Patent Application
20080258849
1. Field of the Invention
The present invention relates generally to filters that selectively pass and attenuate electromagnetic waves and, more particularly, to low pass filters for attenuating high frequency electromagnetic signals.
2. Description of the Prior Art
Various structures commonly known as “filters” are used for suppressing or attenuating, to a desired specification, electromagnetic waves impinging on and propagating through the filter, depending on the signal's or wave's constituent frequencies. The number and scope of fields of communication, entertainment, and industrial equipments and systems requiring electronic filters is essentially indefinable. Therefore, it will be understood that the example applications for the filter described herein are not limiting; the fields are presented to assist the person of ordinary skill better understand the present filter, and to make and use a filter in accordance with described herein, for either an application similar to the example application, or any other of a wide range of applications.
Textbooks, technical journals, and other publications embody a large knowledge base of filters, including their types, structures, guidelines for selection, methods of design, construction, and testing. Within this large existing knowledge base, it is also well known that problems exist in designing and constructing a “low pass” filter, i.e., a filter that attenuates electrical signals above a “cut-off” frequency while, at very high frequencies, both maintaining a given characteristic impedance and adequate attenuation. It is also known that problems exist in designing and/or constructing a filter that meets such impedance and attenuation criteria while operating at very low temperatures.
Stated in reference to particular example requirement, in the existing art of electronic filters it is difficult to construct a low pass filter that can operate at temperatures such as, for example, 4 degrees Kelvin, provide a characteristic impedance of, for example, 50Ω, and provide, for example, −80 dB of attenuation for frequencies above a cutoff frequency of, for example, 100 MHz, while maintaining that attenuation for signals having components over, for example 5-10 GHz.
For purposes of this description, the terms “signal” and “electrical signal” will mean, unless otherwise clear from the context, any electromagnetic energy propagating through, or coupling between, any medium or structure, regardless of informational content in the signal. In other words, the phrases “signal” and “electrical signal” include electromagnetic energy that, for the intended purposes of the invention, are noise, including white noise, or other energy that the filter is intended to attenuate, i.e., not pass.
Further, the phrase “characteristic impedance” is very well known in the electronic filter art and, therefore, further description is omitted except where it is helpful for an understanding of this invention.
An example that reveals certain shortcomings in the prior art of electronic filters is presented by systems and equipment used in research, development and, eventually, manufacture of quantum computers. The present invention is not directed to quantum computing per se. The present invention is a novel method and apparatus for low pass filtering that, in addition to other likely benefits, has very good high frequency attenuation, can be easily built to meet impedance matching requirements, and maintains these attenuation and impedance characteristics at low temperatures. Present and anticipated future quantum computing machines are one, but not the only, system that would benefit from such a filter. However, it is not necessary to describe the theory of quantum computing theory in order to enable construction of a working embodiment of, or to otherwise practice, the invention. Quantum computing methods, equipment and systems are described only where necessary to better understand the example filters described herein, and to assist the user in selecting dimensions, materials and arrangements that fit the user's particular requirements.
In the example field of quantum computing, it is known that decoherence in superconducting qubits is often caused by high frequency noise transmitted along electrical leads connecting the qubit to measurement electronics at room temperature. The term “qubit” is known in the art quantum computing and further description is omitted, as it is not necessary for understanding this invention. One kind of noise comes directly from the measurement electronics at room temperature. In this case the filter can be located anywhere between the measurement electronics and the qubit. The second type of noise is Johnson (“white”) noise that is produced by resistive elements in the electrical connections between the room temperature electronics and the qubit. The location of these resistive elements will usually determine where one or more filters need to be thermally well grounded at one or more carefully chosen temperatures. For purposes of this description, the phrase “thermally well grounded” means a temperature difference of less than approximately 10%, using cooling and connection methods that are well known in the art of low temperature technology.
As an illustrative example of such temperatures, a qubit can be measured in a dilution refrigerator, which attains a typical minimum temperature of about 20 millidegrees Kelvin (“mK”), measured at the mixing chamber within a vacuum can that is immersed in liquid He4, itself at a temperature of 4.2 degrees Kelvin. Before reaching the qubit, all electrical wiring is preferably thermally grounded at, for example, approximately 4.2° K, 1.3° K, 0.7° K, and 0.1° K. These are example temperatures of operating parts of the dilution refrigerator that can handle a sizeable heatload, i.e., the electrical wiring, at that temperature.
There are known methods and structures directed to filtering unwanted noise having frequencies above, for example, 1 MHz at low temperatures. All have shortcomings either in terms of impedance or frequency characteristics. One example is a miniature thin film filter as reported by Vion et al., J. Appl. Phys. 77, 2519 (1995). Another example is a distributed thin film microwave filter reported by Jin et al., Appl. Phys. Lett. 70, 2186 (1997). Still another example is the Philips Thermocoax filter, as discussed in A. Zorin, Rev. Sci. Instrum. 66, 4296 (1995). In most cases these filters were first used to reduce noise in single electron tunneling experiments. Perhaps the simplest and easiest to fabricate “microwave” filter is the bulky metal powder filter. The metal powder filter was first discussed in more detail by Martinis et al., Phys. Rev. B 35, 4682 (1987) and subsequently developed and discussed in detail by others. See K. Bladh et al., Rev. Sci. Instrum. 74, 1323 (2003), and A. Fukushima et al., IEEE Trans. Instrum. Meas. 45, 289 (1997).
The metal powder filters known in the relevant art have a central conductor that is surrounded by metal powder or a metal powder/epoxy mixture. The filter attenuates an incoming electrical signal via eddy current dissipation in the metal powder. The known art teaches, however, that the central conductor is shaped into the form of a spiral to increase the attenuation. This does indeed increase the attenuation but, as observed by the present inventors, these spiral conductor metal powder filters cannot be designed to have a characteristic impedance near 50Ω at high frequencies. The present inventors have identified that such filters cannot provide a 50Ω impedance at high frequencies because each adjoining loop of the spiral is capacitively coupled to the next loop, and if the spiral is “tight” then at high frequency this coupling looks like a short between loops. Stated differently, the physical design of known metal powder low pass filters creates what is technically a short at high frequencies, not 50 ohms.
In many high frequency applications, however, it is necessary to have an all matched 50Ω impedance measurement setup. If low pass filters are used they also must be 50Ω. The known metal powder filters cannot, because of their spiral form, meet this requirement.
It is therefore an object of the invention to provide a method and apparatus for attenuating high frequency signals while maintaining a desired characteristic impedance.
It is a further objective of the invention to provide a method and apparatus that passes signals of a frequency below a given cut-off frequency, attenuates signals above that cut-off frequency, and maintains the attenuation up to a very high frequency.
It is a further objective of the invention to provide a method and apparatus that provides a desired characteristic impedance, passes signals of a frequency below a given cut-off frequency, attenuates signals above that cut-off frequency, and maintains the attenuation and the desired characteristic impedance up to a very high frequency.
It is a further objective of the invention to provide a method and apparatus that provides a desired characteristic impedance, passes signals of a frequency below a given cut-off frequency, attenuates signals above that cut-off frequency, and maintains the attenuation and the desired characteristic impedance up to a very high frequency, and over a very wide temperature range.
It is a further objective of the invention to provide an easy-to-manufacture filter structure that provides a desired characteristic impedance, passes signals of a frequency below a given cut-off frequency, attenuates signals above that cut-off frequency, and maintains the attenuation and the desired characteristic impedance up to a very high frequency.
It is a further objective of the invention to provide an easy-to-manufacture filter structure that provides a desired characteristic impedance, passes signals of a frequency below a given cut-off frequency, attenuates signals above that cut-off frequency, and maintains the attenuation and the desired characteristic impedance up to a very high frequency, over a very wide temperature range.
It is a further objective of the invention to provide an easy-to-manufacture filter structure that provides a 50Ω characteristic impedance, passes signals of a frequency below a given cut-off frequency, attenuates signals above that cut-off frequency, and maintains the attenuation and the desired characteristic impedance up to a very high frequency, at temperatures down to approximately 4 degrees Kelvin.
The foregoing and other features and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention, which is further illustrated in the accompanying drawings.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the claims appended to this specification. The subject matter, features and advantages of the invention will be apparent from the following detailed description viewed together with the accompanying drawings, in which:
The value of the center conductor 14 diameter CD and the outer tube 12 inner diameter ID are dictated in part by the following well-known equation governing the characteristic impedance of a coaxial line:
(Equation No. 1), where K is the effective dielectric constant of the material surrounding the inner conductor, i.e., the material 16, the variable a is the diameter of the inner conductor, i.e., the diameter CD of the center conductor 14, and variable b is the inside diameter of the outer conductor, i.e., the inner diameter ID of the outer conducting tube 12. Since there are three variables, i.e., the CD and ID dimensions and the K effective dielectric constant, the solutions to Equation No. 1 that will provide a given impedance are, at least mathematically, infinite. As will be understood from this description, though, there are certain guidelines for selecting a starting point. For example, the universe of achievable values of K is limited by the binder component of the mixture 16 having to meet certain thermal and viscosity requirements, and by the percentage of metal powder. Also, because, as will be understood upon reading this description, the attenuation mechanism of the filter according to this invention is by losing energy to the metal powder in the mixture 16. The present inventors have identified that the more metal powder close to the center conductor 14 the more energy loss. Therefore, the higher the percentage of metal powder in the mixture 16 the greater the attenuation. The target value of K is therefore selected in view of what is achievable when using the necessary percentage of metal powder in the metal powder/binder mixture 16. The selection of the center conductor 14 wire diameter CD will be driven, at least in part, by the ease of working with the wire. Once K is fixed, fixing CD fixes the outer tube's inner diameter ID. Therefore, it is seen that the choice of CD and ID preferably incorporates the relevant needs of the application.
For example, the present inventors constructed filters using a 0.005 inch diameter wire for the center conductor 14 which, in view of a 50Ω impedance, required the outer tube 12 to have an inner diameter ID of about 0.125 inches. This ID value was practical with respect to the scale/size filters that the inventors needed for the example qubit measurement application. It is conceivable, though, that a larger CD and larger ID may result in a structure better able to withstand thermal stresses without fracture. Stated with greater particularity, it is probable when a filter having larger ID and CD values is exposed to low temperature that the metal powder/epoxy mixture 16 may fracture, but this may not cause an unacceptable failure of the filter such as, for example, the center conductor 14 fracturing.
The connector block 18 of the
The connector block 18 of this example has a connector receiving bore 18B, extending perpendicular to 18A, dimensioned to accept a first connector 22. An example first connector 22 is a commercially available SSMA type. These are well known in the art and, therefore, further discussion is omitted. This is not, however, the only type of acceptable connector 22. The specific connector is a design choice, driven by the specific characteristic impedance and frequency characteristic desired for the filter, and readily made by a person skilled in the art upon reading this description. For example, the first connector 22 could be a commercially available SMA type, also well known in the art, but which generally possesses high frequency performance inferior to the SSMA type.
With continuing reference to
The connector block 18 of the
The first function of the second clearance hole 18C in the
The second function of the second clearance hole 18C in the
Referring again to
A small vent hole 28 is formed in the connector adaptor 20, to enable injection of the viscous form of the metal powder/binder mixture 16, via the second clearance hole 18C formed in the connector block 18. As will be better understood by reading the description below of an example assembly operation, the small vent hole 28 enables injection of the viscous form of the mixture 16 by functioning as an air vent, thereby permitting the mixture 16, while still viscous, to flow into the second clearance hole 18C, and fill the space between the center conductor 14 and the outer tube 12—all the way from the hole 18C to the end surface 26A of the second connector 26, and including the chamber volume labeled as 20B.
Still referring to
The present invention, when employing superconducting wire as the center conductor 14, exploits this characteristic of the wire in a manner directly beneficial to, for example, qubit measurements. It is directly beneficial because in qubit measurements it is important to minimize the amount of heat transported directly along electrical wiring, since they are directly connected to the sample holder that contains the qubit being measured. Filters, or attenuators, even according to the present invention, are resistive elements and therefore generate heat. Even though these attenuators are heat sunk, some heat is still transported along the wire to regions at lower temperatures. Using superconducting wire for the center conductor 14 is a way of blocking this heat.
Further, the present inventors have identified that replacing the inner conductor of even conventional filters with a superconducting wire would obtain at least this heat blocking benefit, although without the impedance and attenuation benefits provided by the
Further, for applications of the present invention not requiring low temperature operation, a standard non-superconducting wire could be used for the center conductor 14.
An example material for the center conductor 14 is Cu-clad NbTi superconducting wire. Commercially available examples of such Cu-clad NbTi wire will sometimes have an insulation coating of polyvinyl such as, for example, the polyvinyl very well known and commonly referenced in the relevant arts by the trademarks Formvar™ and Vinylec™. Typical thickness of such insulation, for a wire of having a diameter CD of 0.005 inches, is about 0.001 inches. The present inventors determined that such insulation is acceptable, at least for the example filter characteristics specifically identified by this specification. However, a center conductor 14 consisting of a wire without such insulation may be preferable as it may provide better filter damping.
Referring to
The make-up of the metal powder/binder mixture 16 is critical, because it controls the filter attenuation characteristics, and determines the value of K in Equation No. 1 of this disclosure, repeated below, which is the well-known equation governing the characteristic impedance of a coaxial line:
(Equation No. 1), where K is the effective dielectric constant of the material 16, the variable a is the diameter CD of the center conductor 14, and variable b is the inner diameter ID of the outer conducting tube 12. The material 16 must meet other criteria as well, such as, for example, thermal conductivity, coefficient of thermal expansion, and the ability to hold a sufficient percentage of metal powder in suspension with sufficiently low viscosity to permit injection into the space between the center conductor 14 and the inner surface of the outer tube 12, as will be described in greater detail below.
Preferred constituent materials of the mixture 16 are metal powder and a binder, which may be, for example, epoxy. Binders other than epoxy may be used, but selection must be made in view of the required dielectric constant, the materials from which the center conductor 14 and outer tube 12 are formed, respectively, and the environment in which the filter is intended to operate. For example, if the filter is intended to operate at extremely low temperatures, then the binder component of the metal powder/binder mixture must have thermal characteristics compatible with those of center conductor 14 and outer tube 12, such that stresses are not built up that may fracture the center conductor 14. This will be understood upon reading the present disclosure in its entirety, including the description of specific examples constructed by the present inventors.
Example metal powders include powdered copper and powdered bronze. Powdered copper and powdered bronze oxidize naturally and, therefore, are insulating at DC. The average size of the metal powder particles and the statistical distribution of the particle size determine the cutoff frequency Fc and attenuation characteristic of the filter. Stated with more specificity, the smaller the particle size, the higher the cutoff frequency Fc. The choice of metal, and the particle size and the statistical distribution of the particle size also affect the effective dielectric constant K of the metal powder/binder 16, as described above in reference to Equation No. 1.
Referring again to Equation No. 1, it is seen that upon fixing K at a particular value, the impedance of the filter 10 is entirely determined by two structural parameters of the filter—the inner diameter ID of the tube 12 and the outer diameter CD of the center conductor 14. However, K may not always be picked at random; it should be selected in view of the necessary percentage of metal powder in the mixture 16, the diameter statistics of the particles in the metal powder, the dielectric properties of the binder components of the mixture 16, as well as in consideration of the available dimensions of commercial materials, such as wire and tubing, for making the center conductor 14 and outer tube 12, respectively.
It should be understood that the actual K of the metal powder/binder 16 may differ from the target K value—the value on which the dimensions CD and IC of center conductor 14 and outer tube 12 were selected. Such variances can arise, for example, from manufacturing variances in the epoxy or other binders used in the metal powder/binder 16. The difference between the actual and target K value will likely result in the filter not having the desired characteristic impedance. The solutions are straightforward. One, as described below, is to remake the filter with an outer tube 12 having a different inner diameter ID. Another is to fine tune the relative percentage of the constituent materials of the metal powder/binder 16, and remake the filter. As stated above, though it is preferable to begin with as high a percentage of metal powder as possible, i.e., the highest percentage at which the liquid form of the mixture 16 can be injected, as described below, because the high percentage maximizes the attenuation. Then, if Z is off, one should adjust IC of the outer tube 12, if possible, rather than fine tune the percentage of metal powder in the mixture 16, because the percentage may already be near the maximum for which the mixture can be injected and, therefore cannot be increased, and decreasing the percentage will adversely affect attenuation.
The powders are preferably free of ferromagnetic impurities, which could be a source of noise. Methods of testing from such impurities are known in the art and need not be described but, for purposes of example, testing can be done using a Quantum Design SQUID based magnetic susceptometer. Commercially available products can be used, including (i) an approximately 1-5 μm Cu powder available from Aremco™ Products, (ii) an approximately 37 μm Cu powder, and (iii) an approximately 3 μm bronze powder (30% Sn, 70% Cu) available from Kennametal™.
In view of the inventors' presently formed theory of operation of this invention, which is described in further detail below, it is generally suggested to inspect the actual particle size(s) and/or statistical distribution of particle sizes in the metal powder before mixing it to form the filler 16, regardless of it being obtained from a commercial vendor. For example,
Referring to
Referring to
With respect to the thermally conductive encapsulating epoxy component of the mixture 16, acceptable specifications are, for example, a mixture of approximately 20-35% (weight concentration) epoxy resin, 1-5% butyl glycidyl ether, and less that 0.5% carbon black having, prior to mixing with the catalyst, a density of approximately 2.35-2.45 grams per cubic centimeter, and a Brooksfield viscosity, using test method ASTM-D-2393, 5 rpm, #7, of 200-250 Pa·s, and 200,000-250,000 cP. After mixing with a catalyst as described below, the thermally conductive epoxy can have a set time ranging from approximately one to four hours at 65 degrees Celsius to 16-24 hours at 25 degrees Celsius. After setting, acceptable relevant specifications are α1 and α2 coefficients of thermal expansion, according to the ASTM-D-3386 test, of α1 ranging from approximately 31 to approximately 36 and α2 ranging from approximately 98 to approximately 112 (where α1 and α2 are in the ASTM-D-3386 units of 10−6/° C.), a thermal conductivity, according to the ASTM-D-2214 test, ranging from approximately 1 to approximately 1.3 Watt/m K and from approximately 7 to approximately 9 Btu-in/hr-ft2-° F., and a dielectric constant, under the ASTM-D-150 test, ranging from approximately 5 to approximately 5.4. An example commercially available thermally conductive epoxy meeting these specifications is “Stycast™ 2850 FT” available from Emerson and Cuming™ and/or the National Starch & Chemical™ Company.
With respect to the catalyst for the above-identified thermally conductive encapsulating epoxy, the specification may, for example, be as follows: an aromatic amine such as 4,7,10-trioxytridecane-1,13diamine. An example commercially available catalyst that meets these specifications is “CATALYST 24LV,” available from Emerson and Cuming™ and/or the National Starch & Chemical™ Company. The mixture ratio of the example thermally conductive encapsulating epoxy and the example catalyst is approximately 7.5 parts catalyst per 100 parts epoxy by weight, or 17.5 parts catalyst per 100 parts epoxy by volume.
With respect to the low viscosity epoxy for controlling the viscosity of the liquid form of the mixture, an example of acceptable specification is as follows: a mixture of amine and epoxy, with approximately 28 parts amine per 100 parts epoxy by weight, or 33 parts amine per 100 parts epoxy by volume. Mixed in these proportions, an example acceptable working life for the low viscosity epoxy is approximately 30 minutes to two hours, with “working life” defined in accordance with ERF 13-70. An acceptable density is, for example, approximately 1.12 grams per cubic centimeter, and an acceptable Brookfield viscosity is, for example, 0.65 Pa·s and 650 cP, as defined by the ASTM-D-2393 standard. An acceptable cure time, at 65 degrees Celsius is, for example, approximately 2-4 hours and, at 25 degrees Celsius is, for example, approximately 8-16 hours. Upon curing, the value 3 is an example acceptable dielectric constant for this low-viscosity component, using the ASTM-D-150 standard at 60 Hz. An example commercially available low viscosity encapsulating epoxy meeting these specifications is “Stycast™ 1266 A/B” available from Emerson and Cuming™ and/or the National Starch & Chemical Company.
To lessen repetition in this description, the above-described “Stycast™ 2850 FT” thermally conductive encapsulating epoxy, and its catalyst, “CATALYST 24LV” are hereinafter referenced collectively as “2850 thermally conductive epoxy,” or simply “2850 FT.” Likewise, the above-described “Stycast™ 1266 A/B” low viscosity epoxy will be referenced as “1266 low viscosity epoxy” or simply “1266 A/B.” It will be understood that the labels “2850 FT” and “1266 do not limit the invention to using the identified example vendors, or the identified examples of specific products. Instead, even for the below-described examples of the filter 10, “2850 FT” and “1266 A/B” encompass the particular identified vendors' products, as well as any other epoxies or binders substantially meeting the above-identified example specifications that “2850 FT” and “1266 A/B” meet, and all equivalents thereto.
For the example epoxy mixture of “2850 FT” and “1266 A/B” the mixing proportion may be 80% “2850 FT” and 20% “1266 A/B.” The function of the example type “1266 A/B” was to lower the viscosity of the mixture, and thereby enable injection of mixture 16 having a higher metal powder content. Stated differently, a viscosity-lowering ingredient, such as “1266 A/B,” generally allows a higher percentage of metal powder to be mixed in before the mixture 16 becomes too viscous to inject into a filter, such as the example 10 of
It should be understood, when choosing the binder for the mixture 16 for a filter of the present invention to be used at very low temperatures, that the metal powder of the mixture 16 must be sufficiently mixed with the binder, such as epoxy, such that the metal powder component of the mixture 16 and the center conductor 14 are well thermalized. Stated differently, a filter according to the invention made with a mixture 16 having no binder, i.e., by simply packing metal powder into the space between the center conductor 14 and the outer tube 12, would not likely perform adequately. Illustrating this by example, if the center conductor 14 has a transition temperature of 9.3 degrees Kelvin then the center conductor 14 must be cooled to below that temperature to operate in a qubit measurement device. Also, the bronze (or copper or other metal) powder must be cooled to some low temperature below which the absorption properties of the metal powder do not change. Because of this cooling requirement, metal powder would likely be unacceptable. There are two reasons for this unacceptability. The first, which can be seen from
The above-described epoxy embodiment of the binder in the mixture 16 overcomes this problem because, if picked as specified above, such an epoxy is a reasonable thermal conductor and it fills the voids between the metal particles. The described epoxy therefore provides a medium that allows heat to pass from the warm powder and center conductor 14 to the outer tube 12.
Guidance for selecting the material for the binder of the metal powder/binder mixture 16 is provided by the illustrative example of the
An observed illustration of the reason for matching the thermal conduction and thermal contraction of the binder, e.g., epoxy, of the mixture 16 with that of the outer tube 12 is that, when prototypes using only type “1266 A/B” were cooled, the mixture 16 would shrink at a rate different than the outer tube 12 and/or center conductor 14, thereby causing the center conductor 14 to break.
Referring to
Referring again to
An important criterion in the assembly is to align and maintain alignment of the center conductor 14 in relation to the outer tube 12, and in relation to the center conductor 26A of the second connector. The structure of the example filter 10 significantly assists with these alignment tasks.
First, a length of the center conductor 14 was selected such that if the filter 10 were assembled as shown in
Next, after selecting the length of wire for the center conductor 14, one end of that wire 14 was soldered to the end surface 26B of the center pin 26A of the second connector 26, which for this example is an SSMA connector. This soldering was done prior to the second connector 26 being soldered to the connector adapter tube 20. This soldering must be carefully performed, because it is important that the wire 14 abuts 26B to be aligned on center, as closely as possible. If the alignment is not on-center, the result is an impedance mismatch at the abutment between the end of the wire 14 and the surface 26B. The numerical tolerance for the alignment therefore translates into the tolerance of an impedance mismatch. An example tolerance, which relates to the above-described dimensions used for the described examples, is the center conductor 14 being from approximately 0.003 inches to 0.005 inches of true center of the surface 26B.
The present inventors developed a soldering technique that is sufficient to practice the described invention. The technique is to use an optical microscope, view the abutment of the center conductor 14 and the surfaced 26B from at least one direction perpendicular to the longitudinal axis of the center conductor 14. When adequate alignment is observed, solder the center conduct 14 to the surface 26B. Next, inspect the soldered joint from the two directions and, if the wire 14 does not look properly centered on 26B after soldering, remove the solder and repeat the operation. Using ordinary soldering skills, the number of repeats (if any) required to attain a centered connection is reasonable.
It should be noted that, ultimately, a time domain reflectometer (“TDR”) test identifies how well the assembly has occurred. As known by persons of ordinary skill in the art, the flatness of the TDR trace shows the characteristics of all connections in the completed filter 10, including any misalignments. As also known in the art, the target flatness of the TDR trace is determined by the particular application the filter will be used for.
After the above-describe soldering of one end of the center conductor 14 to the end surface 26B of the center conductor 26A of the connector 26, the other end of the conductor 14 was inserted into the connector adaptor 20 until the second connector 26 extended into the connector receiving bore (not labeled) of the connector adaptor 20. The second connector 26 was then soldered to the connector receiving bore (not numbered) of the connector adaptor 20.
Next, the outer tube 12 was inserted into outer tube receiving bore 20A of the connector adaptor 20 and soldered. Alternatively, the outer tube 12 could have been soldered to the outer tube receiving bore 20A of the connector adaptor 20 prior to soldering the second connector 26 to the connector adaptor 20.
Next, without any specific requirement as to order, the first connector 22 is inserted into the connector receiving bore (not specifically shown) of the connector block 18, and soldered in place. Assuming that the outer tube 12 has already been inserted into the outer tube receiving bore 20A of the adaptor connector 20, as described above, the right end of the outer tube 12 is inserted in the outer tube receiving bore 18A of the connector block 18 and is soldered in place. The center conductor 14 then extends through the second clearance hole 18C of the connector block 18.
Next, the portion (not shown) of the center conductor 14 extending out from the second clearance hole 18C was gripped with a pair of pliers and pulled tightly across the top 22B of the center pin 22A of the first connector 22 and soldered. After the solder set, the tension established by pulling the center conductor 14 remained, thereby urging the center conductor to follow a substantially straight line, from its solder connection to surface 26B to its solder connection to surface 22B, thus minimizing sagging of the center conductor 14 between those two connection points. The portion of the center conductor 14 extending rightward from its solder connection to surface 22B was then clipped.
Preferably, if the filter 10 is to be used at low temperatures such as those relating to qubit measurements, all solder joints are made using non-superconducting silver/tin solder. The reason is that standard lead tin soft solder will go superconducting at such temperatures, which may create a potential for a problem where two parts are joined with solder. The potential problem is that since the superconducting solder does not transport heat well, the two parts are no longer in good thermal contact. Also, the silver/tin solder is stronger. Therefore the solder joints holding the two ends of the center conductor 14 (namely the joint at one end between the conductor 14 and the end surface 26B of the second connector, and the joint at the other end between the center conductor 14 and the top surface 22B of the first connector 22) can maintain sufficient tension on the center conductor 14 such that sagging prior to injection with the metal powder/binder mixture 16 is tolerable.
Regarding guidelines for the tension on the center conductor 14, these are similar to the guideline for alignment between the center conductor 14 and the end 26B of the center conductor 26A of the second connector 26; tension reduces gravity sag, because sag, like misalignment in the center conductor 14 results is unwanted impedance variations. The desired straightness of the center conductor will depend on how flat of a TDR test result the user desires. If the tension is too low, such that there is too much sag in the center conductor 14, then the TDR trace will have a dip in the middle. Stated differently, the requirement of the particular application determines how much sag can be tolerated. For the example application of qubit measurement, variations of alignment and sag of the order of approximately 0.003 inches were acceptable, i.e., yielded acceptable impedance characteristics as indicated by TDR measurements.
The final step was injecting the metal powder/binder mixture 16 into the second clearance hole 18C until it emerged from the small vent hole 28.
Example applications of the filter described herein include quantum computing. A reason is that in many qubit experiments one or more electrical lines transmit pulses having very fast rise times. A typical system for measuring qubits is designed to be 50Ω everywhere, since this is the characteristic impedance of standard measurement equipment and, as known in the relevant arts, impedance mismatches will affect the shaped pulse. The room temperature electronics are a source of noise, and therefore these fast lines will benefit from the presently described metal powder filters located at low temperatures. Therefore, the criteria for this example application of the filter of this invention is that it be a low pass 50Ω characteristic impedance filter. A filter according to the present invention meets these requirements, is easy to fabricate and, equally important, by simply using a high thermal conductivity epoxy binder, is easy to heat sink.
Five illustrative examples will now be described to assist persons of ordinary skill in the art in forming an understanding of the invention. The five examples are labeled “F1,” F2,” “F3, “F4” and “F5,” and their defining parameters are listed in Table I below. The Z (Ω) and A(dB) values are those exhibited at T=four degrees Kelvin.
Filter F1 was made using Aremco™ 1-5 μm Cu powder. The other four example filters F2-F5 were made using bronze powder. Referring to
The temperature of 4 degrees Kelvin was chosen because an example application for the filters of this invention is in measuring qubits at temperature of 4 degrees Kelvin or lower. The F1 filter has 70% copper powder, and the F5 filter has 78% bronze powder. Attenuation A=Vout/Vin and attenuation A(dB)=20 log(Vout/Vin). The attenuation can be measured using, for example, an Agilent™ model “8729” network analyzer or equivalent. The noise floor of this “8729” example network analyzer, however, is such that attenuation A=0.0001 or A(dB)=−80 dB is effectively the maximum measurable attenuation. This is reflected by graph line 302D of graph 302, showing a flattening of attenuation A or A(dB) at that value.
Graph 304 shows the same observed data as Graph 302, but using a linear frequency scale.
When constructing filters according to this invention, test results such as shown in
With continuing reference to
Referring to
The impedance of the filter is calculated using the formula:
(Eq. 2), where E0 is the voltage level of the known 50Ω region, E is the voltage level of the filter region, Z0 is 50Ω and Z is the filter impedance. Referring to
Since the example application of the invention was measuring qubits at temperatures below 4 degrees Kelvin, and the ideal impedance was 50Ω for purposes of minimizing mismatches, the observed impedance of 54Ω could be a matter for concern. Whether or not such a difference between the actual impedance and the desired impedance is a concern is a matter that is specific to the particular application. If it is a concern, a convenient, practical solution is to fine tune the filter impedance. Guidance for the fine tuning is the following well known formula for the characteristic impedance of a coaxial line, presented as Equation No. 1 in this description:
where K is the effective dielectric constant of the metal powder/binder mixture 16, the variable a is the diameter CD of the center conductor 14, and variable b is the inside diameter ID of the outer conducting tube 12.
Using Eq. 1, the inventors found that Z could be reduced from 54Ω to 50Ω simply by reducing the inner diameter ID of the outer tube 12 (which for this example was a brass tube) from 0.095 inches to 0.077 inches.
The measurements in
While certain embodiments and features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will occur to those of ordinary skill in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the spirit of the invention.
This invention was made with Government support under Contract No.: MDA972-01-C-0052 awarded by Defense Advanced Research Projects Agency (DARPA). The Government may have certain rights in this invention.
