Abstract:
Methods and apparatus, including computer program products, are provided for filtering. In some example embodiments, there is provided an apparatus including a first differential amplifier including a first positive input, a first negative input, and a first output, wherein the first positive input is connected to the first output via at least a first capacitor, and wherein the first negative input is connected to the first output via at least a first resistor; and a second differential amplifier including a second input, a third input, and a second output, wherein the second input is connected to the second output via at least a third resistor, wherein the third input is connected to the second output via at least a second capacitor, and wherein an input is connected to the first positive input and the second input via at least a third capacitor. Related apparatus, systems, methods, and articles are also described.

Description:
FIELD 
     The subject matter described herein relates to filtering. 
     BACKGROUND 
     Radio transmitters and/or radio receivers may implement filtering in order to co-exist with other users and systems occupying neighboring channels and frequency bands. Moreover, in some radios a plurality of band selection filters may be used for each receive band and/or transmission band. When devices support several bands, multiple filters may be used for each band. However, these filters may be implemented with bulky and costly technologies not integrated with the rest of radio. 
     Although some cellular standards currently support about 4 or more bands, future cellular standards may support additional bands (for example, up to and exceeding 40 bands). In the case of a transmitter, the corresponding filters may be configured to primarily transmit noise that is outside of the transmit band. There is, however, a strict requirement with cellular radios regarding how much noise a transmitter can radiate inside the transmit band, so tunable narrowband filters may comply with that requirement. In the case of a receiver, a filter may also be used for channel selection inside a receive frequency band. This filter may be located at baseband, so only one filter is needed in a radio. However, some radios may not have traditional baseband processing, so any noise filtering may be done at the radio frequency front end. For example, N-path filter or trans-impedance filter may be used to provide channel selection at radio frequencies rather than at baseband. These filters may introduce low impedance for the interfering signals and high impedance for the desired signal. When this kind of impedance load is driven from a high impedance source, the interfering signals are attenuated. 
     SUMMARY 
     Methods and apparatus, including computer program products, are provided for filtering using a higher-order load circuit. 
     In some example embodiments, there may be provided an apparatus. The apparatus may include a first differential amplifier including a first positive input, a first negative input, and a first output, wherein the first positive input is connected to the first output via at least a first capacitor, and wherein the first negative input is connected to the first output via at least a first resistor; and a second differential amplifier including a second input, a third input, and a second output, wherein the second input is connected to the second output via at least a third resistor, wherein the third input is connected to the second output via at least a second capacitor, and wherein an input is connected to the first positive input and the second input via at least a third capacitor. 
     In some variations, one or more of the features disclosed herein including the following features can optionally be included in any feasible combination. The apparatus may be coupled to at least one of an N-path filter and a trans-impedance filter. The apparatus may be configured to provide a short at one or more frequencies and a high impedance at one or more other frequencies. The one or more frequencies filtered out may be at least an unwanted signal. The second input may have a positive polarity and the third input may have a negative polarity. The second input may have a negative polarity, and the third input may have a positive polarity. The first negative input may be connected to the first output via at least the first resistor and to a ground via a second resistor. The input may be connected to the first positive input and the second input via the third capacitor and to the second output via a fifth resistor. The third input may be connected to the second output via the second capacitor and to the ground via a fourth resistor. 
     The above-noted aspects and features may be implemented in systems, apparatus, methods, and/or articles depending on the desired configuration. The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       In the drawings, 
         FIGS. 1 and 2  depict block diagrams of example implementations of receivers, in accordance with some example embodiments; 
         FIG. 3  depicts a block diagram of an example implementation of transmitter, in accordance with some example embodiments; 
         FIG. 4  depicts a block diagram of an example higher-order load circuit, in accordance with some example embodiments; 
         FIGS. 5 and 6  depict examples of spectrum plots for a higher-order load circuit, in accordance with some example embodiments; 
         FIG. 7  depicts a process flow for an example higher-order load circuit, in accordance with some example embodiments; and 
         FIG. 8  depicts an example of a user equipment, in accordance with some example embodiments. 
     
    
    
     Like labels are used to refer to same or similar items in the drawings. 
     DETAILED DESCRIPTION 
     In some example embodiments, there is provided a higher-order load circuit that can be used in a filter, such as an N-path filter or trans-impedance filter, to provide low impedance for the unwanted (for example, interfering) signal(s) and high impedance for the wanted signal(s). In some example embodiments, the filters including the higher-order load circuit may be tunable across a transmit band of interest or a receive band of interest. 
       FIG. 1  depicts an example of a receive chain  100  of a radio, in accordance with some example embodiments. The receive chain  100  may include an antenna  102 , a transconductance amplifier  104 , sinks  150 A-B for the unwanted signals, an output load  106  for the signal(s) of interest, passive mixers  108 A-B to downconvert the unwanted components of the output signal  107 A, and the rest of the receiver  190  (or another receiver). The mixers may be implemented using components, such as switches coupled to local oscillators  118 A-B. In some example embodiments, receive chain  100  may be implemented using differential signals. 
     In the example of  FIG. 1 , sinks  150 A-B may represent a short circuit at frequencies at or near those of the unwanted signal and an open circuit at or near the wanted signal frequencies. Signal may refer to a signal or a plurality of signals unless explicitly stated otherwise or clear from its context. Accordingly, the transconductance amplifier output signal current  107 A may be split between the load  106  and sinks  105 A-B. The high-impedance characteristics of the sinks  150 A-B at the frequencies of the desired signal allow the desired signals to travel to the load  106  while the unwanted signals travel to sinks  150 A-B. The resulting filtered signal  107 B can be further processed by the rest of the receiver  190 . In some example embodiments, the load  106  can be part of the input impedance of the receiver  190 . In some example embodiments, the sinks  150 A-B may be implemented as a higher-order load circuit, rather than a first order filter that provides a relatively slow transition from pass band to the stop band. The local oscillators (LO) represent clocks used to downconvert signals. 
       FIG. 2  depicts an example of a receive chain  200  of a radio, in accordance with some example embodiments. The receive chain  200  may include an antenna  202 , a transconductance amplifier  204 , sinks  250 A-B, output loads  206 A-B, and the rest of the receiver  290 . The receive chain  200  includes a first passive mixer  208 A and a second passive mixer  208 B to downconvert the RF signal into a lower frequency.  FIG. 2  is similar to  FIG. 1  in some respects, but  FIG. 1  depicts two parallel receivers, one for the wanted signal (for example, receiver  190 ) and one for the unwanted signal (for example, mixers  108 A-B and sinks  150 A-B). 
     In the example of  FIG. 2 , the sinks  250 A-B may be implemented in a manner similar to the sinks  150 A-B. For example, sinks  250 A-B may represent a short (for example, low impedance) at or near frequencies of the unwanted signal and an open circuit (for example, high impedance) at or near the frequencies of the wanted signal. As such, as signals travel from  207 A through the downconversion mixers  208 A-B, the sinks  250 A-B short to ground the unwanted signal while the wanted signals travel to the loads  206 A-B. The filtered signals at loads  207 B-C may be further processed by the rest of the receiver  290 . In some example embodiments, the loads  206 A-B may be part of the input impedance of the receiver  290 . In some example embodiments, the sinks  250 A-B may be implemented as higher-order load circuits, rather than a first order circuits that provides a relatively slow transition from pass band to the stop band. 
       FIG. 3  depicts an example of a transmit chain  300  of a radio, in accordance with some example embodiments. The transmit chain  300  may include a portion of the transmitter  390  providing for example the signal to be filtered  399 , a transconductance amplifier  304 , sinks  350 A-B, an output load  306 , and passive mixers  308 A-B, which provide downconversion based on local oscillators  391 A-B. 
     In the example of  FIG. 3 , the sinks  350 A-B may be implemented in a manner similar to the sinks  150 A-B and  250 A-B. As such, sinks  350 A-B represent a short at or near frequencies of the unwanted signal downconverted by mixers  308 A-B and high impedances at or near the frequencies of the wanted signal. The local oscillators (or clock signals)  391 A-B may be the same used by the rest of the transmitter to upconvert the wanted signal to RF or some other frequency. In the example of  FIG. 3 , the desired signals at the frequencies of interest pass to the output  307 B for transmission via for example an antenna and the like. In some example embodiments, the sinks  350 A-B may be implemented as higher-order load circuits, rather than first order circuits that provide a relatively slow transition from pass band to the stop band. 
     In some example embodiments, the subject matter disclosed herein may implement a higher-order load (or sink) circuit. In addition, the higher-order load (or sink) circuit may be configured to have high impedance at one or more frequencies for a signal of interest and act as a short at one or more other frequencies for an unwanted signal. Furthermore, N-path filters or trans-impedance filters may, in some example embodiments, include the higher-order sink circuit disclosed herein. 
     In some example embodiments, the N-path filters or trans-impedance filters including higher-order sink circuits may provide multiband filters and/or tunable multiband filters at radio frequencies. 
       FIG. 4  depicts an example of a higher-order load circuit  400 , in accordance with some example embodiments. The higher-order load circuit  400  (also referred to as a higher-order sink circuit) may include two high gain differential amplifiers  405 A-B. The two high gain differential amplifiers  405 A-B may have positive inputs  407 A-B coupled together. Specifically, positive inputs  407 A-B may be coupled to corresponding capacitors  409 A-B. 
     In some example embodiments, high gain differential amplifier  405 A may have capacitive feedback  409 A from the output  412 A to the positive input  407 A. In some example embodiments, high gain differential amplifier  405 B may have resistive feedback  416  from the output  412 B to the positive input  407 B. 
     High gain differential amplifier  405 A may have feedback from the output  412 A that provides half of the output voltage into the negative input port, while high gain differential amplifiers  405 B may have feedback from the output  412 B that provides the output signal to the negative input port at high frequencies and attenuates the low frequencies. Thus, high gain differential amplifiers  405 A may be considered to implement negative capacitance, and high gain differential amplifiers  405 B may be considered to implement negative inductance. 
     The input  490  may correspond to an input to, for example, a sink circuit, such as sink circuits  150 A-B,  250 A-B, and/or  350 A-B. For example, input  490  may provide a short (for example, substantially a short or a low impedance) at or near the unwanted frequencies and an open circuit (for example, a high impedance) to signals at or near the frequencies of interest, so that the signals at frequencies of interest pass to other portions of the receiver or transmitter. The input  490  may also include a resistive component  418  to the outputs of the amplifiers  405 A-B. 
     Although the transconductance amplifiers  405 A-B show a certain polarity at the inputs, the polarity may be changed. 
       FIG. 5  depicts an example frequency dependent input impedance  510 ,  520 ,  530  and  590  for higher-order load circuit  400 , in accordance with some example embodiments. A first order spectrum for a capacitive load circuit is also depicted at  599 .  FIG. 5  shows that at a certain frequency the higher-order load circuit may provide a short  590  and low impedance at frequencies  530  but high impedances at frequencies, such as  510 . When the certain frequency is selected or tuned to short the unwanted frequencies, then they are removed before being passed on to an output or another portion of the circuit. Because load circuit  400  is higher-order, the sharpness and roll off  520  associated with higher-order load circuit  400  is better, when compared to first-order circuit impedance  599 . 
     In some example embodiments, component values at  FIG. 4  may be adapted to change the frequency response to correspond any changes in the bandwidth of the wanted signal or any changes in the unwanted signals. 
       FIG. 6  depicts connecting two of the higher-order load circuits  400 A-B in parallel, in accordance with some example embodiments. For example, sink  350 A may be implemented as two sinks  350 A in parallel, each having different frequency responses resulting in more transmission zeros and sharper transition band as in  605  compared to the single load circuit  610 . The parallel higher-order load/sink circuits may also provide higher-order filters. Moreover, multiple pass bands (or shorts) may be implemented as well. 
       FIG. 7  depicts a process  700 , in accordance with some example embodiments. 
     At  710 , a signal may be connected at an input to a higher-order sink circuit. For example, sink circuits  150 A-B,  250 A-B, and/or  350 A-B may receive an input signal current. 
     At  720 , sinking, by the higher-order sink circuit, current at one or more frequencies representing unwanted signals, while allowing one or more other frequencies corresponding to signals of interest to pass and thus allowing them to be processed by other circuits. For example, sink circuits  150 A-B,  250 A-B, and/or  350 A-B may provide low impedance at one or more frequencies, such as frequencies shown at  590  ( FIG. 5) and 605  and  610  ( FIG. 6 ), so that these frequencies do not pass. However, sink circuits  150 A-B,  250 A-B, and/or  350 A-B may provide a high impedance at one or more other frequencies, so these other frequencies may pass the sink and proceed to other portions of a receiver or transmitter. 
       FIG. 8  illustrates a block diagram of an apparatus  10 , in accordance with some example embodiments. For example, apparatus  10  may comprise a user equipment, such as a smart phone, smart object, mobile station, a mobile unit, a subscriber station, a wireless terminal, a tablet, a wireless plug-in accessory, or any other wireless. 
     The apparatus  10  may include at least one antenna  12  in communication with a transmitter  14  and a receiver  16 . Alternatively transmit and receive antennas may be separate. 
     In some example embodiments, the transmitter  14  and/or receiver  16  may include one or more filters, such as N-path filters or transconductance filters, having the higher-order load or sink circuit  150 A-B,  250 A-B, and/or  350 A-B. 
     The apparatus  10  may also include a processor  20  configured to provide signals to and receive signals from the transmitter and receiver, respectively, and to control the functioning of the apparatus. Processor  20  may be configured to control the functioning of the transmitter and receiver by effecting control signaling via electrical leads to the transmitter and receiver. Likewise, processor  20  may be configured to control other elements of apparatus  10  by effecting control signaling via electrical leads connecting processor  20  to the other elements, such as a display or a memory. The processor  20  may, for example, be embodied in a variety of ways including circuitry, at least one processing core, one or more microprocessors with accompanying digital signal processor(s), one or more processor(s) without an accompanying digital signal processor, one or more coprocessors, one or more multi-core processors, one or more controllers, processing circuitry, one or more computers, various other processing elements including integrated circuits (for example, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), and/or the like), or some combination thereof. Accordingly, although illustrated in  FIG. 8  as a single processor, in some example embodiments the processor  20  may comprise a plurality of processors or processing cores. 
     Signals sent and received by the processor  20  may include signaling information in accordance with an air interface standard of an applicable cellular system, and/or any number of different wireline or wireless networking techniques, comprising but not limited to Wi-Fi, wireless local access network (WLAN) techniques, such as Institute of Electrical and Electronics Engineers (IEEE) 802.11, 802.16, and/or the like. In addition, these signals may include speech data, user generated data, user requested data, and/or the like. 
     The apparatus  10  may be capable of operating with one or more air interface standards, communication protocols, modulation types, access types, and/or the like. For example, the apparatus  10  and/or a cellular modem therein may be capable of operating in accordance with various first generation (1G) communication protocols, second generation (2G or 2.5G) communication protocols, third-generation (3G) communication protocols, fourth-generation (4G) communication protocols, Internet Protocol Multimedia Subsystem (IMS) communication protocols (for example, session initiation protocol (SIP) and/or the like. For example, the apparatus  10  may be capable of operating in accordance with 2G wireless communication protocols IS-136, Time Division Multiple Access TDMA, Global System for Mobile communications, GSM, IS-95, Code Division Multiple Access, CDMA, and/or the like. In addition, for example, the apparatus  10  may be capable of operating in accordance with 2.5G wireless communication protocols General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), and/or the like. Further, for example, the apparatus  10  may be capable of operating in accordance with 3G wireless communication protocols, such as Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (CDMA2000), Wideband Code Division Multiple Access (WCDMA), Time Division-Synchronous Code Division Multiple Access (TD-SCDMA), and/or the like. The apparatus  10  may be additionally capable of operating in accordance with 3.9G wireless communication protocols, such as Long Term Evolution (LTE), Evolved Universal Terrestrial Radio Access Network (E-UTRAN), and/or the like. Additionally, for example, the apparatus  10  may be capable of operating in accordance with 4G wireless communication protocols, such as LTE Advanced and/or the like as well as similar wireless communication protocols that may be subsequently developed. 
     It is understood that the processor  20  may include circuitry for implementing audio/video and logic functions of apparatus  10 . For example, the processor  20  may comprise a digital signal processor device, a microprocessor device, an analog-to-digital converter, a digital-to-analog converter, and/or the like. Control and signal processing functions of the apparatus  10  may be allocated between these devices according to their respective capabilities. The processor  20  may additionally comprise an internal voice coder (VC)  20   a , an internal data modem (DM)  20   b , and/or the like. Further, the processor  20  may include functionality to operate one or more software programs, which may be stored in memory. In general, processor  20  and stored software instructions may be configured to cause apparatus  10  to perform actions. For example, processor  20  may be capable of operating a connectivity program, such as a web browser. The connectivity program may allow the apparatus  10  to transmit and receive web content, such as location-based content, according to a protocol, such as wireless application protocol, WAP, hypertext transfer protocol, HTTP, and/or the like. 
     Apparatus  10  may also comprise a user interface including, for example, an earphone or speaker  24 , a ringer  22 , a microphone  26 , a display  28 , a user input interface, and/or the like, which may be operationally coupled to the processor  20 . The display  28  may, as noted above, include a touch sensitive display, where a user may touch and/or gesture to make selections, enter values, and/or the like. The processor  20  may also include user interface circuitry configured to control at least some functions of one or more elements of the user interface, such as the speaker  24 , the ringer  22 , the microphone  26 , the display  28 , and/or the like. The processor  20  and/or user interface circuitry comprising the processor  20  may be configured to control one or more functions of one or more elements of the user interface through computer program instructions, for example, software and/or firmware, stored on a memory accessible to the processor  20 , for example, volatile memory  40 , non-volatile memory  42 , and/or the like. The apparatus  10  may include a battery for powering various circuits related to the mobile terminal, for example, a circuit to provide mechanical vibration as a detectable output. The user input interface may comprise devices allowing the apparatus  20  to receive data, such as a keypad  30  (which can be a virtual keyboard presented on display  28  or an externally coupled keyboard) and/or other input devices. 
     As shown in  FIG. 8 , apparatus  10  may also include one or more mechanisms for sharing and/or obtaining data. For example, the apparatus  10  may include a short-range radio frequency (RF) transceiver and/or interrogator  64 , so data may be shared with and/or obtained from electronic devices in accordance with RF techniques. The apparatus  10  may include other short-range transceivers, such as an infrared (IR) transceiver  66 , a Bluetooth (BT) transceiver  68  operating using Bluetooth wireless technology, a wireless universal serial bus (USB) transceiver  70 , a Bluetooth Low Energy transceiver, a ZigBee transceiver, an ANT transceiver, a cellular device-to-device transceiver, a wireless local area link transceiver, and/or any other short-range radio technology. Apparatus  10  and, in particular, the short-range transceiver may be capable of transmitting data to and/or receiving data from electronic devices within the proximity of the apparatus, such as within 10 meters, for example. The apparatus  10  including the WiFi or wireless local area networking modem may also be capable of transmitting and/or receiving data from electronic devices according to various wireless networking techniques, including 6LoWpan, Wi-Fi, Wi-Fi low power, WLAN techniques such as IEEE 802.11 techniques, IEEE 802.15 techniques, IEEE 802.16 techniques, and/or the like. 
     The apparatus  10  may comprise memory, such as a subscriber identity module (SIM)  38 , a removable user identity module (R-UIM), an eUICC, an UICC, and/or the like, which may store information elements related to a mobile subscriber. In addition to the SIM, the apparatus  10  may include other removable and/or fixed memory. The apparatus  10  may include volatile memory  40  and/or non-volatile memory  42 . For example, volatile memory  40  may include Random Access Memory (RAM) including dynamic and/or static RAM, on-chip or off-chip cache memory, and/or the like. Non-volatile memory  42 , which may be embedded and/or removable, may include, for example, read-only memory, flash memory, magnetic storage devices, for example, hard disks, floppy disk drives, magnetic tape, optical disc drives and/or media, non-volatile random access memory (NVRAM), and/or the like. Like volatile memory  40 , non-volatile memory  42  may include a cache area for temporary storage of data. At least part of the volatile and/or non-volatile memory may be embedded in processor  20 . The memories may store one or more software programs, instructions, pieces of information, data, and/or the like which may be used by the apparatus for performing functions of the user equipment/mobile terminal. The memories may comprise an identifier, such as an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus  10 . The functions may include one or more of the operations disclosed with respect to the higher-order load/sink circuits including process  700  and the like. The memories may comprise an identifier, such as an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus  10 . In the example embodiment, the processor  20  may be configured using computer code stored at memory  40  and/or  42  to operations disclosed herein with respect to process  700  and the like. 
     Some of the embodiments disclosed herein may be implemented in software, hardware, application logic, or a combination of software, hardware, and application logic. The software, application logic, and/or hardware may reside on memory  40 , the control apparatus  20 , or electronic components, for example. In some example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a “computer-readable medium” may be any non-transitory media that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer or data processor circuitry, with examples depicted at  FIG. 8 , computer-readable medium may comprise a non-transitory computer-readable storage medium that may be any media that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer. 
     Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is providing filters with sharper filters with nulls in a frequency response and providing multiple bands. 
     If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined. Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. It is also noted herein that while the above describes example embodiments, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications that may be made without departing from the scope of the present invention as defined in the appended claims. Other embodiments may be within the scope of the following claims. The term “based on” includes “based on at least.” The use of the phase “such as” means “such as for example” unless otherwise indicated.