Patent Application
20070182512
The present invention is directed to radio frequency or microwave communications systems or subsystems that require integration of multiple passive circuits in limited spaces.
Passive elements are used in all manner of electronic circuits. A wide variety of implementations are known for implementing passive circuits, including lumped element and distributed element designs. In certain applications microstrip and stripline designs have also been employed. For example, a stripline is a printed signal path disposed between two ground planes in a printed circuit board. Many single-element stripline circuits are known and used in the industry (e.g., as implemented by the Anaren Xinger couplers), and disclosures have been made about extending a single element stripline into two layers (see, e.g., U.S. Pat. No. 5,929,729 to Swarup (where coupling was an objective), and U.S. Pat. No. 5,359,304 to Fujiki.
In certain high frequency applications, prior approaches towards designing passive elements have proved unsatisfactory. For example, lumped-element harmonic filters tend to result in poor manufacturing yield, and board-to-board performance variation is typical, requiring expensive hand-tuning. On the other hand, distributed element designs are not typically used where there is limited board space and the possibility of re-entrant bands (e.g., passbands repeated at harmonic frequency, to suppress out-of-band rejection). If the application has to be low loss, the design constraints become even more difficult. For example, where the radio-frequency (RF) is above 1 GHz, many applications will require limiting the radiation of unintended signals (spurs, harmonics, etc) to maximize frequency spectrum reutilization. Such requirements drive the need for passive circuits like low pass filters that provide the lowest possible insertion loss and maximum out-of-band rejection. The conventional lumped-element elliptic low pass filters are very difficult to implement at frequencies above 1 GHz due to small component values and extreme sensitivity to component tolerance. Stripline circuits would not be considered by a typical designer, since the physical dimensions of distributed elements at frequencies below 2 GHz are prohibitively large. This rules out the use of distributed element filters when more than one filter has to be implemented in a constrained board space.
There remains, therefore, a need for a better approach to circuit design for multiple high frequency (i.e., greater than 1 GHz), low loss passive elements for use in constrained-size applications. Just such an approach is now possible by the invention described in more detail in connection with the following embodiment.
The present invention provides such a method and apparatus for design of low loss, size restricted high frequency circuits. In a preferred 2-circuit embodiment, an electronic device includes: a first circuit layer located comprising a first stripline passive circuit between a top and a bottom ground layer; and a second circuit layer located above the first circuit, the second layer comprising a second stripline circuit between a top and a bottom ground layer; the bottom ground layer is shared with the first circuit. The two stripline circuits can be separately coupled to leads, or coupled to each other and other leads using vias through the ground layer(s) separating each stripline. The stacked stripline elements can be used together with other circuits, and the stacked circuit board can be conveniently joined together with other assemblies, e.g., by surface mounting to a main board. An N-circuit implementation extends this 2-circuit embodiment over more circuit layers.
While the invention is defined by the appended claims, as an aid to understanding it, together with certain of its objectives and advantages, the following detailed description and drawings are provided of an illustrative, presently preferred embodiment thereof, of which:
In a preferred embodiment of the invention, a system is provided for high frequency passive elements, where plural passive elements are formed in stripline topology stacked vertically in relation to each other. This arrangement permits implementation of multiple high frequencies, low loss circuits in a small footprint.
Turning now to
The circuit layer 130 is preferably a stripline circuit 132 sandwiched between ground planes 133 and 134. The stripline circuit 132 is isolated from the ground planes 133 and 134 by dielectric substrates 135 and 136, respectively. The stripline circuit 132 is connected to the circuit on the main board 110 by a signal via line 131. Similarly, the second circuit layer 140 comprises a circuit 142 sandwiched between substrate layers 145 and 146, and ground layers 134 and 144. The stripline circuit 140 is connected to signal trace on the main board 110 by via connections 141. Ground plane 133 is connected to the mainboard 110 by solder connections 107. The stripline topology provides power handling capability better than microstrip and built-in shielding.
Below the mainboard 110,
In a 2-circuit embodiment of the invention we implement two elliptic low pass filters in stripline structure in a single package, again with one filter “stacked” on top of the other. In this embodiment, stripline circuit layer 130 comprises the first elliptic low pass filter and stripline circuit layer 140 comprises the second elliptic low pass filter, each having different cut-off frequency.
By stacking two distributed-element filters thus, it is possible to have both filters in the space normally required for one filter. The ground plane between the two filters, an integral part of stripline circuit structure, provides isolation between the two filters. This also eliminates the need to provide external shielding which would add additional parts to the end-assembly. In addition, the use of a distributed element filter allows for more repeatable performance compared to lumped-element filters.
The distributed element filters are synthesized from the lumped-element circuit model using available filter element value tables. To optimize the performance of the filters, electromagnetic simulation is used to tune the dimensions of prototype filters. The filter using multiple cascaded hairpin resonators provides a very sharp cutoff frequency response with low insertion loss. Furthermore, to increase the rejection-band bandwidth, additional attenuation poles are added to the filter. The filters are evaluated by building a few prototypes and ascertaining that the measurement is in agreement with the simulation. This embodiment realizes a wide stop-band analogous to the ideal lumped element version. The measurement shows that the second filter (on circuit layer 130) had typically 30 dB up to 14 GHz and 25 dB out-of-band rejections up to 18 GHz. The cut-off frequency of the second filter is 2 GHz. The circuit can be designed as a separate board that can be surface-mounted to the main board 110.
This embodiment may be used to replace two different elliptic low pass filters used in an RF (radio frequency) transmitter, particularly where very limited board space is available. The lumped-element version may comprise 20 different inductors and capacitors. The performances of the lumped-element version could vary substantially due to component value tolerances. Tin shields might also be required to mitigate the performance degradation caused by coupling between elements and the two filters. One integrated component in accordance with the above embodiment with dual function; may thus replace 20 pick-and-place components with one and eliminate the need for separate RF shielding.
The utility of this embodiment is not limited to two circuits or filters. More than two passive circuits realizable in stripline structure can be integrated in a single package, such as shown in
Of course, one skilled in the art will appreciate how a variety of alternatives are possible for the individual elements, and their arrangement, described above, while still falling within the scope of the invention. Thus, while it is important to note that the present invention has been described in the context of an implementation for plural stripline filters, those of ordinary skill in the art will appreciate that the present invention applies equally regardless of the particular type of passive element actually used in implementing the desired circuit. Further, while this has been found particularly useful for circuits in the 1-2 GHz range, and more generally in the 1-4 GHz range, it should have equal application to all high frequency (i.e., greater or equal to 1 GHz) integrated circuits depending on the desired application and other design considerations.
In conclusion, the above description has been presented for purposes of illustration and description of an embodiment of the invention, but is not intended to be exhaustive or limited to the form disclosed. This embodiment was chosen and described in order to explain the principles of the invention, show its practical application, and to enable those of ordinary skill in the art to understand how to make and use the invention. Many modifications and variations will be apparent to those of ordinary skill in the art. Thus, it should be understood that the invention is not limited to the embodiments described above, but should be interpreted within the full spirit and scope of the appended claims.
