Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
1999-09-28
2001-11-27
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S219000, C333S203000, C333S246000
Reexamination Certificate
active
06323745
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to bandpass filters. In particular, the present invention is a planar bandpass filter.
II. Related Art
Filters are essential to the operation of most electronic circuits. Filters are implemented in electronic circuits to alter the amplitude and/or phase characteristics of a signal with respect to frequency. The design of bandpass filters for RF receivers/transceivers requires numerous tradeoffs with respect to circuit topology, bandwidth, clement realization, insertion loss, rejection level, etc. to arrive at a bandpass filter that satisfies all requirements. RF receivers/transceivers require bandpass filters that provide minimum insertion loss over the desired frequency band. In many cases, the design of conventional filters is driven by the range of capacitor values over the frequency band of interest.
Existing ceramic filters do not provide the rejection levels required by many RF applications without cascading the ceramic filters with other types of filters. Cascading filters results in increased insertion loss levels, and higher costs.
What is desired is a bandpass filter that does not require cascading ceramic filters to obtain the rejection level specifications needed. What is also needed is a bandpass filter that provides a repeatable design that does not require sampling and testing of multiple lumped element passive components such as capacitors or other filters, or tuning of the capacitors to achieve the desired performance levels of the filter. What is needed is a planar bandpass filter topology that provides a broadband response that eliminates reentrance frequencies at the desired rejection bands, while meeting the other requirements above.
One type of filter that satisfies some of these requirements is the combline filter. It can be realized in a planar form, is compact in size, and can be designed to provide a desired broad-band rejection characteristic.
Combline filters, however, have other problems associated with them. These problems arise when the production of mass quantities of filters is required, one of which is the use of lumped element capacitors in a conventional realization of the filter. Each filter usually includes a plurality of lumped element capacitors. Capacitor values vary over lot size. Variations in capacitor values can result in varied filter performance. To remedy the variation in capacitor values, a large quantity of capacitors could be tested to select capacitors with values that fall within a certain tolerance. Another alternative would be to build more filters than are needed, test each filter, and discard those filters that do not meet specifications. These remedies are very time consuming as well as expensive. Slight variations in the placement of the capacitors from filter to filter can also affect filter performance. Slight variations in filter topology from filter to filter, such as variations in tap points for tapped input and output resonators, can cause broadband response reentrance frequencies to occur at undesired harmonics of the passband.
SUMMARY OF THE INVENTION
The present invention satisfies the above mentioned needs by providing a planar bandpass filter having a repeatable or reproducible design that eliminates the need for lumped capacitor elements as well as the need for tuning the capacitors. The present invention is a planar bandpass filter that comprises a substrate having a ground plane on one side and a plurality of resonators on the other side. Each resonator includes an elongated inductive portion and a capacitive portion. The elongated inductive portions are coupled through the substrate at the end opposite the capacitive portion to the ground plane. The planar bandpass filter also includes a first tap and a second tap. The first tap is connected to a first elongated portion to serve as an input to the bandpass filter. The second tap is connected to a last elongated portion to serve as an output to the bandpass filter.
The present invention eliminates the need for lumped element capacitors by implementing loading capacitors using transmission lines. Thus, the loading capacitors of the present invention do not require tuning. Loading capacitors result in the reduction of the length of the associated resonators to approximately 45 degrees in electrical length. Resonator lengths of 45 degrees result in a compact structure with excellent stopband performance. Another advantage of this topology is that the filter's reentrance frequencies occur at much higher frequencies, and multiples of the reentrance frequencies are spaced farther apart. The present topology places the reentrance frequencies at portions of the frequency band where the harmonics do not appear.
Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.
REFERENCES:
patent: 4551696 (1985-11-01), Moutrie et al.
patent: 5344695 (1994-09-01), Hirai et al.
patent: 5521564 (1996-05-01), Kaneko et al.
patent: 5986525 (1999-11-01), Sasaki et al.
Jones Stephen E.
Ogrod Gregory D.
Pascal Robert
Qualcomm Inc.
Wadsworth Philip R.
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