|Publication number||USRE40890 E1|
|Application number||US 10/437,616|
|Publication date||Sep 1, 2009|
|Filing date||May 14, 2003|
|Priority date||Feb 16, 1999|
|Also published as||US6232852, US6529104, WO2000049676A1|
|Publication number||10437616, 437616, US RE40890 E1, US RE40890E1, US-E1-RE40890, USRE40890 E1, USRE40890E1|
|Inventors||Derek J. Small, John A. Lunn|
|Original Assignee||Electronics Research, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (15), Non-Patent Citations (1), Referenced by (2), Classifications (11), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention relates generally to the field of electromagnetic signal communication and, more particularly, to the filtering of high power signals for broadcast communications.
In the field of broadcast communications, electrical filters are required to separate a desired signal from energy in other bands. These bandpass filters are similar to bandpass filters in other fields. However, unlike most other electrical bandpass filters, filters for broadcast communication must be capable of handling a relatively high input power. For example, a signal input to a broadcast communications filter might have an average power between 5 and 100 kilowatts (kW). Many electronic filters do not have the capacity for such large signal powers.
For many years, high power electrical bandpass filtering has included the use of waveguide cavity filters. In particular, the introduction of dual-mode cavities for microwave filters in 1971 made a significant contribution to the art. Dual-mode filters allowed for a reduction in filter size and mass, and could realize more complex filter functions by their ability to easily couple non-adjacent resonators. Later reductions in size and mass were achieved with the introduction of triple and quadruple mode filters.
While dual-mode waveguide cavity filters have been used often for space and satellite communications, they have also been used for terrestrial television broadcast applications. Indeed, for transmitters operating in a common amplification mode (i.e., a mode in which both audio and video signals are being amplified together), dual-mode filters have become predominant because of their low loss and ability to realize complex filter functions. Moreover, dual-mode filters have been favored for the transmission of analog television signals because of their flexibility in realizing wide pass bandwidths to compensate for frequency drift due to RF heating and ambient temperature changes. However, with the advent of digital television, system requirements have changed. The FCC emissions mask for digital television broadcast stations is very restrictive for power radiated into adjacent channels or out-of-band frequencies. These requirements will not be satisfied by filters that have wide pass-bands that are allowed to drift.
In the past, waveguide cavities have been developed that are adjustable to compensate for thermal expansion. Paul Goud in Cavity Frequency Stabilization with Compound Tuning Mechanisms, Microwave Journal, March 1971 discloses a waveguide cavity that may be adjusted to compensate for thermal expansion. In
More recently, filter design has addressed the need for narrower bandwidth filters by constructing filters from materials with lower thermal expansion coefficients to minimize the effect of heating on the filter dimensions. In particular, the nickel/steel alloy InvarŽ (a registered trademark of Imphy, S.A., Paris, France) has been used as a cavity material. Because of its extremely low degree of thermal expansion, the cavities built with InvarŽ suffer less of a dimensional change with heating, and therefore maintain a narrower, more stable passband. However, Invar is also very expensive, and consequently drives up the overall cost of the filter.
In accordance with the present invention, a bandpass filter is provided that uses the deformation of a cavity surface in response to thermal changes to compensate for the resonant frequency shifting effects of thermal expansion. The filter has at least one waveguide cavity in which an input electrical signal resonates at a desired resonant frequency, and a plurality of surfaces, each with a predetermined geometric shape. For example, in a preferred embodiment, the filter has a cylindrical outer surface and a circular end plate. A thermal compensator is provided that responds to thermally induced changes in dimensions of the cavity by distorting the shape of one of the cavity surfaces, thereby minimizing any resulting drift in the resonant frequency.
Typically, the thermally induced changes in the cavity are an increase in cavity dimensions, and the thermal compensator deflects one of the cavity surfaces inward, such as in the case of a concave deflection of the cavity end plate. In the preferred embodiment, the thermal compensator includes a control rod that limits the movement of at least a first point on an end plate of the cavity in a first direction. That is, the control rod prevents movement of that point in the direction of thermal expansion. Thus, as the cavity expands, outer portions of the end plate move in the direction of the expansion, but the first point is restricted by the control rod. As a result, the end plate is deformed from its original shape. The control rod has a coefficient of thermal expansion that is significantly different (typically lower) than that of a material from which the cavity is constructed.
In one embodiment, the control rod fixes a point on the cavity end plate relative to a different location on the filter. This different location may be such that the control rod spans more thermally expanding material than that which defines the waveguide cavity. In such a case, the thermal expansion causes the point of deflection to be moved relative to its original position. In other words, whereas the deflection point initially resides in a first plane perpendicular to the direction of thermal expansion, the expansion of the thermally expanding material spanned by the control rod forces the deflection point out of its original plane toward an interior of the cavity. In another embodiment, a similar inward movement of the deflection point may be accomplished by using an end deflecting rod that connects the control rod to the deflection point. If the end deflecting rod has a coefficient of thermal expansion that is significantly higher than that of the control rod, its expansion will force the deflection point inward relative to the control rod. Naturally, these two techniques may also be combined.
In determining the appropriate amount that a cavity surface point should be deflected, a theoretical model may be used to first establish how far a movable end plate would have to be moved to compensate for an expansion of the waveguide cavity without the end plate being distorted. The resulting deflection distance may then be augmented to compensate for the fact that, in the present invention, the entire surface is not being moved. This additional deflection may be determined empirically, and can provide a more accurate compensation for control of the cavity resonant frequency.
In a preferred embodiment, the waveguide cavity is one of two cavities, which are coupled together via an iris plate. Each of the cavities may be thermally compensated in the manner described herein. One particularly preferred embodiment is a six section filter consisting of two thermally compensated waveguide cavities, each with two orthogonal resonant modes, and two coaxial resonators, each coupled to one of the waveguide cavities via an impedance inverter. The signal to be filtered is input through one of the coaxial resonators to one of the waveguide cavities and output through the other coaxial resonator.
The filter 10 is shown in cross section in
Impedance inverters are found in most microwave RF filter designs, and not discussed in any great detail herein. In the filter of
As mentioned above, control rods provide thermal stability to the cavity waveguide. In the embodiment of
As shown, each control assembly has two side bracing rods 20, each of which is secured at one end by a mounting clip 32 to the edge of the cavity housing. At the opposite end, the bracing rods 20 are fixed to a lateral support 34. The side bracing control rods 22 each reside within a pair of “pass-through” holes in mounting plates 25. Mounting plates 25 provide the means by which to fasten the two cavity housings 12, 14 together and to secure the iris plate 36 separating the cavities. The center of each of the lateral supports 34 is secured to an end deflecting rod 23 that maintains a fixed distance between its respective support and the center of the adjacent end plate 22. Thus, a first bracing assembly establishes a bracing frame between the edge of cavity 12 and the center of the end plate of cavity 14, while the other bracing assembly maintains a bracing frame between the edge of cavity 14 and the center of the end plate of cavity 12.
Because of its relatively small thickness in the axial dimension of the filter (i.e., in a direction parallel to the longitudinal axis of the control rods), the thermal expansion of the lateral supports is negligible for the expected operating temperature range of the filter. Furthermore, the embodiment of
It is known in the art that the resonant frequency f of a cylindrical TE11 n cavity may be expressed as:
where c is the speed of light, D is the cavity diameter, L is the cavity length, n is the number of half wavelengths that contained in the distance L, and x is a zero of a Bessel function dependent on the mode being considered. For example, if n=1 (i.e., the cavity is a T111 cavity), x=1.841. It has also been shown that this equation may be differentiated with respect to temperature to give the relationship:
From this, some of the desired parameters of the waveguide may be determined.
Since the equation above represents the frequency changes in a cylindrical cavity filter with changes in temperature, a stable cavity construction may be determined by setting this equation equal to zero. In other words, when
the filter cavity is stable with temperature. By substitution and reduction, the following relationship results:
Notably, the coefficient of thermal expansion for the length of the cavity (CTEL) is proportional to:
and the coefficient of thermal expansion for the cavity diameter (CTED) is proportional to:
Thus, for a thermally stable cylindrical cavity, the ratio of CTEL to CTED may be expressed as:
The relationship above may be used to modify the length of the cavity to compensate for changes in cavity diameter so as to keep the resonant frequency of the cavity stable. A particular cavity design has a predetermined length and diameter, as well as a particular value for each of the mode-specific variables x and n that make up A. Thus, for that cavity, a particular value for the ratio of CTEL to CTED can be found. Given that ratio, one may determine how one of those parameters must be changed relative to the other in order to maintain a stable resonant frequency. This provides the basis for the thermal compensation of the cavity. For example, if a cavity had a diameter D=17″ and a length L=18″, and a value for A of 1.172 (given, e.g., x=1.84 and n=1), then the ratio of CTEL to CTED would be −1.54. Therefore, to maintain the resonant frequency of the cavity, an increase in its diameter must be met with a reduction its length (since the ratio is negative), where the length change has a magnitude of 1.54 times the diameter change.
While an adjustment mechanism might be used to physically move one or both of the end plates of the filter cavity in response to changes in its diameter, this would require the use of chokes or “bucket shorts” so that the mechanical changes in the cavity shape could be made. Such movable end plates tend to reduce the performance of the filter, and are therefore undesirable. Therefore, in the present invention, rather than moving the cavity end plates, the cavity shape is deformed to compensate for the frequency shifts. The preferred embodiment accomplishes this by using a combination of materials having different coefficients of thermal expansion in such a way as to force a particular deformation in response to temperature changes.
Because of the use of cavity deformation, the mathematical analysis provided above may not apply precisely for temperature compensation. In the preferred embodiment, empirical data is used to augment an initial determination of how the cavity would be modified if a cylindrical shape were maintained. The following example demonstrates such a design, and represents a preferred embodiment of the invention.
One prominent area of use for waveguide cavity filters is in broadcast communications. In particular, ultra-high frequency (UHF) channels for digital television (DTV) have frequency allocations in the United States from approximately 473 MHz (channel 14) to 749 MHz (channel 60). It is known in the art that the optimum Q is achieved in TE111 mode cavity filters with a D/L ratio of approximately 1 to 3. Given this characteristic, it has been found that reasonable performance may be achieved using a filter cavity having a diameter of 17″ for channels 14 through 40 (frequencies from 473 MHz to 629 MHz). In these filters, the length of the cavity is dependent on the desired center frequency. Similarly, it has been found that a cavity filter having a diameter of 15″ is satisfactory for channels 41-60 (frequencies from 635 MHz to 749 MHz). The ranges for desirable filter parameters for UHF communications systems is shown in the following table:
As shown, the ratios of CTEL to CTED for these filters range from −0.62 to −2.80. Thus, using the formulae above, the change in length to compensate for diametric expansion can be calculated. However, because the preferred embodiment relies on cavity deflection, rather than a movable end plate, an adjustment must be made to the calculated value.
The foregoing analysis may be applied to a filter construction as shown in
As can be seen from
To determine an optimum length for the two materials given a filter having a particular center frequency, an approximation is first made using the filter adjustment relationships described above for a cavity in which end plate position may be adjusted without cavity deformation. Known filter parameters are also used, such as those shown above in Table 1, to optimize for the desired frequency. This is demonstrated by the following example.
If a filter having a center frequency of 749 Mhz is desired, a 15″ cavity may be used. From Table 1, the ratio of CTEL to CTED for this frequency is −0.62. Substituting this into the equation above gives the following relationship:
The thermal expansion coefficient for aluminum is CTEALUM=24.7×10−6, while the thermal expansion coefficient for Invar is CTEINVAR=1.6×10−6. Since the cavity is aluminum, CTED=CTEALUM. The foregoing equation may therefore be written as:
or, if (lalum+L) is substituted for lINVAR,
Given the D/L ratio from table 1, L=10 may be used, and the equation solved to give a value of lALUM=9.25. For an initial cavity length L=10, this corresponds to an Invar rod length of lINVAR=19.25.
These values could be used in the filter of
As mentioned above, the present invention currently makes use of some empirical steps in determining an appropriate degree of deformation to be applied to the cavity end plate. The formulaic method above may be used to determine what an appropriate adjustment to the position of the end plate would be if no deformation of the surface was taking place. This provides a cavity parameter, in this case length, that serves as a starting point for determining the appropriate degree of cavity deformation. Thereafter, heating of the cavity and minor adjustment in the deformation, combined with measurement of the filter characteristics, allow fine-tuning of the degree of deformation. Given the description herein, such modifications are well within the ability of one skilled in the art. An example of this process is described below.
After determining an initial deflection amount from the formulae, a low power signal from a network analyzer is input to one port of the filter, and received at the other port. The scattering parameters (“S-parameters”) and temperature of the filter are then measured and recorded. From the S-parameters, the center frequency is found and recorded. The filter unit is then heated in a chamber in order to obtain a change in temperature. Once the frequency response and temperature of the filter have stabilized, the S-parameters and filter temperature are again recorded. At this point, the resonant frequency of the filter will have drifted down a small amount. To compensate, the value of lALUM is increased relative to lINVAR. To increase lALUM, the length of the end deflecting rod 23 may be increased. Alternatively, the length of the invar rods 20 may be increased. This has the same effect, since the larger the distance between the end plate being deflected and the opposite connection point of the rods 20 on the housing, the more length of the aluminum housing there is to move the outer portions of the end plate as it expands.
By readjusting the length of lALUM relative to lINVAR according to the measured resonant frequencies at different temperatures, the optimum length may be determined. As mentioned, for the embodiment above, this required an additional 15% deflection of the end plate. However, those skilled in the art will recognize that for other filter dimension, resonant frequencies, or even types and locations of cavity deformation, different degrees of variation may apply. Nevertheless, by applying empirical modifications, as described above, to a theoretically ideal surface movement model, the appropriate filter characteristics may be achieved.
In one variation of the preferred embodiment, the effective length of lALUM is increased by attaching an extension, such as a disk, to the outside of the end plate being deflected. For example, as shown in
While the invention has been shown and described with regard to a preferred embodiment thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US9128121 *||Sep 28, 2012||Sep 8, 2015||Intel Corporation||Mechanism for facilitating a dynamic electro-mechanical interconnect having a cavity for embedding electrical components and isolating electrical paths|
|US20140091824 *||Sep 28, 2012||Apr 3, 2014||Evan M. Fledell||Mechanism for facilitating a dynamic electro-mechanical interconnect having a cavity for embedding electrical components and isolating electrical paths|
|U.S. Classification||333/208, 333/229, 333/212, 333/234, 333/230|
|International Classification||H01P1/20, H01P1/208, H01P7/00, H01P7/06|