Abstract
Impedance tuners used in high power measurements suffer fast heating and consequently mostly linear thermal expansion of the central conductor, which has a very small mass and is thermally isolated from the slabline walls and the tuner housing. This leads to false measurements or catastrophic tuner failure (short) of either the DUT or the tuner. Gold plated INVAR and SUPER-INVAR center conductor material is preferred to traditional stainless-steel rod. The body of the airline is made of high conductivity low cost Aluminum. INVAR type alloys quasi eliminate the thermal expansion, reducing it by a factor between 10 and 40 compared to Steel. Practical tests have shown significant improvement in thermal behavior.
Claims
1. Slotted airline (slabline) for high power slide screw impedance tuner, comprising two sidewalls, a test port and an idle port with coaxial connectors on each port and a cylindrical center conductor anchored on either coaxial connector between the ports, wherein the center conductor is made of gold-plated lowest thermal expansion alloy SUPER INVAR 32-5.
2. The slotted airline of claim 1, wherein the sidewalls are made of high conductivity Aluminum.
3. The slotted airline of claim 1 having characteristic impedance Zo=50 Ohms.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
(1) The invention and its mode of operation will be more clearly understood from the following detailed description when read with the appended drawings in which:
(2) FIG. 1 depicts prior art: a typical automated transistor load pull test system.
(3) FIG. 2 depicts prior art: a front view of an automated slide screw impedance tuner using a single vertical axis and RF tuning probe (slug).
(4) FIG. 3 depicts Prior Art: cross section of tuning probe inside a slotted airline (slabline) approaching the center conductor.
(5) FIG. 4 depicts center conductor deformation due to heating and expansion and associated position of center conductor related to tuning probe.
(6) FIGS. 5A through 5C depict prior art: possible center conductor deflections due to thermal expansion: FIG. 5A depicts original position (unheated center conductor); FIG. 5B depicts sidewise deflection (and possible electrical short); FIG. 5C depicts downward deflection changing ||and creating measurement error.
(7) FIG. 6 depicts prior art: the measured dependence of tuner VSWR as a function of distance between tuning probe and center conductor.
(8) FIG. 7 depicts prior art; the thermal expansion coefficient of Nickel/Iron (NiFe) alloys as a function of Nickel content.
(9) FIG. 8 depicts prior art, thermal expansion coefficient of Super Invar 32-5 versus ordinary or Carpenter Invar 36.
(10) FIGS. 9A through 9B depict prior art: FIG. 9A depicts physical data of Super invar 32-5 for a temperature range.; FIG. 9B depicts composition of Super Invar 32-5.
(11) FIG. 10 depicts the temperature of the center conductor of a high-power slide screw tuner, as a function of dissipated power.
(12) FIG. 11 depicts prior art: Extracts from a data base for common good electrical conductor metals that can be used, if cost is disregarded, as bulk material for the sidewalls of tuner slablines. Aluminum offers, obviously, the best quality over cost compromise (see ref 7); Aluminum 0.2-1.60$/kg, Copper 4-6$/kg, Silver 473$/kg.
BRIEF DESCRIPTION OF THE INVENTION
(13) Slide screw tuners are designed with three main objectives: (a) generating high GAMMA, (b) having low RF and DC loss and (c) handling high injected and dissipated RF and DC power. This is achieved by (i) using minimal thermal expansion material for the center conductor, (ii) employing high conductivity material for the sidewalls of the tuner airline (slabline) and (iii) Gold or Silver surface plating the center conductor. These measures ensure low RF and DC loss, minimizing the amount of absorbed and dissipated injected RF and traversing DC power and maximizing GAMMA. Thermal expansion of the center conductor leads to false measurements and possible damage of the tuner, whereas high RF loss leads also to reduced tuning range (GAMMA).
DETAILED DESCRIPTION OF THE INVENTION
(14) This invention discloses simple and easily employable techniques destined to allow automated slide screw impedance tuners to have repeatable pre-calibrated impedances, generate high GAMMA and operate without risking destructive behavior and/or systemic measurement errors. The tuner is an RF two-port, best described by its scattering (s-) parameters (see ref. 8) for RF performance, and by the residual DC resistance of the center conductor of its slabline for DC performance. In a typical load pull configuration (FIG. 1) the DUT is DC-biased through the tuner, i.e. the bias tees (10) and (11) are inserted (in signal flow) before the input (2) and after the output (4) tuner. This reduces the insertion loss in the section between tuner and DUT, that would be caused if the DC bias tees were inserted between tuner and DUT; this insertion loss would reduce the tuning range |GAMMA|. The tuner absorbs power, both RF and DC. The dissipated power P.sub.DIS is calculated as the sum of dissipated RF power P.sub.RF and DC power P.sub.DC, wherein P.sub.RF=P.sub.INJ*|S21|.sup.2/(1|S11|.sup.2) {eq. 3} and the dissipated DC power is P.sub.DC=I.sub.DC.sup.2*R.sub.S {eq. 4}, wherein P.sub.INJ is the injected RF power into the input tuner either by the signal source but, more importantly, into the output tuner by the amplified outgoing power from the DUT; I.sub.DC is the DC current through the DUT and R.sub.S is the residual DC resistance of the center conductor of the slabline, including the coaxial connector contact resistances at the test and idle ports of the tuner.
(15) All this dissipated power leads to heating and linear thermal expansion of the thin center conductor of the slabline; to reduce and quasi eliminate this phenomenon the solution is to use the lowest available, but expensive, thermal expansion metal (INVARFIG. 7 or SUPER INVARFIG. 8) but only for the heated tuner part, i.e. the center conductor of the slabline; the body of the slabline itself, including the sidewalls, can be made using much cheaper and high electrical conductivity material, such as aluminum or any other, not necessarily temperature stable, but low or moderate cost material, like copper, since the body of the slabline is not heated. Using INVAR type of alloy for the entire slabline (center conductor and sidewalls) is meaningless for two main reasons: (i) Cost (INVAR is expensive) and (ii) Electrical Conductivity (INVAR as a NiFe (Nickel-Iron) alloy and has approximately 4 times higher electrical resistance and loss than Aluminum and 5 to 6 times higher than Copper, FIG. 11).
(16) The reason why all this invention concerns mostly the center conductor of the airline (slabline) of the tuner is because the center conductor is sensitive to self-heating (FIG. 10); this is for two main reasons:
(17) (a) the center conductor is a long thin metallic rod with small mass, and
(18) (b) the center conductor is suspended freestanding between the coaxial connectors of the two slabline ports and thermally isolated from the environment, since any physical contact with a heat dissipating metallic radiator would create a short circuit and incapacitate the tuner's RF behavior.
(19) The plot of FIG. 10 shows a saturation of the temperature curve above 90 Watt dissipated power, obviously because, above a certain temperature, the center conductor rod radiates heat into the environment. This is facilitated by the fact that the slabline (FIG. 3) is open and allows heat to escape upwards. For this reason, subsequent observations of the thermal expansion coefficient (TEC or ) are restricted to temperatures up to 100 C. For the above reason (b) it is obvious that the heating center conductor does not transfer heat to the slabline itself. There is, therefore, no practical advantage and reason to extend the use of expensive low thermal expansion metallic alloy, such as INVAR or SUPER INVAR to the body and sidewalls of the slabline. The additional reason for not using INVAR: INVAR is electrical conductivity: the Iron-Nickel alloy has high electrical resistivity RT (FIG. 9A). Since the sidewalls of the slabline must (i) have very low electrical resistivity, and (ii) be as low cost as allowable, the best choice is Aluminum, followed by Copper and excluding, of course other, technically possible but economically meaningless alternatives, such as Gold or Silver.
(20) The metallic alloys to be used are known as INVAR (see ref. 5 and FIG. 7) or SUPER-INVAR (see ref. 6 and FIGS. 8 and 9). INVAR is a simple Iron-Nickel alloy of which the thermal expansion coefficient, TEC, or [ppm/ C.] varies strongly, depending on Nickel content (FIG. 7). Whereas full Nickel has a TEC of 13 ppm/ C. (72, 73) and pure iron over 20 ppm/ C. (71), an alloy of 36% Nickel and 64% Iron (also called INVAR 36) has a TEC of only 1.2 to 1.3 ppm/ C. (74). The linear thermal expansion of the metallic center conductor can be calculated as L=*L*T, {eq. 5}, wherein L is the length of the center conductor rod, and T the temperature increase above room temperature. An issue appears, though, since is also temperature dependent (see trace for INVAR 36 in FIG. 8). This observation leads to two basic embodiments of the invention. In a first embodiment, for low to medium power tuners, ordinary INVAR 36 can be used. For high power tuners, in a second embodiment, choosing SUPER INVAR 32-5 (FIG. 8), is the preferred solution.
(21) However, these values are valid at room temperature only; as FIG. 8 shows, normal INVAR (or Carpenter Invar 36) has a strongly temperature dependent TEC varying from 0.50 to over 1.2 ppm/ C. when the center conductor temperature varies from 20 C. to 100 C. Therefore, ordinary INVAR 36 is not the best solution for very high-power tuners. It takes the more sophisticated SUPER INVAR 32-5 alloy (FIG. 8) to ensure temperature independence and stronger reducing thermal expansion. SUPER INVAR 32-5 is a complex alloy of 10 metals with 31.75% Nickel and approximately 62.5% Iron, having quasi constant TEC between 0.3 and 0.6 ppm/ C. in the same temperature range (FIG. 9). Using INVAR and SUPER INVAR reduces thermal expansion of the center conductor by a factor between 10 and 40, compared with ordinary Steel, which is used today for tuner center conductors. This is enough to avoid damage and measurement inaccuracies at medium to high power.
(22) Iron/Nickel alloys do not have superior electrical conductivity. As can be seen from FIG. 9A the electrical resistivity of SUPER INVAR is 80 cm whereas Silver, Copper, Gold and Aluminum have 1.6 to 2.7 cm. The thermally stable center conductor must therefore be silver- or gold-plated to minimize electrical resistance and RF electrical loss. In general Gold is easy to deposit by electrolysis, creates a better passivation and is preferred to Silver for electrical components.
(23) Obvious alternative embodiments to the herein disclosed method of minimizing the thermal expansion of the center conductor of slide screw impedance tuners are imaginable and possible but shall not impede on the validity of the basic idea of the present invention.
