What is generally perceived as a very important parameter in circuit design is a device's transconductance, “gm”. Transconductance (short for “transfer conductance”, and also infrequently called “mutual conductance”) is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciprocal of resistance. Calculating gm is important when conducting AC (alternating current) analysis of a transistor circuit. “gm*Vbe” represents the gain of the AC signal at a transistor after it undergoes amplification. Gain can also be represented as “gm*Vgs” in the case of metal oxide semiconductor (MOS) transistors.
Quadrature signals, also called IQ signals, IQ data or IQ samples, are often used in radio frequency (RF) applications. They form the basis of complex RF signal modulation and demodulation, both in hardware and in software, as well as in complex signal analysis. Polyphase/image rejection (IR) filters are widely used in wireless receivers when I and Q signals are required. They generally operate with polyphase/image reject mixers.
Various implementation methods are available for on-chip filters, including active RC (resistor-capacitor), active RLC (resistor-inductor-capacitor), MOSFET-C (metal-oxide-semiconductor field-effect transistor-capacitance), gm-C and switched capacitor. Using a low noise transconductance, gm-C implementation is believed to have the best power consumption versus noise trade-off and presents good high frequency performance. Using active RC filtering has been popular for high linearity applications (e.g., WI-FI 802.11). RC filters also provide much better Image Rejection (IR), in the range of 15-50 dB depending on the application and design requirements. Coupling components are typically provided in the form of resistors for typical RC filters.
Gm-C filters are good for low linearity, low noise and high-speed applications. They take less area and consume less power than other filtering components. Component mismatch is common, however, because coupling components that turn low pass filters to polyphase (Band Pass) filters are of the gm variety (therefore “gmcomponents”), which are active components instead of active RC (resistor-capacitor) polyphase filters which use passive components. This can be a problem in that coupling gm components tend to achieve less IR compare to active RC polyphase filters in range of 7-25 dB (i.e., lower IR). This problem is common in Gm-C filtering circuitry because different types of, or values for, gm cells are used.
It is a feature of the embodiments to simplify the polyphase gm-C filter by using only one type of gm cell (i.e., similar cells) for its active components by matching their gm value. Using gm cells with a matching gmu value can avoid component mismatch and diminished lower IR performance that current results in currently available gm-C filters.
It is a feature of the present embodiments to simplify the design of the gm-C filter by using gm cell components matched by gmu value, and essentially improving performance by using matching gm components.
It is another feature of the embodiments to provide a method for providing a polyphase gm-C filter using gm cells that include an integer multiplication of one type of gm cell gmu, achieving a matching gmu value.
It is another feature of the embodiments to provide a method for operating a filter, including steps of receiving signals into a polyphase gm-C filter comprising gm cell components matched by gmu value, and filtering the signals through the polyphase gm-C filter.
It is also a feature of the embodiments that a matching gmu value can be implemented on prefabricated gm components of a traditional gm-C filter with prior different gm values to cause all the prefabricated gm components of a traditional gm-C filter to match in gmu value.
It is a feature of the embodiments that gm cell components of the polyphase gm-C filter are all matched by incorporating one gmu value into each of the gm components incorporated in the polyphase gm-C filter.
It is a feature of the embodiments that a matching gmu value can be obtained for each of the gm components that can be incorporated in the polyphase gm-C filter by the process of: calculating coupling of gmi, gmij by gmi=Ciω0 and gmij=Czijω0 for i,j; calculating Ki=gmi/gmu; rounding Ki to an integer number, Ni=round(Ki), KiNi and Nij=round(Kij), KijNij; calculating a scaling factor for circuit capacitors by Δi=(Ni−Ki)/Ki and Δij=(Nij−Kij)/Kij; and the adjusting capacitors Ci and Czij by CiCi*(1+Δi) and CzijCzij*(1+Δij).
It is also a feature of the embodiments that the process is completed for i,j and the result can be implemented in gm cell components to match the gmu value of all the gm cell components.
It is also a feature of the embodiments that the process is completed for i,j and the result is implemented in gm cell components to match the gmu value of the gm cell components and the matching gmcomponent are incorporated on an integrated circuit.
It is also a feature of the embodiments that the process is completed for i,j and the result is implemented on prefabricated gmcomponents of a traditional gm-C filter with prior different gm values to cause all the prefabricated gmcomponents of a traditional gm-C filter to match in gmu value.
It is another feature of the embodiments that the improvement of using similar gm cells for gm-C filters can correct problems encountered with low range Image Rejection (IR). This can be accomplished because most of the polyphase performance drop typically caused by gm cell mismatching that can result by coupling diverse gm cells across a filtering process, voltage and temperature (PVT) can be eliminated. It is a feature of the embodiments to make coupling gm cells an “integer” multiplication of the core gm cell, gmu, which can be possible where all AC parameters of filter are proportional to gm/C, and all the gm cells can then have the same gmu value. Use of one type of gm cell translates into minimum mismatch between cells over circuit PVT.
It is also a feature of the embodiments that the improved gm-C design can be applied to any gm-C filter to accomplish filter normalization. The improvement can be used for any application that may use polyphase gm-C filtering, including use in RF receivers utilizing standards such as GPS (global position system), WIFI (Wireless fidelity), CDMA (code division multiple access), and Bluetooth™.
These and other aspects in accordance with embodiments will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the embodiments.
It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the Figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the embodiments is, therefore, indicated by the appended claims rather than by this detailed description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.
Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.
Reference throughout this specification to “one embodiment”, “an embodiment”, or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment”, “in an embodiment”, and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
Low noise, moderate linearity and low power are the reasons to choose the gm-C method for the implemented filter in accordance with the disclosed embodiments. Referring to
Referring to
To make a polyphase (i.e., image reject) filter, a frequency transformation is needed.
I=(jωC)(V1−V2)−(jω0C)(V1−V2) (1)
In a quadrature system, V1, V2, jV1 and jV2 are available. This function can easily be implemented in hardware as shown by complex function 322 by adding two coupling transconductors, 323, 324, whose inputs are connected to the quadrature nodes (i.e., one fully differential gm cell in fully differential implementation).
Referring to
Referring to
gm1=C1*ω0,gm12=Cz*ω0 (2)
To maintain easier implementation and good matching among the transconductance stages of the improved filter, all gm cell components 520 can be implemented using integer multiples of a unit transconductance stage, gmu. Value for each gm cell component 520 to match can be obtained via the determination of a unit gmu value for each of the gm cell components 520. With the main low pass filter already using gmu, the gm cell components 520 need to be scaled to match the same gmu. This means that the gm cell components 520 should be configured in the form of:
gm1=C1*ω0=K1gmu, gm12=Cz*ω0=K12gmu (3)
where K1 and K12 are not necessarily an integer number. K can be rounded to the closer integer number, which implies some adjustment on the values of the circuit capacitors connected in the circuit to the node V1. C1 and/or Cz may need to be adjusted slightly to make K an integer number. This adjustment is similar to a post layout adjustment of circuit capacitors for a high frequency gm-C filter that keeps the AC response of the overall filter unchanged, while the middle nodes can be slightly changed.
Referring again to formulas (2) and (3), coupling gm values of nodes 1 to 5 of a polyphase filter provided in accordance with features of the embodiment can be calculated as follows:
gm1=ω0*C1=K1gmu (4)
gm2=ω0*C2=K2gmu (5)
g3=ω0*C3=K3gmu (6)
g4=ω0*C4=K4gmu (7)
g5=ω0*C5=K5gmu (8)
g13=ω0*Cz13=K13gmu (9)
g35=ω0*Cz35=K35gmu (10)
The described method (e.g., equation 1) for achieving optimum coupling transconductors can be employed to change coupling gm values of gm cell components to multiples of a unit transconductance, gmu, where K1, K2, K3, K4, K5, K13 and K35 can be replaced with integer numbers, N1, N2, N3, N4, N5, N13 and N35. Furthermore, C1, C2, C3 (and Cz with extra percussion) can be adjusted iteratively to compensate for the rounding coupling gm multiplication factors.
Referring to the flowchart of
Referring to
Equal gm-C filter parameters, (which can be the filter that has been shown in
A frequency shift of fc=4.1 MHz (megahertz), defined as ω0=2*π*fc which is good for an IF frequency of 4.1 MHz, is shown being applied. This can achieve gmi as shown in the following table. Ki (Kij) is being defined as gmi/gmu (gmij/gmu). Ni (Nij) can be derived from Ki (Kij) (i=1, 2, 3, 4, 5, ij=13, 35). A summary of numbers that can be obtained from the disclosed process is listed in the following table:
The embodiments simplify polyphase gm-C filter design because only one type of gm cell adapted by the process described herein needs to be used. The improved design can make frequency shifting modular for variable IF frequencies, if needed. The improved design can save area in the filter circuit by rounding coupling gm cells to an integer factor of a gm unit, and thereby achieving a matching gmu value. The improved design can also reduce the number of the coupling gm cells, which can translate into less parasitic effect and which can be important for MHz range applications while being able to use small size gm cells. The improved design provides less complexity in circuitry due to using “one type of gm cell”, gm cells with a matching gmu value, for the whole polyphase filter. The improved design can achieve better image rejection and less layout effort when incorporated on an integrated circuit.
Referring to
Referring to
Referring to
In accordance with the embodiments, a device and methods have be disclosed that can provide a polyphase gm-C filter using one type of gm cell that includes gm cell components having a matching gmu value, for a polyphase gm-C filter. Gm cells components of the polyphase gm-C filter can all be matched by incorporating a matching gmu value obtained for each of the gm components incorporated in a polyphase gm-C filter. The gm value can be obtained via a number value for each of the gm components which can be determined by: calculating coupling of gmi, gmij by gmi=Ciω0 and gmij=Czijω0 for i,j; calculating Ki=gmi/gmu; rounding Ki to an integer number, Ni=round(Ki), KiNi and Nij=round(Kij), KijNij; calculating a scaling factor for circuit capacitors by Δi=(Ni−Ki)/Ki and Δij=(Nij−Kij)/Kij; and adjusting capacitors Ci and Czij by CiCi*(1+Δi) and CzijCzij*(1+Δij). Once the process is completed for i,j, the result can be implemented in a preexisting (e.g., a traditional gm-C filter) or new polyphase filter to match its gm components with a matching gmu value.
It should be appreciated that a matching gmu value can also be implemented in prefabricated gm cell components of a traditional gm-C filter with prior different gm values to cause all the prefabricated gm components of a traditional gm-C filter to match. It should be appreciated that the improvement can be applied to any gm-C filter to make all gm cells an integer multiplication of one gmu. It should also be appreciated that embodiments of the invention can be implemented entirely in hardware or in an implementation containing both hardware and software elements. In embodiments that use software, the software may include but is not limited to firmware, resident software, microcode, etc.
Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.
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