Apparatus and method for limiting spurious resonant cavity Apparatus and method for limiting spurious resonant cavity effects in electronic equipment.

Abstract: Spurious cavity resonance effects in a cabinet housing electronics circuitry are suppressed by determining the maximum repetition rate or frequency for legitimate signals which can appear in the circuitry and then establishing the dimensions of each cavity within the cabinet such that each cavity's resonant frequency is less than the critical repetition rate/frequency. Since a given cavity in a cabinet is typically block-shaped (such as the space between a cabinet door and the circuit panel facing the door), a special purpose formula may be employed to obtain a good approximation of the cavity's resonant frequency, and the cavity dimensions then adjusted to raise the cavity resonant frequency above the critical frequency. For the still more particular cavity configuration in which the length is greater than the width which is much greater than the depth, a further simplified formula can be employed to find an approximate cavity resonant frequency. In addition, for the common configuration in which one or more significant intrusions reduce the cavity volume, a more complex formula may be employed to find its approximate resonant frequency with the cavity dimensions then being adjusted to raise that approximate resonant frequency above the critical frequency. The method may be employed either in the design stage or at a remedial stage.

Description: Field of the Invention This invention relates to the art of electronics and, more particularly, to a method for limiting spurious resonant cavity effects in electronics circuitry operating at high repetition rates and housed within an electrically conductive cabinet. A typical environment for practicing the invention is in digital computer systems. Background of the Invention The progress of the art of electronics, particularly the advances in information processing systems, have been characterized in one aspect by ever increasing speeds of operation at the state of the art. As the speed of operation of such systems has increased, hitherto unencountered problems have arisen such that, in many instances, extremely transient and elusive failures occur, for no known reason. The present invention is directed to the solution of an exceptionally subtle problem which, itself, does not appear to have been recognized previously. The problem to whose solution the present invention is directed constitutes the inadvertent creation of resonant cavities within a cabinet housing electronic equipment operating within a critical frequency range whereby standing waves may transiently develop and sweep across vulnerable regions of the circuitry, such as contact pads, to thereby raise the possibility of randomly introducing spurious signals into the circuitry. Objects of the Inventions It is therefore a broad object of my invention to provide a method for determining whether cavities in a cabinet housing electronic circuitry, whether existing or planned, may be susceptible to introducing resonant conditions inadvertently and to take corrective action at either planning or remedial stages. It is another object of my invention to provide such a method by which any cavities which may be present in a cabinet housing electronic circuitry may be planned or revised to substantially eliminate the possibility of decreasing the reliability of the housed electronic system which might otherwise result from the inadvertent establishment of resonance conditions in the cavity. In a more specific aspect, it is an object of my invention to determine (using known operational criteria for a given housed electronic system and employing certain formulas) the maximum allowable dimension for a given cavity in a cabinet and ensure that the actual largest dimension of the cavity is smaller. In another aspect, it is an object of my invention to make such a maximum dimension determination for any suspect cavities within an existing cabinet housing electronics circuitry and to subdivide the cavity employing electronically conductive baffles in order to raise the resonant frequency of any cavity remaining within the cabinet above the critical value. Summary of the Invention Briefly, these and other objects of my invention are achieved by determining the maximum repetition rate or frequency for legitimate signals which can appear in the circuitry disposed within the cabinet and then establishing the dimensions of each cavity within the cabinet such that each cavity's resonant frequency is higher than the critical repetition rate/frequency. Since a given cavity in a cabinet is typically block-shaped (such as the space between a cabinet door and the circuit panel facing the door), a special purpose formula may be employed to obtain a good approximation of the cavity's resonant frequency, and the cavity dimensions then adjusted to raise the cavity resonant frequency above the critical frequency. For the still more particular, but prevalent, cavity configuration in which the length is greater than the width which is much greater than the depth, a still more simplified formula can be employed to find an approximate cavity resonant frequency. In addition, for the common configuration in which one or more intrusions, such as block-shaped packs of electronic circuitry, reduce the cavity volume, a more complex formula may be employed to find the approximate resonant frequency of the cavity with the cavity dimensions then being adjusted to raise that approximate resonant frequency above the critical frequency. The method may be employed either in the design stage or at a remedial stage and, in either case, electrically conductive baffles may be appropriately emplaced within the cavity to divide it into smaller dimension subcavities and thereby raise the worst case resonant frequency above the maximum repetition rate for legitimate signals. The method of the invention is defined in claims 1 and 5 with claims dependence thereon, the apparatus according to the invention is defined in claim 7. Description of the Drawing The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, may best be understood by reference to the following description taken in conjunction with the subjoined claims and the accompanying drawing of which: FIG. 1 is a perspective view of the exterior of an exemplary computer system cabinet incorporating several access doors and which is representative of the environments in which the subject invention may be practiced; FIG. 2 is a partial view of the cabinet shown in FIG. 1 and illustrates an upper, center door in its opened position to reveal a cavity behind the door and electronic circuitry facing outwardly in the more or less conventional fashion; FIG. 3 is a cross sectional view taken along the lines 3-3 of FIG. 2 to more clearly illustrate the exemplary cavity; FIG. 4 is a laid-out representation of the exemplary cavity and its boundaries as conceptually removed from the cabinet for purposes of analysis and discussion; FIG. 5 is a view similar to FIG. 4, but also shows an exemplary volume-reducing intrusion into the cavity; FIG. 6 is a view similar to FIG. 2 illustrating the addition of compartment baffles which have been added in accordance with the present invention; FIG. 7 is a view taken along the lines 7-7 of FIG. 6 illustrating the vertical baffle employed; and FIG. 8 is a cross sectional view taken along the lines 8-8 of FIG. 6 illustrating the horizontal baffle employed to effect the desired resonant suppression. Detailed Description of the Invention Referring now to FIG. 1, there is shown a cabinet 1 which, for purposes of example, houses a computer system or subsystem operating at a high clock rate. In the typical configuration shown, the left and right doors 2, 3 are full-height whereas the center section includes a lower door panel 4 and an upper door panel 5 which will be examined more particularly in the explanation of the invention to follow. Thus, FIG. 2 is an enlarged view of the region of the cabinet 1 occupied by the door 5 and is illustrated with the door 5 opened to reveal electronic circuitry, such as circuit packs 6, emplaced on an outwardly circuit panel 7. Referring also to FIG. 3, it will be observed that the region between the circuit panel 7 and the rear 8 of the cabinet 1 is substantially filled with closely packed electronic, mechanical and other structure and, specifically for purposes of explaining the present invention, does not contain any cavities of sufficient size to be a cause for concern. However, the cavity 10 disposed between the circuit panel 7 and the door 5 will, when the door 5 is closed, present a region at which undesired and potentially deleterious resonance conditions may arise. This is the exemplary cavity to be discussed below. As those skilled in the art will appreciate, cavities of this nature are usually found behind access doors and may also be present in other regions of a cabinet containing electronic circuitry operating at high signal rates. It will be understood that the cavity 10 has definite physical boundaries; i.e., the door 5, the circuit panel 7, intra- or inter-cabinet separators 11 effecting left and right physical boundaries and top 12 and bottom 13 cabinet sections. Thus, the most common configuration for a cavity of sufficient size to be of concern may be deemed to be generally block-shaped and is likely to have one or more "intrusions" into the cavity volume such as the circuit packs 6 which are themselves typically shallow blocks in configuration. Consider now, with reference to FIG. 4, that, for purposes of analysis, the cavity 10 has been isolated, complete with its physical boundaries 11, 12, 13, and reoriented for convenience by "laying it down". It will be seen that the cavity is substantially block-shaped and that its length c. is greater than its width a. which is considerably greater than its depth b. Again, those skilled in the art will appreciate that most cavities in cabinets housing electronic circuitry and of sufficient size to be of concern can be reasonably approximated by this particular shape. It can be shown that the resonant frequency of a cavity of this configuration is sufficiently closely approximated, for the purpose and considering the TE₁₀₁ mode, by the simplified formula: f r = 1.5 X 10⁸ ((1/a)² + (1/c)²) 1/2 Hz where a is the cavity width dimension in meters and c is the cavity length dimension in meters. Consider a specific example in which width dimension a is one meter and length dimension c is two meters. The approximate resonant frequency of the cavity is therefore: f r = 1.5 X 10⁸ ((1/1)² + (1/2)²) 1/2 Hz f r = 1.5 X 10⁸ + (1.707) 1/2 Hz f r = 196 MHz Thus, if the cabinet 1 is housing electronic circuitry such as an information processing system employing a clock circuitry of, merely by way of example, 200 MHz, the possibility of establishing a spurious resonance conditions within the cabinet because of the presence of cavity 10 exists and must be addressed. The fundamental preventative or remedial action is to subdivide a cavity of suspect size into a plurality of subcavities. Referring to FIGS. 6, 7 and 8, in the example in which the resonant frequency of the cavity 10 is about 196 MHz, and the clock frequency of the electronic circuitry enclosed in the cabinet 1 is 200 MHz, horizontal baffle 14 may be provided and situated intermediate the top 12 and bottom 13 boundaries of the cavity between the circuit panel 7 and the door 5 into two smaller components, each of which has a resonant frequency higher than the critical frequency. If the critical frequency were higher yet, vertical baffle 15 may be employed to further subdivide this space into a plurality of smaller cavities. In effect, the cavity dimensions are revised to raise the subcavities' resonant frequencies above the critical frequency. Thus, if the single horizontal baffle 14 is used to subdivide the cavity 10 equally (which is not a constraint) the resonant frequency of each subcavity is: f r = 1.5 X 10⁸ ((1/1)² + (1/1)²) 1/2 Hz f r = 1.5 X 10⁸ + (2.0) 1/2 Hz f r = 212 MHz If the vertical baffle 15 is added, the resonant frequency of each subcavity is: f r = 1.5 X 10⁸ ((1/0.5)² + (1/1)²) 1/2 Hz f r = 1.5 X 10⁸ + (2.414) 1/2 Hz f r = 233 MHz As those skilled in the art will appreciate, the baffles employed should be fabricated from electrically conductive material (or at least have an external or internal electrically conductive layer), and all should preferably be at the same potential (such as ground) to effectively constitute cavity boundaries. Care must also be taken that, in the case of a baffle edge abutting a door or other movable panel effecting a cavity boundary, none or very little space exists between the baffle edge and the movable panel. As shown in FIGS. 7 and 8, an elastic conductive material, such as conductive elastomer strips 16, 17, may be adhesively secured to the baffle edges to ensure that no cavities of very small depth remain since, as will be understood from the simplified equation discussed above, such a cavity could still be the source of spurious resonance conditions. For some cavities, of course, other modes, such as TE₁₁₁, must be considered, and a more exact formula must be employed as follows: f r = 1.5 X 10⁸ ((1/a)² + (1/b)² + (1/c)²) 1/2 Hz where a is the cavity width dimension in meters, b is the cavity depth dimension in meters and c is the cavity length dimension in meters. For example, if a is one meter, b is one-half meter and c is two meters: f r = 1.5 X 10⁸ + (3.121) 1/2 Hz f r = 265 MHz The formulas employed in the examples given above are simplifications of the more complete expression for the transverse electric field case (in which the E-field is transverse the z-direction and the H-field has components H x and H z ): (f r ) mnp = (1/(2 Pi)(mu e) 1/2 ) ((m Pi/a)² + (n Pi/b)² + (p Pi/c)²) 1/2 where: n is the number of half cycles of variation in the X direction; m is the number of half cycles of variation in the Y direction; p is the number of half cycles of variation in the Z direction; 1/(mu e) 1/2 is the propagation rate in air = 3 X 10⁸ m/s; a is the cavity width in meters; b is the cavity depth in meters; and c is the cavity length in meters. The case in which there are one or more significant intrusions into the cavity must also be considered. For example, as shown in FIGS. 7 and 8, the circuit packs 6 constitute such intrusions which somewhat decrease the cavity volume. Referring to FIG. 5, consider the case in which an intrusion having boundaries defined by dimensions a₁, b₁ and c₁ intrude upon the cavity 10 having boundaries defined by dimensions a, b and c as previously described. ,The resonant frequency of the reduced-volume cavity 10′ may be approximated by the formula: An expression* for the perturbed resonant frequency is ω: perturbed resonant frequency Δτ: indentation volume τ: original unindented volume For the TE₁₀₁ mode: E x = E z = H y =0 E y is of the form H x is of the form H z is of the form At resonance-- H o =peak H of unperturbed field The denominator of ω-ω o ω becomes: (1/8)ε o |A n |² abc For the numerator of ω-ω o ω : At x=a/2, y=0, z=c/2 Therefore H x , H y , and H z are zero at the center and approximately throughout the perturbed region. Then-- Numerator of ω-ω o ω becomes -(1/4)ε o |A n |³a₁b₁c₁ Finally: ω-ω o ω o = -(1/4)e o |A n |²a₁b₁c₁ (1/8)ε o |A n |²abc ω-ω o ω o = -2Δτ τ or ω = ωo 1 + 2Δτ τ Perturbation example: if Δτ=.1τ then ω= ω o 1.2 =.83ω o --->139MHz It will be seen that an intrusion (and there may be several of significance for a given cavity) lowers the resonant frequency of a cavity and therefore tends to aggravate the potential for problems of the nature under consideration. Consequently, care must be taken to account for such intrusions if they are present, and a sufficient number of appropriately placed baffles employed to raise the resonant frequency of all subcavities above the critical frequency. Those skilled in the art will appreciate that various cabinet and cavity configurations may indicate that cavity resonance in modes other than TE may have to be considered and calculated in the manner set forth above with the adjustments well known in the art in order to insure that the cavity resonant frequency in all modes is able the critical frequency. For a more complete exposition of the derivation and basis of the full (i.e., unsimplified) formulas discussed above and for modes in addition to TE, one may refer to Time Harmonic Electromagnetic Fields by Roger F. Harrington, (McGraw-Hill, New York, 1961), Chapter 7, pp. 317-321. Thus, while the principles of the invention have now been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications of structure, arrangements, proportions, the elements, materials, and components, used in the practice of the invention which are particularly adapted for specific environments and operating requirements without departing from those principles.

Claims: A method for suppressing resonance conditions in a cabinet housing digital electronics circuitry, which method comprises: A) determining the maximum repetition rate for legitimate signals which can appear in the circuitry disposed within the cabinet; and B) establishing the dimensions of each cavity within the cabinet such that each cavity's resonant frequency is higher than the repetition rate determined in step A). The method of Claim 1 in which: A) a given cavity is generally block-shaped; B) its length dimension is greater than its width dimension; and C) its width dimension is considerably greater than its depth dimension; in which the cavity resonant frequency is calculated according to the formula: f r = 1.5 X 10⁸ ((1/a)² + (1/c)²) 1/2 Hz where a is the cavity width dimension in meters and c is the cavity length dimension in meters; and in which: D) the cavity dimensions are revised to raise f r above the repetition rate. The method of Claim 1 in which: A) a given cavity is generally block-shaped; in which: B) the cavity resonant frequency is calculated according to the formula: fr = 1.5 X 10⁸ ((1/a)² + (1/b)² + (1/c)²) 1/2 Hz where a is the cavity width dimension in meters, b is the cavity depth dimension in meters and c is the cavity length dimension in meters; and in which: C) the cavity dimensions are revised to raise f r above the repetition rate. The method of Claim 1 in which: A) a given cavity includes at least one intrusion into at least one cavity boundary, thereby decreasing the volume of the cavity; B) the cavity resonant frequency is approximated by the formula: ω = ω o 1 + 2Δτ τ where: ω o = original cavity resonant frequency; ω = perturbed resonant frequency; τ = original volume; and Δτ = the change in volume; and in which: C) the cavity dimensions are revised to raise above the repetition rate. A method for determining the possible presence of resonance conditions in a cabinet housing electronics circuitry and for applying corrective measures to substantially eliminate such resonance conditions if they are determined to possibly exist, which method comprises: A) determining the maximum repetition rate for legitimate signals which can appear in the circuitry disposed within the cabinet; B) determining the dimensions of each cavity within the cabinet; and C) as necessary, adjusting the dimensions of cavities within the cabinet such that each cavity's resonant wavelength is less than the wavelength of the repetition rate determined in step A). The method of Claim 5 in which step C) is carried out by introducing at least one electrically conductive baffle into a cavity in order to establish a plurality of subcavities which are characterized by the largest dimension of each being less than the wavelength of the repetition rate determined in step A of Claim 5. A hollow rectangular conductive cabinet (1⊘) having electronic circuits (6) disposed therein, wherein said circuits operate at a particular maximum effective frequency, said cabinet being characterized by comprising: at least one conductive planar baffle (14) mounted within and transverse to a pair of the walls (11) of said cabinet for dividing the part or those parts of the total internal volume of said cabinet not occupied by said electronic circuits into at least two parts, each of said parts having a minimum resonant frequency which is greater than said maximum effective frequency of said circuits.

Inventors: PERSON GEORGE A, PERSON, GEORGE A.
Applicants: BULL HN INFORMATION SYST, BULL HN INFORMATION SYSTEMS INC.
Application year: 1989
Main classification: H01P7
IPC codes: H01P1, H01P7, G06F1, H05K5