (911) around the center conductor (98) changes and the capture of the electric field (99) between the center conductor and the side-walls of the airline changes (FIG. 9); therefore also the reflection factor GAMMA changes; by consequence by simply rotating the disc-probe we can control the amplitude of the reflection factor. In order to avoid accidental galvanic contact between the probe and the center conductor, which might destroy the DUT, the concave surface of the groove is coated using a thin layer of insulating material. In order to establish reliable ground contact between the probe and the sidewalls of the slabline a probe design can be used, in which a slot (103,901) is cut into the body of the disc, parallel to the disc surface, allowing the two halves of the probe to apply a spring contact to the sidewalls (97).

Having two segments of disc remaining (78) and (715) we create the equivalent of a long and a short slug. The segment (78) corresponds to a short (high frequency) slug (71) and the segment (715) to a long (low frequency) slug (75). In short, by just rotating the disc-probe we capture the same effect as having two prior art vertical slugs ofFIG. 7a).

The effect of the rotation angle on coupling between center conductor and multi-segment disc-probe is best shown inFIGS. 16 and 17for the example of a two-segment probe (the same concept is valid for probes with three or four segments—FIG. 15): the disc is rotated around axis (164) using remotely controlled stepper motor (163); when the disc-channel wings (162) are enveloping tightly the center conductor and the body (161) of the disc is as close as a few micrometers (FIGS. 12 and 16) to the center conductor (165) a maximum portion of the electric field lines is captured, leading to maximum reflection factor.

When inFIG. 17the disc is rotated by the motor (173) around the axis (174) away from the maximum state (FIG. 17), then coupling of the disc wings (176) with the center conductor (175) is minimum and so is GAMMA.

By controlling Φ (FIG. 8) one can control GAMMA; additional benefit from a disc-probe is the progressive disturbance of the electric field, compared with prior art probes (FIGS. 7(a) and7(b); it can be seen the reflection of the injected electromagnetic wave is progressive (717) in a disc-probe, whereas it is abrupt in rectangular probes (73). In general abrupt disturbances create multiple reflections which lead to ripple in the response, of any wave propagation, including microwaves. As the probe (71) moves (72) closer to the center conductor (76) a portion of the travelling wave is reflected (73); in the case of the disc probe (FIG. 7(b)), as the sidewalls of the probe-channel (75) envelop the center conductor (712) progressively both in direction of the incoming (714) and the leaving (711) wave, this field disturbance (reflection) occurs progressively. This creates fewer ripples. Similar transformers (tapers) are common in physics in general (Megaphones) and in the microwave art (see ref. 4).

A disc-shaped tuner probe can be divided in more than two segments (FIG. 15); depending on the disc and the airline channel width (which is associated with the diameter of the center conductor for determining the characteristic propagation impedance Zo) as well as the frequency range targeted, three or four segments can be carved out of the disc-probe. InFIG. 15a four segment disc probe is shown with probes covering low frequencies (typically 0.6 to 2 GHz (151)), medium frequencies (1-6 GHz (155)), high frequencies (3-12 GHz (154)) and very high frequencies (8-18 GHz (153)); or for a narrower airline and center conductor of smaller diameter the corresponding maximum frequency can be 50 GHz (segment (153) covering 18-50 GHz), then segment (154) covering 9-26 GHz, segment (155) covering 4-12 GHz and segment (151) covering 2-6 GHz. This way a multi-octave tuner can be made using a single multi-segment probe structure; by rotating this probe, as shown inFIG. 18, the coupling can be reduced for the actual medium frequency segment (155) and by continuing the rotation the very high frequency segment (153) will engage.

Further-on the over-hanging walls (127,128, respectively134,137) of the disc channel can be made short or long, depending on the application and the overall disc diameter (FIGS. 12 and 13); by making said over-hanging longer we achieve higher coupling, but reduce the available segment area (152) that can be carved out of the disc to allow for distinct segments covering multiple frequency ranges (FIG. 15).

FIG. 10offers a perspective view of the tuning area of the tuner, the disc-probe (102) and the center conductor (101);FIG. 11shows the overall tuner (without a ‘prior art’ horizontal carriage control), the input and output ports (112,116), the airline (115), the center conductor (114), the disc-probe (113) and the stepper motor (111).

A further advantage of the here disclosed disc-probes (shown inFIG. 19), compared with prior art block-probes (shown inFIG. 7a), is the avoidance of edge effects and associated peak electric fields and corona arcing. The difference can be seen betweenFIGS. 5 and 7(a) on one hand and7(b),16,17and more clearly inFIG. 19(195) on the other: At the position (52) at the edge (corner) of the block-probe the voltage between center conductor and ground will cause an electric field, which is higher than the average field due to the RF power injected, and thus possible premature arcing (corona discharge); this is shown inFIG. 7(a) by arrow (73); in the case of a disc-probe (FIGS. 7(b),16,17and19) there are no edges creating peak electric fields: the transition from the center conductor to grounded probe is gradual: (166) inFIG. 16, (714) inFIG. 7and (195) inFIG. 19. This lowers the electric field and thus the risk for arcing.

The rotation of the bottom point of the disc-probe as it approaches the center conductor causes it to travel horizontally (89) inFIG. 8. The disc-probe does not approach the center conductor at one point (FIG. 8); the increase of the capacitance, as a prior art block shaped probe (slug) approaches the center conductor creates a negative phase turn of GAMMA on the Smith chart (see FIG. 1 in ref. 5); in the case of a disc-probe as disclosed here, the horizontal shift of the closest point, as the probe rotates (84), when viewed from the test port (801) of the tuner (from the left inFIG. 8) will create a compensation (linearization) of the original phase shift as a function of increasing reflection factor, which also improves the interpolation accuracy of the algorithms used to calculate GAMMA between calibrated points (see ref. 5).

Finally a three-dimensional view of a three-segment disc-probe is shown inFIG. 19. Segment (191) covers low frequency band, segment (193) covers medium frequencies and segment (192) high frequencies. The rotation axis (194) is approximately in the geometrical center of the disc.

An alternative disc-probe design is shown inFIG. 12: in the case ofFIG. 12(b) the protruding over-hang wings (126) of the probe are longer than the overhang wings (123) of the probe inFIG. 12(a). The coupling is here also controlled by rotating the probe, but in the case ofFIG. 12(b) more electric field is captured (see also shorter overhang, item (134) versus longer overhang, item (137), both inFIGS. 13(a) and13(b)). So the two segments of the probe (124) can be used to operate a tuner with certain GAMMA limitation: Long overhang means higher GAMMA.

The tuner calibration process uses a setup as inFIG. 14: the stepper motor (143) is directed by the control computer (142) to rotate the disc probes into the slot of the airline in order to increase the GAMMA value, while reading the four scattering parameters (s-parameters) from the Vector Network Analyzer-VNA (140) using standard communication cable (141) and protocol. S-parameters of the tuner are measured at typically 10 to 20 angles of the disc-probe, corresponding to minimum and maximum GAMMA, and saved in a calibration file in the form Sij(Φi, Xi); where Φi is the angle relative to the vertical direction (FIG. 7) and horizontal position Xi of each probe-carrying carriage between a starting (zero) position and one half of a wavelength at the selected frequency. This procedure is repeated for each disc-probe segment (715) or (78) inFIG. 7and all corresponding segments in the other Figures, at each frequency of interest covered by the corresponding segment and the data are saved in successive calibration files for later use.

When an impedance synthesis (tuning) is requested by a user, the computer loads the calibration data from the hard-disk into its active memory and scans through them to find the closest match for the parameter S11 in the s-parameter matrix. After the first step a second search is performed, in which interpolated data between calibration points are used and a final match is found, usually very close or identical within approximately a vector difference of close to 1% or less of the requested impedance value.

Impedance synthesis using s-parameters of tuners is a specific procedure related to specific tuners which shall not be elaborated further here (see ref. 5). This does not, however, limit the scope of the invention itself, since the invention relates to the new disc-probes used in the tuner apparatus rather than the tuning technique and presumes appropriate control software allowing calibration and tuning to be available. The calibration procedure has been laid out briefly only in order to manifest the fact that this wideband tuner is being used for impedance synthesis, when previously calibrated and the calibration data are used accordingly.

Obvious alternatives to the disclosed multi-segment disc-shaped RF probes for slide screw tuners are possible but shall not impede on to the validity of the present invention.