WO2001029962A1

WO2001029962A1 - Wideband vector feedback apparatus

Info

Publication number: WO2001029962A1
Application number: PCT/US1999/024363
Authority: WO
Inventors: Brian L. Baskin
Original assignee: Baskin Brian L
Priority date: 1999-10-18
Filing date: 1999-10-18
Publication date: 2001-04-26

Abstract

Linearity of a microwave amplifier (12) is enhanced by a phase and amplitude distortion correction mechanism based on baseband envelope feedback, wherein the phase and amplitude distortion of the output signal are detected by comparing an output signal to an input signal to downconvert the output distortion signal. The downconverted distortion signal is filtered, amplified and used as a modulation signal to modulate a sample of the input signal. The modulation of the sample of the input signal eliminates the input signal, leaving only the modulation sidebands which are amplified to cancel the distortion signals at the output.

Description

WIDEBAND VECTOR FEEDBACK APPARATUS

FIELD OF INVENTION

The present invention relates to communication systems, and particularly relates to amplification systems wherein an RF amplifier is corrected for phase and amplitude distortion by way of a feedback mechanism.

BACKGROUND OF INVENTION

Increasing demand for mobile and personal communication services has renewed interest in spectrally efficient modulation schemes. Utilization of these high efficiency modulation techniques requires a high level of spectral purity within the means of transmission. Ideally, the input to output transfer characteristic of an amplifier is linear. In other words, a perfect replica of increased amplitude is present at the output of the amplifier. When reduced to practice, however, all amplifiers have nonlinear components comprised in the transfer function.

When a plurality of RF signals propagate through a nonlinear amplifier, in first approximation, an intermodulation product is introduced. These intermodulation products introduce interference signals, crosstalk, and spectral regrowth, limiting the use of efficient modulation schemes. Moreover, when RF amplifiers are cascaded in order to increase signal gain, the above mentioned nonlinearity is increased.

Conventional linear amplifiers (e.g. Class A) are both inefficient and costly devices, resulting in a decrease of battery life in battery dependant systems, and increased cost of system infrastructure and operation. Improvements in RF amplifier efficiency would lead to an increase in talk time, shorter recharging intervals, and reduced size and weight of the overall wireless unit.

U.S. Patent 5,742,201 entitled "Polar Envelope Correction Mechanism for Enhancing Linearity of RF/Microwave Power Amplifier" discloses a feedback mechanism utilized to correct for amplifier distortion. The system disclosed by U.S. Patent 5,742,201, however, is very limited in the bandwidth that can be corrected by the feedback loop which includes the delay of power amplifier 113.

U.S. Patent 5,781,069, entitled "Pre-Post Distortion Amplifier" disclosed by Brian Baskin, specifically incorporated by reference herein, addresses this problem.

In the U.S. Patent application "Pre-Post Distortion Amplifier", an apparatus and a method for linearization of a cascade of nonlinear amplifiers is disclosed. The linearization is achieved by matching the distortion characteristics of each pair of amplifiers, and by taking into account the variation of the distortion characteristics of each amplifier over frequency, temperature, and other variables.

The above-described linearization, however, does not completely eliminate nonlinear distortion introduced by the amplifiers within. A technique, which provides a more complete elimination of distortion products, is needed.

SUMMARY OF THE INVENTION

The present invention discloses a unique apparatus and method for elimination of distortion signals.

One aspect of the invention is directed to a wideband vector feedback apparatus. The apparatus comprises: (1) an input nonlinear amplifier responsive to an input carrier frequency signal, wherein the input nonlinear amplifier outputs a composite signal and an output distortion signal; and (2) A feedback loop configured to eliminate the output distortion signal.

In one embodiment, the feedback loop further comprises: (1) a first automatic gain control circuit (AGC_) configured to provide a constant power level of the output composite signal; (2) a delay circuit configured to delay the input carrier frequency signal; (3) a second automatic gain control circuit (AGC₂) configured to provide a constant power level of the delayed input carrier frequency signal and a constant phase relationship between the delayed input carrier signal and the output composite signal; (4) a multiplier configured to multiply the output composite signal and the delayed carrier frequency input signal in order to generate an intermediate double frequency phase and amplitude signal, an intermediate double frequency phase and amplitude distortion signal and a modulation band phase and amplitude error signal; (5) an oscillator suppression circuit (OSC) configured to filter out the intermediate double frequency phase and amplitude signal and an intermediate double frequency phase and amplitude distortion signal; (6) a double sideband suppressed carrier frequency modulator (DSSM) configured to up convert the modulation band phase and amplitude error signal to a carrier frequency; and (7) a loop amplifier configured to amplify the carrier frequency phase and amplitude error signal in order to cancel out the output distortion signal. In one embodiment, the OSC circuit further comprises: (1) a dynamic automatic gain control circuit (DAGC) configured to detect an oscillation signal outside the modulation frequency band and attenuate the signal gain in order to limit the amplitude of the oscillation signal; (2) an amplifier configured to amplify a signal within the modulation frequency band; and (3) a low pass filter configured to filter out the intermediate double frequency phase and amplitude signal and an intermediate double frequency phase and amplitude distortion signal, and configured to pass the modulation band phase and amplitude error signal.

In one embodiment, the DAGC circuit further comprises: (1) a filter configured to pass the oscillation signal outside the modulation frequency band; (2) a detector configured to detect the oscillation signal outside the modulation frequency band; and (3) an attenuator configured to attenuate the oscillation signal outside the modulation frequency band by adjusting the gain of the DAGC circuit. The filter can be implemented using an active or passive, high pass or band pass filter. The dynamic amplitude gain controlled circuit (DAGC) can be implemented using an operational amplifier.

In one embodiment, the first automatic gain control circuit (AGCi) further comprises an amplitude control circuit configured to control the amplitude of the composite output carrier frequency signal.

In one embodiment, the second automatic gain control circuit (AGC₂) further comprises: (1) a phase control circuit configured to control the phase of the delayed input single carrier frequency signal; and (2) an amplitude control circuit configured to control the amplitude of the delayed input single carrier frequency signal.

In yet another embodiment, separate amplitude detectors detect the amplitude at the output of (AGCi) and at the output of (AGC₂) and are utilized as a balanced amplitude error signal. A phase detector is constructed by adding the two signals of (AGC and (AGC₂) twice. In the first addition of signals, the signal at the output of (AGd) leads, in phase to that of (AGC₂). In the second addition of signals, (AGC_) lags, in phase to that of (AGC₂). The two added signals are then separately amplitude detected and the detected outputs are used as a balanced phase error signal.

Another aspect of the present invention is directed to a method of eliminating an output distortion signal introduced by a non-linear amplifier while amplifying a carrier frequency input signal, The method comprises the following steps: (1) generating a carrier frequency phase and amplitude error signal using a feedback loop; (2) amplifying a carrier frequency phase and amplitude error signal using a feedback loop amplifier; and (3) adding the output distortion signal to the amplified carrier frequency error signal in order to cancel out the output distortion signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a wideband vector feedback apparatus. FIG. 2 shows an oscillation suppression circuit (OSC).

FULL DESCRIPTION QF THE.PREFERRED EMBODIMENTS

FIG. 1 illustrates a wideband vector feedback apparatus (10). An input non-linear amplifier G__N (12) amplifies an input carrier frequency signal IΠM (ωc) and outputs a composite signal comprising a carrier frequency output signal IOUT (ωc) and a carrier frequency output distortion signal α_ (ωc). In general the carrier frequency output distortion signal α. (ωc) includes two spectral components at (COC-COM) and (COC+G)M) frequencies respectively, wherein CO_M is modulation frequency of a distortion component generated by the non-linear amplifier G_IN (12). The basic idea of the present invention is to eliminate the carrier frequency output distortion signal αi (ωc) by generating an exact copy 0.5 (ωc) of the a_ (ωc) signal that is out of phase with the cti (ωc) signal and to add those two signals α₅ (ωc) and αi (ωc) at point (14) in order to eliminate the distortion αi (ωc) from the output of the non-linear amplifier GI_N (12). In the prior art conventional feedback circuits, the composite output signal is fed back to the input of the input amplifier in order to eliminate undesirable distortion components. The feedback of the composite signal to the input of the amplifier utilized in a conventional feedback circuit creates two problems. First, the conventional feedback circuit reduces the gain of the amplifier because the inverted composite feedback signal is amplified along with the input signal. Secondly, the conventional feedback circuit path includes the significant delay of the corrected amplifier which limits the correction bandwidth and causes instability.

In the current invention, the carrier frequency error signal is fed back to the output point of the input amplifier G_IN (12). Therefore, there is substantially no reduction in the effective gain of the circuit. Moreover, because a relatively small loop amplifier is needed to amplify the error signal, there is little delay in the feedback path, thus a wide correction bandwidth is possible. This is due to the fact that the delay of a power amplifier is substantially proportional to output power capability. Typically, for an input amplifier with 20 watt output power capability, the loop amplifier can have as little as 200 milliwatt output power capability. Moreover, the distortion introduced by the loop amplifier itself can be controlled by the feedback circuit due to the control loop architecture (see discussion below).

In one embodiment, the circuit of the present invention includes the feedback loop (16) connected to the input non-linear power amplifier GΓN (12). The feedback loop is designed to eliminate the output distortion signal αi (ωc).

In one embodiment, the feedback loop further comprises a first automatic gain control circuit (AGCi) (18) configured to provide a constant power level of the output composite signal (14) comprising the output signal lour (ωc) and the carrier frequency output distortion signal α_ (ωc).

In one embodiment, the first automatic gain control circuit (AGC_) (18) includes only an amplitude control circuit configured to control the amplitude of the composite output carrier frequency signal (14) including the output I_OUT (ωc) signal, the distortion cti (ωc) signal, and the cancellation signal α₅ (ωc)- The delay circuit (24) is connected to the input of the non-linear amplifier G_IN (12) in order to delay the input carrier frequency signal I_I (ω_c) (22).

The delay circuit (24) is utilized to compensate for differences in delay between the two electrical paths the two signals travel before they meet at the multiplier (34) (see discussion below).

A second automatic gain control circuit (AGC₂) (26) is connected to the delay element (24) to provide a constant power level of the delayed input carrier frequency signal Delayed_I_N (ω ) (25).

In one embodiment, the second automatic gain control circuit (AGC₂) includes: (1) a phase control circuit configured to control the phase of the delayed input carrier frequency signal Phase_ Delayed_I_N (ωc), and an amplitude control circuit configured to control the amplitude of the delayed input carrier frequency signal Amplitude_Delayed_ II_N (ωc). As a result, signals (30) and (32) on average are equal in phase and amplitude. A multiplier (34) is configured to multiply the output composite signal (30) and the delayed carrier frequency input signal (32) in order to generate an intermediate composite signal (38) comprising the double frequency intermediate signal at frequency (2ωc), the double frequency distortion signal at frequencies (2<BC-OOM) and

and the modulation band signal at frequencies (-CUM), 0, and (+CO_M)- This result is easy to understand if one takes into account that the multiplier (34) introduces into the loop circuit the signals at frequencies equal to the sum and minus of frequencies [(COC-CO_M), ωc, (ODC+COM)] (signal 30) and [ωc](signal 32),

In the preferred embodiment, the multiplier comprises two channels; the in phase channel to process the amplitudes of the signals (30) and (32), and the quadrature channel to process the phases of the signals (30) and (32). The quadrature channel multiplies the two signals at (AGC_) and (AGC₂) twice. In the first multiplication, the signal of (AGCi) leads the signal of (AGC₂) by substantially 90°. In the second multiplication, the signal of (AGC₂) leads the signal of (AGCi) by substantially 90°. The two multiplication outputs are utilized as a balanced phase error signal output. The in phase channel multiplies the same two signals substantially in phase two times, the first in phase multiplication uses the output of (AGCi) as the LO (switching) input and the output of (AGC₂) as the RF (detected) input, the second in phase multiplication uses the output of (AGC₂) as the LO (switching) input and the output of (AGCi) as the RF (detected) input. The two multiplication outputs are utilized as a balanced amplitude error signal output. A frequency mixer or a gilbert cell can be used for multiplication. An oscillator suppression circuit (OSC) (36) is configured to filter out the d.c. signal at frequency 0, intermediate double carrier frequency signal I_INTER (2ωc) and the intermediate distortion signal at frequencies (2COC-COM) and (2COC+COM), and to pass the modulation band signal (40) at frequencies (-COM) and (+COM). An active low pass filter accomplishes this in the preferred embodiment.

In the preferred embodiment, the OSC also comprises two channels: the in phase channel in order to process the amplitude of the signal (38), and the quadrature channel to process the phases of the signal (38). The double sideband suppressed carrier frequency modulator (DSSM) (42) up converts the modulation band signal (40) at frequencies (-CO_M) and (+COM) to the amplitude and phase error signal at frequencies (CO_C- UM) and (COC+ OM) by multiplying the modulation band signal (40) at frequencies (-CO_M) and (+COM) and the delayed carrier frequency input signal (28) at frequency (ωc). In the preferred embodiment, the DSSM circuit (42) comprises two channels: the in phase channel in order to process the amplitudes of the signals (28) and (40), and the quadrature channel to process the phases of the signals (28) and (40). In the preferred embodiment, the delayed input signal (28) is substantially evenly split in two. The two signals are then phase and amplitude modulated at opposite phase by the signals at 40. The outputs of these modulators are then combined, out of phase in order to cancel the unmodulated carrier (28).

Finally, a loop amplifier GL_OOP (46) amplifies the amplitude and phase error signal (44) at frequencies (GO_C-COM) and (CO_C+CO_M) in order to generate a cancellation α₅ (ωc) signal (48) that cancels out the original output distortion signal αi (ωc) at point (14). As was mentioned above, the distortion introduced by the loop amplifier GLO_OP (46) is corrected by the control loop (16).

At some frequency the negative feedback loop (16) of FIG. 1 would oscillate. The oscillation frequency is determined by the delay of the feedback loop (16). FIG. 2 depicts the oscillator suppression circuit (OSC) (36) designed to suppress these oscillations.

In the preferred embodiment, the OSC circuit includes a dynamic automatic gain control circuit (DAGC) (62), The DAGC circuit is designed to detect the oscillation signal outside the modulation frequency band [0, CO_M] by way of the band pass filter (68) and the detector (70). The DAGC is also designed to reduce the gain of the OSC circuit for a such oscillation signal outside the modulation frequency band by adjusting the gain of the attenuator (72) at very high speed determined by the modulation frequency CO_M. In one embodiment, the DAGC can be implemented using an operational amplifier. The OSC circuit further includes an amplifier (64) configured to amplify a signal within the modulation frequency band. A low pass filter (66) filters out the intermediate double carrier frequency signal I_INTER (2ωc) and the intermediate distortion signal at frequencies (2<BC-COM) and (2COC+COM), and passes the modulation band signal (40) at frequencies (-ωM) and (+COM)- The band pass or high pass filter (68) as well as the low pass filter (66) can be implemented using an active or a passive filter.

The present invention also discloses a method of eliminating an output distortion signal introduced by a non-linear amplifier while amplifying a carrier frequency input signal. In one embodiment, the method comprises the following steps: (1) generating a carrier frequency phase and amplitude error signal using a feedback loop; (2) amplifying carrier frequency phase and amplitude error signal using a feedback loop amplifier; and (3) adding the output distortion signal to the amplified carrier frequency error signal in order to cancel out the output distortion signal. This method of operation results in the non-linear amplifier yielding an output signal with high spectral purity. The description of the preferred embodiment of this invention is given for purposes of explaining the principles thereof, and is not to be considered as limiting or restricting the invention since many modifications may be made by the exercise of skill in the art without departing from the scope of the invention.

Claims

LA method of amplifying an input signal, said method comprising: amplifying a function of the input signal to output an amplified input signal; generating an error signal, wherein said error signal is a function of a sample of the input signal and a sample of an output signal; filtering the error signal to output a filtered error signal; multiplying the filtered error signal by the sample of the input signal to output an up-converted signal; amplifying the up-converted signal to output a cancellation signal; and summing the amplified input signal by the cancellation signal to output the output signal.

2. The amplification method of claim 1, wherein an oscillation signal is detected, whereby said oscillation signal is a fiinction of the method of amplifying an input signal; and amplitude modulating the error signal, whereby modulation of the error signal is a function of a detected oscillation signal.

3. The amplification method of claim 1, wherein the input signal is multiplied by the filtered error signal by way of double sideband suppressed carrier modulation.

4. The amplification method of claim 2, wherein the input signal is multiplied by the filtered error signal by way of double sideband suppressed carrier modulation.

5. An amplifier system comprising: an input signal port for providing an input signal; a first amplifier for amplifying the input signal, said first amplifier outputting an amplified input signal; a feedback loop coupled to the input signal port and the first amplifier, said feedback loop multiplying a function of the input signal by a function of the amplified input signal to generate an error signal, filtering the error signal to generate a filtered error signal, modulating the function of the input signal by the filtered error signal to generate a modulated function of the input signal, amplifying the modulated function of the input signal to output an amplified modulated function of the input signal, and summing the amplified modulated function of the input signal and the amplified input signal to output a corrected signal; and an output port for receiving the corrected signal, said output port outputting an output signal that is a function of the amplified modulated function of the input signal and the amplified input signal.

6. the amplifier system of claim 5, wherein the feedback loop generates an oscillation signal, whereby the oscillation signal is a function of signal delay and signal attenuation within the feedback loop; and wherein the amplitude of said oscillation signal is used to amplitude modulate the feedback loop.

7. An amplifier system comprising: an input port configured to receive an input signal; a first amplifier coupled to the input port, said first amplifier amplifying the input signal and outputting an amplified input signal; and a feedback loop comprising: a multiplication means coupled to the input port and the first amplifier, said multiplication means receiving a sample of the input signal and a sample of an output signal, multiplying the sample of the input signal by the sample of the output signal and outputting an error signal, wherein the error signal is a function of the input signal and the output signal; a filtering means coupled to the multiplication means, said filtering means receiving the error signal and outputting a filtered error signal; a modulation means coupled to said filtering means, said modulation means receiving the sample of the input signal and a sample of the filtered error signal and outputting a cancellation signal; a second amplifier coupled to the modulation means, said second amplifier amplifying the cancellation signal and outputting an amplified cancellation signal; and an output port coupled to the first amplifier and the second amplifier, said output port receiving a signal from the first amplifier and a signal from the second amplifier and outputting the output signal, wherein the output signal is a function of the amplified input signal and the amplified cancellation signal.

8. The amplifier system of claim 7, wherein the feedback loop further comprises: a means of amplitude detection configured to detect an oscillation signal, whereby the oscillation signal is a function of signal delay and signal attenuation within the feedback loop; a means of amplitude modulating a signal within the feedback loop, wherein modulation of the signal is a function of an amplitude detected oscillation signal.

9. The amplifier system of claim 7, wherein the modulation means is a double sideband suppressed carrier modulator.

10. The amplifier system of claim 8, wherein the modulation means is a double sideband suppressed carrier modulator.

11. A method of controlling instability in a feedback loop comprising: generating an oscillation frequency signal, whereby said oscillation frequency signal is a function of signal delay and signal attenuation of the feedback loop; amplitude detecting the oscillation frequency signal in order to generate an attenuation signal; adjusting attenuation of the feedback loop in order to control the amplitude of said oscillation frequency signal.

12. The method of claim 11, wherein the oscillation frequency signal is filtered prior to amplitude detection.

13. A feedback stabilization circuit comprising: a feedback circuit; an amplitude detector configured to detect an oscillation frequency signal, wherein said oscillation frequency signal is a function of signal delay and signal attenuation within the feedback circuit, said amplitude detector outputting a detection signal that is a function of the oscillation frequency signal; a means of attenuation coupled to said amplitude detector and said feedback circuit, said means of attenuation amplitude modulating a signal that is a function of the feedback circuit.

14. The feedback stabilization circuit of claim 13, wherein the amplitude detector further includes a filter, said filter filtering said oscillation frequency signal.

15. The feedback stabilization circuit of claim 14, wherein said filter is configured to pass the oscillation frequency signal.

16. The feedback stabilization circuit of claim 15, wherein said filter is a high pass filter.

17. The feedback stabilization circuit of claim 15, wherein said filter is a band pass filter.

18) A wideband vector feedback apparatus comprising: an input non-linear amplifier responsive to an input carrier frequency signal, wherein said input non-linear amplifier outputs a composite signal comprising a carrier frequency output signal and an output distortion signal, a delay circuit connected to said input non-linear amplifier, wherein said delay circuit is configured to delay said input carrier frequency signal, and a feedback loop connected to said input non-linear amplifier and said delay circuit , wherein said feedback loop is configured to receive signals from said delay circuit and said input nonlinear amplifier and output a signal to output of said input non-linear amplifier so as to eliminate said output distortion signal; a first automatic gain control circuit (AGCi) configured to provide a constant power level of said output composite signal; a second automatic control circuit (AGC₂) connected to said delay circuit, wherein said automatic control circuit (AGC₂) is configured to provide a constant power level and a constant phase of said delayed input carrier frequency signal in reference to the phase of said output composite signal; a multiplier connected to said first automatic gain control circuit (AGCi) and connected to said second automatic control circuit (AGC₂), wherein said multiplier is configured to multiply said output composite signal and said delayed carrier frequency signal; an oscillator suppression circuit (OSC) connected to said multiplier, wherein said OSC is configured to filter out signals outside the modulaton frequency band; a double sideband suppressed carrier frequency modulator (DSSM) connected to said OSC circuit and connected to said AGC₂, wherein said DSSM circuit is configured to generate a carrier frequency error signal; and a loop amplifier connected to said DSSM circuit, wherein said loop amplifier is configured to amplify said carrier frequency error signal in order to cancel out said output distortion signal.

19. The apparatus of claim 17, wherein said OSC further comprises: a dynamic automatic gain control circuit (DAGC) configured to detect oscillation signals outside the modulation frequency band and to reduce the gain of said OSC circuit for oscillation signals outside the modulation frequency band; an amplifier connected to said DAGC, wherein said amplifier is configured to amplify signals within the modulation frequency band; and a low pass filter connected to said amplifier, wherein said low pass filter is configured to filter out signals outside the modulation frequency band.

20. The apparatus of claim 18, wherein said multiplier further comprises: an in phase channel for amplitude processing; and a quadrature channel for phase signal processing.

21. An amplifier system comprising: an input signal port for providing an input signal; a first amplifier for amplifying the input signal, said first amplifier outputting an amplified input signal; a feedback loop coupled to the input signal port and the first amplifier, said feedback loop generating an error signal, wherein said error signal is a function of a sample of the input signal and a sample of an output signal, filtering the error signal to generate a filtered error signal, modulating the function of the input signal by the filtered error signal to generate a modulated function of the input signal, amplifying the modulated function of the input signal to output an amplified modulated function of the input signal, and summing the amplified modulated function of the input signal and the amplified input signal to output a corrected signal; and an output port for receiving the corrected signal, said output port outputting an output signal that is a function of the amplified modulated function of the input signal and the amplified input signal.

22. the amplifier system of claim 21, wherein the feedback loop generates an oscillation signal, whereby the oscillation signal is a function of signal delay and signal attenuation within the feedback loop; and wherein the amplitude of said oscillation signal is used to amplitude modulate the feedback loop.