Adaptive frequency equalizer for wide modulation bandwidth envelope tracking

Abstract

An adaptive frequency equalizer for wide modulation bandwidth envelope tracking (ET) is provided. In this regard, an ET integrated circuit (ETIC) provides an ET power signal for one or more power amplifiers (PAs). A voltage error can occur in the ET power signal due to variable impedance sources, such as a variable load impedance at the PA and a variable trace inductance between the ETIC and the PA. The adaptive frequency equalizer disclosed herein works to adaptively correct for such voltage errors to provide improved overall power signal tracking at the PA, especially where there is a large trace inductance from the ETIC being located several centimeters (cm) away from the PA. Thus, embodiments of the adaptive frequency equalizer enhance ET performance for radio frequency (RF) systems having a modulation bandwidth of 100 megahertz (MHz) or above.

Claims

1. A radio frequency (RF) circuit, comprising: a power supply; an envelope tracking integrated circuit (ETIC) coupled to the power supply and configured to provide an envelope tracked power signal to a power amplifier (PA); and an adaptive frequency equalizer configured to inject an error correcting signal into the ETIC, the error correcting signal compensating for a voltage tracking error in the envelope tracked power signal due to a variable load impedance at the PA and a variable trace inductance between the ETIC and the PA; wherein the adaptive frequency equalizer comprises an impedance compensation circuit and a frequency filter which inject the error correcting signal as a function of a modulated target voltage signal which controls an envelope tracking (ET) function of the ETIC.

2. The RF circuit of claim 1, wherein: the adaptive frequency equalizer further comprises a PA resistance estimator circuit configured to estimate a combined impedance effect of the variable load impedance at the PA and the variable trace inductance; and the error correcting signal is adjusted based on an output of the PA resistance estimator circuit.

3. The RF circuit of claim 2, wherein the PA resistance estimator circuit estimates the combined impedance based on a current through the ETIC and a voltage representative of the envelope tracked power signal at the PA.

4. The RF circuit of claim 3, wherein the PA resistance estimator circuit is coupled to an input voltage of a tracking amplifier in the ETIC.

5. The RF circuit of claim 3, wherein the PA resistance estimator circuit is coupled to a scaled ideal voltage derived from the modulated target voltage signal.

6. The RF circuit of claim 2, wherein the adaptive frequency equalizer further comprises an equalizer settings correction circuit coupled to the PA resistance estimator circuit and is configured to update settings of a transfer function of the impedance compensation circuit based on the output of the PA resistance estimator circuit.

7. The RF circuit of claim 1, further comprising transmitter control circuitry configured to provide the modulated target voltage signal; wherein: the ETIC provides the envelope tracked power signal as a function of the modulated target voltage signal; and the adaptive frequency equalizer is coupled to the modulated target voltage signal and defines a transfer function to compensate for the voltage tracking error in the envelope tracked power signal.

8. The RF circuit of claim 7, wherein the transfer function is a function of changes in a PA resistance and a delay in the modulated target voltage signal through the ETIC.

9. The RF circuit of claim 7, further comprising a signal processing circuit configured to generate an RF amplifier input signal to be amplified by the PA using the envelope tracked power signal; wherein the signal processing circuit comprises an adaptive in-phase/quadrature (I/Q) memory digital pre-distortion (mDPD) circuit configured to compensate for I/Q errors in a transmission path through the PA.

10. The RF circuit of claim 9, wherein the adaptive I/Q mDPD circuit is adapted in conjunction with the adaptive frequency equalizer to correct the voltage tracking error in the envelope tracked power signal.

11. A method for correcting errors in wide modulation bandwidth envelope tracking (ET), the method comprising: receiving a modulated target voltage signal; providing an envelope tracked power signal using an ET integrated circuit (ETIC) for a power amplifier (PA) based on the modulated target voltage signal; generating an error correcting signal as a function of the modulated target voltage signal, a current through the ETIC, and a voltage representative of the envelope tracked power signal at the PA; and injecting the error correcting signal into the ETIC, the error correcting signal compensating for a voltage tracking error in the envelope tracked power signal due to a variable load impedance at the PA and a variable trace inductance between the ETIC and the PA.

12. The method of claim 11, wherein the error correcting signal is generated from a transfer function (H(s)) for a voltage of the envelope tracked power signal at the PA (V.sub.ccPA) as a function of the modulated target voltage signal (V.sub.ccideal).

13. The method of claim 12, wherein the transfer function compensates for a source impedance (Z.sub.source) for the PA as a function of changes in a PA resistance (R.sub.Icc) and a delay in the modulated target voltage signal (V.sub.ccideal) through the ETIC.

14. The method of claim 13, further comprising: estimating changes in the PA resistance by comparing the voltage representative of the envelope tracked power signal with a voltage derived from the current through the ETIC and a representation of the PA resistance; and adjusting the transfer function from the estimated changes in the PA resistance.

15. The method of claim 13, wherein the transfer function (H(s)) is calculated from: $\frac{V_{ccPA}}{V_{ccideal}}=H\left(s\right)*AAF\left(s\right)*\mathrm{tr}\mathrm{kamp}\left(s\right)*\frac{Z_{\mathrm{ltrace}}\text{/}{Z}_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}*Z_{\mathrm{zout}}}{Z}_{\mathrm{source}}-\frac{Z_{\mathrm{source}}}{R_{\mathrm{Icc}}}{e}^{-j\omega *delay}$ wherein AAF(s) is a transfer function of an anti-aliasing filter, trkamp(s) is a transfer function of a tracking amplifier, Z.sub.ltrace corresponds to the variable trace inductance, Z.sub.ETIC is an impedance of the ETIC, and Z.sub.zout is an output impedance of the ETIC.

16. The method of claim 11, further comprising generating the error correcting signal from a first transfer function (H.sub.1(s)) comprising a frequency filter and a second transfer function (H.sub.2(s)) which compensates for a source impedance (Z.sub.source) for the PA as a function of changes in a PA resistance (R.sub.Icc).

17. The method of claim 16, wherein the first transfer function (H.sub.1(s)) is set to achieve a desired frequency response of the envelope tracked power signal as (V.sub.ccPA) a function of the modulated target voltage signal (V.sub.ccideal).

18. The method of claim 17, wherein the first transfer function (H.sub.1(s)) and the second transfer function (H.sub.2(s)) are calculated from: $\frac{V_{ccPA}}{V_{\mathrm{ccideal}}}=H_1\left(s\right)*AAF\left(s\right)*\mathrm{trkamp}\left(s\right)*\frac{\frac{Z_{ltrace}}{Z_{\mathrm{ETIC}}}}{Z_{\mathrm{ltrace}}*Z_{zout}}{Z}_{source}-\frac{Z_{source}}{R_{\mathrm{Icc}}}{e}^{-j\omega *\mathrm{delay}}+H_2\left(s\right)\star \mathrm{AAF}\left(s\right)*\mathrm{trkamp}\left(s\right)*\frac{\frac{Z_{ltrace}}{Z_{\mathrm{ETIC}}}}{Z_{\mathrm{ltrace}}*Z_{zout}}{Z}_{source}-\frac{Z_{source}}{R_{\mathrm{Icc}}}{e}^{-j\omega *\mathrm{delay}}$ wherein AAF(s) is a transfer function of an anti-aliasing filter, trkamp(s) is a transfer function of a tracking amplifier, Z.sub.ltrace corresponds to the variable trace inductance, Z.sub.ETIC is an impedance of the ETIC, Z.sub.zout is an output impedance of the ETIC, and the delay is a delay in the modulated target voltage signal (V.sub.ccideal) through the ETIC.

19. A radio frequency (RF) circuit, comprising: a power supply; transmitter control circuitry configured to provide a modulated target voltage signal; an envelope tracking integrated circuit (ETIC) coupled to the power supply and configured to provide an envelope tracked power signal to a power amplifier (PA) as a function of the modulated target voltage signal; and an adaptive frequency equalizer coupled to the modulated target voltage signal and configured to inject an error correcting signal into the ETIC, the error correcting signal compensating for a voltage tracking error in the envelope tracked power signal due to a variable load impedance at the PA and a variable trace inductance between the ETIC and the PA; wherein the adaptive frequency equalizer defines a transfer function to compensate for the voltage tracking error in the envelope tracked power signal.

20. The RF circuit of claim 19, further comprising a signal processing circuit configured to generate an RF amplifier input signal to be amplified by the PA using the envelope tracked power signal; wherein: the signal processing circuit comprises an adaptive in-phase/quadrature (I/Q) memory digital pre-distortion (mDPD) circuit configured to compensate for I/Q errors in a transmission path through the PA; and the adaptive I/Q mDPD circuit is adapted in conjunction with the adaptive frequency equalizer to correct the voltage tracking error in the envelope tracked power signal.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

(2) FIG. 1 is a block schematic diagram of an exemplary radio frequency (RF) communications system.

(3) FIG. 2 is a schematic model of an exemplary envelope tracking (ET) system including an ET integrated circuit (ETIC) for the RF communications system of FIG. 1.

(4) FIG. 3 is a schematic model of the ET system of FIG. 2, modeled as a single input system.

(5) FIG. 4 is a schematic model of an exemplary embodiment of the ET system of FIG. 2 including a frequency equalizer for the ETIC.

(6) FIG. 5 is a schematic model of the ET system of FIG. 4 illustrated with combined impedances.

(7) FIG. 6 is a schematic model of the overall transfer function of the ET system of FIG. 4.

(8) FIG. 7 is a schematic model of the ET system of FIG. 4 illustrating a delay in the overall transfer function.

(9) FIG. 8 is a schematic model of another exemplary embodiment of the ET system of FIGS. 2 and 4 with an adaptive frequency equalizer using a tracking amplifier sense current and a tracking amplifier input voltage to update settings of the adaptive frequency equalizer.

(10) FIG. 9 is a schematic model of another exemplary embodiment of the ET system of FIGS. 2 and 4 with an adaptive frequency equalizer using the tracking amplifier sense current and a scaled ideal voltage to update settings of the adaptive frequency equalizer.

(11) FIG. 10 is a schematic model of an exemplary embodiment of a PA resistance estimator circuit and an equalizer settings correction circuit for the adaptive frequency equalizer of FIG. 9.

(12) FIG. 11 is a schematic model of another exemplary embodiment of the PA resistance estimator circuit and the equalizer settings correction circuit for the adaptive frequency equalizer of FIG. 9.

(13) FIG. 12 is a block schematic diagram of an embodiment of the RF transmitter circuitry of FIG. 1 with the ETIC having the adaptive frequency equalizer and the transmitter control circuitry having an adaptive in-phase/quadrature (I/Q) memory digital pre-distortion (mDPD) circuit.

(14) FIG. 13 is a block schematic diagram of another embodiment of the RF transmitter circuitry of FIG. 12 with a group delay assist circuit.

(15) FIG. 14 is a block schematic diagram of the adaptive frequency equalizer integrated into a displaced tracking amplifier (DTA).

DETAILED DESCRIPTION

(16) The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

(17) It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

(18) It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

(19) The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

(20) Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

(21) An adaptive frequency equalizer for wide modulation bandwidth envelope tracking (ET) is provided. In this regard, an ET integrated circuit (ETIC) provides an ET power signal for one or more power amplifiers (PAs). A voltage error can occur in the ET power signal due to variable impedance sources, such as a variable load impedance at the PA and a variable trace inductance between the ETIC and the PA. The adaptive frequency equalizer disclosed herein works to adaptively correct for such voltage errors to provide improved overall power signal tracking at the PA, especially where there is a large trace inductance from the ETIC being located several centimeters (cm) away from the PA. Thus, embodiments of the adaptive frequency equalizer enhance ET performance for radio frequency (RF) systems having modulation bandwidth of 100 megahertz (MHz) or above.

(22) FIG. 1 is a block schematic diagram of an exemplary RF communications system 10. The RF communications system 10 includes RF transmitter circuitry 12, RF system control circuitry 14, RF front-end circuitry 16, an RF antenna 18, and a direct current (DC) power source 20. The RF transmitter circuitry 12 includes transmitter control circuitry 22, an RF PA 24, an ETIC 26, and PA bias circuitry 28.

(23) In one embodiment of the RF communications system 10, the RF front-end circuitry 16 receives via the RF antenna 18, processes, and forwards an RF receive signal RFRX to the RF system control circuitry 14. The RF system control circuitry 14 provides a transmitter configuration signal TXCS and an RF input signal RFI to the transmitter control circuitry 22. Based on the RF input signal RFI and the transmitter configuration signal TXCS, the transmitter control circuitry 22 provides an RF amplifier input signal RFAI to the RF PA 24. The DC power source 20 provides a DC source signal VDC to the RF transmitter circuitry 12 (e.g., via transmitter control circuitry 22). In this manner, the DC power source 20 provides power to components of the RF transmitter circuitry 12, such as the transmitter control circuitry 22, the RF PA 24, the ETIC 26, and the PA bias circuitry 28. In one embodiment of the DC power source 20, the DC power source 20 is a battery.

(24) In an exemplary aspect, the transmitter control circuitry 22 includes an ET look-up table (LUT) for generating an envelope power supply control signal VRMP based on the RF input signal RFI and the transmitter configuration signal TXCS. The ET LUT may include storage elements (e.g., registers) for storing predetermined target voltages in association with amplitudes of the transmitter configuration signal TXCS. The ET LUT may further include an application-specific integrated circuit (ASIC) or another appropriate processor to generate the envelope power supply control signal VRMP (e.g., a modulated target voltage signal) for the ETIC 26, which may be a differential voltage signal.

(25) The ETIC 26 provides an envelope power supply signal EPS to the RF PA 24 based on the envelope power supply control signal VRMP. The envelope power supply control signal VRMP is representative of a set point of the envelope power supply signal EPS. The RF PA 24 receives and amplifies the RF amplifier input signal RFAI to provide an RF transmit signal RFTX using the envelope power supply signal EPS. The envelope power supply signal EPS provides power for amplification. The RF front-end circuitry 16 receives, processes, and transmits the RF transmit signal RFTX via the RF antenna 18. In one embodiment of the RF transmitter circuitry 12, the transmitter control circuitry 22 configures the RF transmitter circuitry 12 based on the transmitter configuration signal TXCS.

(26) The transmitter control circuitry 22 is coupled to the PA bias circuitry 28. The PA bias circuitry 28 provides a PA bias signal PAB to the RF PA 24. In this regard, the PA bias circuitry 28 biases the RF PA 24 via the PA bias signal PAB. In one embodiment of the PA bias circuitry 28, the PA bias circuitry 28 biases the RF PA 24 based on the transmitter configuration signal TXCS. In one embodiment of the RF front-end circuitry 16, the RF front-end circuitry 16 includes at least one RF switch, at least one RF amplifier, at least one RF filter, at least one RF duplexer, at least one RF diplexer, the like, or any combination thereof. In one embodiment of the RF system control circuitry 14, the RF system control circuitry 14 is RF transceiver circuitry, which may include an RF transceiver IC, baseband controller circuitry, the like, or any combination thereof.

(27) In one embodiment of the RF communications system 10, the RF communications system 10 communicates with other RF communications systems (not shown) using multiple communications slots, which may include transmit communications slots, receive communications slots, simultaneous receive and transmit communications slots, or any combination thereof. Such communications slots may utilize the RF transmit signal RFTX, the RF receive signal RFRX, other RF signals (not shown), or any combination thereof. In one embodiment of an RF communications slot, the RF communications slot is a time period during which RF transmissions, RF receptions, or both, may occur. Adjacent RF communications slots may be separated by slot boundaries, in which RF transmissions, RF receptions, or both, may be prohibited. As a result, during the slot boundaries, the RF communications system 10 may prepare for RF transmissions, RF receptions, or both.

(28) In an exemplary aspect, the RF transmitter circuitry 12 is configured to provide the RF transmit signal RFTX having a wide modulation bandwidth, such as a 100 MHz or greater bandwidth. Because of this wide modulation bandwidth, a voltage error can occur in the envelope power supply signal EPS due to variable impedance sources, as discussed further below with respect to FIGS. 2 and 3. Accordingly, the ETIC 26 can include a frequency equalizer to compensate for these impedance sources, as discussed further below with respect to FIGS. 4-7. In some embodiments, the frequency equalizer is adaptive to compensate for changes in the variable impedance sources, as discussed further below with respect to FIGS. 8-11. Additional embodiments may enhance the adaptive frequency equalizer, as discussed further below with respect to FIGS. 12-14.

(29) FIG. 2 is a schematic model of an exemplary ET system 30 including the ETIC 26 for the RF communications system 10 of FIG. 1. The ET system 30 is configured to couple to one or more PAs (e.g., the RF PA 24 of FIG. 1), and provides an envelope tracked power signal (e.g., power supply signal EPS of FIG. 1) having an ET modulated voltage V.sub.ccPA and a modulated PA load current I.sub.ccPA for the one or more PAs. The PA, in turn, amplifies an RF signal (e.g., RF amplifier input signal RFAI of FIG. 1) from an input power to an output power based on an ET modulated voltage V.sub.ccETIC generated by the ETIC 26.

(30) In an exemplary aspect, the ET system 30 is modeled as a two input system which outputs the ET modulated voltage V.sub.ccPA. The first input is a modulated target voltage signal V.sub.ccideal which is what drives the ETIC 26 (e.g., the envelope power supply control signal VRMP generated by the transmitter control circuitry 22 using the ET LUT). The second input is the modulated PA load current I.sub.ccPA. The ET modulated voltage V.sub.ccPA is therefore a response of the ET system 30 relative to these inputs.

(31) The modulated PA load current I.sub.ccPA creates a voltage error across the ET modulated voltage V.sub.ccPA based on the load modulation (modeled as an ETIC impedance Z.sub.ETIC) and the source impedance Z.sub.source presented to the PA. The ETIC impedance Z.sub.ETIC includes various impedance sources. For example, the modulated target voltage signal V.sub.ccideal can control a tracking amplifier 32 after being filtered, such as with an anti-aliasing filter 34 (with transfer function 1/(1+s/w.sub.aaf)) and/or a tracking amplifier bandwidth filter 36 (with transfer function 1/(1+s/w.sub.trkamp)) The tracking amplifier 32 can be modeled as a current source 38 (dependent on the filtered modulated target voltage signal V.sub.ccideal), a series resistance R.sub.szout between the current source 38 and a parallel inductance L.sub.zout, parallel resistance R.sub.pzout, and parallel capacitance C.sub.pzout, a shunt capacitance C.sub.szout, and a shunt resistance R.sub.s2zout. The ETIC impedance Z.sub.ETIC can include additional impedance sources, such as a shunt auxiliary capacitance C.sub.aux, a shunt pulldown capacitance C.sub.pulldown, and a shunt notch filter 40.

(32) The source impedance Z.sub.source includes various impedance sources, modeled as a series trace inductance L.sub.trace (e.g., inductance of a conductive trace between the ETIC 26 and the PA), a shunt resistance R.sub.p_pa (which represents an incremental resistance around root mean square (RMS) ET modulated voltage over current, V.sub.ccRMS/I.sub.ccRMS), and a shunt capacitance C.sub.PA. The modulated PA load current I.sub.ccPA is a function of the input power of the PA (i.e., I.sub.ccPA=f.sub.2(P.sub.in)). In addition, the modulated target voltage signal V.sub.ccideal is generated from the ET LUT (e.g., an isogain ET LUT, a low-slope ET LUT, or other appropriate ET LUT) using the envelope power supply control signal VRMP, which is a scaled digital input representing envelope input power P.sub.in (i.e., V.sub.ccideal=ƒ.sub.1(P.sub.in)) Thus, the modulated PA load current I.sub.ccPA can be expressed as I.sub.ccPA=ƒ.sub.2(ƒ.sub.1.sup.−1(V.sub.ccideal)) where ƒ.sub.1.sup.−1 is the inverse of function ƒ.sub.1. This combined function leads to I.sub.ccPA=ƒ(V.sub.ccideal). (in some examples, there may be multiple PAs, each of which has a corresponding source impedance Z.sub.source).

(33) FIG. 3 is a schematic model of the ET system 30 of FIG. 2, modeled as a single input system. The modulated PA load current I.sub.ccPA=ƒ(V.sub.ccideal) can generally be represented as a function of the modulated target voltage signal V.sub.ccideal, DC voltage V.sub.0, and PA resistance R.sub.Icc (representing a PA load-line equation) by I.sub.ccPA=(V.sub.ccideal−V.sub.0)/R.sub.Icc. Focusing on the non-DC components, the DC voltage V.sub.0 can be removed such that the modulated PA load current I.sub.ccPA represents alternating current (AC) input current components and the modulated target voltage signal V.sub.ccideal represents AC input voltage components in the model of FIG. 2.

(34) Under this approach, I.sub.ccPA˜=V.sub.ccideal(delay)/R.sub.Icc, where the delay represents a delay the modulated target voltage signal V.sub.ccideal will see within the signal path through the ET system 30. Thus the transfer function block diagram becomes a single input based system using only the modulated target voltage signal V.sub.ccideal as the input to the ET system 30. Because the PA resistance R.sub.Icc represents the PA load-line equation, it can change as a function of VSWR operation of the PA. The PA resistance R.sub.Icc can be estimated as discussed below with respect to FIGS. 8-11 and used to adjust settings of an adaptive frequency equalizer.

(35) FIG. 4 is a schematic model of an exemplary embodiment of the ET system 30 of FIG. 2 including a frequency equalizer 42 for the ETIC 26. The frequency equalizer 42 injects an error correcting signal into the ETIC 26 (e.g., as a function of the modulated target voltage signal V.sub.ccideal) to compensate for a voltage tracking error in the ET modulated voltage V.sub.ccPA provided to the PA. In some examples, the frequency equalizer 42 is implemented as an analog circuit which uses a single block to create a transfer function H(s) to achieve this error compensation (e.g., to inject the error correcting signal into the ETIC 26).

(36) In some examples, the frequency equalizer 42 is implemented as two blocks to create two transfer functions H.sub.1(s) and H.sub.2(s). A Z.sub.source/R.sub.Icc compensation circuit 44 uses a second transfer function H.sub.2(s) to compensate for the Z.sub.source(s)/R.sub.Icc effect (as a function of changes in the PA resistance R.sub.Icc) and a frequency filter 46 uses a first transfer function H.sub.1(s) to equalize the remaining transfer function.

(37) FIG. 5 is a schematic model of the ET system 30 of FIG. 4 illustrated with combined impedances. Mathematical calculations can be performed for the various transfer functions of the ET system 30, such as combining the effect of impedance sources in the ETIC impedance Z.sub.ETIC. The series resistance R.sub.szout and the parallel inductance L.sub.out, parallel resistance R.sub.pzout, and parallel capacitance C.sub.pzout of the tracking amplifier 32 can be combined as a tracking amplifier output impedance Z.sub.zout. The shunt capacitance C.sub.szout and shunt resistance R.sub.s2zout of the tracking amplifier 32 can be combined with the shunt auxiliary capacitance C.sub.aux, the shunt pulldown capacitance C.sub.pulldown, and the shunt notch filter 40 as a shunt impedance Z.sub.shunt.

(38) In addition, elements of the source impedance Z.sub.source can be combined. The series trace inductance L.sub.trace can be modeled as a trace impedance Z.sub.ltrace. The shunt resistance R.sub.p_pa and the shunt capacitance C.sub.PA can be combined as a PA load impedance Z.sub.loadPA. Each of these combined impedances (the tracking amplifier output impedance Z.sub.zout, the shunt impedance Z.sub.shunt, the trace impedance Z.sub.ltrace, and the PA load impedance Z.sub.loadPA) can include real components and/or or equivalent models.

(39) FIG. 6 is a schematic model of the overall transfer function of the ET system 30 of FIG. 4. The network and transfer function can be calculated based on the illustrated model, where an overall network impedance 48 is derived from the tracking amplifier output impedance Z.sub.zout, the shunt impedance Z.sub.shunt, the trace impedance Z.sub.ltrace, and the PA load impedance Z.sub.loadPA:

(40) $\mathrm{Network}\mathrm{Impedance}=\frac{Z_{\mathrm{ltrace}}/Z_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}*Z_{\mathrm{zout}}}{Z}_{\mathrm{source}}$
where the ETIC impedance

(41) $Z_{\mathrm{ETIC}}=\frac{Z_{shunt}*Z_{zout}}{Z_{\mathrm{shunt}}+Z_{zout}}$
and the source impedance

(42) $Z_{\mathrm{source}}=\frac{Z_{loadPA}*\left(Z_{ltrace}+Z_{\mathrm{ETIC}}\right)}{Z_{loadPA}+Z_{ltrace}+Z_{\mathrm{ETIC}})}.$

(43) The overall transfer function for the ET system 30, expressed as V.sub.ccPA/V.sub.ccideal, is calculated as:

(44) $\frac{V_{ccPA}}{V_{ccideal}}=H\left(s\right)*AAF\left(s\right)*\mathrm{trkamp}\left(s\right)\star \frac{Z_{\mathrm{ltrace}}\text{/}{Z}_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}*Z_{\mathrm{zout}}}{Z}_{\mathrm{source}}-\frac{Z_{source}}{R_{\mathrm{Icc}}}{e}^{-j\omega *\mathrm{delay}}$$\frac{V_{ccPA}}{V_{\mathrm{ccideal}}}=H_1\left(s\right)*AAF\left(s\right)*\mathrm{trkamp}\left(s\right)*\frac{Z_{ltrace}\text{/}{Z}_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}*Z_{zout}}{Z}_{source}-\frac{Z_{source}}{R_{\mathrm{Icc}}}{e}^{-j\omega *\mathrm{delay}}+H_2\left(s\right)\star \mathrm{AAF}\left(s\right)*\mathrm{trkamp}\left(s\right)*\frac{Z_{ltrace}\text{/}{Z}_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}*Z_{zout}}{Z}_{source}-\frac{Z_{source}}{R_{\mathrm{Icc}}}{e}^{-j\omega *\mathrm{delay}}$
where

(45) $Z_{\mathrm{ETIC}}=\frac{Z_{shunt}*Z_{zout}}{Z_{shunt}+Z_{zout}}\mathrm{and}{Z}_{\mathrm{source}}=\frac{Z_{loadPA}*\left(Z_{ltrace}+Z_{\mathrm{ETIC}}\right)}{Z_{loadPA}+Z_{ltrace}+Z_{\mathrm{ETIC}})}.$

(46) FIG. 7 is a schematic model of the ET system 30 of FIG. 4 illustrating a delay 50 in the overall transfer function. The delay term in the transfer function is the delay 50 on the ET path from the modulated target voltage signal V.sub.ccideal to the ET modulated voltage V.sub.ccPA without a modulated PA load current V.sub.ccPAwoIccPA, as shown in FIG. 7.

(47) In one embodiment, the second transfer function H.sub.2(s) is set such that the Z.sub.source/R.sub.Icc compensation circuit 44 cancels the effect of Z.sub.source(s)/R.sub.Icc:

(48) $\begin{array}{cc}{H}_2\left(s\right)*AAF\left(s\right)*\mathrm{trkamps}\left(s\right)*\frac{Z_{\mathrm{ltrace}}\text{/}{Z}_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}*Z_{\mathrm{zout}}}{Z}_{\mathrm{source}}=\frac{Z_{\mathrm{source}}}{R_{\mathrm{Icc}}}{e}^{-j\omega *delay}{H}_2\left(s\right)*AAF\left(s\right)*trkamp\left(s\right)*\frac{Z_{ltrace}\text{/}{Z}_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}\star Z_{\mathrm{zout}}}=\frac{1}{R_{\mathrm{Icc}}}{e}^{-j\omega *delay}& \mathrm{Equation}1\end{array}$
where the parameters of H.sub.2(s) are tuned via changes in the PA resistance R.sub.Icc.

(49) In addition, the first transfer function H.sub.1(s) is set to achieve an overall desired frequency response H.sub.desired:

(50) $\begin{array}{cc}{H}_1\left(s\right)*AAF\left(s\right)*\mathrm{trkamp}\left(s\right)\star \frac{Z_{ltrace}\text{/}{Z}_{\mathrm{ETIC}}}{Z_{\mathrm{ltrace}}\star Z_{\mathrm{zout}}}{Z}_{\mathrm{source}}=H_{\mathrm{desired}}& \mathrm{Equation}2\end{array}$
It should be noted that H.sub.1(s) is not a function of R.sub.Icc as a first approximation.

(51) FIG. 8 is a schematic model of another exemplary embodiment of the ET system 30 of FIGS. 2 and 4 with an adaptive frequency equalizer 42 using a tracking amplifier sense current I.sub.trkamp_sense and a tracking amplifier input voltage V.sub.cc_inputtrkamp to update settings of the frequency equalizer 42. In an exemplary aspect, the frequency equalizer 42 includes a PA resistance estimator circuit 52 which estimates the PA resistance R.sub.Icc (representing the PA load-line equation) or its inverse 1/R.sub.Icc from the tracking amplifier sense current I.sub.trkamp_sense and the tracking amplifier input voltage V.sub.cc_inputtrkamp.

(52) An equalizer settings correction circuit 54 maps the estimated PA resistance R.sub.Icc or its inverse 1/R.sub.Icc to different values of settings for the frequency equalizer 42 that can primarily affect the H.sub.2(s) response of the Z.sub.source/R.sub.Icc compensation circuit 44. The corrected settings for the frequency equalizer 42 eventually affects the H.sub.1(s) response of the frequency filter 46 to equalize the overall ET system 30 response V.sub.ccPA/V.sub.ccideal under varying PA resistance R.sub.Icc conditions.

(53) FIG. 9 is a schematic model of another exemplary embodiment of the ET system 30 of FIGS. 2 and 4 with an adaptive frequency equalizer 42 using the tracking amplifier sense current I.sub.trkamp_sense and a scaled ideal voltage Scaled_V.sub.ccideal to update settings of the frequency equalizer 42. The PA resistance estimator circuit 52 uses the tracking amplifier sense current I.sub.trkamp_sense and the scaled ideal voltage Scaled_V.sub.ccideal (e.g., derived from the modulated target voltage signal V.sub.ccideal) to estimate the PA resistance R.sub.Icc or any function of the PA resistance R.sub.Icc (e.g., 1/R.sub.Icc) that the PA has. This results in a highly accurate estimation of the PA resistance R.sub.Icc for very wide modulation bandwidth ET signals because most of the PA load current I.sub.ccPA is provided by the tracking amplifier bandwidth filter 36.

(54) FIG. 10 is a schematic model of an exemplary embodiment of the PA resistance estimator circuit 52 and the equalizer settings correction circuit 54 for the adaptive frequency equalizer 42 of FIG. 9. The PA resistance estimator circuit 52 can first estimate the AC component of the PA load current I.sub.ccPA by removing an estimated current due to capacitance to a ground reference C*dV/dt. The estimated PA load current I.sub.ccPA is then converted into a voltage by multiplying by a programmable resistor R.sub.Iccx. The slope of the scaled ideal voltage Scaled_V.sub.ccideal (with an estimated delay delay.sub.x) is subtracted from the slope of R.sub.Iccx*I.sub.ccPA, filtered through a low pass filter 56, and compared with a comparator 58. The value of the programmable resistor R.sub.Iccx is adjusted (e.g., digitally incremented or decremented) with an up/down counter 60 until the slopes match.

(55) The value of the programmable resistor R.sub.Iccx which results in this match is the estimated PA resistance R.sub.Icc and is passed to the equalizer settings correction circuit 54, which can use a digital mapping table 62 (e.g., an LUT implemented in an ASIC or other logic circuit) to provide different settings for the frequency equalizer 42. It should be understood that this embodiment is illustrative in nature, and other approaches can be used to estimate the slope between the estimated PA load current I.sub.ccPA and the scaled ideal voltage Scaled_V.sub.ccideal. Accordingly, other embodiments may implement the PA resistance estimator circuit 52 and the equalizer settings correction circuit 54 differently. In some examples, the functions of the of the PA resistance estimator circuit 52 and the equalizer settings correction circuit 54 can be combined in a single circuit or divided into additional sub-circuits.

(56) FIG. 11 is a schematic model of another exemplary embodiment of the PA resistance estimator circuit 52 and the equalizer settings correction circuit 54 for the adaptive frequency equalizer 42 of FIG. 9. In this example, the derivative and low-pass filter 56 of FIG. 10 are replaced by a single combination filter 64 that combines both functions. In other examples, additional functions of the PA resistance estimator circuit 52 can be combined into a single operational amplifier and with appropriate resistors and capacitors for biasing.

(57) In this manner, the adaptive frequency equalizer 42 described with respect to FIGS. 8-11 can use internal information in the ETIC 26 to detect changes in the PA resistance R.sub.Icc at the PA. The adaptive frequency equalizer 42 can further use this information to update its transfer function(s) to compensate for these changes relative to a nominal PA resistance R.sub.Iccnom value.

(58) Returning to FIG. 9, the transfer functions H.sub.1(s) and H.sub.2(s) and/or the single transfer function H(s) of the frequency equalizer 42 can be defined in a number of ways to be adaptive relative to the PA resistance R.sub.Icc and improve ET performance of the ET system 30. Several exemplary cases are described below, along with the parameters of the frequency equalizer 42 which can be adjusted using the PA resistance estimator circuit 52 and the equalizer settings correction circuit 54.

(59) Case 1) Predefined Complex poles frequency equalizer for H(s): The frequency equalizer 42 includes a single predefined equalizer that is based on 2nd order complex poles, defined by:

(60) $H\left(s\right)=\frac{1}{1+\frac{s}{Q_{eq}*{\omega }_{0eq}}-{\left(\frac{s}{{\omega }_{0eq}}\right)}^2}$
where Q.sub.eq is the quality (Q) factor and ω.sub.0eq is the natural pulsation frequency of the frequency equalizer 42. It should be understood that this transfer function can also be decomposed into two equivalent terms H.sub.1(s)+H.sub.2(s)=H(s) where H.sub.2(s) is set to a transfer function having a zero only transfer function with a scaling factor a.sub.xR.sub.Icc and H.sub.1(s)=H(s)−H.sub.2(s).

(61) In this case, the overall ET system 30 response V.sub.ccPA/V.sub.ccideal can be equalized by tuning the Q factor Q.sub.eq of the frequency equalizer 42 as a function of changes in the PA resistance R.sub.Icc. For example, the Q factor Q.sub.eq can be defined as a function of a new PA resistance R.sub.Icc* as follows:

(62) $Q_{eq}\left(R_{\mathrm{Icc}}^{*}\right)=Q_{e\mathrm{qnom}}\star \sqrt{\frac{R_{\mathrm{Iccnom}}}{R_{\mathrm{Icc}}^{*}}}$
where Q.sub.eqnom is a nominal Q factor and R.sub.Iccnom is a nominal PA resistance of the transfer function H(s) of the frequency equalizer 42.

(63) Case 2) Frequency equalizer using independent H.sub.1 and H.sub.2 equalizer: The frequency equalizer 42 includes two transfer functions H.sub.1 and H.sub.2 (e.g., for the Z.sub.source/R.sub.Icc compensation circuit 44 and the frequency filter 46, respectively) which are predefined independently resulting in an overall transfer function H(s) predefined equalizer response.

(64) The second transfer function H.sub.2(s) is based on a zero-transfer function with a scaling term a.sub.xR.sub.Icc that is scaled relative to 1/R.sub.Icc changes, where the scaling term a.sub.xR.sub.Icc is proportional to the nominal PA resistance over the new PA resistance

(65) 0$\frac{R_{\mathrm{Iccnom}}}{R_{\mathrm{Icc}}^{*}}.$

(66) $H_2\left(s\right)=a_x{R}_{\mathrm{Icc}}\left(1+\frac{s}{{\omega }_{\mathrm{zero}}}\right)$
where ω.sub.zero is a zero frequency of the transfer function.

(67) The first transfer function H.sub.1(s) is based on a second order complex zero transfer function and a real pole:

(68) $H_1\left(s\right)=\left(1+\frac{s}{Q_{eq}*{\omega }_{0eq}}\right)-\frac{{\left(\frac{s}{{\omega }_{0\mathrm{eq}}}\right)}^2}{1+\frac{S}{{\omega }_{0eq}}}$

(69) In this case, the overall ET system 30 response V.sub.ccPA/V.sub.ccideal can be equalized by scaling the coefficient term of the second transfer function H.sub.2(s). By adjusting the scaling term a.sub.xR.sub.Icc relative to changes in the PA resistance R.sub.Icc, while keeping the first transfer function H.sub.1(s) unchanged, the overall frequency versus PA load-line is tuned for wide modulation bandwidth signals as follows:

(70) $a_x{R}_{\mathrm{Icc}}\left(R_{\mathrm{Icc}}^{*}\right)=a_x{R}_{\mathrm{Iccn}om}\frac{R_{\mathrm{Iccn}om}}{R_{\mathrm{Icc}}^{*}}$

(71) Case 3) Frequency equalizer using real pole/real zero H(s) predefined equalizer: In this case, a predefined equalizer H(s) has a real pole and a real zero (from which H.sub.1(s) and H.sub.2(s) can be derived mathematically from H(s)). The transfer function H(s) is defined as:

(72) $H\left(s\right)=\frac{1+\frac{s}{{\omega }_{\mathrm{eqzero}}}}{1+\frac{S}{{\omega }_{eqpole}}}$
where ω.sub.eqzero is a zero frequency of the frequency equalizer 42 and ω.sub.eqpole is a pole frequency of the frequency equalizer 42.

(73) In this case, the overall ET system 30 response V.sub.ccPA/V.sub.ccideal can be equalized to compensate for PA load-line variation by scaling the frequency of the frequency equalizer 42 zero ω.sub.eqzero and scaling the frequency of the frequency equalizer 42 pole ω.sub.eqpole as a function of changes in the PA resistance R.sub.Icc as follows:

(74) ${\omega }_{\mathrm{eqzero}}\left(R_{\mathrm{Icc}}^{*}\right)={\omega }_{\mathrm{eqzeronom}}\frac{R_{\mathrm{Icc}}^{*}}{R_{\mathrm{Iccn}om}}{\omega }_{eqpole}\left(R_{\mathrm{Icc}}^{*}\right)={\omega }_{eqpolenom}\frac{R_{\mathrm{Icc}}^{*}}{R_{\mathrm{Iccn}om}}$
where ω.sub.eqzeronom is a nominal zero frequency of the frequency equalizer 42 and ω.sub.eqpolenom is a nominal pole frequency of the frequency equalizer 42.

(75) In summary, various analog frequency equalizers can be used in embodiments of the adaptive frequency equalizer 42. In some embodiments, only one or two parameters need to be tuned relative to changes in the PA resistance R.sub.Icc versus the nominal value, resulting in simple and effective adaptive tuning for very wide modulation bandwidth ET.

(76) FIG. 12 is a block schematic diagram of an embodiment of the RF transmitter circuitry 12 of FIG. 1 with the ETIC 26 having the adaptive frequency equalizer 42 and the transmitter control circuitry 22 having an adaptive in-phase/quadrature (I/Q) memory digital pre-distortion (mDPD) circuit 66. The modulated target voltage signal V.sub.ccideal is provided by the transmitter control circuitry 22 based on an ET LUT gain signal et_lut_gain (e.g., received as part of, or derived from, the transmitter configuration signal TXCS of FIG. 1) and the RF input signal RFI. In this regard, the transmitter control circuitry 22 includes a target voltage LUT 68, which in some examples is an ASIC and includes storage elements (e.g., registers) for storing predetermined digital target voltages in association with received digital amplitudes 70. The target voltage LUT 68 may also include processing elements (e.g., microprocessors) to generate a digital target voltage based on the digital amplitudes 70.

(77) The transmitter control circuitry 22 further includes a signal processing circuit 72 configured to generate the RF amplifier input signal RFAI based on the RF input signal RFI, which is a digital signal. In a non-limiting example, the RF input signal RFI, which can be a digital baseband signal, includes a digital in-phase (I) signal and a digital quadrature (Q) signal, corresponding to an in-phase amplitude I and a quadrature amplitude Q, respectively. The in-phase amplitude I and the quadrature amplitude Q collectively define a number of digital amplitudes √{square root over (I.sup.2+Q.sup.2)}. The transmitter control circuitry 22 may include a combiner 74, which combines the digital amplitudes √{square root over (I.sup.2+Q.sup.2)} with the ET LUT gain signal et_lut_gain (e.g., received as part of, or derived from the transmitter configuration signal TXCS of FIG. 1) to generate the digital amplitudes 70.

(78) The signal processing circuit 72 further includes the adaptive I/Q mDPD circuit 66, which is configured to perform mDPD on the I/Q components of the RF input signal RFI. The signal processing circuit 72 includes an in-phase digital-to-analog converter (DAC) 76I and a quadrature DAC 76Q that convert the digital in-phase signal and the digital quadrature signal into an analog in-phase signal and an analog quadrature signal, respectively. The signal processing circuit 72 may include an in-phase filter 78I and a quadrature filter 78Q for passing the analog in-phase signal and the analog quadrature signal in a desired frequency band, respectively. The signal processing circuit 72 may include an in-phase multiplexer 80I and a quadrature mixture 80Q configured to convert the analog in-phase signal and the analog quadrature signal to appropriate frequencies (e.g., carrier frequency or intermediate frequency). The in-phase multiplexer 80I and the quadrature mixture 80Q may be configured to operate based on a reference frequency provided by an oscillator 82. The signal processing circuit 72 includes a signal combiner 84 configured to combine the analog in-phase signal and the analog quadrature signal to generate the RF amplifier input signal RFAI.

(79) The transmitter circuitry 12 includes the ETIC 26, which uses the adaptive frequency equalizer 42 to compensate for the effect of load-line variations (e.g., PA resistance R.sub.Icc) on the ET modulated voltage V.sub.ccPA provided to the RF PA 24. In an exemplary aspect, the adaptive I/Q mDPD circuit 66 may further enhance performance of the transmitter circuitry 12 by compensating for I/Q errors created by the RF PA 24 and/or multiplexer filters 86 in the transmission path. The adaptive I/Q mDPD circuit 66 can receive an RF feedback signal RFF through a feedback circuit 88, which is coupled to the RF transmit signal RFTX through one or more bidirectional couplers 90. Using the RF feedback signal RFF, the adaptive I/Q mDPD circuit 66 works in conjunction with the adaptive frequency equalizer 42 to correct ET errors in the ET modulated voltage V.sub.ccPA.

(80) FIG. 13 is a block schematic diagram of another embodiment of the RF transmitter circuitry 12 of FIG. 12 with a group delay assist circuit 92. In this example, in addition to the PA resistance estimator circuit 52, the adaptive frequency equalizer 42 includes the group delay assist circuit 92, which uses the tracking amplifier sense current I.sub.trkamp_sense and the tracking amplifier input voltage V.sub.cc_inputtrkamp to estimate a group delay between and provide a mismatch correction between the ET modulated voltage V.sub.ccPA and the modulated PA load current I.sub.ccPA.

(81) It should be understood that the embodiments described and depicted herein are illustrative in nature, and improvements and modifications are considered within the scope of the concepts disclosed herein. For example, FIG. 14 is a block schematic diagram of the adaptive frequency equalizer 42 integrated into a displaced tracking amplifier (DTA) 94. In this example, the adaptive frequency equalizer 42 in the DTA 94 can track variations versus VSWR in addition to the group delay assist circuit 92.