High rejection wideband bandpass N-path filter

Certain aspects of the present disclosure provide an N-path filter implemented using a generalized impedance converter (GIC) circuit. The GIC circuit is configured such that the N-path filter has a desired frequency response, which may include a wide passband with steeper rejection than a conventional N-path filter with only a single pole in each filter path. Certain aspects of the present disclosure provide an N-path filter having a frequency response with multiple concurrent passbands. In certain aspects, the N-path filter with multiple passbands is implemented using the GIC circuit. In other aspects, the N-path filter may include a bandpass response circuit where an inductance of the bandpass response circuit may be implemented using gyrators.

TECHNICAL FIELD

Certain aspects of the present disclosure generally relate to electronic circuits and, more particularly, to N-path filters configured as bandpass filters.

BACKGROUND

Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. A wireless communication network may include a number of base stations that can support communication for a number of mobile stations. A mobile station (MS) may communicate with a base station (BS) via a downlink and an uplink. The MS and/or BS may include any of various suitable types of filters, such as an N-path filter. N-path filters have a number N of parallel impedance sections and a switching arrangement to connect each impedance section periodically with an input signal path. N-path filters may also be used for other applications in addition to wireless communications.

SUMMARY

Certain aspects of the present disclosure generally relate to N-path filters with wider passbands and steeper rejection than conventional N-path filters with only a single pole in each filter path. Certain other aspects of the present disclosure generally relate to N-path filters with multiple concurrent passbands.

Certain aspects of the present disclosure provide an N-path filter configured as a bandpass filter. The N-path filter generally includes a plurality of branches selectively connected with a common node, each branch of the N-path filter comprising a switch connected in series with an impedance, wherein the impedance in each branch of the N-path filter includes a bandpass response circuit.

According to certain aspects, the bandpass filter has multiple concurrent passbands.

According to certain aspects, each bandpass response circuit comprises an inductive element connected in parallel with a first capacitive element. The inductive element may include a gyrator circuit. The gyrator circuit may include a first gyrator, a second gyrator, a second capacitive element, and a third capacitive element. For certain aspects, the gyrator circuit further includes a first node and a second node; the first node is coupled to a first port of the first gyrator and to a first port of the second gyrator; the second node is coupled to a second port of the first gyrator and to a second port of the second gyrator; a first terminal of the second capacitive element is coupled to a third port of the first gyrator and to a third port of the second gyrator; a first terminal of the third capacitive element is coupled to a fourth port of the first gyrator and to a fourth port of the second gyrator; and a second terminal of the second capacitive element and a second terminal of the third capacitive element are coupled to a reference potential of the N-path filter. For certain aspects, a first terminal of the inductive element is connected with a first terminal of the first capacitive element, and a second terminal of the inductive element and a second terminal of the first capacitive element are connected with a reference potential of the N-path filter.

According to certain aspects, the bandpass response circuits in each non-overlapping pair of the branches are connected with the switches of the pair of branches. For certain aspects, the bandpass response circuits in each pair of branches include an inductive element, which may include: (1) a first node connected with a first capacitive element and with a first switch of the pair of branches and (2) a second node connected with a second capacitive element and with a second switch of the pair of branches. For certain aspects, the inductive element includes a gyrator circuit. The gyrator circuit may include a first gyrator, a second gyrator, a third capacitive element, and a fourth capacitive element. In this case, the first node may be coupled to a first port of the first gyrator and to a first port of the second gyrator; the second node may be coupled to a second port of the first gyrator and to a second port of the second gyrator; a first terminal of the third capacitive element may be coupled to a third port of the first gyrator and to a third port of the second gyrator; a first terminal of the fourth capacitive element may be coupled to a fourth port of the first gyrator and to a fourth port of the second gyrator; and a second terminal of the third capacitive element and a second terminal of the fourth capacitive element may be coupled to a reference potential of the N-path filter. For certain aspects, no two switches of the N-path filter are concurrently closed, and another switch in another pair of branches is closed between the first and second switches of the pair of branches being closed.

According to certain aspects, the multiple concurrent passbands are associated with different component carriers of an intra-band carrier aggregation scheme.

According to certain aspects, the N-path filter is configured to implement a concurrent dual-bandpass filter.

According to certain aspects, the N-path filter is configured to filter an output of an amplifier in a receive path of a transceiver. In this case, the N-path filter may be further configured to suppress leakage from a transmit path of the transceiver.

Certain aspects of the present disclosure provide a method for filtering a signal. The method generally includes selectively connecting each of a plurality of branches of an N-path filter with a circuit node carrying the signal, each branch of the N-path filter comprising a switch connected in series with an impedance, wherein the impedance in each branch of the N-path filter includes a bandpass response circuit. The N-path filter may have multiple concurrent passbands.

Certain aspects of the present disclosure provide an apparatus for filtering a signal. The apparatus generally includes means for carrying the signal and means for selectively connecting each of a plurality of branches of the apparatus with the means for carrying the signal, wherein each branch of the apparatus comprises means for providing a bandpass response coupled to the means for selectively connecting. For certain aspects, the apparatus has multiple concurrent passbands.

Certain aspects of the present disclosure provide an N-path filter configured as a bandpass filter. The N-path filter generally includes a plurality of branches selectively connected with a common node, each branch of the N-path filter comprising a switch connected in series with an impedance converter.

The impedance converter in each branch of the N-path filter may be configured to implement a bandpass impedance response.

According to certain aspects, the impedance converter in each branch of the N-path filter includes one or more amplifiers and a plurality of passive components such that each branch of the N-path filter has at least two poles. The impedance converter in each branch of the N-path filter may be configured to implement a bandpass impedance response.

According to certain aspects, the impedance converter includes first, second, third, fourth, and fifth impedances connected in series; a first amplifier having a positive input, a negative input, and an output, wherein the positive input of the first amplifier is connected with the first impedance, wherein the negative input of the first amplifier is connected with a node between the second impedance and the third impedance, and wherein the output of the first amplifier is connected with a node between the third impedance and the fourth impedance; and a second amplifier having a positive input, a negative input, and an output, wherein the positive input of the second amplifier is connected with a node between the fourth impedance and the fifth impedance, wherein the negative input of the second amplifier is connected with the node between the second impedance and the third impedance, and wherein the output of the second amplifier is connected with a node between the first impedance and the second impedance. For certain aspects, the first impedance is coupled to the switch of the branch, and the fifth impedance may be coupled to a reference potential of the N-path filter. For certain aspects, the first, third, and fifth impedances have matching values. For certain aspects, the first, third, and fifth impedances are each implemented with a resistive element connected in parallel with a capacitive element, and the second and fourth impedances may each be implemented with a resistor.

According to certain aspects, the N-path filter is configured to filter an output of an amplifier in a receive path of a transceiver. In this case, the N-path filter may be further configured to suppress leakage from a transmit path of the transceiver.

According to certain aspects, the bandpass filter has multiple concurrent passbands.

Certain aspects of the present disclosure provide a method for filtering a signal. The method generally includes selectively connecting each of a plurality of branches of an N-path filter with a circuit node carrying the signal, each branch of the N-path filter comprising a switch connected in series with an impedance converter.

Certain aspects of the present disclosure provide an apparatus for filtering a signal. The apparatus generally includes means for carrying the signal and means for selectively connecting each of a plurality of branches of the apparatus with the means for carrying the signal, wherein each branch of the apparatus comprises means for converting an impedance coupled to the means for selectively connecting.

DETAILED DESCRIPTION

Certain aspects of the present disclosure provide an N-path filter implemented using an impedance converting circuit. The impedance converting circuit may be configured such that the N-path filter has a desired frequency response, which may include a wide passband with steeper rejection than a conventional N-path filter with only a single pole in each filter path. Certain aspects of the present disclosure provide an N-path filter having a frequency response with multiple concurrent passbands. In certain aspects, the N-path filter with multiple passbands is implemented using an impedance converting circuit. In other aspects, the N-path filter with multiple passbands may include a bandpass response circuit where an inductance of the bandpass response circuit may be implemented using gyrators.

As used herein, the term “connected with” in the various tenses of the verb “connect” may mean that element A is directly connected to element B or that other elements may be connected between elements A and B (i.e., that element A is indirectly connected with element B). In the case of electrical components, the term “connected with” may also be used herein to mean that a wire, trace, or other electrically conductive material is used to electrically connect elements A and B (and any components electrically connected therebetween).

The techniques described herein may be used in combination with various wireless technologies such as Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiplexing (OFDM), Time Division Multiple Access (TDMA), Spatial Division Multiple Access (SDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), Time Division Synchronous Code Division Multiple Access (TD-SCDMA), and so on. Multiple user terminals can concurrently transmit/receive data via different (1) orthogonal code channels for CDMA, (2) time slots for TDMA, or (3) sub-bands for OFDM. A CDMA system may implement IS-2000, IS-95, IS-856, Wideband-CDMA (W-CDMA), or some other standards. An OFDM system may implement Institute of Electrical and Electronics Engineers (IEEE) 802.11, IEEE 802.16, Long Term Evolution (LTE) (e.g., in TDD and/or FDD modes), or some other standards. A TDMA system may implement Global System for Mobile Communications (GSM) or some other standards. These various standards are known in the art.

An Example Wireless System

FIG. 1illustrates a wireless communications system100with access points110and user terminals120, in which aspects of the present disclosure may be practiced. For simplicity, only one access point110is shown inFIG. 1. An access point (AP) is generally a fixed station that communicates with the user terminals and may also be referred to as a base station (BS), an evolved Node B (eNB), or some other terminology. A user terminal (UT) may be fixed or mobile and may also be referred to as a mobile station (MS), an access terminal, user equipment (UE), a station (STA), a client, a wireless device, or some other terminology. A user terminal may be a wireless device, such as a cellular phone, a personal digital assistant (PDA), a handheld device, a wireless modem, a laptop computer, a tablet, a personal computer, etc.

Access point110may communicate with one or more user terminals120at any given moment on the downlink and uplink. The downlink (i.e., forward link) is the communication link from the access point to the user terminals, and the uplink (i.e., reverse link) is the communication link from the user terminals to the access point. A user terminal may also communicate peer-to-peer with another user terminal. A system controller130couples to and provides coordination and control for the access points.

System100employs multiple transmit and multiple receive antennas for data transmission on the downlink and uplink. Access point110may be equipped with a number Napof antennas to achieve transmit diversity for downlink transmissions and/or receive diversity for uplink transmissions. A set Nuof selected user terminals120may receive downlink transmissions and transmit uplink transmissions. Each selected user terminal transmits user-specific data to and/or receives user-specific data from the access point. In general, each selected user terminal may be equipped with one or multiple antennas (i.e., Nut≥1). The Nuselected user terminals can have the same or different number of antennas.

Wireless system100may be a time division duplex (TDD) system or a frequency division duplex (FDD) system. For a TDD system, the downlink and uplink may share the same frequency band. For an FDD system, the downlink and uplink use different frequency bands. System100may also utilize a single carrier or multiple carriers for transmission. Each user terminal120may be equipped with a single antenna (e.g., in order to keep costs and/or size down) or multiple antennas (e.g., where the additional cost or size can be supported).

In certain aspects of the present disclosure, the access point110or user terminal120may include an N-path filter configured as a bandpass filter. Each branch of the N-path filter may include a switch connected in series with a second-order or higher impedance. For certain aspects, the impedance may be implemented with an impedance converter, as described below. For certain aspects, the bandpass filter may have multiple concurrent passbands.

On the uplink, at each user terminal120selected for uplink transmission, a TX data processor288receives traffic data from a data source286and control data from a controller280. TX data processor288processes (e.g., encodes, interleaves, and modulates) the traffic data {dup} for the user terminal based on the coding and modulation schemes associated with the rate selected for the user terminal and provides a data symbol stream {sup} for one of the Nut,mantennas. A transceiver front end (TX/RX)254(also known as a radio frequency front end (RFFE)) receives and processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) a respective symbol stream to generate an uplink signal. The transceiver front end254may also route the uplink signal to one of the Nut,mantennas for transmit diversity via an RF switch, for example. The controller280may control the routing within the transceiver front end254. Memory282may store data and program codes for the user terminal120and may interface with the controller280.

A number Nupof user terminals120may be scheduled for simultaneous transmission on the uplink. Each of these user terminals transmits its set of processed symbol streams on the uplink to the access point.

At access point110, Napantennas224athrough224apreceive the uplink signals from all Nupuser terminals transmitting on the uplink. For receive diversity, a transceiver front end222may select signals received from one of the antennas224for processing. The signals received from multiple antennas224may be combined for enhanced receive diversity. The access point's transceiver front end222also performs processing complementary to that performed by the user terminal's transceiver front end254and provides a recovered uplink data symbol stream. The recovered uplink data symbol stream is an estimate of a data symbol stream {sup} transmitted by a user terminal. An RX data processor242processes (e.g., demodulates, deinterleaves, and decodes) the recovered uplink data symbol stream in accordance with the rate used for that stream to obtain decoded data. The decoded data for each user terminal may be provided to a data sink244for storage and/or a controller230for further processing.

On the downlink, at access point110, a TX data processor210receives traffic data from a data source208for Ndnuser terminals scheduled for downlink transmission, control data from a controller230and possibly other data from a scheduler234. The various types of data may be sent on different transport channels. TX data processor210processes (e.g., encodes, interleaves, and modulates) the traffic data for each user terminal based on the rate selected for that user terminal. TX data processor210may provide a downlink data symbol streams for one of more of the Ndnuser terminals to be transmitted from one of the Napantennas. The transceiver front end222receives and processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) the symbol stream to generate a downlink signal. The transceiver front end222may also route the downlink signal to one or more of the Napantennas224for transmit diversity via an RF switch, for example. The controller230may control the routing within the transceiver front end222. Memory232may store data and program codes for the access point110and may interface with the controller230.

In certain aspects of the present disclosure, the transceiver front end222of access point110and/or the transceiver front end254of user terminal120may include an N-path filter configured as a bandpass filter. Each branch of the N-path filter may include a switch connected in series with a second-order or higher impedance. For certain aspects, the impedance may be implemented with an impedance converter, as described below. For certain aspects, the bandpass filter may have multiple concurrent passbands.

At each user terminal120, Nut,mantennas252receive the downlink signals from access point110. For receive diversity at the user terminal120, the transceiver front end254may select signals received from one of the antennas252for processing. The signals received from multiple antennas252may be combined for enhanced receive diversity. The user terminal's transceiver front end254also performs processing complementary to that performed by the access point's transceiver front end222and provides a recovered downlink data symbol stream. An RX data processor270processes (e.g., demodulates, deinterleaves, and decodes) the recovered downlink data symbol stream to obtain decoded data for the user terminal.

Those skilled in the art will recognize the techniques described herein may be generally applied in systems utilizing any type of multiple access schemes, such as TDMA, SDMA, Orthogonal Frequency Division Multiple Access (OFDMA), CDMA, SC-FDMA, TD-SCDMA, and combinations thereof.

FIG. 3is a block diagram of an example transceiver front end300, such as transceiver front ends222,254inFIG. 2, in which aspects of the present disclosure may be practiced. The transceiver front end300includes at least one transmit (TX) path302(also known as a transmit chain) for transmitting signals via one or more antennas and at least one receive (RX) path304(also known as a receive chain) for receiving signals via the antennas. When the TX path302and the RX path304share an antenna303, the paths may be connected with the antenna via an interface306, which may include any of various suitable RF devices, such as a duplexer, a switch, a diplexer, and the like.

Receiving in-phase (I) or quadrature (Q) baseband analog signals from a digital-to-analog converter (DAC)308, the TX path302may include a baseband filter (BBF)310, a mixer312, a driver amplifier (DA)314, and a power amplifier (PA)316. The BBF310, the mixer312, and the DA314may be included in a radio frequency integrated circuit (RFIC), while the PA316may be included in the RFIC or external to the RFIC. The BBF310filters the baseband signals received from the DAC308, and the mixer312mixes the filtered baseband signals with a transmit local oscillator (LO) signal to convert the baseband signal of interest to a different frequency (e.g., upconvert from baseband to RF). This frequency conversion process produces the sum and difference frequencies between the LO frequency and the frequencies of the baseband signal of interest. The sum and difference frequencies are referred to as the beat frequencies. The beat frequencies are typically in the RF range, such that the signals output by the mixer312are typically RF signals, which may be amplified by the DA314and/or by the PA316before transmission by the antenna303.

The RX path304may include a low noise amplifier (LNA)322, a mixer324, and a baseband filter (BBF)326. The LNA322, the mixer324, and the BBF326may be included in a radio frequency integrated circuit (RFIC), which may or may not be the same RFIC that includes the TX path components. RF signals received via the antenna303may be amplified by the LNA322, and the mixer324mixes the amplified RF signals with a receive local oscillator (LO) signal to convert the RF signal of interest to a different baseband frequency (i.e., downconvert). The baseband signals output by the mixer324may be filtered by the BBF326before being converted by an analog-to-digital converter (ADC)328to digital I or Q signals for digital signal processing.

In certain aspects of the present disclosure, the RX path304may include an N-path filter configured as a bandpass filter. Each branch of the N-path filter may include a switch connected in series with a second-order or higher impedance. For certain aspects, the impedance may be implemented with an impedance converter, as described below. For certain aspects, the bandpass filter may have multiple concurrent passbands.

While it is desirable for the output of an LO to remain stable in frequency, tuning to different frequencies indicates using a variable-frequency oscillator, which involves compromises between stability and tunability. Contemporary systems may employ frequency synthesizers with a voltage-controlled oscillator (VCO) to generate a stable, tunable LO with a particular tuning range. Thus, the transmit LO may be produced by a TX frequency synthesizer318, which may be buffered or amplified by amplifier320before being mixed with the baseband signals in the mixer312. Similarly, the receive LO may be produced by an RX frequency synthesizer330, which may be buffered or amplified by amplifier332before being mixed with the RF signals in the mixer324.

Example Bandpass N-Path Filter

Carrier aggregation (CA) is used in some radio access technologies (RATs), such as Long Term Evolution Advanced (LTE-A), in an effort to increase the bandwidth, and thereby increase bitrates. In carrier aggregation, multiple frequency resources (i.e., carriers) are allocated for sending data. Each aggregated carrier is referred to as a component carrier (CC). In LTE Rel-10, for example, up to five component carriers can be aggregated, leading to a maximum aggregated bandwidth of 100 MHz. The allocation of resources may be contiguous or non-contiguous. Non-contiguous allocation may be either intra-band (i.e., the component carriers belong to the same operating frequency band, but have one or more gaps in between) or inter-band, in which case the component carriers belong to different operating frequency bands. To implement CA in radio frequency front ends (RFFEs), various CA transceivers have been developed.

In contemporary CA architectures, transmit (TX) leakage is one of the main performance limitations during concurrent downlink CA operations. TX leakage is the leakage from a TX chain (e.g., TX path302) into a receive (RX) chain (e.g., RX path304) in a transceiver front end. Signals at the local oscillator (LO) frequency and multiples thereof (e.g., two and three times the LO frequency) coupling into the RX chain together with the TX leakage can be modulated and down-converted to the baseband (BB). Large voltage swings can saturate the BB output. This may be mitigated by decreasing the BB gain, although this may degrade the signal-to-noise ratio (SNR).

In non-CA applications, TX leakage or blockers and jammers are limiting factors to improving noise figure (NF) and linearity. Large blockers and TX leakage may prevent employing some of the architectures that are well-suited to achieving the increased NF.

Thus, circuits used to reject TX leakage or blockers to maintain sensitivity during concurrent CA or non-CA operation are important. However, the bandwidth of desired signals is becoming larger and larger (e.g., on the order of 80 MHz). Therefore, it is becoming increasingly challenging to implement a high quality factor (high-Q) bandpass filter at radio frequencies to reject out-of-band jammers and TX leakage while avoiding attenuation of desired signals in the RX band.

N-path filters may be used to provide high-Q bandpass filters at radio frequencies. An N-path filter may be composed of N identical linear time invariant (LTI) networks and 2N frequency mixers driven by time/phase-shifted versions of a clock signal. If the LTI networks exhibit a low-pass characteristic around DC, mixing by the mixers results in a bandpass filter response with a passband centered around the mixing frequency. That is, the input signal is down-converted to baseband, filtered by the LTI network, and then up-converted again to the original band of the input signal. The center frequency is determined by the mixing frequency and is insensitive to filter component values. A high mixing frequency combined with a narrow low-pass filter bandwidth provides a very high filter Q.

However, many N-path filters are implemented with a single pole (e.g., a resistor-capacitor (RC) load) in each branch. Due to the first-order nature of such N-path filters, these filters may not be able to provide sufficient out-of-band rejection for TX leakage and jammers when being used as a filter for a wideband signal.

FIG. 4Aillustrates an example receive chain (e.g., RX path304) of an RF front end comprising an N-path filter402, in accordance with certain aspects of the present disclosure. In certain aspects, the LNA322ofFIG. 3may comprise two separate LNAs322A and322B. For certain aspects, the first LNA322A may be external to an RF integrated circuit (RFIC), while the second LNA322B may be included in the RFIC, along with other circuits (e.g., the mixer324and the BBF326). For other aspects, the first LNA322A and the second LNA322B may both be included in the RFIC, along with other circuits (e.g., the mixer324and the BBF326). The N-path filter402may be connected with a node406between the LNAs322A and322B. In this manner, the N-path filter402may function as a shunt filter having frequency response410in an effort to pass signals in the desired RX band and reject signals having frequencies outside this band (including TX leakage and jammers).

The N-path filter402has a number N=4 of parallel branches selectively connected with the node406, which is a common node for the plurality of branches. Those having ordinary skill in the art of N-path filters will understand that there may be more or less than N=4 branches in any of the various aspects of the present disclosure provided herein. For ease of description and understanding by the reader, the remainder of the disclosure will present circuits with N=4 branches.

The N-path filter402may include a number of switches404(e.g., N switches, one in each filter branch), which may be implemented with n-channel metal oxide semiconductor (NMOS) transistors, individually labeled as transistors M1, M2, M3, and M4inFIG. 4A. For other aspects, the switches404in the N-path filter may be implemented with p-channel metal-oxide-semiconductor (PMOS) transistors. However, for ease of description and understanding by the reader, the remainder of the disclosure will use NMOS transistors to implement the branch switches404of the N-path filters.

The four transistors M1, M2, M3, and M4may be controlled using four 25% duty cycle signals P1, P2, P3, and P4, respectively, as illustrated in the timing diagram412ofFIG. 4B. In this manner, one switch404may be opened before or as the next switch in the control signal sequence is closed. That is, each of the transistors M1, M2, M3, and M4may be driven such that the transistors are activated in sequence and periods during which each transistor is activated (i.e., each switch404is closed) ideally do not overlap, although a small amount of overlap may be tolerated for practical implementations. The duty cycle of the control signals may be a function of the number N of filter branches (e.g., equal to 1/N). The amount of overlap, if any, in the control signals P1, P2, P3, etc. may be a small fraction (e.g., 1/10th) of the duty cycle.

Each switch404may connect a corresponding impedance ZA, ZB, ZC, or ZDwith the node406when closed. Impedances ZA, ZB, ZC, and ZDmay all have the same impedance value. One end of each impedance ZA, ZB, ZC, or ZDmay be connected with a corresponding switch404, and the other end of each impedance may be connected with a reference potential (e.g., electrical ground, a power supply voltage, or a bias voltage) for the N-path filter402.

In this configuration, the frequency response410of the N-path filter402may have a center frequency approximately equal to the switching frequency of the control signals P1, P2, P3, and P4for the transistors M1, M2, M3, and M4, respectively. For example, the switching frequency may be considered as the inverse of the period between rising edges of the control signal P1, shown by vertical dashed lines in timing diagram412. The control signals P1, P2, P3, and P4may have the same frequency (i.e., the switching frequency), but different phases. Moreover, the bandwidth of the frequency response410may be twice the bandwidth of a pole of the branch impedance (ZAZB, ZC, or ZD).

As described above, each of the impedances ZA, ZB, ZC, and ZDmay have only one pole (RC load), which may provide a narrow-band high-Q bandpass N-path filter. However, N-path filters with only one pole may not provide suitable out-of-band rejection when being used as a filter for a wideband signal. Certain aspects of the present disclosure provide an N-path filter, where each of the impedances ZAZB, ZC, and ZDis composed of a second-order or higher impedance to increase rejection. In certain aspects, the second-order (or higher) impedances for ZA, ZB, ZC, and ZDmay be implemented using a generalized impedance converter (GIC) circuit. An impedance converter is used to implement a particular impedance using, for example, one or more different types of impedances.

FIG. 5Aillustrates an example GIC circuit500that may be used to provide each of the branch impedances (e.g., impedances ZA, ZB, ZC, and ZD) in the N-path filter402, in accordance with certain aspects of the present disclosure. As illustrated, the GIC circuit500comprises impedances Z1, Z2, Z3, Z4, and Z5and operational amplifiers502and504connected with feedback loops506and508. Operational amplifier (op amp)502has a positive input, a negative input, and an output. The positive input of op amp502is connected with one terminal510of the GIC circuit500and impedance Z1. The negative input of op amp502is connected with a node514between impedance Z2and impedance Z3. The output of op amp502is connected via feedback loop506with a node516between impedance Z3and impedance Z4. Op amp504also has a positive input, a negative input, and an output. The positive input of op amp504is connected with a node518between impedance Z4and impedance Z5. The other side of impedance Z5is the other terminal512of the GIC circuit500and may be connected with a reference potential (e.g., electrical ground as shown) for the N-path filter402. The negative input of op amp504is connected with the node514between impedance Z2and impedance Z3. The output of op amp504is connected via feedback loop508with a node520between impedance Z1and impedance Z2.

The input impedance ZINof the GIC circuit500may be a function of all the impedances (Z1, Z2, Z3, Z4, and the Z5) of the GIC circuit500, and therefore, the order of the N-path filter402may be increased. For example, the impedance ZINof the GIC circuit500ofFIG. 5Amay be calculated using the following equation:

ZIN=Z1⁢Z3⁢Z5Z2⁢Z4
Each of the impedances Z1, Z2, Z3, Z4, and Z5can be any desired impedance and may be implemented using any combination of various suitable components, such as resistors, capacitors, and/or inductors.

FIG. 5Billustrates a differential implementation of an example GIC circuit530, in accordance with certain aspects of the present disclosure. In the GIC circuit530, impedance Z1is implemented with resistor R1connected in parallel with capacitor C1, impedance Z2is implemented using resistor R2, impedance Z3is implemented using resistor R3connected in parallel with capacitor C3, impedance Z4is implemented using resistor R4, and impedance Z5is implemented using resistor R5connected in parallel with capacitor C5. Therefore, the input impedance ZINof the GIC circuit530ofFIG. 5Bmay be calculated using the following equation:

The resistance and capacitance values of the resistors and capacitors in the GIC circuit530ofFIG. 5Bmay be selected to meet stability and frequency response goals. For example, the resistances R1, R3, and R5and the capacitances C1, C3, and C5may be selected to provide three real poles at a desired cutoff frequency of the N-path filter. In certain aspects, resistances R1and R5and capacitances C1and C5may be selected to provide real poles at a desired cutoff frequency. Resistance R3and capacitance C3may be selected such that a time constant based on R3and C3(e.g., product of R3and C3) may be at least ten times a time constant based on R1and C1. In this manner, resistance R3and capacitance C3may provide some help with TX swing (i.e., the amplitude of the undesired transmit or other blocker signal incident on the receive chain) inside the GIC circuit, but may not provide significant filtering.

Using two operational amplifiers in the GIC circuit500may result in stability issues in the different feedback loops506and508. To overcome these stability issues, the capacitance C3may be selected to be very small and the resistances R3and R4may be designed to be approximately equal. In certain aspects, the impedance Z3may not include a capacitor C3, as illustrated inFIG. 6. In this manner, the rejection of the N-path filter may be limited (e.g., to 26 dB overall) since one pole may be lost due to the removal (or reduction in capacitance) of capacitor C3. However, a zero may be added to the transfer function at a frequency of the TX leakage frequency to improve rejection, which may be accomplished by adding series resistance between node406and the GIC circuit500.

FIG. 6illustrates the differential input impedance (ZIN) looking into two filter branches whose impedances are implemented with GIC circuit630, in accordance with certain aspects of the present disclosure. Resistors RSWrepresent the on-resistance of the switches404. GIC circuit630is similar to GIC circuit530, where impedance Z3is implemented with a resistor R3, instead of with resistor R3connected in parallel with capacitor C3. A notch may be added to the frequency response of the N-path filter402by using the passive mixer switch resistance (RSW). For example, the control voltage at a gate of each of the transistors M1, M2, M3, and M4may be selected such that the on-resistance (RSW) of the transistors M1, M2, M3, and M4adds a zero to the frequency response of the N-path filter402at the TX leakage frequency. The series resistance RSWmay also be designed by proper choice of transistor width (W) and length (L), resulting in a smaller transistor area. By using the on-resistance of the transistors M1, M2, M3, and M4, a large series resistance RSWbetween node406and a respective GIC circuit may be obtained. The zero at baseband frequency may be added to the transfer function of the N-path filter402in accordance with the following equation:

Zero=s2⁢RSW⁢C1⁢C5⁢R2⁢R3R4+1
By adding resistance RSW, rejection of the N-path filter402may be improved. Moreover, a gate capacitance of each of the transistors M1, M2, M3, and M4may be reduced, which may result in lower power consumption for the N-path filter402.

FIG. 7illustrates the N-path filter402where each of the impedances ZA, ZB, ZC, and ZDis replaced with a GIC circuit530, in accordance with certain aspects of the present disclosure. Although the GIC circuit530is used, other impedance converter circuits may be used as alternatives for the impedances ZAZB, ZC, and ZD. In certain aspects, a capacitor CBBmay be coupled between sources of transistors M1and M3, and another capacitor CBBmay be coupled between sources of transistors M2and M4to increase rejection at offsets from an RX frequency larger than the duplex frequency. This may also help mitigate any peaking due to the zero introduced by resistance RSW.

With an N-path filter implemented with impedance converters as described above, coupled signals in the TX band can be rejected significantly (e.g., by at least 17 dB). Moreover, since the bandwidth of this N-path filter is wide enough, no attenuation may be added in-band (in the RX band).

FIG. 8is a graph800of amplitude versus frequency, illustrating multiple channels in an RX band801and a TX band804offset from the RX band, in accordance with aspects of the present disclosure. A filter having a frequency response802in the RX chain may be used to filter out TX signals in the TX band804coupling into the RX chain, while preventing degradation of signals in the RX band801. However, a filter with frequency response802may be a complex, high-order filter to have such a relatively wide bandwidth and level of rejection.

In intra-band non-contiguous CA operations, multiple CCs (e.g., two) may be received concurrently. Thus, a receiver may be configured to handle a wideband signal (by having a wideband transfer function) in order to receive the multiple CA signals. A single wideband filter (such as that having frequency response802) may be sufficient when there is no jammer in between the two non-contiguous CA signals. However, when a jammer is added in between the two non-contiguous CA signals, there can be a degradation in the receiver's performance since there is no selectivity between the two signals. Thus, certain aspects of the present disclosure provide an N-path filter having multiple concurrent passbands808and810. Such an N-path filter may have a frequency response806with dual passbands, one for each CA signal. The two passbands808,810of the frequency response806may be centered around the LO frequency (fLO) (i.e., the frequency of the control signals P1, P2, P3, and P4for the switches404).

FIG. 9illustrates a GIC circuit900that may be implemented in the N-path filter402, in accordance with certain aspects of the present disclosure. The GIC circuit900adds impedances Z6and Z7in the feedback loops508and506, respectively, to the GIC circuit500ofFIG. 5A. With the addition of impedances Z6and Z7, the impedance of the GIC circuit may realize a bandpass impedance response approximating the response of a parallel RLC (resistor, inductor, capacitor) circuit. Consequently, the frequency response of the N-path filter402may include separate, multiple bandpass center frequencies, similar to the frequency response806ofFIG. 8. Thus, all jammers and blockers in between and outside the two passbands (e.g., the passbands808,810of frequency response806) may be suppressed. Therefore, the receiver trans-impedance amplifier (TIA) (e.g., LNA322B) and baseband filter326may not suffer from large voltage swings, and thus, may not lower gain to recover linearity.

FIG. 10illustrates an N-path filter1000with a frequency response having multiple concurrent passbands, in accordance with certain aspects of the present disclosure. The N-path filter1000may include bandpass response circuits, which may be implemented with inductor-capacitor (LC) tank circuits1002A,1002B,1002C, and1002D (also referred to as “resonant circuits”), each coupled to one of the switches404(e.g., to a source of one of the transistors M1, M2, M3, and M4). Each of the tank circuits1002A,1002B,1002C, and1002D may include a respective capacitor1004A,1004B,1004C, and1004D (labeled “Cbb”) and a respective inductor Lp. The N-path filter1000is illustrated as being connected with a Thévenin equivalent circuit having an input signal (e.g., voltage source Vin) and input impedance (e.g., series resistance RS), which may represent the equivalent of the signal received by the antenna303and amplified by the first LNA322A, for example.

FIG. 11illustrates an N-path filter1100that is an equivalent circuit for the N-path filter1000ofFIG. 10, in accordance with certain aspects of the present disclosure. As illustrated, inductors1102A and1102B of the N-path filter1100have an inductance equal to twice the inductance of inductors Lp ofFIG. 10. Inductor1102A may be coupled between sources of transistors M2and M4, and inductor1102B may be coupled between sources of transistors M1and M3. The N-path filter1000ofFIG. 10, or the equivalent circuit thereof inFIG. 11, may provide the frequency response806having two passbands808,810.

FIG. 12illustrates an example frequency response1200of the N-path filter1000ofFIG. 10or the N-path filter1100ofFIG. 11, in accordance with certain aspects of the present disclosure. As illustrated, the frequency response1200includes two passbands1202,1204centered around the LO frequency (flo). The offset frequency fbbmay be calculated using the following equation:

fbb=12⁢π⁢Lp⁢Cbb
The rejection of the N-path filters may be calculated using the following equation:

Rej=RSWRS+RSW
where RSis the Thévenin equivalent series resistance and RSWis the switch resistance of one of the switches404(e.g., the on-resistance of one of the transistors M1, M2, M3, or M4) as described above. Moreover, the bandwidth (BW) of the frequency response1200may be calculated using the following equation:

As shown in this equation, the bandwidths of N-path filters1000and1100are not a function of the inductance Lp. Therefore, fbbcan be adjusted by changing the inductance Lpwithout changing the bandwidth of the frequency response1200. The quality factor of the inductor Lpmay be greater than the quality factor of the N-path filters1000and1100at the baseband frequency. The inductance Lpcan be large, and thus, the inductor Lpmay be synthesized using a gyrator circuit for certain aspects, as described in more detail below.

FIG. 13illustrates an example gyrator circuit1300used to simulate an inductor Lp, in accordance with certain aspects of the present disclosure. The gyrator circuit1300may include a feedforward gyrator1302and a feedback gyrator1304, coupled to capacitors Cbb2. As shown, one terminal1310of the gyrator circuit1300is connected with a negative input of the feedforward gyrator1302and a negative output of the feedback gyrator1304. The other terminal1312of the gyrator circuit1300is connected with a positive input of the feedforward gyrator1302and a positive output of the feedback gyrator1304. One side of one capacitor Cbb2is connected with a node1308, and the other side of this capacitor is connected with a reference potential for the N-path filter402, for example. Node1308is connected with a positive output of the feedforward gyrator1302and with a negative input of the feedback gyrator1304. One side of another capacitor Cbb2is connected with a node1306, and the other side of this capacitor is connected with a reference potential for the N-path filter402, for example. Node1306is connected with a negative output of the feedforward gyrator1302and with a positive input of the feedback gyrator1304.

The equivalent inductance of the gyrator circuit1300may be calculated using the following equation:

Lp=Cbb⁢⁢22×gmff×gmfb
where gmffis the transconductance of the feedforward gyrator1302, gmfbis the transconductance of the feedback gyrator1304, and Cbb2is the capacitance of capacitor Cbb2.

FIG. 14illustrates an example N-path filter1400incorporating the gyrator circuit1300ofFIG. 13, in accordance with certain aspects of the present disclosure. One gyrator circuit1300A is coupled between the sources of transistors M2and M4, and another gyrator circuit1300B is coupled between the sources of transistors M1and M3, in place of inductors1102A and1102B, respectively, ofFIG. 11. With the gyrator circuits1300A,1300B, the offset frequency fbbof the frequency response1200may be calculated using the following equation:

fbb=gmff⁢gmfbπ⁢Cbb⁢⁢2⁢Cbb
The bandwidth of the frequency response1200does not depend on the equivalent inductance of the gyrator circuits1300A,1300B and remains as follows:

BW⁡(Hz)=18⁢π⁡(RS+RSW)⁢Cbb
Thus, the bandwidth may be controlled by adjusting capacitance Cbb. The offset frequency fbbof the N-path filter1400may be adjusted independently by changing Cbb2, gmff, and/or gmfb. In certain aspects, one or more operational transconductance amplifiers (OTAs) of the gyrators1302,1304may be degenerated (e.g., using source degeneration techniques) to achieve improved linearity and noise performance.

In certain aspects, the NF of the N-path filter1400may be reduced while maintaining a constant offset frequency fbb. To reduce the NF, gmfbmay be reduced, and gmffmay be increased, by the same factor β. The NF may also be reduced by reducing capacitance Cbband increasing capacitance Cbb2by the same amount α.

By adding a filter having multiple concurrent passbands, the receiver may remain as a wideband receiver to save power and area while filtering signals caused by the jammers in between the CA signals. In case of a single CA signal, the N-path filter1400can converge into a single-CA N-path filter by turning off (i.e., powering down) or disconnecting the OTAs of the gyrators1302and1304. This technique may also relax constraints for wideband filters with large out-of-band rejection that may be desired for rejecting TX jammers.

FIG. 15illustrates an example gyrator circuit1500including gyrators1302and1304, in accordance with certain aspects of the present disclosure. The gyrator circuit1500includes a capacitor1502that is coupled in a differential fashion to save area. That is—instead of having separate capacitors Cbb2coupled to an output (e.g., node1308) of gyrator1302and to an input (e.g., node1306) of gyrator1304as illustrated inFIG. 13—a single capacitor1502may be coupled between the output and input of the gyrators1302and1304, respectively, in a differential fashion, as illustrated inFIG. 15. In this configuration, a single capacitor1502having a capacitance equal to half of capacitance Cbb2may be used, for example.

FIG. 16illustrates an N-path filter1600, in accordance with certain aspects of the present disclosure. As shown, a capacitor1602may replace capacitors1004A and1004C (Cbb), and another capacitor1604may replace capacitors1004B and1004D (Cbb). The capacitors1602and1604may be coupled in a differential fashion, as illustrated inFIG. 16, and may have half the capacitance of capacitors1004A and1004C, or capacitors1004B and1004D ofFIG. 11.

With aspects of the present disclosure, instead of selecting the entire RX band (e.g. 80-100 MHz of RF BW) using a wideband high-order filter, only channels on which CA signals are received may be selected. Other in-band and out-of-band jammers may be rejected (e.g., filtered out) with lower bandwidth and lower-order N-path filters. In this manner, the trade-offs between having a wider bandwidth or a steeper rejection in N-path filters for a wideband receiver may be relaxed.

Example Filtering Operations

FIG. 17is a flow diagram of example operations1700for filtering a signal, in accordance with certain aspects of the present disclosure. The operations1700may be performed by an N-path filter, such as those described herein. The signal may be, for example, the output of an amplifier, such as the first LNA322A.

The operations1700may begin, at block1702, by selectively connecting each of a plurality of branches of an N-path filter (e.g., N-path filter402) with a circuit node (e.g., node406) carrying the signal. Each branch of the N-path filter includes a switch (e.g., switch404) connected in series with an impedance (e.g., one of the impedances ZA, ZB, ZC, and ZD).

According to certain aspects, selectively connecting at block1702may involve connecting one of the plurality of branches of the N-path filter according to a control signal (e.g., one of signals P1, P2, P3, and P4). In this case, a switching frequency of the control signal may establish a center frequency of a bandwidth for the N-path filter.

According to certain aspects, selectively connecting at block1702entails connecting a first one of the plurality of branches of the N-path filter with the circuit node at block1704; disconnecting the first one of the plurality of branches of the N-path filter from the circuit node at block1706; and after the disconnecting at block1706, connecting a second one of the plurality of branches of the N-path filter with the circuit node at block1708. For certain aspects, a period between connecting and disconnecting the first one of the plurality of branches at blocks1704and1706(e.g., the pulse width of a control signal P1, P2, P3, or P4) is determined based on an inverse of a center frequency of a bandwidth for the N-path filter divided by a number of the plurality of branches.

According to certain aspects, the signal is an output of an amplifier (e.g., the first LNA322A) in a receive path (e.g., RX path304) of a transceiver (e.g., transceiver front end300). In this case, selectively connecting each of the plurality of branches of the N-path filter at block1702suppresses leakage from a transmit path (e.g., TX path302) of the transceiver.

According to certain aspects, the impedance is implemented with an impedance converter. For certain aspects, the impedance converter in each branch of the N-path filter includes one or more amplifiers and a plurality of passive components such that each branch of the N-path filter has at least two poles. For certain aspects, the impedance converter includes: (1) first, second, third, fourth, and fifth impedances (e.g., impedances Z1, Z2, Z3, Z4, and Z5) connected in series; (2) a first amplifier (e.g., op amp502) having a positive input, a negative input, and an output, wherein: the positive input of the first amplifier is connected with the first impedance; the negative input of the first amplifier is connected with a node (e.g., node514) between the second impedance and the third impedance; and the output of the first amplifier is connected with a node (e.g., node516) between the third impedance and the fourth impedance; and (3) a second amplifier (e.g., op amp504) having a positive input, a negative input, and an output, wherein: the positive input of the second amplifier is connected with a node (e.g., node518) between the fourth impedance and the fifth impedance; the negative input of the second amplifier is connected with the node between the second impedance and the third impedance; and the output of the second amplifier is connected with a node (e.g., node520) between the first impedance and the second impedance. In this case, the first impedance may be connected with the switch of the branch and/or the fifth impedance may be connected with a reference potential of the N-path filter. The first, third, and fifth impedances may have matching values. The first, third, and fifth impedances may each be implemented with a resistive element connected in parallel with a capacitive element. The second and fourth impedances may each be implemented with a resistor.

According to certain aspects, the bandpass filter has multiple concurrent passbands. For certain aspects, the impedance in each branch of the N-path filter includes an inductive element connected in parallel with a capacitive element. In this case, center frequencies of the multiple concurrent passbands are offset from the center frequency of the bandwidth for the N-path filter by an offset frequency, and the offset frequency is based on an inductance of the inductive element and a capacitance of the capacitive element. For certain aspects, the operations1700further entail adjusting the inductance of the inductive element to change the offset frequency.

According to certain aspects, the impedance in each branch of the N-path filter includes a bandpass response circuit. Each bandpass response circuit may include an inductive element (e.g., inductor Lp) connected in parallel with a first capacitive element (e.g., capacitor Cbb). For certain aspects, the inductive element includes a gyrator circuit (e.g., gyrator circuit1300), which may include a first gyrator (e.g., gyrator1302), a second gyrator (e.g., gyrator1304), a second capacitive element (e.g., capacitor Cbb2), and a third capacitive element (e.g., capacitor Cbb2). The gyrator circuit may further include a first node (e.g., terminal1310) and a second node (e.g., terminal1312); the first node may be coupled to a first port (e.g., negative input) of the first gyrator and to a first port (e.g., negative output) of the second gyrator; the second node may be coupled to a second port (e.g., positive input) of the first gyrator and to a second port (e.g., positive output) of the second gyrator; a first terminal of the second capacitive element may be coupled to a third port (e.g., positive output) of the first gyrator and to a third port (e.g., negative input) of the second gyrator; a first terminal of the third capacitive element may be coupled to a fourth port (e.g., negative output) of the first gyrator and to a fourth port (e.g., positive input) of the second gyrator; and/or a second terminal of the second capacitive element and a second terminal of the third capacitive element may be coupled to a reference potential of the N-path filter. For certain aspects, a first terminal of the inductive element is connected with a first terminal of the first capacitive element, and/or a second terminal of the inductive element and a second terminal of the first capacitive element are connected with a reference potential of the N-path filter.

According to certain aspects, the bandpass response circuits in each non-overlapping pair of the branches of the N-path filter are connected with the switches of the pair of branches (e.g., at least some of the components of the bandpass response circuits are shared between the pair of branches). As used herein, a non-overlapping pair of branches generally refers to a pair of branches in an N-path filter that do not share a branch with any other pair of branches. Thus, an N-path filter with four branches will have only two non-overlapping pairs of branches. For example, in the N-path filter1400ofFIG. 14, the branches comprising transistors M1and M3form one non-overlapping pair of branches, while the branches comprising transistors M2and M4for another non-overlapping pair of branches.

For certain aspects, the bandpass response circuits in each pair of branches include an inductive element having: (1) a first node connected with a first capacitive element and with a first switch of the pair of branches; and (2) a second node connected with a second capacitive element and with a second switch of the pair of branches. The inductive element may include a gyrator circuit, which may include a first gyrator, a second gyrator, a third capacitive element, and a fourth capacitive element. In this case, the first node may be coupled to a first port of the first gyrator and to a first port of the second gyrator; the second node may be coupled to a second port of the first gyrator and to a second port of the second gyrator; a first terminal of the third capacitive element may be coupled to a third port of the first gyrator and to a third port of the second gyrator; a first terminal of the fourth capacitive element may be coupled to a fourth port of the first gyrator and to a fourth port of the second gyrator; and/or a second terminal of the third capacitive element and a second terminal of the fourth capacitive element may be coupled to a reference potential of the N-path filter. For certain aspects, no two switches of the N-path filter are concurrently closed, and another switch in another pair of branches is closed between the first and second switches of the pair of branches being closed.

According to certain aspects, the multiple concurrent passbands are associated with different component carriers of an intra-band carrier aggregation scheme.

According to certain aspects, the N-path filter is configured to implement a concurrent dual-bandpass filter.

For example, means for transmitting may comprise a transmitter (e.g., the transceiver front end254of the user terminal120depicted inFIG. 2or the transceiver front end222of the access point110shown inFIG. 2) and/or an antenna (e.g., the antennas252mathrough252muof the user terminal120mportrayed inFIG. 2or the antennas224athrough224apof the access point110illustrated inFIG. 2). Means for receiving may comprise a receiver (e.g., the transceiver front end254of the user terminal120depicted inFIG. 2or the transceiver front end222of the access point110shown inFIG. 2) and/or an antenna (e.g., the antennas252mathrough252muof the user terminal120mportrayed inFIG. 2or the antennas224athrough224apof the access point110illustrated inFIG. 2). Means for processing or means for determining may comprise a processing system, which may include one or more processors, such as the RX data processor270, the TX data processor288, and/or the controller280of the user terminal120illustrated inFIG. 2or the RX data processor242, the TX data processor210, and/or the controller230of the access point110shown inFIG. 2.

Furthermore, means for carrying a signal may comprise a wire, trace, a circuit node (e.g., node406as illustrated inFIG. 4A), an optical fiber, or any other suitable electrical or optical conductor, which may be reflected in a schematic circuit diagram. Means for selectively connecting may comprise a switching arrangement (e.g., switches404depicted inFIG. 4A). Means for converting an impedance may comprise an impedance converting circuit (e.g., the GIC circuit500illustrated inFIG. 5). Means for providing a bandpass response may comprise an inductive element (e.g., inductor LpinFIG. 10or a gyrator circuit1300inFIG. 13) in parallel with a capacitive element (e.g., capacitor CbbinFIG. 10) or an impedance converting circuit, for example.