Tunable wideband bandpass filter, tunable multi-band bandpass filter using the same, and methods therefore

[0001] This application claims benefit under 35 U.S.C. § 119 from Korean Patent Application No. 2004-11670, filed on Feb. 21, 2004, the content of which is incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to a frequency-tunable filter, and more particularly to digitally-controlled, small-sized, low power-consuming tunable wideband filter and tunable multi-band wideband filter using switched capacitors and inductors in use of the same, which are suitable for wideband communication systems and multi-mode communication systems.

[0004] 2. Description of the Related Art

[0005] In general, frequency-tunable bandpass filters with freely variable center frequencies are required in wideband communication systems, such as television (“TV”), ultra highband (“UWB”), and multi-mode communication systems, such as cellular phones, personal communication service (“PCS”), and wideband code-division multiple access (“WCDMA”).

[0006] FIG. 1 is a block diagram showing a communication system of a conventional RF receiver. In FIG. 1, an RF band signal received through an antenna is input to an RF bandpass filter 10. The RF bandpass filter 10 filters the signal of the antenna to a wireless frequency band, and outputs the filtered signal to a low-noise amplifier (LNA) 20. The low-noise amplifier 20 receives a signal output from the RF bandpass filter 10, low-noise-amplifies the signal by a predetermined gain, and outputs the amplified signal to a mixer 30. The mixer 30 mixes the signal of the low-noise amplifier 20 with a sinusoidal signal generated from a local oscillator, and outputs a signal having a center frequency f_IF shifted in a center frequency band. The signal of the mixer 30 is input to the base band modem 60 through an intermediate frequency filter 40 and an intermediate frequency/automatic gain control amplifier 50.

[0007] As such, the conventional RF receiver has the RF bandpass filter 10 with fixed frequency characteristics.

[0008] FIG. 2 illustrates a general multi-band and multi-mode receiver that switches receivers fit to a band and mode. Thus, the multi-band and multi-mode receiver requires additional receivers and space.

[0009] The development of integration circuit (“IC”) technology is facilitating the implementation of wideband low noise amplifiers (“LNAs”) and local oscillators. As shown in FIG. 3, a frequency-tunable radio frequency (“RF”) bandpass filter enables a single receiver to serve as a multi-band and multi-mode receiver.

[0010] The frequency-tunable RF bandpass filter enables a user to select a desired RF band channel. The frequency-tunable RF bandpass filter removes interference signals outside of the desired band and interference signals caused by neighboring channels which alleviates the linear characteristics required for an RF circuit and the phase-noise requirements of a local oscillator so that power consumption is reduced.

[0011] As previously discussed, the conventional frequency-tunable bandpass filter is constructed in the hybrid form using a varactor diode controlled by an analog tuning signal or a pin diode switch. However, the conventional frequency-tunable bandpass filter described above has severe non-linear characteristics, a large size, increased power consumption, and is costly.

[0012] On the other hand, a frequency-tunable RF bandpass filter using only active elements is easily controlled; however, it consumes a lot of power, has a low yield, and performs worse than the passive filter in terms of linearity or noise factor. As such, the frequency-tunable bandpass filter using only active elements is difficult to apply to the wideband communication systems or multi-mode communication systems.

[0013] An aspect of the invention provides an on-complimentary metal-oxide semiconductor (CMOS) chip, digitally-controlled, small-sized, low power-consuming tunable wideband bandpass filter and tunable multi-band wideband bandpass filter using the same.

[0014] The foregoing and/or other objects and advantages are substantially realized by providing a tunable wideband bandpass filter capable of varying a center frequency and bandwidth of a signal to be filtered, comprising a plurality of switched-capacitor banks (SCBs) for varying capacitance and converting the center frequency and bandwidth into a digital signal; and a plurality of inductors provided for inductance to generate the center frequency, wherein the plurality of switched-capacitor banks (SCBs) are integrated into a signal chip.

[0015] According to an aspect of the invention, the chip is formed of complementary metal oxide semiconductor.

[0016] According to an aspect of the invention, the plurality of inductors are preferably integrated into the chip.

[0017] According to an aspect of the invention, the plurality of inductors are preferably implemented as individual elements or in a hybrid form outside of the chip.

[0018] According to an aspect of the invention, the switched-capacitor banks each include a plurality of switched capacitors connected in parallel, and a plurality of switching elements respectively connected in series to the plurality of switched capacitors.

[0019] According to an aspect of the invention, the switched-capacitor banks further include a varactor diode, respectively, connected in parallel with the plurality of switched capacitors and switching elements.

[0020] According to an aspect of the invention, an equivalent capacitance of the switched-capacitor banks preferably varies by turning on or off the plurality of switching elements.

[0021] According to an aspect of the invention, the plurality of switching elements are metal oxide semiconductor transistors.

[0022] According to an aspect of the invention, the plurality of switching elements are micro electro mechanical system (MEMS) switches.

[0023] According to an aspect of the invention, a tunable multi-band wideband bandpass filter capable of varying a center frequency and bandwidth of a signal to be filtered, may include: a plurality of switched-capacitor banks for varying capacitance and converting the center frequency and bandwidth into a digital signal; a plurality of switched inductors for varying inductance and converting the center frequency and bandwidth into a digital signal; and a plurality of switches for switching the plurality of switched inductors, wherein the plurality of switched-capacitor banks and switches are integrated into a single chip, and the plurality of switched inductors are connected in parallel with the switched capacitors.

[0024] According to an aspect of the invention, the plurality of switched inductors are implemented as individual elements or in a hybrid form outside of the chip.

[0025] According to an aspect of the invention, the plurality of switched inductors are integrated into the chip.

[0026] According to an aspect of the invention, the chip is formed of Complementary Metal Oxide Semiconductor (CMOS).

[0027] According to an aspect of the invention, the switched-capacitor banks each include a plurality of switched capacitors connected in parallel and a plurality of switching elements respectively connected in series to the plurality of switched capacitors.

[0028] According to an aspect of the invention, the switched-capacitor banks each further includes a varactor diode connected in parallel with the plurality of switched capacitors and switching elements.

[0029] According to an aspect of the invention, an equivalent capacitance of the switched-capacitor banks preferably varies by turning on or off the plurality of switching elements.

[0030] According to an aspect of the invention, the plurality of switching elements are Metal Oxide Semiconductor (MOS) transistors.

[0031] According to an aspect of the invention, the plurality of switching elements are micro electro mechanical system (MEMS) switches.

[0032] Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

[0044] Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

[0045] FIG. 4 is an illustration of a structure of a tunable multi-band wideband bandpass filter according to an embodiment of the invention. In FIG. 4, a tunable multi-band wideband bandpass filter 300 has a plurality of switched-capacitor banks (“SCB”) SCB1, SCB2, SCB3, and SCB4 and a plurality of switched-inductor banks (L1a, L1b, and L1c), (L2a, L2b, and L2c), (L3a, L3b, and L3c), and (L4a, L4b, and L4c), respectively integrated on a side of a CMOS chip 100. For example, the plurality of SCBs and switched-inductor banks are provided on an upper side of the CMOS chip 100. The capacitors and inductors are switched in order to improve the performance of the filter by expanding the tuning range of the filter.

[0046] The first switched-capacitor bank SCB1 is connected with a plurality of switched inductors L1a, L1b, and L1c that are connected in parallel, and the other ends of the switched inductors L1a, L1b, and L1c are connected in series to a switch respectively to turn the switched inductors L1a, L1b, and L1c on or off. Likewise, the second, third, and fourth switched-capacitor banks SCB2, SCB3, and SCB4 are connected to switched inductors L2a to L2c, L3a to L3c, and L4a to L4c, respectively, which are connected in parallel with one another, and the other ends of the switched inductors L2a to L2c, L3a to L3c, and L4a to L4c are connected in series to a switch, respectively.

[0047] The tunable multi-band wideband bandpass filter switching on and off the inductors structured as above controls the equivalent capacitance of a circuit by the switched-capacitor banks SCB1 to SCB4 and the equivalent inductance of a circuit by turning on and off the switches of the switched inductors, to enable bandwidth extension.

[0048] FIG. 5 is a view of the tunable multi-band wideband bandpass filter according to an embodiment of the invention. In FIG. 5, when a high quality factor is required, the inductors L1a to L1c, L2a to L2c, L3a to L3c, and L4a to L4c are preferably implemented as individual elements or in a hybrid form outside of the CMOS chip 100.

[0049] FIG. 6 is a view of a tunable wideband bandpass filter according to an aspect of the invention. In FIG. 6, the tunable wideband bandpass filter 20 has a plurality of switched-capacitor banks SCB1, SCB2, SCB3, and SCB4 integrated on a side of the CMOS chip 100 and a plurality of inductors L1, L2, L3, and L4 implemented outside of the CMOS chip 100. For example, the plurality of SCB's are provided on an upper side of the CMOS chip 100. In order to reduce the deterioration of the quality factor of the switched inductors due to the resistance component of the switches and the number of pads for the inductor switches, external inductors L1 to L4 without a switch have to be used. However, when the high quality factor is not required, the inductors L1 to L4 are preferably integrated in the form of a spiral inductor into the CMOS chip 100.

[0050] As seen in the embodiment illustrated in FIG. 6, the first switched-capacitor bank SCB1 is connected in series to one end of the first inductor L1, and the other end of the first inductor L1 is connected to additional inductors L2, L3, and L4, wherein L1 through L4 are connected in parallel with one another. Further, the other end of the second inductor L2 is connected in series to the second switched-capacitor bank SCB2, and the other end of the second switched capacitor bank SCB2 is grounded. The other end of the third inductor L3 is connected in series to the one end of the third switched-capacitor bank SCB3, and the other end of the third switched-capacitor bank SCB3 is grounded. The other end of the fourth inductor L4 is connected in series to the one end of the fourth switched-capacitor bank SCB4.

[0051] An RF analog signal applied to the first switched-capacitor bank SCB1 through an input terminal is tuned to a center frequency and a bandwidth, and then output throughout a low-noise amplifier through the fourth switched-capacitor bank SCB4. The present embodiment is not limited to four SCBs and four inductors. For example, an RF analog signal applied to SCB1 may be output to a low noise amplifier 20 through a fifth SCB.

[0052] FIG. 7 is a view of the internal circuit of the switched-capacitor banks SCB1 to SCB4 according to an embodiment of the invention. In FIG. 7, the switched-capacitor banks SCB1 through SCB4 each have n switched capacitor C1, C2, . . . , and Cn, with one end of each switched capacitor being connected in parallel with the input terminal, and with the other end of each switched capacitor being is connected in series to n transistors Q1, Q2, . . . , and Qn. The transistors Q1 to Qn may include metal oxide semiconductor (MOS) transistors and micro electro mechanical system (MEMS) switch elements.

[0053] According to an embodiment of the invention, the gate of each transistor is applied with a digital control signal Si consisting of bits, and the transistor properly controls the digital control signal Si, in order to convert the capacitance of the switched capacitors C1 to Cn into a digital signal. Thus, the entire capacitance of the tunable wideband bandpass filter 200 is controlled.

[0054] Equation 1 shows the relationship among capacitance, inductance, and a center frequency, according to an embodiment of the invention:

[0055] $\begin{array}{cc}{f}_r=\frac{1}{2\pi \sqrt{\mathrm{LC}}},& \left[\mathrm{Equation}1\right]\end{array}$

[0056] In Equation 1, fr denotes the center frequency, L is an equivalent inductance of the tunable wideband bandpass filter, and C is an equivalent capacitance of the tunable wideband bandpass filter.

[0057] As described above, when the equivalent capacitance of the tunable wideband bandpass filter is controlled, the center frequency of the signal applied to the tunable wideband bandpass filter and the signal bandwidth can be adjusted.

[0058] FIG. 8 illustrates an embodiment of the present invention having a switched-capacitor bank SCB with a varactor diode. As shown in FIG. 8, the varactor diode is connected in parallel with a plurality of switched capacitors Csw1, Csw2, . . . , and Cswn and a plurality of transistors Q1, Q2, . . . , and Qn. The plurality of transistors are MEMS switch elements.

[0059] The addition of the varactor diode enables the equivalent capacitance of the respective switched-capacitor banks SCB1 to SCB4 to be more precisely controlled. For example, the varactor diode enables fine frequency tuning to be achieved.

[0060] As described above, the transistors are turned on or off when digital control signal voltages are set lower or higher than a predetermined threshold voltage of the transistors, which enables the entire capacitance of the respective switched-capacitor banks SCB1 to SCB4 to be adjusted. For example, when the capacitance of the respective switched-capacitor banks SCB1 to SCB4 is adjusted as described above, the center frequency and bandwidth of an RF analog signal applied to the filter can be adjusted.

[0061] FIG. 9 illustrates a simulation for the tunable multi-band wideband bandpass filter shown in FIG. 4. As shown in FIG. 9, the values of the switched inductors are set in order for three bands to have center frequencies between 50 MHz and 110 MHz, 110 MHz and 500 MHz, and 500 MHz and 900 MHz, respectively. The tunable multi-band wideband bandpass filter varies its band range from 50 MHz to 900 MHz as shown in the simulation result.

[0062] FIG. 10 illustrates a simulation result for the tunable wideband bandpass filter shown in FIG. 6. In FIG. 10, the tunable wideband bandpass filter varies its center frequency from 50 MHz to 900 MHz as shown in the simulation result.

[0063] As discussed above, the invention can implement a digitally controlled, small-sized, low power-consumed, and manufacturing cost-lowered tunable wideband bandpass filter.

[0064] Further, the invention improves the performance of a tunable multi-band wideband bandpass filter since the bandwidth of the filter can be widened.

[0065] Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.