WIDE BANDWIDTH R.F. LOAD

Abstract: A wide bandwidth R.F. load includes high-frequency, intermediate-frequency, and low-frequency loads. In the high-frequency load, a continually-tapered section includes concentric inner and outer elements matching in taper but differing in R.F. material constants, and a stepwise-tapered section includes a number of two-­ element loads each composed of cylindrical inner and outer elements having matching, and stepwise decreasing, outer and inner diameters, respectively, but differing in R.F. materials constants. The intermediate-frequency load is composed of tantalum resistors; the low-­ frequency load is composed of ferrite beads.

Description: The present invention relates to a wide bandwidth R.F. load. Wide bandwidth R.F. loads or precision terminations are used for several high frequency applications, and are, for example, required as terminations of vector network analyzers. For this field of application, precision terminations are required having an essentially contant impedance over a very wide range of frequencies, the return loss being 40 dB or less from 1 GHz to 26.5 GHz and less than 34 dB from 45 MHz to 16 Hz. Thus, the present invention has as its object of providing a precision termination or wide bandwidth R.F. load which can be used over a broader frequency range at and essentially constant impedance than has previously been possible. This object is achieved by a wide bandwidth R.F. load in accordance with the prior art portion of claim 1 having the features indicated in the characterizing portion thereof. An embodiment of the invention will now be discussed in detail with reference to the accompanying drawings. In the drawings, Fig. 1 shows a reference load according to the invention; Fig. 2A, 2B, 2C, 2D, 2E and 2G show expanded views of the reference load and its elements; and Fig. 2F shows a graph of electrical characteristics of beads shown in Fig. 2C. A wide bandwith R.F. load is a precision element with a flat response over the entire frequency range of interest, which in the preferred embodiment is generally from 45 MHz to 26.5 GHz. In order to provide such a constant impedance over this large bandwidth, the R.F. load RO is typically a composite system as depicted in Figure 1. As illustrated, RO has three distinct elements. The first, RO1, is a high frequency distributed load for preventing signals at the reference port from seeing any shunt impedance other than RO. Unlike an isolated lumped resistor which provides virtually no isolation, such a distributed load can be made to provide almost any degree of isolation commensurate with acceptable dimensions. Such isolation requires, however, that there be sufficient loss per unit length to avoid introducing shunt impedance at the frequencies of interest. Losses of 20-25 dB along the length of the load (40-50 dB isolation) are generelly sufficient to avoid bridge imbalance. The second stage of the reference impedance, RO2, is a lumped load placed at the end of RO1 to provide a termination at lower frequencies where losses in the distributed load, RO1, are insufficient to achieve isolation. To avoid parasitic shunt impedances at these lower frequencies, the third stage, RO3, a ferrite core (balun), is added to keep the shunt reactance high. The magnitude of the impedances of RO2 and RO3 are chosen so that their combined parallel impedance is equal to RO since RO1 is transparent at low frequencies. Shown in Figure 2A is an expanded view of distributed load RO1 and lumped load RO2. For purposes of discussion, all elements not already described previously will be numbered in the 700's to associate that element with Figure 2. Element 701 is a bottom ground plane, typically aluminum, having a groove therein to accommodate the distributed load material of the triaxial structure. Tapered loads 711 and 712 typically provide a constant impedance distributed load for frequencies above about 8 GHz, while the balance of the loads 711 and 713 through 740 are for achieving a flat response in the intermediate frequency ranges. As illustrated in Figure 2B, tapered loads 711 and 712 are physically mated to achieve a snug fit. Tapered load 711 is typically constructed of a relatively lossless material such as cross-linked polystyrene having a relative dielectric constant of about 2.54 (e.g., STYCAST 0005, which is manufactured by Emerson & Cuming) so that the distributed load is dielectrically loaded on the front end rather that relying on air as the dielectric. This dielectric loading is very important in achieving the desired matching characteristics with the lumped load. At the front end of tapered load 711 the preferred diameter is about 0.1497 inches (0,3802 cm), while at the distal end the preferred diameter is about 0.0797 inches (0,2024 cm). The preferred length for tapered load 711 is generally about 1.959 inches (4,9759 cm) with taper beginning about 0.230 inches (0,5842 cm) from the front. Tapered load 712 is typically constructed of a machineable lossy material such as MF112 which is also manufactured by Emerson & Cuming. Table I provides a list of the electromagnetic characteristics of MF112 relative to the vacuum as a function of frequency. The balance of the distributed load, i.e., loads 713 through 740, carries through this tapered load concept, except that the loads are stepwise tapered. Generally, step tapered terminations have a relatively narrow bandwidth and critical dimensionality. However, by using lossy loads in multiple steps (i.e., multiple loads) the bandwidth can be increased while at the same time reducing the overall length of the distributed load section. In addition, the use of lossy loads provides a forgiving structure, since subsequent losses in successive loads tends to obscure reflections at interfaces between loads. As indicated in Figure 2A in the first stepwise taper, inner loads 713, 715, 717, and load spacer 720 are typically constructed of STYCAST 0005 material, while outer loads 714, 716, 718, and 719, and inner loads 721, 723, 725, 727, 729, 731, 733, 735, and 737 are typically constructed of MF112 material. The second stepwise taper begins at load 722 and continues through load 740, with loads 722, 724, 726, 728, 730, 732, 734, 736, 738, 739, and 740 being constructed of MF124, a material produced by Emerson & Cuming which has a substantially higher loss than MF112. Table I also lists the electromagnetic characteristics of MF124 as a function of frequency. MF112 and MF124 were chosen because they are a mixture of lossy elements in a base material which is the same as STYCAST 0005. Hence, at low frequencies where the lossy elements have a minimal loss contribution, the outer taper resembles the inner taper, so that the distributed portion and the lumped load termination together are essentially matched to the reference impedance. Table II is a list of the inside and outside diameters of each of the loads. The linear dimension of each load is 0.25 inches (0,635 cm), except for load 719 which is 0.230 inches (0,5842 cm) long and load 719 which together with load spacer 720 totals 0.25 inches (0,635 cm). As indicated in drawings 2A, 2B, and 2C, the loads are strung in close contact on a center conductor 705. The diameter of center conductor 705 is nominally 0.0474 inches (0,1204 cm). Holding loads 710 through 740 in place is an adjustable top ground plane 741, typically constructed of gold-plated berylliumcopper and nominally having a width W of 0.237 inches (0,6020 cm), a depth D of 0.197 inches (0,5004 cm), and a length L of 5.735 inches (14,567 cm) (see Figures 2A and 2E). Holes in the top of ground plane 741 permit press blocks 748 to extend through the top surface to contact conductive shoes 747 which in turn contact the lower flanges of the top ground plane, in order to push them into firm contact with the load elements (see Figure 2D for a cross-section showing the deformation of the top ground plane on the outer surface of the load). The force for press blocks 748 is furnished by set screws 755 and 756 mounted in a top housing 753. This ability to tune the individual load elements via the top ground plane is particularly important in achieving a precision load since step tapered loads can be particularly sensitive to small changes in magnetic and dielectric properties. It should also be noted that throughout the system, RFI round strip conductive elastomeric gaskets 708 are provided to insure good electrical contact. Shown in Figure 2C is the tail end of the reference load illustrating the single loop of the inner coax threaded through a sequence of nine (9) ferrite shielding beads 757. Such beads are used because they have a high magnetic permeability and are very lossy, so that when the inner coax is eventually shorted to ground, there will be enough series loss to isolate the short and provide a precision reference termination. A typical material for beads 757 is Ferroxcube material 3B (available from Hewlett-Packard Company as stock number 9170-0016), having an initial magnetic permeability of 900 relative to the vacuum at 20 degrees Celsius. Beads 757 typically have an inner diameter of 0.051 inches (0,1295 cm), and outer diameter of 0.138 inches (0,3505 cm), and are 0.159 inches (0,4039 cm) long, so as to achieve an impedance of 300 ohms in this balun section. Figure 2F is a plot of the electrical characteristics of beads 757. Shown in Figure 2G is an expanded view of lumped resistor 709 (the equivalent of RO2 of Figure 1). It is generally constructed by connecting inner and outer rings of gold-coating saphire with eight identical, equally spaced, resistors 758 to obtain a combined resistance of 61.5 ohms. The outer diameter of the outer ring of the saphire substrate is typically 0.345 inches (0,8763 cm) and the inner diameter is typically 0.270 inches (0,6858 cm). The outer diameter of the inner ring is typically 0.135 inches (0,3429 cm) and the inner diameter of the inner ring is typically 0.075 inches (0,1905 cm). Resistors 758 are generally constructed of tantalum.

Claims: Wide bandwidth R.F. load having a substantially constant impedance over a range of frequencies comprising: an input port; and a first and second conductor coupled to said input port; characterized by a first distributed load (RO1) which is distributed along and between said first (705) and second (701, 741) conductors, said distributed load (RO1) consisting of inner loads (odds 711-737) having an inner taper and of outer loads (evens (712-719, 721-738)) having an outer taper with said inner loads comprising a first material having a first inner dielectric constant and a first inner attenuation coefficient and said outer loads comprising a second material having first outer dielectric constant and a first outer attenuation coefficient, with said first inner dielectric constant being smaller in magnitude than the magnitude of said first outer dielectric constant and said first inner attenuation coefficient being smaller in magnitude than the magnitude of said first outer attenuation coefficient; a lumped load (RO2) coupled to said distributed load (RO1) opposite said input port; and a second distributed load (RO3) coupled to said lumped load (RO2) opposite said distributed load (RO1) relative to said lumped load, constructed of a lossy material having a magnetic permeability which is substantially larger than the magnetic permeability of both said first material and said second material. R.F. load as claimed in claim 1, characterized in that said first inner dielectric constant is larger than the dielectric constant of air over substantially all of said range of frequencies. R.F. load as claimed in claim 1 or 2, characterized in that a first portion of said inner loads comprises a single continuously tapered loads (711) of said first material and a first portion of said outer load comprises a single continuously tapered load (712) of said second material. R.F. load as claimed in one of claims 1 through 3, characterized in that a second portion of said inner and outer loads comprises a plurality of stepwise tapered loads (713-719, 721-738). R.F. load as claimed in claim 4, characterized in that said inner loads in said second portion comprise said first material. R.F. load as claimed in claim 5, characterized in that said outer loads in said second portion comprise a second material. R.F. load as claimed in one of claims 4 through 6, characterized in that a third portion of said inner and outer loads are stepwise tapered, with said inner loads comprising said second material. R.F. load as claimed in claim 7, characterized in that said outer loads in said third portion comprise a third material. R.F. load as claimed in claim 8, characterized in that the magnitudes of the dielectric constant and attenuation coefficient of said second material are smaller than the dielectric constant and attenuation coefficient of said third material respectively. R.F. load as claimed in one of claims 1 through 9, characterized in that said second conductor (701, 741) is concentric with said first conductor (705) to form a coaxial system about said first conductor (705) with said inner loads being substantially concentrically located within said outer loads, with said outer loads themselves being located within said second conductor. R.F. load as claimed in claim 10 characterized in that said second conductor (701, 741) comprises a plurality of conductors, at least one of which can be adjusted to provide independent ground plane proximity to each load element for tuning said first distributed load. R.F. load as claimed in one of claims 6 through 11 characterized in that said second conductor (701, 741) is concentric with said first conductor (705) to form a coaxial system about said first conductor (10) with said first distributed load (RO1) located therein. R.F. load as claimed in one of claims 6 through 12 characterized in that said first conductor (705) is itself an outer conductive sheath for a second coaxial line (Fig. 2D).

Inventors: BOTKA, JULIUS K., DONECKER, S. BRUCE
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1984
Main classification: H01P1
IPC codes: G01R27, G01R1, H01P1