BROADBAND RADIO FREQUENCY DIRECTIONAL BRIDGE

Abstract: A broadband radio frequency (R.F.) directional bridge and a reference load for combination therewith are suitable for R.F. vector network analysis and R. F. time domain re­ flectometry measurements up to 26.5 GHz in a terminated Wheatstone configuration. The signal path length between the input, reference and test ports of the bridge microcircuit is less than one-tenth of the wavelength at the maximum frequency of operation. The reference load includes high-­ frequency, intermediate-frequency, and low-frequency loads. In the high-frequency load, a continually-tapered sec­ tion includes concentric inner and outer elements matching in taper but differing in R.F. material constants, and a step­ wise-tapered section includes a number of two-element loads each composed of cylindrical inner and outer ele­ ments having matching, and stepwise decreasing, outer and inner diameters, respectively, but differing in R. F. material 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 radio frequency vector bridge in accordance with the precharacterizing portions of the independent claims 1 and 6. A class of instruments called vector network analyzers is commonly employed for characterizing circuits in the frequency domain by providing plots of gain or loss, phase shift and reflection coefficients versus frequency. At the heart of such measuring instruments is a directional bridge for separating reflected and transmitted signals from incident signals, e.g., in order to characterize the S-parameters of a device under test (D.U.T.). Such a prior art bridge is shown in Figure 1. Here the bridge is typically symmetric relative to the R.F. input, with the resistance of resistors R1 and R2 being equal to each other, and equal in value to that of load resistor RO, i.e., R1 = R2 = RO. The bridge is typically implemented with a balun B between reference port (R) and test port (T). and with a termination resistor R3 = RO for extracting the signal. As a practical matter, however, the circuit of Figure 1 is highly idealized. If the balun and termination are replaced by equivalent realistic impedances, the difficulties in constructing such a device become readily apparent. A corresponding equivalent circuit is shown in Figure 2. Here, R1 = R2 = RO as before. However, for satisfactory measurements the following requirements must be satisfied: (1) the parasitic shunt impedances ZB and ZC must be very large, i.e., ZB >> RO and Zc >> Z D.U.T. where Z D.U.T. is the impedance of the device under test; or (2) ZB and ZC must be extremely well balanced, i.e., ZB = ZC exactly; or (3) ZB and ZC must satisfy some relation which is a compromise between (1) and (2) above. For measuring instruments which are restricted to a few octaves or less in bandwidth, these restrictions can usually be met without extreme measures. However, above 1 to 2 GHz, it becomes very difficult to produce a broadband impedance greater than about 1 K ohm, since the impedance decreases rapidly with increasing frequency. Hence, higher frequencies impose very stringent requirements for balancing of the two shunt impedances. As a result, for very wide bandwidth devices, e.g., over the entire range from 45 MHz to 26,5 GHz, it does not appear physically possible with the present state of the art to provide such high impedances or such precise balancing. Nevertheless, wide bandwidth directional bridges are highly desirable for automated test equipment using broadband sweepers to characterize circuit parameters and are particularly important for performing Fourier transforms for accurate time domain reflectometry, a technique which has proven extremely useful in solving intricate design problems. To date, the practical difficulties of extracting the desired differential signal over such a wide bandwidth without introducing unbalanced parasitic impedances have eluded the industry. Symmetrical bridge-type structures are known for providing an A.C. signal separation circuit to compare a known impedance and an unknown impedance. Such bridges are disclosed, for example, in U.S. Patents No. 2,976,497 and 3,278,840. However, U.S. Patent No. 3,278,840 is limited to operation at relatively low frequencies (less than approximately 300 MHz). Furthermore, U.S. Patent No. 2,976,497, while potentially operative at higher frequencies depending upon the physical size of the components, is limited to a two-to-one frequency range (one octave) absent repeated adjustment. Therefore, the known prior art does not permit comparison of known and unknown impedances over a broad range of frequencies extending upwardly from 45 MHz to 26,5 GHz (nine octaves). Relative to this prior art, the invention is based on the object of providing a radio frequency vector bridge which can be used over a broader frequency range than has previously been possible. This object is, according to the invention, achieved by the features characterized in the independent claims 1 and 6. The dependent claims 2 to 5 characterize advantageous developments of claim 1. The dependent claims 7 to 18 characterize advantageous developments of claim 6. The Wheatstone bridge is made up of a microcircuit which has a test port return loss of less than or equal to 20 dB over the entire range for frequencies from 45 MHz to 26,5 GHz. With a nominal impedance between the various ports (i.e., test port, reflection port, and reference port) of 50 ohms, this low return loss is accomplished by keeping the electrical path length in the various arms between ports at or below 0,1 the wavelength at the highest frequency. A precision termination or wide bandwidth R.F. load which is particularly suitable for the radio frequency vector bridge in accordance with the present invention is described in detail in a divisional application (application number EP-A-89 103 821.8) published under nr. EP-A-0327 138 on 09.08.1989. An embodiment of the invention will now be discussed in detail with reference to the accompanying drawings. In the drawings, Fig. 1 shows a typical prior art bridge; Fig. 2 shows an equivalent circuit for an actual bridge in accordance with Figure 1; Fig. 3 shows a terminated Wheatstone bridge in accordance with the invention; Fig. 4 shows a reference load according to the invention; Fig. 5A shows a reflectometer configuration using a bridge according to the invention; Fig. 5B shows a reflection/transmission setup for use in an automated network analyzer; Figs. 6, 6A and 6B show an exploded view of the bridge system and reference port; Figs. 7A, 7B depict a microcircuit in the terminated Wheatstone bridge according to the invention; and Fig. 8 is a cross-sectional view of the bridge housing. Shown in Figure 3 is a design for an R.F. terminated Wheatstone bridge which does not require either high shunt impedance or delicate balancing. Here all the elements and relationships are the same as those in Figure 2, except that the shunt impedance ZC is substantially eliminated and ZB is made to be RO, where RO is a constant impedance (typically 50 ohms) over the frequencies of interest, thus eliminating the separate resistor for that function and achieving a balanced bridge. with this structure, it can be shown by simple circuit analysis that the floating voltage VT, generated across the terminating impedance ZA, is proportional to the reflection coefficient of the D.U.T. While the built-in detector of a scalar bridge can easily convert the floating R.F. voltage VT to a single-ended D.C. output, the vector bridge of Figure 3 is faced with a much more severe challenge. It must extract the floating R.F. signal and feed it to a single ended vector detection system, without upsetting the delicate symmetry and impedance level of the bridge. For this reason, it should be readily apparent that RO must be 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, RO is typically a composite system as depicted in Figure 4. 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 generally 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 of 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. Another important feature of this structure is its triaxial nature, i.e., having a center conductor C1 inside a coaxial conducting sheath C2, which is itself inside a second coaxial conductor C3. Because the input and detector ports of a vector bridge are functionally interchangeable, either port may be driven while the other extracts the reflected signal. Hence, by reversing the orientation from that shown in Figure 4 and driving from the balun end RO3, the test port T can be D.C. biased by means of center conductor C1 (e.g., to provide power to transistors and I.C.'s in the D.U.T.). Such a reflectometer design is illustrated schematically in Figure 5A. Here the balun and R' represent the reference impedance RO. The system is fed through an R.F. power splitter 11 to provide a reference signal and to supply power to the bridge. b1 corresponds to the signal at port R and b2 corresponds to the signal after transmission through the D.U.T. The vector ratio b1/a1 is proportional to the input reflection coefficient while b2/a1 is proportional to the transmission coefficient of the D.U.T.. Also, included is a D.C. block 12 for isolating the R.F. power input from the bias voltage provided by bias input 13 on the center conductor of the reference impedance RO. Shown in Figure 5B is a typical reflection/transmission test setup which is particularly adapted for use by an automated R.F. network analyzer. A practical implementation of the bridge and the accompanying precision broadband load assembly is depicted in a somewhat exploded view in Figure 6. The system is made up of a bridge assembly housing 1, which contains a bridge microcircuit 6 (shown in enlarged view in Figure 6A) for providing electrical connections between the reflection port, the test port and the reference port. Also included in housing 1 is an adjustable spring loaded top ground plane 2. Also shown is a coax assembly 3 for connecting the high frequency portion RO1 of the reference load to the reference port of the microwave bridge. This is accomplished by providing a tapered transition from center coax 10 of RO1 to center coax 7 of the reference port which feeds microcircuit 6, the transition being accomplished by means of an inner taper 9 and an outer taper 8. At the other end of the distributed portion of the RO1 reference load is the lumped load RO2 which is made up of a 61,5 ohm resistor 22 and an inner conductor 24 and outer conductor 23. Attached to lumped load RO2 is the balun section RO3 made up of a series of ferrite beads, such as bead 21, located on the coax. The impedance of RO3 is generally chosen to be about 285 ohms in order to provide a combined shunt impedance with RO2 of 50 ohms. Balun RO3 is typically formed into a single loop for mechanical stress relief purposes, and a pad of polyurethane foam 26 is formed around it for protection from impact. Coax assembly 16 then provides connection for R.F. input and D.C. bias, and a grommet 5 is used to stabilize the positions of loads RO2 and RO3 in a load shroud assembly 17. Shown in Figure 7A is a magnified view of microcircuit 6 which was described briefly in reference to Figure 6. The microcircuit is formed on a hexagonal sapphire substrate 801 (shown in detail in Figure 7B), which has a nominal thickness of 10 mils (0,254 mm). Geometrically, the circuit has reflection symmetry in a plane passing through the center of reflection port 823 and the center of the circuit 6. However, the hexagon is not regular. Opposite sides are parallel, sides 802, 803, and 804 are equal in length, and sides 805 and 807 are equal in length. However, side 802 is not the same length as side 805 or 806. Table I gives a list of preferred dimensions for the microcircuit. Typically, the gold pattern and tantalum resistors 809, 810, 811, and 812 shown in Figures 7A and 7B are put down on both faces of the substrate with resistors on one face being disposed immediately opposite resistors on the other face. The resistance of each resistor 809, 810, 811, and 812 and their counterparts on the opposite face of the substrate is nominally 50,0 ohms, so that the combined resistance in each leg, i.e., between the reflection port and the reference port is 50 ohms and between the reflection port and the test port is 50 ohms. This results from the fact that each leg has a series resistance of 100 ohms on each face, and the two faces provide parallel connections to the ports. The use of two (2) parallel paths per arm helps to pull down the inductance of the system. Four (4) barium titanate single layer dielectric capacitors 814 (two (2) on each face of the substrate with each one opposite an identical one on the opposite face of the substrate) are epoxied in place in series with the resistors, and gold wire mesh is used to make electrical contact between gold pads 819 and capacitors 814. Each capacitor is nominally 350 picofarads, and capacitors on opposite faces of the substrate are oriented in opposite direction for symmetry reasons. Arrowheads 821, 822, and 823 for providing electrical and mechanical connections to each port are typically constructed of beryllium copper turned on a numerically controlled screw machine to a diameter of 0.060 inches (0,1524 cm). The overall length of the arrowheads is 0.080 inches (0,2032 cm) and includes a cone portion having a length of 0.037 inches (0,0940 cm) to avoid introducing unwanted capacitance into the system. Each cone portion is slotted to a width of about 11 mils (0,279 mm) to accomodate the 10 mil (0,254 mm) substrate and is soldered to the substrate via solder holes such as 824. Each arrowhead is gold plated to insure good electrical contact. Also shown in, Figure 7A is inner coax 825 which extends through the reference port arrowhead. Its center conductor 826 bridges a gap 827 of about 0.003 inches (0.076 mm) and makes contact with a dummy coax 829 which electrically connects center conductor 826 directly to the test port. Gap 827 then acts as an R.F. generator midway between the test and reference ports. The diameter of conductor 829, typically 0.013 inches (0,330 mm) is the same as the diameter of inner coax 825 in order to maintain electrical and physical symmetry as to both the test port and reference port. Despite the efforts at maintaining electrical and physical symmetry between ports, it is important to note that such symmetry alone is not sufficient to achieve an adequate port match. This can be most easily seen by noting that looking into the microcircuit from the generator with the test port terminated in 50 ohms, the transmission lines from the generator to the test port and reflection port should be 25 ohms for best termination (since the reflection port looks like a virtual ground because of the symmetry of the device). However, on terminating the reflection port in 50 ohms instead of the test port and looking into the microcircuit from the test port, the desired impedance should look like 100 ohms in each branch, i.e., from the test port to ground through the reflection port, and from the test port to ground through the reference port. To accommodate this poor port match, the microcircuit itself is made very small in order that the parasitic impedances introduced by the microcircuit are small relative to the 50 ohm operating impedance. By deciding on a design criterion for test port return loss, the corresponding maximum desired electrical path length between ports can be calculated using the following relationship: Z in = Z o (Z LOAD + Z o tan B) - (Z o + j Z LOAD tan B) where B is the electrical path length. By choosing the small interconnecting transmission lines between ports to be approximately 50 ohms and with a design of 20dB test port return loss, the maximum desired electrical path length in any one of the three arms is found to be 0.10 of the wavelength at the highest frequency. For the geometry described above, this corresponds to 0.045 inches (1,143 mm) at 26,5 GHz. Another unique feature of microcircuit 6 is that it is mechanically supported totally by its electrical connections. As illustrated in Figure 8, which is a cross-section of housing 1, outer coax 901, supports the microcircuit at the reference port. The end most tip of outer coax 901 is typically hemispherical in shape with a hole therethrough to accommodate inner coax 825, and contacts the drilled end 921 of arrowhead 821. The test port of the microcircuit is supported by a spring loaded ball joint 903 which contacts the drilled end 922 of arrowhead 822. The reflection port is supported by a spring loaded piece of slabline 905 which rests in a slot 923 cut in the end of arrowhead 823. This manner of three element mechanical support permits very stable mounting which also permits electrical adjustment within the housing by placing adjusting screws 907, 908, and 909 through bottom ground plane 910 of housing 1, as shown in figure 6, directly under the joint between each arrowhead and its electrical/mechanical connection.

Claims: Radio frequency vector bridge for use in a terminated Wheatstone configuration forming a symmetrical bridge-type structure for providing an A.C. signal separation circuit to compare a known impedance and an unknown impedance, said radio frequency vector bridge comprising: a housing; and a microcircuit located within said housing, characterized in that said microcircuit comprises: a substrate (801) having two parallel faces; a patterned layer of conductive material on each face of said substrate defining primarily three areas, one a reflection port (823), one a reference port (821) and one a test port (822); a first pair of capacitors (814), one on each face of said substrate (801) between said test port (822) and said reflection port (823) for blocking DC into the said ports and to permit test port biasing; a second pair of capacitors (814), one on each face of said substrate (801) located between said reference port (821) and said reflection port (823) for blocking DC into the said ports and to permit test port biasing; four resistors (809-812) on each face of said substrate, with a first pair (809, 810) of said resistors coupled to said test port (822) and to said reflection port (823), said first pair (809, 810) of resistors connected in series with one of said first pair of capacitors (814), and with a second pair (811, 812) of said resistors coupled to said reference port (821) and to said reflection port (823), said second pair of resistors (811, 812) connected in series with one of said second pair of capacitors (814); first coupling means (905) attached to said reflection port (823) for coupling said reflection port to other electrical components; second coupling means (901) attached to said reference port (821) for coupling said reference port to a reference load (RO); third coupling means (903) attached to said test port (822) for coupling said test port to a device under test (D.U.T.); and an R.F. signal source being coupleable to one of said reflection and reference port means (823, 821) for applying an R.F. input signal thereto; said resistors (809-812), capacitors (814), and said first (905), second (901) and third (903) coupling means being arranged symmetrically about said reflection port (823) to provide equal signal transit times between said reflection port (823) and said reference port (821) and between said reflection port (823) and said test port (822), with a maximum electrical path length between any two ports being less than or equal to 0,1 wavelength at the maximum frequency of operation. Radio frequency vector bridge as claimed in claim 1, characterized in that said substrate (801) comprises sapphire in the shape of an irregular hexagon. Radio frequency vector bridge as claimed in claim 1 or 2, characterized in that the combined resistance between any two ports is substantially 50 ohms. Radio frequency vector bridge as claimed in one of claims 1 through 3, characterized in that said second coupling means (901) is adapted to receive in an abutting arrangement a spring-loaded ball joint which serves as both an electrical connection and a mechanical support without the use of other independent connecting devices between said ball joint and said second coupling means (901). Radio frequency vector bridge as claimed in one of claims 1 through 4, characterized in that said first coupling means (905) is adapted to receive in an abutting arrangement a spring-loaded piece of slabline which serves as both an electrical connection and a mechanical connection without the use of other independent connecting devices between said slabline and said first coupling means (905). Radio frequency vector bridge having a microcircuit forming a symmetrical bridge-type structure for providing an A.C. signal separation circuit to compare a known impedance and an unknown impedance, characterized in that said microcircuit comprises: a substrate (801) having two opposing parallel faces; a patterned layer of conductive material on each face of said substrate defining a reflection port area (823), a reference port area (821) and a test port area (822); first impedance means (811, 812) for providing a first impedance between said reflection port area (823) and said reference port area (821); second impedance means (809, 810) for providing a second impedance between said reflection port area (823) and said test port area (822); reflection port means (905) for coupling said reflection port area (823) to other electrical components; reference port means (901) for coupling said reference port area (821) to a reference load (RO); test port means (903) for coupling said test port area (822) to a device under test (D.U.T.); and an R. F. signal source being able to be coupled to one of said reflection and reference port means (905, 901) for applying an R. F. input signal thereto; said first (811, 812) and second (809, 810) impedance means and said microcircuit (6) having reflection symmetry in a plane passing through the center of said reflection port area (823) and the center of said microcircuit (6) to provide equal times for signal transit between said reflection port means (905) and said test port means (903) and between said reflection port means (905) and said reference port means (901), with a maximum electrical path length between any two of said port means of at most 0,1 wavelength at a maximum R.F. frequency F. Radio frequency vector bridge as claimed in claim 6. characterized in that said reflection port area (823) on each face of said substrate (801) is symmetric with respect to said reference port area (821) and said test port area (822). Radio frequency vector bridge as claimed in claim 6 or 7, characterized in that said first impedance (811, 812) is equal to said second impedance (809, 810). Radio frequency vector bridge as claimed in claim 8, characterized in that said first impedance comprises two equal third impedances (811, 812 front; 811, 812 back), with one of said third impedances attached to said microcircuit between said reflection port area (823) and said reference port area (821) on each face of said substrate (801). Radio frequency vector bridge as claimed in claim 9, characterized in that said second impedance comprises two equal fourth impedances (809, 810 front; 809, 810 back), with one of said fourth impedances attached to said microcircuit between said reflection port area (823) and said test port area (822) on each face of said substrate (801). Radio frequency vector bridge as claimed in one of the claims 6 to 10, characterized in that said reference port means (901) comprises coaxial input means (C₁, C₂, C₃) for coupling said microcircuit to the said source of RF power and for creating a fifth impedance in said microcircuit. Radio frequency vector bridge as claimed in claim 11, characterized in that said coaxial input means (C₁, C₂, C₃) comprises biasing means for providing a bias potential to said test port means (903). Radio frequency vector bridge as claimed in one of the claims 11 and 12, characterized in that said test port means (903) comprises balancing means for providing a sixth impedance to balance said fifth impedance caused by said coaxial input means, (C₁,C₂,C₃) thereby preserving impedance balance between said reflection port means (905) and said reference port means (901) and between said reflection port means (905) and said test port means (903). Radio frequency vector, bridge as claimed in claim 6, characterized by further comprising first capacitive reactance means (814) connected in series with said first impedance means (811, 812) between said reflection port area (823) and said reference port area (821); and second capacitive reactance means (814) connected in series with said second impedance means (809, 810) between said reflection port area (823) and said test port area (822). Radio frequency vector bridge as claimed in claim 14, characterized by further comprising a housing (1) which comprises reflection port coupling means (905) for attaching said housing to said reflection port means, reference port coupling means (901) for attaching said housing to said reference port means and test port coupling means (903) for attaching said housing to said test port means. Radio frequency vector bridge as claimed in claim 15, characterized in that said reflection port coupling means includes a spring-loaded piece of slabline (905) and said reflection port means includes a slot (923) adapted to receive said slabline (905). Radio frequency vector bridge as claimed in claim 15 or 16, characterized in that said test port coupling means includes a spring-loaded ball joint (903) and said test port means is adapted to receive said ball joint (903). Radio frequency vector bridge as claimed in one of the claims 15 to 17, characterized in that said reflection port coupling means (905), said test port coupling means (903) and said reference port coupling means (901) serve as mechanical support elements for said microcircuit in said housing and also as electrical leads to said microcircuit from outside said housing.

Inventors: BOTKA, JULIUS K., DONECKER, S. BRUCE
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1984
Main classification: G01R27
IPC codes: G01R27, H01P1, H01P5