Patent Grant
8760236
Yttrium iron garnet (YIG) filters are magnetically tunable bandpass filters that can be found in a variety of test and measurement systems. For example, YIG filters are commonly included in front-end sections of microwave spectrum analyzers as a preselector for applied input signals.
YIG belongs to a broader class of microwave band ferrimagnetic materials used to make microwave filters and oscillators. These materials, as applied to such applications, are referred to generally as “ferrimagnetic resonators”. Other types of garnets include YIG doped with aluminum, gallium, gadolinium, or aluminum and gadolinium, and calcium vanadium. In addition to garnets, magnetic ferrites can be used, such as magnesium, magnesium-zinc, magnesium-aluminum, nickel, nickel-aluminum, nickel-zinc, lithium, and hexagonal ferrites made with barium, for example.
Referring to
The output signal of YIG filter 100 has a frequency spectrum determined by the frequency passband of YIG sphere 105. The center frequency of the passband can be raised or lowered by increasing or decreasing the strength of magnetic field “H”, and the width of the passband can be increased or decreased by adjusting other factors such as the geometry and configuration of input and output coils 110 and 115. The passband can also be modified by varying the number of YIG spheres in the filter. For instance, many applications use three or four YIG spheres, although any number of spheres is possible. In addition, as alternatives to YIG spheres, other types of ferrite materials can be used for the filter element, such as barium hexi-ferrite, nickel zinc, or various other materials.
In some applications, YIG filter 100 is placed in a gap along a magnetic pole of an electromagnet to allow precise focusing of magnetic field “H”. In such applications, the passband and the center frequency of YIG filter 100 varies according to the magnetic flux density “B” within the magnetic gap. The magnetic flux density “B” can be modified by changing the strength of magnetic field “H” or by changing the size of the magnetic gap.
Referring to
During operation, input band switch 205 receives an input signal and transmits it to a designated one of the filters 210 or 220 according to an operating mode of the spectrum analyzer. The input signal is filtered by the designated filter and then transmitted to a corresponding one of frequency mixers 215 and 225. The respective passbands of preselectors 210 and 220 are typically designed to match to the respective mixing modes of frequency mixers 215 and 225.
YIG filters can generally provide high frequency selectivity and broad frequency tuning ranges. However, they can also suffer from frequency drift, making it difficult to accurately set and maintain a passband center frequency at a desired value. Where the passband center frequency of a YIG filter is inaccurately set or maintained in a preselector of a microwave spectrum analyzer, amplitude errors can occur in the spectrum analyzer's response.
One cause of frequency drift is heat dissipated by an electromagnet used to tune the YIG filter. The electromagnet dissipates heat through conductive coils that generate the magnetic field for tuning. The dissipated heat causes non-uniform thermal expansion of the electromagnet, which can gradually modify the passband center frequency by changing the magnetic field density “B” applied to the YIG filter. This frequency drift tends to stabilize as the thermal expansion approaches an equilibrium state. However, a typical electromagnet structure can take several minutes to reach equilibrium.
Another cause of frequency drift is thermal expansion due to changes in ambient temperature. This type of thermal expansion can be less predictable than that caused by the electromagnet, and the ambient temperature may not have a reliable equilibrium state.
Frequency drift can be especially problematic in YIG filters designed for high frequency ranges, such as 50 GHz, because these YIG filters are generally placed in a smaller magnetic gap in order to increase magnetic flux density. The small size of the magnetic gap can magnify the effects of thermal expansion in the electromagnet, which can lead to unacceptable levels of frequency drift.
What is needed, therefore, are improved techniques and technologies for stabilizing drift in YIG filters. Such improvements are especially needed for high frequency applications such as microwave spectrum analyzers.
In accordance with a representative embodiment, an electromagnet structure, comprises: a magnetic shell comprising a cavity; a magnetic pole located within the cavity and having a magnetic gap for focusing a magnetic field on a magnetically tunable filter; a conductive coil located within the cavity of the magnetic shell and forming multiple turns around the magnetic pole; and a heater located within the cavity of the magnetic shell and configured to maintain the conductive coil at a substantially constant temperature when the magnetically tunable filter is tuned to different frequencies.
In accordance with another representative embodiment, a method of controlling an electromagnet structure comprising an electronic filter is disclosed. The method comprises: energizing an electromagnet coil to tune the filter to a target frequency range; determining a set point of a parameter to maintain the filter in the target frequency range; receiving feedback indicating a state of the parameter; and adjusting a power level of an input signal supplied to the electromagnet structure to maintain the parameter at the set point.
The described embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatuses are clearly within the scope of the present teachings.
The terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. The defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings. In addition, unless expressly so defined herein, terms are not to be interpreted in an overly idealized fashion. For example, the terms “isolation” or “separation” are not to be interpreted to require a complete lack of interaction between the described features.
As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices.
As used in the specification and appended claims, and in addition to their ordinary meanings, the terms ‘substantial’ or ‘substantially’ mean to within acceptable limits or degree.
As used in the specification and the appended claims and in addition to its ordinary meaning, the term ‘approximately’ means to within an acceptable limit or amount to one having ordinary skill in the art. For example, ‘approximately the same’ means that one of ordinary skill in the art would consider the items being compared to be the same.
The described embodiments relate generally to frequency drift stabilization in magnetically tunable filters. In some embodiments, frequency drift is stabilized by incorporating a heater into a conductive coil used to magnetize a magnetically tunable filter. The heater can be adjusted to maintain the conductive coil at a substantially constant temperature. This reduces frequency drift due to thermal expansion, as will be described below.
In other embodiments, frequency drift is stabilized by mechanically isolating the conductive coil from a shell encompassing the YIG filter. The mechanical isolation can be accomplished, for instance, by placing the conductive coil on a pedestal that is mechanically separated from the shell. The pedestal can be connected to a heat sink to dissipate thermal energy from the coil. The mechanical isolation of the coil can prevent it from placing stress on the shell when it undergoes thermal expansion, as will be described below.
Certain embodiments can be implemented using a YIG filter such as that illustrated in
One way to evaluate the performance of the described embodiments is by measuring post-tuning frequency drift of a tuned filter. Post-tuning frequency drift is the amount of change in the filter's center frequency after it is tuned to a new passband. As an example, suppose a filter is changed from a center frequency of 3 GHz to a center frequency of 50 GHz. At the 50 GHz center frequency, an electromagnet must supply a much larger current to a conductive coil compared to the 3 GHz frequency. Consequently, the conductive coil tends to heat up after the filter is tuned to 50 GHz. This heat creates thermal expansion in the electromagnet, which can cause the filter's center frequency to drift. However, by using a heater and/or coil isolation pedestal to mitigate the effects of the heat, the amount of post-tuning frequency drift can be reduced to less than 10 MHz of a 45 MHz passband.
Referring to
Conductive coil 320 comprises several loops of a conductive material such as copper. These loops are wound around magnetic pole 310 in the form of a solenoid, as shown, for instance in
Shell 305 and magnetic pole 310 are typically fabricated from a magnetic alloy, such as 50% nickel and 50% iron. Shell 305 and magnetic pole 310 can be made from the same blank or from separate blanks. If made from separate blanks, they can be made from different alloys and can be joined by screw attachment, by welding, or other means. Together, shell 305 and magnetic pole 310 form a self-shielding structure for containing magnetic fields.
Pole gap 315 is used to focus the magnetic field on a YIG filter such as that illustrated in
In a modified embodiment shown in
In alternative embodiments, conductive coil 320 can be modified by forming multiple coils around the lower portion of magnetic pole 310, or by forming one or more coils around each of the upper and lower portions of magnetic pole 310. In addition, pole gap 315 can be modified by placing it at a lower position within shell 305. These and other configurations of conductive coil 320 and magnetic pole 310 can be used in conjunction with a heater or coil pedestal such as those illustrated in
As illustrated in
During operation of YIG filter 405, an electrical current is applied to conductive coil 320 to create a magnetic field in magnetic pole 310. The magnetic field is driven across pole gap 315 to control the passband of YIG filter 405. More specifically, the passband is controlled by varying the intensity of the magnetic field applied to YIG filter 405. This is generally accomplished by varying the electrical current applied to conductive coil 320.
As illustrated in
A significant amount of thermal expansion can occur if the current is increased by a large amount, for example, to tune the YIG filter from a lowest frequency to a highest frequency. Moreover, this thermal expansion can cause significant frequency drift in the YIG filter. As an example, in the spectrum analyzer front-end shown in
Thermal expansion can also occur due to changes in ambient temperature, such as the temperature of a room in which electromagnet structure 300 is located. Changes in ambient temperature tend to occur more slowly than changes in the temperature conductive coil 320. Nevertheless, it can be beneficial to compensate for the effects of those changes.
As YIG filter 405 is operated at higher frequencies, it becomes more sensitive to thermal expansion of electromagnet structure 300. In other words, at higher frequencies, the same amount of thermal expansion in electromagnet structure 300 causes a greater amount of frequency drift in YIG filter 405. This increased sensitivity occurs at higher frequencies because a stronger magnetic field exists in pole gap 315. In the presence of a stronger magnetic field, expansion or contraction of pole gap 315 causes a proportionally larger change in the magnetic flux density applied to YIG filter 405. For example, the same change in pole gap 315 will result in almost twice the frequency drift at 50 GHz than at 26.5 GHz because the magnetic flux density is almost twice as high, as illustrated by the equation B50GHz/B26.5GHz=50 GHz/26.5 GHz=1.89.
Referring to
Heater 520 is attached between lower and upper conductive coils 505 and 510 by an adhesive such as epoxy. In some embodiments, heater 520 is formed on a flex circuit having resistive elements. The flex circuit can be placed on top of lower conductive coil 505, and then upper conductive coil 510 can be placed on top of the flex circuit. The flex circuit can be controlled through an electrical connection passing through a slot in shell 305.
Heater 520 maintains electromagnet structure 500 at a substantially constant temperature by modifying the amount of heat that it generates based on the amount of heat generated by lower and upper conductive coils 505 and 510. For example, heater 520 may generate less heat when lower and upper conductive coils 505 and 510 generate more heat, and it may generate more heat when lower and upper conductive coils 505 and 510 generate less heat. In this manner, the combined heat generated by lower and upper conductive coils 505 and 510 and heater 520 remains substantially constant.
One way to control the amount of heat generated by heater 520 is to use temperature sensor 515 to detect the temperature of lower and upper conductive coils 505 and 510, and to adjust the amount of electrical power supplied to heater 520 according to the detected temperature. In the embodiment of
Another way to control the amount of heat generated by heater 520 is to ensure that the sum of the electrical power supplied to heater 520 and lower and upper conductive coils 505 and 510 remains substantially constant. For instance, where less power is supplied to lower and upper conductive coils 505 and 510, more power can be supplied to heater 520, and vice versa.
Still another way to control the amount of heat generated by heater 520 is to monitor the ambient temperature of electromagnet structure 500 using temperature sensor 525. Heater 520 can be controlled to generate more heat as the ambient temperature drops, or less heat as the ambient temperature rises.
Heater 520 can take various forms, such as discrete resistor elements or a distributed resistor network. In the distributed resistor network, heater 520 is formed from a resistive foil. Where the network comprises at least two resistors, the resistors can be arranged such that current flows in opposite directions around magnetic pole 310 in order to minimize unwanted magnetic fields.
Heater 520 can also be implemented by an independent magnetic coil coupled to magnetic pole 310. In other words, heater 520 can be one of two independent magnetic coils used to tune a YIG filter. The two magnetic coils can be controlled independently such that the net magnetic field applied to magnetic pole 310 is varied linearly and the heat generated by the two coils is substantially constant. The use of these two magnetic coils can eliminate the need to include separate resistive elements in electromagnet structure 500. However, requires two clean current sources and a control algorithm to achieve linear tuning current and constant power.
Heater 520 can be energized in various ways, such as applying a linearly modulated current source or by pulse-width modulated signal. The pulse-width modulated signal can be filtered and/or dithered to minimize unwanted magnetic fields in magnetic pole 310.
In the method of
In the method of
In the method of
For convenience of explanation, the methods of
Referring to
Next, the method determines or identifies an amount of power supplied to conductive coil 320 to tune filter 405 (S910). This can be performed in various ways, such as measuring an amount of current in conductive coil 320, measuring a voltage across conductive coil 320, or measuring a resistance value of conductive coil 320.
Finally, the method adjusts an amount of power supplied to heater 520 according to the amount of power supplied to conductive coil 320 (S915). Where a greater amount of power is supplied to conductive coil 320, a smaller amount of power is supplied to heater 520, and vice versa.
Referring to
Next, the method detects a temperature of a portion of electromagnet structure 500 (S1010). As illustrated, for example, by
Finally, the method adjusts the amount of power supplied to heater 520 according to the detected temperature (S1015). Where the detected temperature is below the set point, the method increases the amount of power supplied to heater 520, and where the detected temperature is above the set point, the method decreases the amount of power supplied to heater 520.
Referring to
Next, the method monitors the magnetic flux density in pole gap 315 (S1110). This can be accomplished, for example, by using feedback regarding the center frequency of YIG filter 405. For instance, a decrease in magnetic flux density can be inferred from a decrease in the center frequency of YIG filter 405.
Finally, the method adjusts the amount of power supplied to conductive coil 320 according to a detected change in magnetic flux density (S1115). This adjustment of the coil power can stabilize the passband of YIG filter 405 without requiring heater 520 in electromagnet structure 500.
Referring to
Coil isolation pedestal 1205 separates lower and upper conductive coils 505 and 510 and its temperature effects from shell 305. In particular, it conducts heat away from lower and upper conductive coils 505 and 510 to heat spreader so that shell 305 is less sensitive to changes in the power applied to lower and upper conductive coils 505 and 510. In addition, it mechanically separates conductive coil 320 from shell 305 so that mechanical variations in conductive coil 320, such as those from thermal expansion, do not distort the shape of shell 305.
Heat spreader 1215 is typically made from an engineering alloy with a higher thermal conductivity than shell 305. For example, heat spreader 1215 can be made from aluminum. In addition, heat spreader 1215 is generally larger than shell 305 and it can be attached to a chassis. This configuration tends to reduce stresses imposed on electromagnet structure 1200. Moreover, because heat spreader 1215 has a larger plan area and is made from a higher thermally conducting alloy than the magnet shell, it spreads heat generated by conductive coil 320 to the instrument chassis.
Heat spreader 1215 and coil isolation pedestal 1205 can also reduce radial strain on shell 305 due to the expansion of conductive coil 320. Radial strain tends to occur in electromagnet structure 300 of
Because shell 305 typically has a thin walled shape, the radial strain can cause axial bowing at the ends of shell 305. Moreover, because magnetic pole 310 is attached to the ends of shell 305, the axial bowing can change the size of pole gap 315, resulting in frequency drift. The radial strain can be reduced by omitting an adhesive layer such as that illustrated in
Coil isolation pedestal 1205 and pedestal legs 1210 can be fabricated from an engineering alloy such as aluminum, brass, or copper. The engineering alloy can be selected to have a thermal expansion match to conductive coil 320, allowing coil isolation pedestal 1205 to be connected to conductive coil 320 by a relatively thin layer of adhesive. This can reduce the effects of thermal resistance of the adhesive. In addition, the engineering alloy can have significantly higher thermal conductivity than shell 305. For example, aluminum alloy 6061 has approximately 13 times higher thermal conductivity than a 50% Ni 50% Fe magnetic alloy. The higher thermal conductivity of coil isolation pedestal 1205 can maintain conductive coil 320 at a cooler temperature, which can prevent pole gap 315 from changing.
In certain alternative embodiments, coil isolation pedestal 1205 is mounted directly to shell 305, and shell 305 is mounted directly to heat spreader 1215. In such embodiments, through holes can be formed in shell 305 to connect pedestal legs 1210 to coil isolation pedestal 1205. In these embodiments, if heat spreader 1215 is made from a material with a significantly different thermal coefficient from shell 305, the combination of materials can create a bi-metal device that produces a change in pole gap 315. Accordingly, heat spreader 1215 is typically formed of a material with a similar coefficient of thermal expansion to shell 305.
Because coil isolation pedestal 1205 is located in a magnetic field formed by conductive coil 320, eddy currents can form in coil isolation pedestal 1205 in response to changes in the magnetic field of conductive coil 320. These eddy currents can slow the sweep speed of a filter within electromagnet structure 1200 through induced magnetic fields created by the eddy current.
The effects of these eddy currents can be reduced in a number of ways. In one example, these effects are reduced can be reduced by forming coil isolation pedestal 1205 of a zero susceptibility material such as a ceramic. Such a material can be formed by plating a diamagnetic material onto a paramagnetic material. In another example, the effects of eddy currents are reduced by forming coil isolation pedestal 1205 of a thin metallic material, and slotting the material so a continuous loop is not formed around magnetic pole 310. In yet another example, the effects of eddy currents are reduced by forming coil isolation pedestal 1205 of separate pedestal pieces so that an eddy current loop is not formed.
In various alternative embodiments, different coil and heater configurations, such as those described with reference to
While example embodiments are disclosed herein, one of ordinary skill in the art will appreciate that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. The invention therefore is not to be restricted except within the scope of the appended claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3973204 | Keiter | Aug 1976 | A |
| 4465974 | Sparks | Aug 1984 | A |
| 4506240 | Shores et al. | Mar 1985 | A |
| 4555683 | Sorger et al. | Nov 1985 | A |
| 4827230 | Harris | May 1989 | A |
| 5221912 | Keane et al. | Jun 1993 | A |
| 5291163 | Finkle et al. | Mar 1994 | A |
| 5298872 | Trump et al. | Mar 1994 | A |
| 5517161 | Andersson et al. | May 1996 | A |
| 5959513 | Parrott et al. | Sep 1999 | A |
| 6727775 | Basawapatna et al. | Apr 2004 | B2 |
| 7378926 | Abe et al. | May 2008 | B2 |
| Number | Date | Country |
|---|---|---|
| 2573923 | May 1986 | FR |
| 2161653 | Jan 1986 | GB |
| Number | Date | Country | |
|---|---|---|---|
| 20130027152 A1 | Jan 2013 | US |
