Methods and apparatus for signal filtering

#The present invention relates generally to wireless communication. More particularly, the invention relates to improved systems and techniques for tuning of center frequencies, fractional bandwidths and transmission nulls, or zeroes, in bandpass filters.

#As the popularity of wireless cellular data communications devices, and the demands made on them, continue to increase, the demands on the frequency spectrum available for such communication become greater and greater, with network operators and device manufacturers under increasing pressure to use frequencies as efficiently as possible in order to avoid saturating the available frequency bands.

#In one embodiment of the invention, an apparatus comprises a cross-coupled bandpass filter characterized by a tunable center frequency, and one or more programmable zeroes.

#FIG. 1 illustrates a graph showing forward transmission coefficient against frequency for coupled and cross-coupled filters;

#FIGS. 2A and 2B illustrate filters according to embodiments of the present invention;

#FIGS. 3A and 3B illustrate electrical models of filters according to embodiments of the present invention;

#Embodiments of the present invention recognize that modern wireless communication devices use numerous frequency bands that are closely spaced in order to accommodate as many devices in close physical proximity to one another as possible. At least one anticipated implementation of third generation partnership project (3GPP) long term evolution (LTE) systems is expected to support 29 frequency division duplex (FDD) and 11 time division duplex (TDD) single-carrier bands. Embodiments of the present invention recognize that however many bands are supported by network implementations, it is desirable for communication devices to be able to operate in as many bands as possible in order to allow selection of a band that minimizes interference between devices operating in close physical proximity to one another. In order for devices to operate in multiple bands, they must be able to filter out undesired frequencies, and must be capable of a substantial level of stop-band rejection. Frequently, higher levels of stop-band rejection are accompanied by higher levels of insertion loss and, therefore, lower receiver sensitivity. Insertion loss can be reduced while maintaining stop-band rejection by employing a cross-coupled filter structure. FIG. 1 illustrates, as an example, forward transmission provided by coupled resonator and cross-coupled resonator transmit filters for LTE Band 25. FIG. 1 presents curves 102 and 104, each showing the relationship between forward transmission coefficient (S21), measured in dB, and frequency, measured in GHz. The curve 102 presents the relationship for a prior-art coupled filter, and the curve 104 presents the relationship for a prior-art cross-coupled filter. In the presently illustrated example, the cross-coupled filter is shown (as compared to the coupled filter) to reduce insertion loss and increase stop-band rejection, for one attenuation skirt (in this case, the high-side skirt). Stop-band rejection is reduced by introducing two transmission zeroes 106 and 108, at the expense of reducing attenuation on the low-side skirt. The improved attenuation skirt, with no increase in the filter's unloaded quality factor, attenuates transmit power leakage emanating into the receiver path, and can represent an important approach to improving receiver sensitivity in FDD cellular systems such as LTE. Cross-coupling (understood in the art as encompassing a filter design that provides for undesired frequencies at both ends of the filter to cancel one another out) can substantially improve performance, while substantially reducing the size and complexity of filters as compared to a design that uses additional filtering components to remove such frequencies.

#To the extent that the ability to adapt filtering is limited, increasing the number of frequency bands in a system increases the likelihood of corrupted reception due to interfering carriers, or blockers, in addition to self-corruption, such as transmitter power leakage into a receiver during fixed division duplex operation. As illustrated in FIG. 1 and discussed above, the use of cross-coupled filtering helps reduce transmitter power leakage and other undesirable effects, but prior-art cross-coupled filters are difficult to vary in such a way that a single such filter can be used with numerous different bands.

#Current filter technologies, in fact, are only able to support one single-carrier band per filter count, and discrete component filters are bulky and costly and complicate front-end design for both FDD and TDD devices. Increasing the band count, that is, the number of frequency bands to be supported, greatly complicates the front-end design if a separate filter must be implemented for each band. Particularly if discrete components are to be used, design of a device with multiple filters to accommodate multiple bands is likely to be so complex and costly that the device will be able to operate only in relatively few bands.