Abstract
Coaxial microwave transistor test fixtures provide lowest insertion loss possible and include, as part of the input and output sections, transformer networks either in form of single stage /4 segments, or, for larger bandwidth, multiple step segments or ramped transitions from 50 to the impedance closer to the internal impedance of the power transistor. The transforming networks are flat or cylindrical and can be made exchangeable in order to accommodate various transforming ratios using the same fixture body and coaxial adapters. The fixtures can be calibrated using standard TRL method.
Claims
1. A coaxial pre-matched RF transistor (DUT) test fixture comprising an input section attached to an input port, an output section attached to an output port, a metallic block (INSERT), which is inserted between the sections and carries the DUT, and a metallic cover extending from the input port to the output port, wherein the input and the output sections are slabline segments, each comprising a bottom metallic block, an associated section of the metallic cover, a coaxial connector attached to the corresponding port, and a center conductor attached to the connector and reaching from the connector to the INSERT, and wherein the center conductors of the slabline segments comprise a 50 section and at least one non-50 section, whereby the 50 sections are attached to the connectors and the non-50 sections are inserted between the 50 sections and the DUT.
2. The test fixture of claim 1, wherein a length of the at least one non-50 section is one quarter wavelength long at an operation frequency.
3. The test fixture of claim 1, wherein at least one non-50 section comprises a number N1 of cascaded segments 1, 2, . . . N with characteristic impedances Z1, Z2, . . . ZN, wherein segment 1 is attached to the 50 section and segment N is attached to the DUT, and wherein 50>Z1>Z2 . . . >ZN, and wherein a length of the segments is optimized for DUT impedance matching and frequency bandwidth.
4. The test fixture of claim 1, wherein the center conductor of at least one slabline segment has a contour form of a lateral ramp, the ramp having a narrow end and a wide end, wherein the narrow end approximately matches a cross section of and is attached to the center conductor of a 50 section, which said 50 section is attached to the coaxial connector, and wherein the wide end has a width approaching a width of and is attached to a DUT terminal lead, and wherein a contour form and a length of the ramp are optimized to cover a frequency bandwidth.
5. The test fixture of claim 1 or 3, wherein the non-50 section has a form of a parallelepiped block.
6. The test fixture of claim 1 or 3, wherein the non-50 section is a co-axial cylinder.
7. The test fixture of claim 1 wherein the non-50 segment is a co-axial ellipsoid.
8. The test fixture of claim 1, wherein at least one non-50 segment is exchangeable.
9. The test fixture of claim 1, wherein at least one non-50 segment is co-axial ellipsoidal ramp.
Description
BRIEF DESCRIPTION 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 traditional load pull measurement setup.
(3) FIGS. 2A through 2B depict prior art; FIG. 2A depicts top view of a micro-strip transistor test fixture; FIG. 2B depicts side view of the micro-strip transistor test fixture.
(4) FIG. 3 depicts prior art, a 3D view of coaxial 50 test fixture.
(5) FIGS. 4A through 4B depict prior art; FIG. 4A depicts side view of 50 coaxial test fixture; FIG. 4B depicts front view of cross section A-B of 50 coaxial test fixture.
(6) FIG. 5 depicts prior art, a packaged RF power transistor.
(7) FIGS. 6A through 6B depict a pre-matched coaxial test fixture using /4 (quarter lambda) impedance transforming sections; FIG. 6A depicts a side view, FIG. 6B depicts a top view of the left section of the fixture in FIG. 6A with the cover removed.
(8) FIGS. 7A through 7B depict a pre-matched coaxial test fixture using wideband transformer sections; FIG. 7A depicts a side view, FIG. 7B depicts a top view of the left section of the fixture in FIG. 7A with the cover removed.
(9) FIGS. 8A through 8B depict 3D views of mounting the packaged transistor of FIG. 5 in the fixture of FIG. 7; FIG. 8A depicts the packaged transistor (DUT) and FIG. 8B depicts the DUT mounted into the transforming section of the center conductor of the fixture.
(10) FIG. 9 depicts a 3D view of the packaged transistor of FIG. 5 being mounted into the transforming section of the center conductor of the fixture of FIG. 6.
(11) FIG. 10 depicts a 3D view of the packaged transistor being mounted in the cylindrical transforming center conductor sections of the test fixture.
(12) FIG. 11 depicts a 3D view of exchangeable /4 transmission segments of center conductor of coaxial test fixture with characteristic impedances Za<Zb<Zc.
(13) FIG. 12 depicts a 3D view of wideband transforming structure of input or output section of center conductor of coaxial test fixture.
(14) FIG. 13 depicts the frequency response of the reflection factor of the input and output sections of the test fixture when terminated with the low internal impedance of the DUT; multi step transformer covers a higher frequency band.
(15) FIG. 14 depicts a front view of the multi-step transforming coaxial test fixture.
(16) FIGS. 15A through 15B depict elliptical transforming segments; FIG. 15A depicts a 3D view of transforming segment with elliptical cross section and mounted DUT; FIG. 15B depicts the associated component contours in a view from the fixture test port and connector.
DETAILED DESCRIPTION OF THE INVENTION
(17) FIGS. 3 and 4 show the prior art coaxial transistor test fixture (see ref. 1). Two coaxial connectors (40, 42) are attached to vertical walls (412, 413) and the extensions of the center conductors (31, 47, 48) form with the two blocks (25, 414, 415) and the cover (11, 416) an open transmission line (slabline), which is interrupted in the center to INSERT a block (21, 44), referred hitherto as insert, which carries the packaged DUT (43). In this case the transistor package (43) is placed on the INSERT (44) which is part of the horizontal slabline structure formed of the bottom ground planes (49, 412, 413) and the top ground planes (46, 416) in FIGS. 4A and 4B and the center conductor (47, 48). The signal enters in the input port (40), leaves at the output port (42) and is being conducted to and from the transistor DUT by the two coaxial center conductor segments (47, 48). The transistor leads (41) are inserted into horizontal slots (411) of the center conductor segments facing the DUT. The test fixture INSERT (44, 410) holds the transistor package (43, 45) secured with two screws (412) to ensure good RF grounding and heat dissipation. The advantage of this type of test fixture is lower insertion loss between the transistor leads (41) and the input (40) and output (42) ports and by consequence it allows higher tuning range created by the tuners.
(18) In a first embodiment of a pre-matching coaxial test fixture the impedance transformers are quarter wavelength (/4) sections of center conductor creating characteristic impedance Z1 lower than 50 in the slabline. These sections can be cylindrical or parallelepiped, for which electromagnetic simulation and analysis determines the correct dimensions. Typically, in a slabline structure with a wall-to-wall gap of 20 mm, a cylindrical center conductor of 11 mm diameter creates Zo=50.3 and a center conductor of 17 mm diameter creates a characteristic impedance of 24.2. A rectangular center conductor of 17 mm width and 5 mm thickness creates a Zo of 50.6 and a 13 mm7 mm center conductor creates a Zo of 21 in the same slabline. It is therefore possible to adjust the characteristic impedance of the transforming sections to electrically match the DUT impedance and to mechanically match the width of the package leads (51, 52) as well. FIG. 6 shows a side (FIG. 6A) and top (FIG. 6B) view of a pre-matching coaxial test fixture using parallelepiped transforming sections. The transforming sections are /4 long.
(19) Further on in FIG. 6A, beyond the /4 transforming sections (60) attached to the 50 sections of center conductor (61), one recognizes the DUT leads (63) inserted into the transformer section slots (62) and secured by the vertical screws (64). FIG. 6B shows the top view of the left half part of the fixture without the cover. It shall be noticed that the two halves of the fixture do not have to be identical. Existing calibration techniques allow characterizing accurately non-symmetrical test fixtures as well (see ref. 5).
(20) Using single stage /4 transformers is narrowband (see FIG. 13); it works best at the specific frequency F in gigahertz, F(GHz)=75/L(mm), whereby L is the length of the transforming section in millimeters. In order to increase the effective bandwidth one has two choices: (i) in a first embodiment use multiple transforming steps with decreasing characteristic impedance Z1>Z2>Z3 . . . from the 50 center conductor to the DUT terminals (FIGS. 12 and 14) or (ii) in a second embodiment use ramped transformers (see ref 6, FIGS. 7 and 8), whereby the center conductor becomes larger (the characteristic impedance becomes smaller) as we get closer to the DUT. In both cases the transforming sections become substantially longer, and the determination of their exact dimensions require specific network calculation software (see ref. 7).
(21) In the second embodiment (FIG. 7A) the leads (70, 73) of the DUT (71) are inserted and secured into slots of the transmission transformer (72) and the body (71) of the package is secured on the INSERT (74); the transforming center conductor is ramped (FIG. 7B) and is dimensioned to present to the DUT an impedance of 10 over a wide frequency band (FIG. 13). To achieve this the transforming section increases in width (and/or also in thickness) gradually and the characteristic impedance Z1 decreases gradually from 50 (see FIG. 8B). In particular the width of the transforming section can be designed to match the width of the package leads (51, 52). FIG. 8 shows the packaged DUT (FIG. 8A) before and after (FIG. 8B) insertion into the slot (81) of the transforming center conductor, on which it is secured using one or more vertical screws (80). This choice of transformer is particularly easy to manufacture, because it only takes a piece of sheet metal properly contoured. If the width of the transformer at the DUT end must match the leads of the DUT (51, 52) and create a given lower characteristic impedance, the transformer can also be made thicker close to the DUT (thicker transformer allows narrower width for the same characteristic impedance). In this case the transformer will have a pyramidal form and looks more like a horn antenna (see ref. 8).
(22) The 3D view of the single stage /4 flat transformer is shown in FIG. 9. One can distinguish the holding screws (90) that secure the DUT lead which is inserted into the slot (91) of the transforming section (92). It is desirable but not necessary to select the width of the transforming section (92) to match the width of the DUT leads (51, 52). If mostly the same package of DUT is used, then the choice of thickness (65) of the transforming section (92) can be chosen to account for this. As an example: in a slabline with a top to bottom channel width of 20 mm a /4 transforming section 20 mm wide and 10 mm thick creates a characteristic impedance Z1 of 30.5. The same Z1 can be obtained if the same section is, instead, 25 mm wide and 8.25 mm thick, or 15 mm wide and 11.75 mm thick. In all cases the impedance seen by the DUT, when the fixture is terminated with 50 is Z2=Z1.sup.2/5018.6.
(23) FIG. 10 shows single stage /4 cylindrical transformers, forming the core of the coaxial test fixture, attached to both leads of the DUT. The leads of the DUT are inserted into slots cut parallel to the axis of the transformers and secured with screws (100).
(24) FIG. 11 shows a useful variant of the coaxial fixture, in which a number of /4 transformers, each having a different characteristic impedance (Za<Zb<Zc . . . ) can be used and exchanged, easily mounted and dismounted on the extension of the center conductor (110) using a centered screw with a fine thread (111). The body of the fixture remains the same and uses the same adapters (112) and coaxial lines (110). The impedance Z.sub.dut seen by the DUT follows the above outlined rule: Z.sub.dut=Zt.sup.2/Zo, whereby Zo=50 and Zt is the characteristic impedance of the transformer (Zt=Za, Zb, Zc . . . ), as an example, if Zt=20, the DUT will see an impedance of 400/50=8 when the test fixture is terminated with 50 on the other end. Starting tuning from this point will easily create impedances of 1 or 2.
(25) In a third embodiment (FIG. 12) the 3D view of the multi-section wideband cylindrical transformer in step-form is shown, which also allows larger instantaneous bandwidth (FIG. 13). This is also shown in the front view of FIG. 14. It is obvious that this technique also applies to the flat transformers of FIGS. 6 and 9. In FIG. 13 the Reflection Factor (Fx) is related to the target impedance Zx, to be presented to the DUT. If this impedance is Zx then Fx is defined as: Fx=(ZZx)/(Z+Zx), whereby Z is the actual impedance presented by the transforming fixture to the DUT (i.e. if the generated impedance Z is equal to the target Zx then Fx=0).
(26) Simple calculations show that the various characteristic impedances of the transforming sections (152), cascaded with the 50 sections (151) and the connectors (156), can be generated by a combination of width versus height of the cross section of the airlines (in the case of a cylinder width=height=diameter). If the impedance requirement is such as to dictate a maximum diameter (150) fitting into the fixture walls (154, 155) of the slabline height (153 and 66, FIG. 6), associated with an optimum width matching the package leads (50, 51, FIG. 5), then an elliptical cross section (152) offers a suitable compromise; as an example: in a 20 mm large cavity (153, 66) the cylindrical airline required to generate a 24.3 characteristic impedance must have a diameter (150) of 17 mm, leaving only 1.5 mm gap C (Cylindrical) on each side towards the bottom (154) and cover (155) of the slabline; a parallelepiped cross section structure (not shown in FIG. 15) with a width of 17 mm, which is equal to the diameter of the cylindrical structure, needs only be 13 mm thick for the same result, leaving 3.5 mm space at the top and bottom. The elliptical solution using flattened cylinders or ellipsoid bodies (152) (best seen in cross section in FIG. 15B) lies in-between, allowing a higher gap E (Elliptical) at the same or slightly larger width; it is obvious that it is preferable to keep dimensions and gaps as large as possible to allow for higher manufacturing tolerances.
(27) Although the present invention has been explained hereinabove by way of a number of preferred embodiments, it should be pointed out that other combinations of the described components are possible and, as far as obvious to a person skilled in the art, they shall not limit the scope of the present invention.
