WO2014018861A1

WO2014018861A1 - Programmable rf notch filter for envelope tracking

Info

Publication number: WO2014018861A1
Application number: PCT/US2013/052277
Authority: WO
Inventors: Nadim Khlat
Original assignee: Rf Micro Devices, Inc.
Priority date: 2012-07-26
Filing date: 2013-07-26
Publication date: 2014-01-30
Also published as: WO2014018861A4; US9020451B2; CN104662792A; US20140028368A1; CN104662792B

Abstract

A parallel amplifier (14), a switching supply (12), and a radio frequency (RF) notch filter (18) are disclosed. The parallel amplifier has a parallel amplifier output, such that the switching supply is coupled to the parallel amplifier output. Further, the RF notch filter is coupled between the parallel amplifier output and a ground. The RF notch filter has a selectable notch frequency, which is based on an RF duplex frequency.

Description

PROGRAMMABLE RF NOTCH FILTER FOR ENVELOPE TRACKING

Related Applications

[0001] This application claims the benefit of U.S. provisional patent

application number 61 /675,898, filed July 26, 2012, the disclosure of which is incorporated herein by reference in its entirety.

Field of the Disclosure

[0002] The present disclosure relates to direct current (DC)-DC converters and circuits that use DC-DC converters.

Background

[0003] DC-DC converters often include switching power supplies, which may be based on switching at least one end of an energy storage element, such as an inductor, between a source of DC voltage and a ground. As a result, an output voltage from a DC-DC converter may have a ripple voltage resulting from the switching associated with the energy storage element. Typically, the ripple voltage is undesirable and is minimized as much as sizes and costs permit.

Thus, there is a need to minimize ripple voltage using techniques that minimize sizes and costs.

Summary

[0004] A parallel amplifier, a switching supply, and a radio frequency (RF) notch filter are disclosed. The parallel amplifier has a parallel amplifier output, such that the switching supply is coupled to the parallel amplifier output. Further, the RF notch filter is coupled between the parallel amplifier output and a ground. The RF notch filter has a selectable notch frequency, which is based on an RF duplex frequency.

[0005] In one embodiment of the present disclosure, the parallel amplifier and the switching supply combine to provide a first envelope power supply signal to an RF power amplifier (PA), such that the first envelope power supply signal at least partially envelope tracks an RF transmit signal, which is provided by the RF PA. In one embodiment of the RF notch filter, the RF notch filter reduces noise introduced by the first envelope power supply signal into RF receive circuitry. As such, the selectable notch frequency is selected to target specific receive frequencies.

[0006] Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings. Brief Description of the Drawings

[0007] The accompanying drawings 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.

[0008] Figure 1 shows a direct current (DC)-DC converter according to one embodiment of the present disclosure.

[0009] Figure 2 shows the DC-DC converter according to an alternate embodiment of the DC-DC converter.

[0010] Figure 3 is a graph illustrating a frequency response of a radio frequency (RF) notch filter illustrated in Figure 1 according to an additional embodiment of the DC-DC converter.

[0011] Figure 4 shows the DC-DC converter according to another

embodiment of the DC-DC converter.

[0012] Figure 5 shows the DC-DC converter according to a further

embodiment of the DC-DC converter.

[0013] Figure 6 shows a radio frequency (RF) communications system according to one embodiment of the present disclosure.

[0014] Figure 7 shows the RF communications system according to an alternate embodiment of the RF communications system.

[0015] Figure 8 shows the RF communications system according to an additional embodiment of the RF communications system. [0016] Figure 9 shows the RF communications system according to another embodiment of the RF communications system.

[0017] Figure 10 shows the DC-DC converter according to one embodiment of the DC-DC converter.

[0018] Figure 1 1 shows the DC-DC converter according to an alternate embodiment of the DC-DC converter.

[0019] Figure 12 shows the RF communications system according to one embodiment of the RF communications system.

[0020] Figure 13 shows details of the DC-DC converter illustrated in Figure 12 according to one embodiment of the DC-DC converter.

[0021] Figure 14 shows details of an RF notch filter used in the DC-DC converter illustrated in Figure 13 according to one embodiment of the RF notch filter.

[0022] Figure 15 shows the details of the RF notch filter used in the DC-DC converter illustrated in Figure 13 according to an alternate embodiment of the RF notch filter.

[0023] Figure 1 6 shows the details of the RF notch filter used in the DC-DC converter illustrated in Figure 13 according to an additional embodiment of the RF notch filter.

[0024] Figure 17 is a graph illustrating frequency behavior of the RF communications system illustrated in Figure 12 according to one embodiment of the RF communications system.

Detailed Description

[0025] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, 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. [0026] Figure 1 shows a direct current (DC)-DC converter 10 according to one embodiment of the present disclosure. The DC-DC converter 10 includes a switching supply 12, a parallel amplifier 14, and a radio frequency (RF) notch filter 18. The switching supply 12 includes switching circuitry 1 6 and a first inductive element L1 . The parallel amplifier 14 has a feedback input FBI and a parallel amplifier output PAO. In general, the switching supply 12 is coupled to the parallel amplifier output PAO. The switching circuitry 1 6 has a switching circuitry output SCO. The first inductive element L1 is coupled between the switching circuitry output SCO and the feedback input FBI. The RF notch filter 18 is coupled between the parallel amplifier output PAO and a ground. In one embodiment of the DC-DC converter 10, the parallel amplifier output PAO is directly coupled to the feedback input FBI, as shown.

[0027] In one embodiment of the DC-DC converter 10, the parallel amplifier 14 partially provides a first power supply output signal PS1 via the parallel amplifier output PAO based on a voltage setpoint. The switching supply 12 partially provides the first power supply output signal PS1 via the first inductive element L1 . The switching supply 12 may provide power more efficiently than the parallel amplifier 14. However, the parallel amplifier 14 may provide a voltage of the first power supply output signal PS1 more accurately than the switching supply 12. As such, in one embodiment of the DC-DC converter 10, the parallel amplifier 14 regulates the voltage, called a first voltage V1 , of the first power supply output signal PS1 based on the voltage setpoint of the first power supply output signal PS1 . Further, the switching supply 12 regulates the first power supply output signal PS1 to minimize an output current, called a parallel amplifier output current IP, from the parallel amplifier 14 to maximize efficiency. In this regard, the parallel amplifier 14 behaves like a voltage source and the switching supply 12 behaves like a current source. Additionally, the switching circuitry 1 6 provides a switching output voltage VS and an inductor current IL to the first inductive element L1 via the switching circuitry output SCO.

[0028] In one embodiment of the DC-DC converter 10, the DC-DC converter 10 receives a DC source signal VDC, such that the parallel amplifier 14 partially provides the first power supply output signal PS1 using the DC source signal VDC and the switching supply 12 partially provides the first power supply output signal PS1 using the DC source signal VDC.

[0029] Figure 2 shows the DC-DC converter 10 according to an alternate embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in Figure 2 is similar to the DC-DC converter 10 illustrated in Figure 1 , except the DC-DC converter 10 illustrated in Figure 2 further includes power supply control circuitry 20 and the switching supply 12 further includes a filter capacitive element CF. The filter capacitive element CF is coupled between the parallel amplifier output PAO and the ground. As such, the filter capacitive element CF may significantly reduce unwanted noise, ripple, or both from the first power supply output signal PS1 . The power supply control circuitry 20 receives the DC source signal VDC and is coupled to the parallel amplifier 14 and the switching circuitry 1 6.

[0030] Figure 3 is a graph illustrating a frequency response 22 of the RF notch filter 18 (Figure 1 ) illustrated in Figure 1 according to an additional embodiment of the DC-DC converter 10. The RF notch filter 18 (Figure 1 ) has the frequency response 22 with an RF notch 24 at an RF notch frequency RNF. Therefore, the RF notch filter 18 (Figure 1 ) filters the first power supply output signal PS1 (Figure 1 ) based on the frequency response 22. As such, the RF notch filter 18 (Figure 1 ) may significantly reduce unwanted noise, ripple, or both from the first power supply output signal PS1 (Figure 1 ) at the RF notch frequency RNF. In one embodiment of the RF notch filter 18 (Figure 1 ), the RF notch filter 18 (Figure 1 ) is a programmable RF notch filter, such that the RF notch frequency RNF is a selectable notch frequency. In an alternate

embodiment of the RF notch filter 18 (Figure 1 ), the RF notch filter 18 (Figure 1 ) is a fixed RF notch filter, such that the RF notch frequency RNF is not a selectable notch frequency.

[0031] In a first embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 10 megahertz. In a second embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 20 megahertz. In a third embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 30 megahertz. In a fourth embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 40 megahertz. In a fifth embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 50 megahertz.

[0032] Figure 4 shows the DC-DC converter 10 according to another embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in Figure 4 is similar to the DC-DC converter 10 illustrated in Figure 2, except the DC-DC converter 10 illustrated in Figure 4 further includes an offset capacitive element CO coupled between the parallel amplifier output PAO and the feedback input FBI. As such, the switching supply 12 is coupled to the parallel amplifier output PAO via the offset capacitive element CO and the RF notch filter 18 is coupled to the parallel amplifier output PAO via the offset capacitive element CO. Additionally, the RF notch filter 18 includes a notch filter capacitive element CT and a notch filter inductive element LT coupled in series. The notch filter capacitive element CT and the notch filter inductive element LT form a resonant circuit having a resonant frequency. The RF notch frequency RNF (Figure 3) is based on the resonant frequency. A shape of the frequency response 22 (Figure 3) near the RF notch frequency RNF (Figure 3) may be based on a Q factor of the resonant circuit.

[0033] The parallel amplifier 14 partially provides the first power supply output signal PS1 via the parallel amplifier output PAO and the offset capacitive element CO based on the voltage setpoint. The offset capacitive element CO allows the first voltage V1 to be higher than a voltage at the parallel amplifier output PAO. As a result, the parallel amplifier 14 may properly regulate the first voltage V1 even if the first voltage V1 is greater than a maximum output voltage from the parallel amplifier 14 at the parallel amplifier output PAO. In the embodiment of the DC-DC converter 10 illustrated in Figure 4, the filter capacitive element CF is coupled between the parallel amplifier output PAO and the ground through the offset capacitive element CO. In an alternate embodiment of the DC-DC converter 10, the offset capacitive element CO is omitted. [0034] Figure 5 shows the DC-DC converter 10 according to a further embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in Figure 5 is similar to the DC-DC converter 10 illustrated in Figure 4, except in the DC-DC converter 10 illustrated in Figure 5, the RF notch filter 18 includes the notch filter capacitive element CT, the notch filter inductive element LT, and a notch filter resistive element RT coupled in series. The notch filter capacitive element CT, the notch filter inductive element LT, and the notch filter resistive element RT form a resonant circuit having a resonant frequency. The RF notch frequency RNF (Figure 3) is based on the resonant frequency. A shape of the frequency response 22 (Figure 3) near the RF notch frequency RNF (Figure 3) may be based on a Q factor of the resonant circuit.

[0035] Figure 6 shows a radio frequency (RF) communications system 26 according to one embodiment of the present disclosure. The RF

communications system 26 includes RF transmitter circuitry 28, RF system control circuitry 30, RF front-end circuitry 32, an RF antenna 34, and a DC power source 36. The RF transmitter circuitry 28 includes transmitter control circuitry 38, an RF power amplifier (PA) 40, the DC-DC converter 10, and PA bias circuitry 42. The DC-DC converter 10 functions as an envelope tracking power supply. In an alternate embodiment of the RF communications system 26, the DC power source 36 is external to the RF communications system 26.

[0036] In one embodiment of the RF communications system 26, the RF front- end circuitry 32 receives via the RF antenna 34, processes, and forwards an RF receive signal RFR to the RF system control circuitry 30. In one embodiment of the RF communications system 26, the RF receive signal RFR has an RF receive frequency. Further, the RF notch frequency RNF (Figure 3) is about equal to the RF receive frequency, which may reduce noise, ripple, or both in the receive path from the transmit path or other noise sources. The RF system control circuitry 30 provides a power supply control signal VRMP and a

transmitter configuration signal PACS to the transmitter control circuitry 38. The RF system control circuitry 30 provides an RF input signal RFI to the RF PA 40. The DC power source 36 provides a DC source signal VDC to the DC-DC converter 10. In one embodiment of the DC power source 36, the DC power source 36 is a battery. In one embodiment of the power supply control signal VRMP, the power supply control signal VRMP is an envelope power supply control signal. Specifically, the DC power source 36 provides the DC source signal VDC to the parallel amplifier 14 (Figure 1 ) and to the switching supply 12 (Figure 1 ).

[0037] The transmitter control circuitry 38 is coupled to the DC-DC converter 10 and to the PA bias circuitry 42. The DC-DC converter 10 provides the first power supply output signal PS1 to the RF PA 40 based on the power supply control signal VRMP. In this regard, the DC-DC converter 10 is an envelope tracking power supply and the first power supply output signal PS1 is a first envelope power supply signal EPS. The DC source signal VDC provides power to the DC-DC converter 10. As such, the first power supply output signal PS1 , which is the first envelope power supply signal EPS, is based on the DC source signal VDC. The power supply control signal VRMP is representative of a voltage setpoint of the first envelope power supply signal EPS. The RF PA 40 receives and amplifies the RF input signal RFI to provide an RF transmit signal RFT using the first envelope power supply signal EPS. The first envelope power supply signal EPS provides power for amplification to the RF PA 40.

[0038] In one embodiment of the DC-DC converter 10, the first envelope power supply signal EPS is amplitude modulated to at least partially provide envelope tracking. In one embodiment of the RF PA 40, the RF PA 40 operates with approximately constant gain, called isogain, and with gain compression. In a first embodiment of the gain compression, the gain compression is greater than about one decibel. In a second embodiment of the gain compression, the gain compression is greater than about two decibels. In a third embodiment of the gain compression, the gain compression is equal to about two decibels. In a fourth embodiment of the gain compression, the gain compression is equal to about three decibels. In a fifth embodiment of the gain compression, the gain compression is equal to about four decibels. By operating with higher levels of gain compression, efficiency of the RF PA 40 may be increased, which may help compensate for reduced efficiency in the DC-DC converter 10.

[0039] In a first embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is greater than or equal to about 10 megahertz. In a second embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is less than or equal to about 10 megahertz. In a third embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is greater than or equal to about 20 megahertz. In a fourth embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is less than or equal to about 20 megahertz.

[0040] The RF front-end circuitry 32 receives, processes, and transmits the RF transmit signal RFT via the RF antenna 34. In one embodiment of the RF transmitter circuitry 28, the transmitter control circuitry 38 configures the RF transmitter circuitry 28 based on the transmitter configuration signal PACS. In one embodiment of the RF communications system 26, the RF communications system 26 operates in a full duplex environment, such that the RF transmit signal RFT and the RF receive signal RFR may be active simultaneously. The RF transmit signal RFT has an RF transmit frequency and the RF receive signal RFR has the RF receive frequency. A difference between the RF transmit frequency and the RF receive frequency is about equal to an RF duplex frequency. In one embodiment of the RF communications system 26, the RF notch frequency RNF (Figure 3) is about equal to the RF duplex frequency, which may reduce noise in the receive path from the transmit path.

[0041] In one embodiment of the RF notch filter 18 (Figure 1 ), the RF notch filter 18 (Figure 1 ) is a programmable RF notch filter, such that the RF notch frequency RNF is a selectable notch frequency. In one embodiment of the programmable RF notch filter, the selectable notch frequency is based on the RF duplex frequency. In one embodiment of the programmable RF notch filter, the selectable notch frequency is about equal to the RF duplex frequency. In one embodiment of the RF communications system 26, the transmitter control circuitry 38 selects the selectable notch frequency based on the RF duplex frequency. In one embodiment of the RF communications system 26, the RF system control circuitry 30 provides notch frequency information to the

transmitter control circuitry 38 via the transmitter configuration signal PACS. The notch frequency information is based on the RF duplex frequency. Then, the transmitter control circuitry 38 selects the selectable notch frequency based on the notch frequency information. As such, selection of the selectable notch frequency is based on the notch frequency information.

[0042] In a first embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 10 megahertz. In a second embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 20 megahertz. In a third embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 30 megahertz. In a fourth embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 40 megahertz. In a fifth embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 50 megahertz.

[0043] The PA bias circuitry 42 provides a PA bias signal PAB to the RF PA 40. In this regard, the PA bias circuitry 42 biases the RF PA 40 via the PA bias signal PAB. In one embodiment of the PA bias circuitry 42, the PA bias circuitry 42 biases the RF PA 40 based on the transmitter configuration signal PACS. In one embodiment of the RF front-end circuitry 32, the RF front-end circuitry 32 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 30, the RF system control circuitry 30 is RF transceiver circuitry, which may include an RF transceiver IC, baseband controller circuitry, the like, or any combination thereof. In one embodiment of the RF transmitter circuitry 28, the DC-DC converter 10 provides the first envelope power supply signal EPS, which has switching ripple. In one embodiment of the RF transmitter circuitry 28, the first envelope power supply signal EPS provides power for amplification and at least partially envelope tracks the RF transmit signal RFT.

[0044] Figure 7 shows the RF communications system 26 according to an alternate embodiment of the RF communications system 26. The RF

communications system 26 illustrated in Figure 7 is similar to the RF

communications system 26 illustrated in Figure 6, except in the RF

communications system 26 illustrated in Figure 7, the RF transmitter circuitry 28 further includes a digital communications interface 44, which is coupled between the transmitter control circuitry 38 and a digital communications bus 46. The digital communications bus 46 is also coupled to the RF system control circuitry 30. As such, the RF system control circuitry 30 provides the power supply control signal VRMP (Figure 6) and the transmitter configuration signal PACS (Figure 6) to the transmitter control circuitry 38 via the digital communications bus 46 and the digital communications interface 44.

[0045] Figure 8 shows details of the DC-DC converter 10 illustrated in Figure 6 according to one embodiment of the DC-DC converter 10. The DC-DC converter 10 includes the power supply control circuitry 20, the parallel amplifier 14, and the switching supply 12. The power supply control circuitry 20 controls the parallel amplifier 14 and the switching supply 12. The parallel amplifier 14 and the switching supply 12 provide the first power supply output signal PS1 , such that the parallel amplifier 14 partially provides the first power supply output signal PS1 and the switching supply 12 partially provides the first power supply output signal PS1 .

[0046] Figure 9 shows the RF communications system 26 according to another embodiment of the RF communications system 26. The RF

communications system 26 illustrated in Figure 9 is similar to the RF

communications system 26 illustrated in Figure 6, except in the RF

communications system 26 illustrated in Figure 9, the PA bias circuitry 42 (Figure 6) is omitted and the RF PA 40 includes a driver stage 48 and a final stage 50, which is coupled to the driver stage 48. The DC-DC converter 10 provides the second power supply output signal PS2, which is a second envelope power supply signal, to the driver stage 48 based on the power supply control signal VRMP. Further, the DC-DC converter 10 provides the first power supply output signal PS1 , which is the first envelope power supply signal, to the final stage 50 based on the power supply control signal VRMP. The driver stage 48 receives and amplifies the RF input signal RFI to provide a driver stage output signal DSO using the second envelope power supply signal, which provides power for amplification. Similarly, the final stage 50 receives and amplifies the driver stage output signal DSO to provide the RF transmit signal RFT using the first envelope power supply signal, which provides power for amplification.

[0047] Figure 10 shows the DC-DC converter 10 according to one

embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in Figure 10 is similar to the DC-DC converter 10 illustrated in Figure 1 , except in the DC-DC converter 10 illustrated in Figure 10, the switching supply 12 further includes a second inductive element L2. The second inductive element L2 is coupled between the feedback input FBI and the parallel amplifier output PAO. The switching supply 12 partially provides the first power supply output signal PS1 via the first inductive element L1 and the second inductive element L2. Specifically, the switching supply 12 partially provides the first power supply output signal PS1 via a series combination of the first inductive element L1 and the second inductive element L2.

[0048] In one embodiment of the switching supply 12, a connection node 52 is provided where the first inductive element L1 and the second inductive element L2 are connected to one another. The connection node 52 provides a second voltage V2 to the parallel amplifier 14 via the feedback input FBI. Further, in one embodiment of the parallel amplifier 14, the parallel amplifier 14 has a limited open loop gain at high frequencies that are above a frequency threshold. At such frequencies, a group delay in the parallel amplifier 14 may normally limit the ability of the parallel amplifier 14 to accurately regulate the first voltage V1 of the first power supply output signal PS1 . However, by feeding back the second voltage V2 to the feedback input FBI instead of the first voltage V1 , a phase-shift that is developed across the second inductive element L2 at least partially compensates for the limited open loop gain of the parallel amplifier 14 at frequencies that are above the frequency threshold, thereby improving the ability of the parallel amplifier 14 to accurately regulate the first voltage V1 . In this regard, in one embodiment of the DC-DC converter 10, the parallel amplifier 14 partially provides the first power supply output signal PS1 via the parallel amplifier output PAO based on the voltage setpoint and feeding back a voltage to the feedback input FBI from the connection node 52 between the first inductive element L1 and the second inductive element L2.

[0049] Figure 1 1 shows the DC-DC converter 10 according to an alternate embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in Figure 1 1 is similar to the DC-DC converter 10 illustrated in Figure 10, except the DC-DC converter 10 illustrated in Figure 1 1 further includes the offset capacitive element CO and the switching supply 12 further includes the filter capacitive element CF. The offset capacitive element CO is coupled between the parallel amplifier output PAO and the second inductive element L2. In one embodiment of the DC-DC converter 10, the parallel amplifier 14 partially provides the first power supply output signal PS1 via the parallel amplifier output PAO and the offset capacitive element CO based on the voltage setpoint. The first inductive element L1 and the second inductive element L2 provide a second power supply output signal PS2 via the connection node 52.

[0050] The first inductive element L1 , the second inductive element L2, and the filter capacitive element CF form a first low-pass filter 54 having a first cutoff frequency. The second inductive element L2 and the filter capacitive element CF form a second low-pass filter 56 having a second cutoff frequency. The second cutoff frequency may be significantly higher than the first cutoff frequency. As such, the first low-pass filter 54 may be used primarily to filter the switching output voltage VS, which is typically a square wave. However, the second low- pass filter 56 may be used to target specific high frequencies, such as certain harmonics of the switching output voltage VS.

[0051] In a first embodiment of the first low-pass filter 54 and the second low- pass filter 56, the second cutoff frequency is at least 10 times greater than the first cutoff frequency. In a second embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is at least 100 times greater than the first cutoff frequency. In a third embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is at least 500 times greater than the first cutoff frequency. In a fourth embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is at least 1000 times greater than the first cutoff frequency. In a fifth embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is less than 1000 times greater than the first cutoff frequency. In a sixth embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is less than 5000 times greater than the first cutoff frequency.

[0052] The first inductive element L1 has a first inductance and the second inductive element L2 has a second inductance. In a first embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 10 times greater than a magnitude of the second inductance. In a second embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 100 times greater than a magnitude of the second inductance. In a third embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 500 times greater than a magnitude of the second inductance. In a fourth embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 1000 times greater than a magnitude of the second inductance. In a fifth embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is less than 1000 times greater than a magnitude of the second inductance. In a sixth embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is less than 5000 times greater than a magnitude of the second inductance. [0053] An analysis of improved ripple cancellation performance of the DC-DC converter 10 illustrated in Figure 1 1 is presented. In general, the first power supply output signal PS1 is fed to a load (not shown) having a load resistance RL, such as the RF PA 40 (Figure 6). The switching output voltage VS has a DC component called a DC voltage VD and a ripple component called an AC voltage VA given by EQ. 1 , as shown below.

EQ. 1 : VS = VD + VA. [0054] Further, the inductor current IL has a DC current I D and an AC current IA given by EQ. 2, as shown below.

EQ. 2: IL = ID + IA. [0055] The DC-DC converter 10 regulates the DC voltage VD to be about equal to the voltage setpoint. The first inductive element L1 and the second inductive element L2 appear approximately as short circuits to the DC

component. Further, the filter capacitive element CF appears approximately as an open circuit to the DC component. Therefore, the DC voltage VD is approximately applied to the load resistance RL, as intended. As a result, the DC current ID is based on the DC voltage VD and the load resistance RL, as shown in EQ. 3 below.

EQ. 3: ID = VD/RL.

[0056] Most of the ripple components of the switching output voltage VS is filtered out from the first voltage V1 by the first low-pass filter 54 and the second low-pass filter 56. As a result, most of the AC voltage VA is across the series combination of the first inductive element L1 and the second inductive element L2. The first inductive element L1 has a first inductance 11 and the second inductive element L2 has a second inductance I2. Therefore, the AC current IA is based on the AC voltage VA, the first inductance 11 and the second inductance I2, where s=j2nf, j=V-1 , and f=frequency, as shown in EQ. 4 below.

EQ. 4: IA = VA/[S(I1 +I2)].

[0057] Much of what remains of the ripple component is cancelled out from the first voltage V1 by the parallel amplifier 14. However, to the extent that the parallel amplifier 14 cannot completely cancel out the remains of the ripple component, the first voltage V1 has a first residual ripple voltage VR1 and the second voltage V2 has a second residual ripple voltage VR2. Two approaches to ripple cancellation will be compared against one another. In the first approach, the DC-DC converter 10 is the DC-DC converter 10 illustrated in Figure 10, such that the second voltage V2 is fed to the feedback input FBI, as shown. In this regard, the second residual ripple voltage VR2 drives the parallel amplifier 14 to provide a ripple cancellation current, which is the parallel amplifier output current IP. In the second approach, the DC-DC converter 10 is similar to the DC-DC converter 10 illustrated in Figure 10, except the first voltage V1 is fed to the feedback input FBI instead of the second voltage V2, such that the first residual ripple voltage VR1 drives the parallel amplifier 14 to provide the ripple

cancellation current, which is the parallel amplifier output current IP.

[0058] In the following analysis, the parallel amplifier 14 has a DC open loop gain GO and an open loop bandwidth factor T. As a result, the parallel amplifier 14 has a gain G, as shown in EQ. 5 below. EQ. 5: G = GO/(1 +sT).

[0059] As a result, at frequencies significantly below an open loop bandwidth of the parallel amplifier 14, the open loop bandwidth factor T is small compared to one, such that the gain G approaches the DC open loop gain GO. Conversely, at frequencies significantly above the open loop bandwidth of the parallel amplifier 14, the open loop bandwidth factor T is large compared to one, such that the gain G approaches GO/sT.

[0060] In the first approach, described above wherein the second residual ripple voltage VR2 drives the parallel amplifier 14 and at frequencies significantly above the open loop bandwidth of the parallel amplifier 14, the parallel amplifier output current IP is based on the second residual ripple voltage VR2, as shown in EQ. 6 below.

EQ. 6: IP = G ^* VR2 * (GO ^* VR2)/sT.

[0061 ] In the second approach described above, when the first residual ripple voltage VR1 drives the parallel amplifier 14 and at frequencies significantly above the open loop bandwidth of the parallel amplifier 14, the parallel amplifier output current IP is based on the first residual ripple voltage VR1 , as shown in EQ. 7 below.

EQ. 7: IP = G ^* VR1 ~ (GO ^* VR1 )/sT.

[0062] However, a difference between the first residual ripple voltage VR1 and the second residual ripple voltage VR2 is based on the AC current IA and the second inductance I2, as shown in EQ. 8 and EQ. 9 below.

EQ. 8: (VR2 - VR1 ) = (S)(IA)(I2), or

EQ. 9: VR2 = (s)(IA)(l2) + VR1 .

[0063] Substituting EQ. 9 into EQ. 6 provides EQ. 10 and EQ. 1 1 , as shown below.

EQ. 10: IP * (GO)(VR1 )/sT + (GO)(s)(IA)(l2)/sT, or

EQ. 1 1 : IP * (GO)(VR1 )/sT + (GO)(IA)(l2)/T. [0064] EQ. 1 1 is representative of the first approach and EQ. 7 is representative of the second approach. As a reminder, in the first approach, the second residual ripple voltage VR2 drives the parallel amplifier 14 and in the second approach, the first residual ripple voltage VR1 drives the parallel amplifier 14. In both equations, a smaller first residual ripple voltage VR1 represents better ripple cancellation performance. For comparison purposes, both approaches are assumed to provide the same magnitude of parallel amplifier output current IP. However, in the second approach, the parallel amplifier output current IP is phase-shifted from the first residual ripple voltage VR1 by about 90 degrees. As such, the parallel amplifier output current IP is phase-shifted from the ripple current it is trying to cancel by about 90 degrees, thereby degrading ripple cancellation performance. However, in the first approach, according to EQ. 1 1 , the parallel amplifier output current IP has two terms, namely the

(GO)(VR1 )/sT term and the (G0)(IA)(I2)/T term. The (GO)(VR1 )/sT term has the same phase-alignment shortcoming as in the second approach. But the

(G0)(IA)(I2)/T term phase-aligns the parallel amplifier output current IP with the ripple current it is trying to cancel. Overall, the phase-alignment in the first approach is improved over the second approach. Additionally, to the extent that the (GO)(VR1 )/sT term is smaller than the (GO)(IA)(l2)/T term, the first residual ripple voltage VR1 is reduced, thereby improving ripple cancellation. In this regard, if the (GO)(IA)(l2)/T term is equal to the (GO)(VR1 )/sT term in EQ. 7, then in the (GO)(VR1 )/sT term in EQ. 1 1 , the first residual ripple voltage VR1 is equal to about zero, such that the first approach is greatly improved over the second approach.

[0065] Figure 12 shows the RF communications system 26 according to one embodiment of the RF communications system 26. The RF communications system 26 illustrated in Figure 12 is similar to the RF communications system 26 illustrated in Figure 6, except in the RF communications system 26 illustrated in Figure 12 the transmitter control circuitry 38 provides a filter control signal FCS to the DC-DC converter 10. In one embodiment of the RF communications system 26, the transmitter control circuitry 38 selects the selectable notch frequency based on the RF duplex frequency. As such, the filter control signal FCS is indicative of the selection of the selectable notch frequency.

[0066] Figure 13 shows details of the DC-DC converter 10 illustrated in Figure 12 according to one embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in Figure 13 is similar to the DC-DC converter 10 illustrated in Figure 1 , except in the DC-DC converter 10 illustrated in Figure 13, details of the switching supply 12 are not shown and the RF notch filter 18 receives the filter control signal FCS.

[0067] Figure 14 shows details of the RF notch filter 18 used in the DC-DC converter 10 illustrated in Figure 13 according to one embodiment of the RF notch filter 18. The RF notch filter 18 includes a notch filter inductive element LT, a first notch filter capacitive element CT1 , a second notch filter capacitive element CT2, and a first switching element 58. The notch filter inductive element LT and the second notch filter capacitive element CT2 are coupled in series between the parallel amplifier output PAO and the ground. The first notch filter capacitive element CT1 and the first switching element 58 are coupled in series to form a series coupling, which is coupled across the second notch filter capacitive element CT2.

[0068] A control input to the first switching element 58 receives the filter control signal FCS. As such, the first switching element 58 is in one of an ON state and an OFF state based on the filter control signal FCS. Therefore, the selectable notch frequency is one of a first frequency and a second frequency. When the first switching element 58 is in the ON state, the selectable notch frequency is based on a combination of the notch filter inductive element LT in series with a parallel combination of the first notch filter capacitive element CT1 and the second notch filter capacitive element CT2. As such, when the first switching element 58 is in the ON state, the selectable notch frequency is the first frequency. When the first switching element 58 is in the OFF state, the

selectable notch frequency is based on a series combination of the notch filter inductive element LT and the second notch filter capacitive element CT2. As such, when the first switching element 58 is in the OFF state, the selectable notch frequency is the second frequency.

[0069] In one embodiment of the RF notch filter 18, the first frequency is equal to about 30 megahertz and the second frequency is equal to about 45

megahertz. The filter control signal FCS illustrated in Figure 14 is a single-bit signal, which minimizes control signal complexity. Further, in one embodiment of the RF notch filter 18, the notch filter inductive element LT and the second notch filter capacitive element CT2 are coupled directly in series between the parallel amplifier output PAO and the ground, which maximizes efficiency.

[0070] Figure 15 shows the details of the RF notch filter 18 used in the DC- DC converter 10 illustrated in Figure 13 according to an alternate embodiment of the RF notch filter 18. The RF notch filter 18 includes the notch filter inductive element LT, the first notch filter capacitive element CT1 , the second notch filter capacitive element CT2, the first switching element 58, and a second switching element 60. The notch filter inductive element LT, the second notch filter capacitive element CT2, and the second switching element 60 are coupled in series between the parallel amplifier output PAO and the ground. The first notch filter capacitive element CT1 and the first switching element 58 are coupled in series to form a series coupling, which is coupled across a series combination of the second notch filter capacitive element CT2 and the second switching element 60.

[0071] A control input to the first switching element 58 receives one bit of the filter control signal FCS. A control input to the second switching element 60 receives another bit of the filter control signal FCS. As such, the first switching element 58 is in one of the ON state and the OFF state based on the filter control signal FCS, and the second switching element 60 is in one of an ON state and an OFF state based on the filter control signal FCS. Therefore, the selectable notch frequency is one of a first frequency, a second frequency, and a third frequency. When the first switching element 58 is in the ON state and the second switching element 60 is in the ON state, the selectable notch frequency is based on a combination of the notch filter inductive element LT in series with a parallel combination of the first notch filter capacitive element CT1 and the second notch filter capacitive element CT2. As such, when the first switching element 58 is in the ON state and the second switching element 60 is in the ON state, the selectable notch frequency is the first frequency.

[0072] When the first switching element 58 is in the OFF state and the second switching element 60 is in the ON state, the selectable notch frequency is based on a series combination of the notch filter inductive element LT and the second notch filter capacitive element CT2. As such, when the first switching element 58 is in the OFF state and the second switching element 60 is in the ON state, the selectable notch frequency is the second frequency. When the first switching element 58 is in the ON state and the second switching element 60 is in the OFF state, the selectable notch frequency is based on a series combination of the notch filter inductive element LT and the first notch filter capacitive element CT1 . As such, when the first switching element 58 is in the ON state and the second switching element 60 is in the OFF state, the selectable notch frequency is the third frequency.

[0073] In one embodiment of the RF notch filter 18, the first frequency is equal to about 30 megahertz, the second frequency is equal to about 39 megahertz, and the third frequency is equal to about 47 megahertz. When the first switching element 58 is in the OFF state and the second switching element 60 is in the OFF state, the RF notch filter 18 is disabled.

[0074] Figure 1 6 shows the details of the RF notch filter 18 used in the DC- DC converter 10 illustrated in Figure 13 according to an additional embodiment of the RF notch filter 18. The RF notch filter 18 includes the notch filter inductive element LT, the first notch filter capacitive element CT1 and up to and including an N^TH notch filter capacitive element CTN. The RF notch filter 18 further includes the first switching element 58 and up to and including an N™ switching element 62. The first notch filter capacitive element CT1 and the first switching element 58 are coupled in series to form a first series coupling. The N^TH notch filter capacitive element CTN and the N^TH switching element 62 are coupled in series to form an Nth series coupling. As such, the RF notch filter 18 includes the first series coupling and up to and including the Nth series coupling to form a group of series couplings, such that each of the group of series couplings is coupled in parallel with one another. As such, the notch filter inductive element LT and the group of series couplings are coupled in series between the parallel amplifier output PAO and the ground.

[0075] Figure 17 is a graph illustrating frequency behavior of the RF

communications system 26 illustrated in Figure 12 according to one embodiment of the RF communications system 26. The graph illustrated in Figure 17 shows an RF spectrum associated with the RF transmit signal RFT (Figure 12) and the RF receive signal RFR (Figure 12) The RF receive signal RFR (Figure 12) has an RF receive frequency FRX. In one embodiment of the RF notch frequency RNF (Figure 3), which is the selectable notch frequency, the selectable notch frequency is selected to reduce noise at the RF receive frequency FRX. In one embodiment of the RF transmit signal RFT (Figure 12), the RF transmit signal RFT (Figure 12) is associated with a maximum transmit band 64. In an alternate embodiment of the RF transmit signal RFT (Figure 12), the RF transmit signal RFT (Figure 12) is associated with a selected transmit band 66, which is a subset of the maximum transmit band 64.

[0076] The maximum transmit band 64 has a maximum transmit bandwidth MTXB and the selected transmit band 66 has a selected transmit bandwidth STXB. The selected transmit bandwidth STXB is less than the maximum transmit bandwidth MTXB. As such, the selected transmit band 66 may be used when the maximum transmit bandwidth MTXB is not required. The maximum transmit band 64 has a nominal RF transmit frequency FTXN, which is in the middle of the maximum transmit band 64. The selected transmit band 66 has a selected RF transmit frequency FTXS, which is in the middle of the selected transmit band 66. A selected transmit start offset STXO identifies the bottom edge of the selected transmit band 66 relative to the bottom edge of the maximum transmit band 64.

[0077] An RF duplex frequency FDP is about equal to a difference between the RF receive frequency FRX and the nominal RF transmit frequency FTXN. In one embodiment of the RF notch frequency RNF (Figure 3), which is the selectable notch frequency, the selectable notch frequency is selected to minimize a difference between the selectable notch frequency and the RF duplex frequency FDP, which may reduce noise at the RF receive frequency FRX. A preferred notch frequency FPN is about equal to a difference between the RF receive frequency FRX and the selected RF transmit frequency FTXS. In one embodiment of the RF notch frequency RNF (Figure 3), which is the selectable notch frequency, the selectable notch frequency is selected to minimize a difference between the selectable notch frequency and the preferred notch frequency FPN, which may reduce noise at the RF receive frequency FRX.

[0078] A duplex frequency correction FDC is about equal to a difference between the nominal RF transmit frequency FTXN and the selected RF transmit frequency FTXS. As such, the duplex frequency correction FDC is about equal to a difference between the preferred notch frequency FPN and the RF duplex frequency FDP. In one embodiment of the selected RF transmit frequency

FTXS, the selected RF transmit frequency FTXS is greater than the nominal RF transmit frequency FTXN, as shown in Figure 17. However, in an alternate embodiment of the selected RF transmit frequency FTXS, the selected RF transmit frequency FTXS is less than the nominal RF transmit frequency FTXN. Therefore, the duplex frequency correction FDC may be positive or negative.

[0079] Some of the circuitry previously described may use discrete circuitry, integrated circuitry, programmable circuitry, non-volatile circuitry, volatile circuitry, software executing instructions on computing hardware, firmware executing instructions on computing hardware, the like, or any combination thereof. The computing hardware may include mainframes, micro-processors, microcontrollers, DSPs, the like, or any combination thereof.

[0080] None of the embodiments of the present disclosure are intended to limit the scope of any other embodiment of the present disclosure. Any or all of any embodiment of the present disclosure may be combined with any or all of any other embodiment of the present disclosure to create new embodiments of the present disclosure. [0081] Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.

Claims

Claims What is claimed is:

1 . Circuitry comprising:

· a parallel amplifier having a parallel amplifier output and configured to regulate a voltage of a first power supply output signal based on a voltage setpoint;

• a switching supply coupled to the parallel amplifier output; and

• a radio frequency (RF) notch filter having a selectable notch frequency and coupled between the parallel amplifier output and a ground, wherein the selectable notch frequency is based on an RF duplex frequency.

2. The circuitry of claim 1 wherein the RF notch filter is a programmable RF notch filter.

3. The circuitry of claim 1 wherein the RF duplex frequency is about equal to a difference between an RF transmit frequency and an RF receive frequency.

4. The circuitry of claim 1 wherein the selectable notch frequency is about equal to the RF duplex frequency.

5. The circuitry of claim 1 wherein transmitter control circuitry is configured to select the selectable notch frequency based on the RF duplex frequency.

6. The circuitry of claim 5 further comprising the transmitter control circuitry.

7. The circuitry of claim 5 wherein selection of the selectable notch frequency is further based on notch frequency information, which is based on the RF duplex frequency.

8. The circuitry of claim 1 further comprising an offset capacitive element, wherein the switching supply is coupled to the parallel amplifier output via the offset capacitive element, and the RF notch filter is coupled to the parallel amplifier output via the offset capacitive element.

9. The circuitry of claim 1 wherein the selectable notch frequency is one of a first frequency and a second frequency.

10. The circuitry of claim 9 wherein the first frequency is equal to about 30 megahertz and the second frequency is equal to about 45 megahertz.

1 1 . The circuitry of claim 1 wherein the selectable notch frequency is one of a first frequency, a second frequency, and a third frequency.

12. The circuitry of claim 1 1 wherein the first frequency is equal to about 30 megahertz, the second frequency is equal to about 39 megahertz, and the third frequency is equal to about 47 megahertz.

13. The circuitry of claim 1 wherein the RF notch filter is configured to be disabled.

14. The circuitry of claim 1 wherein:

• the parallel amplifier is further configured to partially provide the first power supply output signal via the parallel amplifier output based on the voltage setpoint; and

• the switching supply is configured to partially provide the first power supply output signal.

15. The circuitry of claim 14 wherein the switching supply is further configured to regulate the first power supply output signal to minimize an output current from the parallel amplifier.

1 6. The circuitry of claim 1 further comprising a direct current (DC)-DC converter, an RF PA, and RF front-end circuitry, wherein:

• the DC-DC converter comprises the parallel amplifier, the switching supply, and the RF notch filter, and is configured to provide a first envelope power supply signal to the RF PA, such that the first envelope power supply signal is configured to at least partially envelope track an RF transmit signal;

• the RF PA is configured to receive and amplify an RF input signal to provide the RF transmit signal using the first envelope power supply signal; and

• the RF front-end circuitry is configured to provide an RF receive signal, which has an RF receive frequency.

17. The circuitry of claim 16 wherein the first envelope power supply signal provides power for amplification to the RF PA.

18. The circuitry of claim 16 wherein a bandwidth of the first envelope power supply signal is greater than or equal to about 20 megahertz.

19. The circuitry of claim 16 wherein the selectable notch frequency is selected to reduce noise at the RF receive frequency.

20. The circuitry of claim 1 wherein the selectable notch frequency is selected to minimize a difference between the selectable notch frequency and the RF duplex frequency.

21 . The circuitry of claim 1 wherein the selectable notch frequency is selected to minimize a difference between the selectable notch frequency and a preferred notch frequency.

22. A method comprising:

• providing a first envelope power supply signal to a radio frequency (RF) power amplifier (PA);

• at least partially envelope tracking an RF transmit signal;

• receiving and amplifying an RF input signal to provide the RF transmit signal using the first envelope power supply signal;

• providing an RF receive signal, which has an RF receive frequency; and

• RF notch filtering the first envelope power supply signal to reduce noise at the RF receive frequency.