Digitally tuned, integrated RF filters with enhanced linearity for multi-band radio applications

An integrated, multi-band radio frequency (RF) filter is capable of modifying a filter response thereof in response to control information. Switching elements within the filter can be changed between on and off conditions to modify the filter response. In one implementation, the integrated, multi-band filter is integrated on a front end module chip to be used within a multi-radio wireless device. In at least one embodiment, linearity enhancement circuitry is provided within a multi-band filter to improve linearity and reduce insertion loss.

TECHNICAL FIELD

The invention relates generally to radio frequency filters and, more particularly, to techniques for providing filters that can be tuned for operation within multiple different frequency bands.

BACKGROUND OF THE INVENTION

Many modern communication and computing devices support wireless communication for multiple different wireless standards. For example, a laptop computer may support wireless networking in accordance with both the IEEE 802.11b,g and IEEE 802.16 wireless networking standards. Often, the various supported standards will involve different operational frequency bands. Typically, separate circuitry would be provided within a device for each of the supported standards. It would be beneficial if one or more circuit components could be shared by multiple different wireless standards to, for example, reduce circuit size and/or cost.

DETAILED DESCRIPTION

The present invention relates to tunable, multi-band filters that can be implemented on-chip within, for example, radio frequency (RF) front end modules (FEMs) and/or other RF and mixed-signal chips. In one possible application, the tunable, multi-band filters may be used in devices and systems that support multiple different wireless standards. That is, the filter can be digitally switched between filter responses associated with multiple different wireless technologies so that a single filter structure may be used to support multiple radios. In the past, multi-band radio devices typically used a separate, off-chip filter for each of the corresponding radios. As will be appreciated, these separate filters may be costly to implement and consume much space within the corresponding device. By implementing the filters on-chip, circuit implementation size and cost can be reduced considerably. As will be described in greater detail, in at least one embodiment, a tunable filter is provided that includes dynamic bias control to improve linearity and reduce insertion loss.

FIGS. 1,2,3, and4are schematic diagrams illustrating example bandpass filter architectures10,30,40,50that may be used in accordance with embodiments of the present invention. Each filter architecture10,30,40,50includes a number of tunable LC tank circuits that are interconnected with series coupling elements. For example, filter10ofFIG. 1includes capacitors18,20as series coupling elements between tunable tank circuits12,14, and16. The tunable tank circuits in the filter architectures10,30,40,50each have at least one reactive element that can be changed in value during circuit operation to change a corresponding response of the filter. For example, the tunable tank circuits12,14, and16ofFIG. 1each include variable inductances for use in changing the filter response. In this manner, the filter10ofFIG. 1uses capacitive coupling and inductive tuning, the filter30ofFIG. 2uses capacitive coupling and capacitive tuning, the filter40ofFIG. 3uses inductive coupling and capacitive tuning, and the filter50ofFIG. 4uses inductive coupling and inductive tuning. In other embodiments, more than one of the parallel branches within each tank circuit in a filter may be tunable. Similarly, in some embodiments, the coupling elements are also tunable. Other filter architectures may also be used in accordance with embodiments of the invention.

In at least one embodiment of the invention, filter tunability is achieved using switching mode transistors and on-chip inductors and capacitors. Techniques for achieving fixed-value inductances and capacitances “on-chip” are well known in the art. To change the response of a filter, the switching mode transistors therein may be switched between on and off states. The change in the states of the switches will modify the values of the inductances and capacitances within the filter in a manner that changes the filter response in a desired way. In at least one embodiment, the parasitic capacitances of the “off” state transistors may be used as tuning elements within a filter to generate a desired filter response.

FIG. 5is a schematic diagram illustrating an example tunable, multi-band filter60in accordance with an embodiment of the present invention. Like the filter40ofFIG. 3, the tunable, multi-band filter60ofFIG. 5uses inductive coupling and capacitive tuning. As shown, the filter60includes multiple tunable tank circuits that each include: a switching transistor62, first and second capacitors64,66, and an inductor68. The first and second capacitors64,66and the transistor62are located in one branch of each tank and the inductor68is located within another, parallel branch of each tank. The switching transistor62may include any type of transistor that can switch between on and off states (e.g., FET, CMOS, pHEMT, etc.). The filter60also includes two coupling inductors70. A DC blocking capacitor74is located at both the input and the output of the filter60.

The filter60is operative for covering two different operational frequency bands. These two bands may be associated with, for example, two different wireless standards being supported by a multi-radio device (e.g., IEEE 802.11 and IEEE 802.16, etc.). In one filter state, all of the switching transistors62will be biased “on” and, in the other filter state, all of the switching transistors62will be biased “off.” The biasing may be carried out using control voltage VCapplied to the gate terminals of the switching transistors62. To bias the transistors62“on,” a voltage level of VCC(the supply voltage) may be applied to the gate terminals of the transistors. To bias the transistors “off,” a voltage level of zero may be applied to the gate terminals of the transistors.

When the switching transistor62within each tank is on, the transistor62appears as a small “on” resistance between its drain and source terminals. To a first approximation, ignoring the “on” resistance of the transistor, the total capacitance (CT) of the tank is the series combination of C1and C2or CT=C1C2/(C1+C2) when the transistor is biased on. The capacitance values of C1and C2, as well as the inductance values of the inductors68,70, will be selected to achieve a first desired band pass filter response when the switching transistors62are on. When the switching transistor62within each tank circuit is biased off, the transistor62appears as a parasitic capacitance CPwithin the tank circuit. The parasitic “off” capacitance is the total parasitic capacitance between the drain and source, the drain and ground, and the source and ground. The capacitance values of the first and second capacitors64,66will have negligible effect on the filter response when the switching transistor62is off. The value of the parasitic capacitance CP, as well as the inductance values of the inductors68,70, will be selected to achieve a second desired band pass filter response when the switching transistors62are off. The size of the switching transistor62will typically determine both the off state parasitic capacitance and the on state resistance. Therefore, given the capacitance tuning ratio dictated by the dual band frequency, the transistor size and the values of C1and C2can be optimized to maximize the Q factor of the circuit.

In the above described embodiment, the tunable, multi-band filter60is capable of operating within two different desired bands. In other embodiments, filters that are capable of switching between three or more different filter responses are provided. In one approach, for example, tunability may be extended by adding more tunable elements in parallel to a fixed element. The number of bands that can be covered will thus be a binary function of the number to tunable elements that are placed in parallel with the fixed element. For example, in the filer60discussed above, there is one tunable element in parallel with one fixed element per tank, thus giving two total bands to select between. If there are two tunable elements in parallel with the fixed element, then there will be 4 bands to select between. If there are three tunable elements in parallel with the fixed element, there will be 8 bands to select between, and so on. Other techniques for increasing the number of tunable bands also exist.

As described above, the filter60ofFIG. 5includes three individual tank circuits. In other embodiments, a higher or lower number of tank circuits may be used. In addition, the filter60ofFIG. 5includes two parallel branches within each of the tank circuits. In other embodiments, one or more additional parallel branches may be added to each tank circuit. For example, in the filter60ofFIG. 5, an additional branch having a capacitor may be added to each tank circuit. In such an embodiment, the overall capacitance of the tank will include a parallel combination of the capacitances of the two capacitive branches.

In high power RF applications, such as an IEEE 802.16 radio, it is very important to provide filters with a high level of linearity. This can be especially important within the post power amplifier filter in the RF transmitter of the radio. Non-linearities within the filter will typically be greatest within the switching transistors themselves. To address these non-linearities, in at least one embodiment of the present invention, a dynamic bias control technique is used to control the biasing of the switching transistors in a way that enhances linear operation and reduces insertion loss.

FIG. 6is a schematic diagram illustrating an example tank circuit80that may be used within a tunable, multi-band filter in accordance with an embodiment of the present invention. The tank circuit80includes linearity enhancement circuitry that is capable of reducing non-linearities within the tank circuit80that can distort a signal being filtered. The tank circuit80may be used within any of the filters inFIGS. 1-5or in other filters. As illustrated, the tank circuit80includes a switching transistor82, first and second capacitors84,86, a fixed inductor90, and an inverter92. As described previously, a control voltage VCis applied to the gate terminal of the switching transistor82to turn the transistor on and off to change the filter response of the associated filter.

The linearity of a filter using the tank circuit80ofFIG. 6will typically depend upon the linearity of the switching transistor82of the tank. This is because the linear amplitude of the signal passing through the filter will often be limited by the maximum linear voltage swing across the “off” state switching transistor. To maximize linearity, the value of |Vg-Vs| should be maximized to the power supply voltage level VCC, where Vgand Vsare voltages applied to the transistor gate and source, respectively. This may be realized by causing the source bias voltage of the switching transistor82to follow an inverted version of the gate control voltage VC. As shown inFIG. 6, in one possible implementation, an inverter92may be provided to invert the gate control voltage VCand apply the inverted signal to the source terminal94of the switching transistor82. In the illustrated embodiment, the inverter92comprises a field effect transistor (FET) connected in a common source configuration. Other types of inverter circuits may alternatively be used. When the switching transistor82is to be turned on, a logic high voltage (e.g., VCC) is applied to the gate thereof and a logic low voltage (e.g., zero volts) is applied to the source terminal94by the inverter92. This effectively minimizes the on state resistance of the switching transistor82which translates into reduced filter insertion loss. When the switching transistor82is to be turned off, a logic low voltage is applied to the gate terminal of the transistor and a logic high voltage is applied to the source terminal94by the inverter92. As described above, this maximizes the allowable voltage swing of the off state transistor, thereby improving circuit linearity.

FIG. 7is a schematic diagram illustrating an example tunable, multi-band band pass filter100that includes linearity enhancement circuitry in accordance with an embodiment of the present invention. The filter100uses the same overall architecture as the filter60ofFIG. 5, but adds linearity enhancement circuitry, such as that shown inFIG. 6. A gate control voltage VCis applied to the gates of three switching transistors62to change the transistors between on and off states. An inverter102inverts the gate control signal and applies the inverted signal to the source terminals of the transistor62within each tank circuit. As described previously, the inverted signal is applied to improve the linearity of the filter and also to reduce insertion loss. The filter100is operative within both a higher frequency band and a lower frequency band. When the higher frequency band is desired, a voltage of zero is applied to the gate terminals of the switching transistors62and a voltage of VCCis applied to the source terminals by the inverter102. When the lower frequency band is desired, a voltage of VCCis applied to the gate terminals of the switching transistors62and a voltage of zero is applied to the source terminals by the inverter102. In at least one implementation, the same transistor technology is used to provide the transistor within the inverter102that is used to provide the switching transistors62(e.g., FETs, CMOS, pHEMT, etc.). This approach facilitates the implementation of the filter circuitry on chip.

In at least one embodiment of the invention, one or more of the tunable, multi-band filters are provided on-chip within a wireless front-end module (FEM). Embodiments within other types of RF chip and within mixed signal chips also exist. The filters may also be provided as separate tunable filter chips that can then be coupled to other chips or circuits. Features of the invention may be used to provide any type of filter response including, for example, bandpass, bandstop, high pass, and low pass. Filters in accordance with the present invention may be used within RF transmitters, RF RF receivers, and/or in other RF circuits.

As described previously, in at least one embodiment of the invention, one or more tunable, multi-band filters are used within a multi-radio device that includes radios following two or more different wireless standards. In this manner, a filter can be designed to achieve two or more distinct filter responses for use with the different wireless standards. When a multi-band radio device is about to use a particular wireless technology (e.g., IEEE 802.16 wireless networking), digital control signals may be delivered to the filter units within the transceiver circuitry to appropriately configure the filters. Communication may then be allowed to proceed in a normal manner. Other applications for the tunable, multi-band filters also exist.

FIG. 8is a block diagram illustrating an example multi-band receiver110that includes a tunable, multi-band filter in accordance with an embodiment of the present invention. As illustrated, the wireless device110includes a radio frequency front end module (FEM)112that is coupled to a controller114. The FEM112is also coupled to one or more antennas128. Any type of antenna may be used including, for example, a dipole, a patch, a helical antenna, an antenna array, and/or others. In at least one embodiment, the antenna128is implemented as part of the FEM112. The FEM112includes receiver circuitry to support two different wireless standards that use different frequency bands. As shown inFIG. 8, the FEM112includes: a tunable, multi-band filter116, a switch118, first and second low noise amplifiers (LNAs)120,122, and first and second mixers124,126. Although not shown, additional receiver circuitry may also be implemented on the FEM112. The first LNA120and the first mixer124are associated with one of the supported wireless standards and the second LNA122and the second mixer126are associated with the other supported standard. The switch118is operative for directing a received signal to appropriate receive circuitry when received. The tunable, multi-band filter116acts as a preselector to filter a received signal to reject signal energy outside of a desired band.

The controller114may configure the filter116by sending control signals thereto based on the wireless standard that is currently active for the multi-band receiver110. The controller114may also configure the switch118to direct the filtered signal to the proper receive channel. One of the LNAs120,122will receive the signal from the switch118and amplify the signal in a low noise manner. The corresponding mixer124,126will then down convert the signal to an intermediate frequency or to baseband. Additional receive processing may then occur.

FIG. 9is a block diagram illustrating an example multi-band transmitter130that includes a tunable, multi-band filter in accordance with an embodiment of the present invention. As illustrated, the multi-band transmitter130is part of a radio frequency front end module (FEM)132that is coupled to a controller134. The FEM132is also coupled to one or more antennas148. As before, any type of antenna may be used including, for example, a dipole, a patch, a helical antenna, an antenna array, and/or others. The FEM132includes: first and second mixers136,138, first and second power amplifiers140,142, a switch144, and a tunable, multi-band filter146. The first mixer136and the first power amplifier140are associated with one of the supported wireless standards and the second mixer138and the second power amplifier142are associated with the other supported standard. Each mixer136,138are operative for up converting transmit signals to appropriate transmit frequencies during transmit operations. The corresponding power amplifier140,142then amplifies the signal to a desired transmit power level. Depending on which radio is transmitting, the controller134will cause the switch144to couple either the first power amplifier140or the second power amplifier142to the tunable, multi-band filter146. The controller134will also send control signals to the tunable, multi-band filter146to configure the filter for the corresponding filter response. The filtered transmit signal is then delivered to the antenna(s)148for transmission.

It should be appreciated that the multi-radio wireless devices ofFIGS. 8 and 9represent two possible applications of a tunable, multi band filter in accordance with the present invention. Many other applications also exist. For example, the filters may be integrated onto FEMs having entirely different architectures. The filters may also be implemented on other type of chips or in additional or alternative locations on a chip. As described previously, in some embodiments, filters are used that are operative within three or more different operational frequency bands. Implementation as a separate filter chip is also possible.

The techniques and structures of the present invention may be implemented in any of a variety of different forms. For example, features of the invention may be embodied within laptop, palmtop, desktop, and tablet computers having wireless capability; personal digital assistants (PDAs) having wireless capability; cellular telephones and other handheld wireless communicators; pagers; satellite communicators; cameras having wireless capability; audio/video devices having wireless capability; network interface cards (NICs) and other network interface structures; base stations; wireless access points; integrated circuits; and/or in other formats.

In the foregoing detailed description, various features of the invention are grouped together in one or more individual embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of each disclosed embodiment.