U.S. patent application number 09/928127 was filed with the patent office on 2002-01-24 for carrier-blanking mechanism for sweeping detector used to measure and correct rf power amplifier distortion.
This patent application is currently assigned to Spectrian Corporation. Invention is credited to Bonds, David Kent, Mucenieks, Lance Todd.
Application Number | 20020008579 09/928127 |
Document ID | / |
Family ID | 23905150 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008579 |
Kind Code |
A1 |
Mucenieks, Lance Todd ; et
al. |
January 24, 2002 |
Carrier-blanking mechanism for sweeping detector used to measure
and correct RF power amplifier distortion
Abstract
RF power amplifier distortion can be accurately measured in the
presence of multi-frequency input signals, by using a swept local
oscillator to tune RF input and output receivers. The power
detected by the input receiver is compared with a threshold
associated with the carrier. Whenever the power detected by the
input receiver exceeds the threshold--indicating that the input
receiver is tuned on carrier energy--the signal path through the
output receiver is blanked. The sweeping action combined with
selective blanking of the output receiver creates an adaptive notch
filter, which allows for the direct measurement of low level
distortion power in the presence of high power carriers. This
distortion power is digitized and can be processed to control
pre-distortion correction circuitry or gain/phase adjustment
circuitry of a feed-forward error correction loop.
Inventors: |
Mucenieks, Lance Todd;
(Boulder Creek, CA) ; Bonds, David Kent; (Quincy,
IL) |
Correspondence
Address: |
ALLEN, DYER, DOPPELT, MILBRATH & GILCHRIST P.A.
1401 CITRUS CENTER 255 SOUTH ORANGE AVENUE
P.O. BOX 3791
ORLANDO
FL
32802-3791
US
|
Assignee: |
Spectrian Corporation
|
Family ID: |
23905150 |
Appl. No.: |
09/928127 |
Filed: |
August 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09928127 |
Aug 10, 2001 |
|
|
|
09479723 |
Jan 7, 2000 |
|
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|
Current U.S.
Class: |
330/149 |
Current CPC
Class: |
H03F 1/3247 20130101;
H03F 1/3229 20130101 |
Class at
Publication: |
330/149 |
International
Class: |
H03F 001/26 |
Claims
What is claimed:
1. An RF power amplifier arrangement comprising: an RF input port
to which an RF input signal is applied; an RF output port from
which an amplified RF output signal is derived; and an RF signal
processing path coupled between said input and output ports, and
containing an RF power amplifier and an RF distortion correction
unit that is controllably operative to adjust one or more
parameters of said RF signal processing path so as to compensate
for distortion introduced by said RF power amplifier, said RF
distortion correction unit being coupled to derive information
representative of said distortion introduced by said RF power
amplifier over a prescribed bandwidth, but excluding the effect of
an RF carrier frequency of said RF input signal.
2. An RF power amplifier arrangement according to claim 1, wherein
said RF distortion correction unit includes a frequency swept
output receiver coupled to monitor energy contained in said
amplified RF output signal, and being coupled to an output signal
path that derives said information representative of said
distortion introduced by said RF power amplifier over said
prescribed bandwidth and excluding said RF carrier frequency.
3. An RF power amplifier arrangement according to claim 2, wherein
said output signal path of said frequency swept output receiver is
configured to be controllably interrupted in response to said RF
output signal containing said RF carrier frequency.
4. An RF power amplifier arrangement according to claim 3, wherein
said RF distortion correction unit includes a threshold detector
that is operative to controllably interrupt said output signal path
of said frequency swept output receiver, in response to said RF
output signal containing RF carrier energy in excess of a
threshold.
5. An RF power amplifier arrangement according to claim 3, wherein
said RF distortion correction unit comprises an RF predistortion
correction unit that controllably imparts distortion onto said RF
input signal upstream of said RF amplifier, so as to cancel the
distortion contributed by said RF amplifier.
6. An RF power amplifier arrangement according to claim 3, wherein
said RF signal processing path includes a feed-forward error
correction and reinjection loop containing an auxiliary RF error
amplifier coupled in circuit with said RF output port of said RF
power amplifier, and wherein said RF distortion correction unit
comprises an RF distortion correction unit installed in said
feed-forward error correction and reinjection loop upstream of said
auxiliary RF error amplifier.
7. An RF power amplifier arrangement according to claim 6, wherein
said output signal path of said frequency swept output receiver is
configured to be controllably interrupted in response to said RF
input signal containing said RF carrier frequency.
8. An RF power amplifier arrangement according to claim 7, wherein
said RF distortion correction unit includes a threshold detector
that is operative to controllably interrupt said output signal path
of said frequency swept output receiver, in response to said RF
input signal containing RF carrier energy in excess of a
threshold.
9. An RF power amplifier arrangement according to claim 7, wherein
said RF distortion correction unit comprises an RF predistortion
correction unit that controllably imparts distortion onto said RF
input signal upstream of said RF amplifier, so as to cancel the
distortion contributed by said RF amplifier.
10. An RF power amplifier arrangement according to claim 7, wherein
said RF signal processing path includes a feed-forward error
correction and reinjection loop containing an auxiliary RF error
amplifier coupled in circuit with said RF output port of said RF
power amplifier, and wherein said RF distortion correction unit
comprises an RF distortion correction unit installed in said
feed-forward error correction and reinjection loop upstream of said
auxiliary RF error amplifier.
11. An RF power amplifier arrangement according to claim 1, wherein
said RF distortion correction unit includes a frequency swept input
receiver coupled to monitor energy contained in said RF input
signal, a frequency swept output receiver coupled to monitor energy
contained in said RF output signal, and being coupled in an output
signal path coupled that derives said information representative of
said distortion introduced by said RF power amplifier, and wherein
said RF distortion correction unit includes a threshold detector
that is operative to controllably interrupt said output signal path
of said frequency swept output receiver, in response to said RF
input signal containing RF carrier energy in excess of a
threshold.
12. An RF power amplifier arrangement according to claim 11,
wherein said RF distortion correction unit comprises an RF
predistortion correction unit that controllably adjusts the
amplitude and phase of said RF input signal upstream of said RF
amplifier.
13. An RF power amplifier arrangement according to claim 1, wherein
said RF signal processing path includes a feed-forward error
correction and reinjection loop containing an auxiliary RF error
amplifier coupled in circuit with said RF output port of said RF
power amplifier, and wherein said RF distortion correction unit
comprises an RF distortion correction unit installed in said
feed-forward error correction and reinjection loop upstream of said
auxiliary RF error amplifier.
14. A method of measuring and compensating for distortion in an RF
power amplifier to which an RF input signal is coupled and from
which an amplified RF output signal is derived, said method
comprising the steps of: (a) deriving information representative of
said distortion introduced by said RF power amplifier over a
prescribed bandwidth, but excluding the effect of an RF carrier
frequency present in said RF input signal; and (b) controllably
adjusting one or more parameters of said RF signal processing path
so as to compensate for distortion introduced by said RF power
amplifier, in accordance with said information derived in step
(a).
15. A method according to claim 14, wherein step (a) comprises
varying the frequency of operation of an output receiver that is
coupled in an output signal path through which energy contained in
said amplified RF output signal may be monitored, and extracting
therefrom said information representative of said distortion
introduced by said RF power amplifier over said prescribed
bandwidth and excluding said RF carrier frequency.
16. A method according to claim 14, wherein step (a) comprises
simultaneously varying the frequency of operation of each of an
input receiver coupled to monitor energy contained in said RF input
signal, and an output receiver coupled in an output signal path to
said RF power amplifier and being operative to monitor energy
contained in said RF output signal, and controllably interrupting
said output signal path of said output receiver, in response to
said RF input signal containing RF carrier energy in excess of a
threshold.
17. A method according to claim 15, wherein step (a) further
includes controllably interrupting said output signal path of said
output receiver in response to said RF output signal containing RF
carrier energy in excess of a threshold.
18. A method according to claim 14, wherein step (b) comprises
controllably adjusting the amplitude and phase of said RF input
signal upstream of said RF amplifier.
19. A method according to claim 14, wherein a feed-forward error
correction and reinjection loop containing an auxiliary RF error
amplifier is coupled in circuit with said RF output port of said RF
power amplifier, and wherein step (b) comprises controllably
adjusting the amplitude and phase of an RF signal input to said
auxiliary RF error amplifier.
20. A method of measuring and compensating for distortion in an RF
power amplifier to which an RF input signal is coupled and from
which an amplified RF output signal is derived, said method
comprising the steps of sweeping an oscillator to locate and
isolate an RF carrier component in said amplified RF output signal,
and thereby detect distortion energy produced at the output of said
RF power amplifier exclusive of said RF carrier component, and
controllably adjusting the operation of one of a pre-distortion
unit and a post-distortion unit installed in a respective input and
output path of said RF power amplifier in accordance with said
distortion energy.
21. An arrangement for measuring distortion in an amplifier
comprising: a first receiver coupled to measure carrier power at
one of an input and output of said amplifier; a second receiver,
having an operational bandwidth less than that of said first
receiver, and being coupled to measure distortion energy at said
output of said amplifier, except in response to said first receiver
being tuned to carrier energy; and a swept local oscillator that is
operative to sweep the frequency of operation of each of said first
and second receivers in common.
22. An arrangement according to claim 21, further including a
pre-distortion correction unit coupled in an input path of said
amplifier, and being operative to controllably impart distortion
onto an input signal upstream of said amplifier, so as to cancel
distortion contributed by said amplifier, in accordance with said
distortion energy measured by said second receiver.
23. An arrangement according to claim 22, further including a
feed-forward error correction and reinjection loop containing an
auxiliary error amplifier coupled in circuit with said output of
said amplifier, and a post-distortion correction unit installed in
said feed-forward error correction and reinjection loop upstream of
said auxiliary error amplifier, and being operative to controllably
impart distortion onto an output signal of said amplifier and
coupled through said auxiliary error amplifier, so as to cancel
distortion in accordance with said distortion energy measured by
said second receiver.
Description
FIELD OF THE INVENTION
[0001] The present invention relates, in general to radio frequency
(RF) communication systems, and is particularly directed to an RF
power amplifier distortion correction mechanism, that employs a
swept oscillator to locate and isolate the RF carrier component in
the RF power amplifier output, so that distortion energy produced
at the output of the amplifier may be detected. Once detected, the
distortion energy may be controllably removed by a digital signal
processor-controlled distortion cancellation device, such as
pre-distortion unit installed in the input path of the RF power
amplifier, or a gain/phase adjustment unit installed in the error
path of a feed-forward RF amplifier.
BACKGROUND OF THE INVENTION
[0002] The specifications and regulations of the Federal
Communications Commission (FCC) mandate that communication service
providers comply with very strict bandwidth constraints, including
the requirement that the amount of energy spillover outside a
licensed channel or band of interest, be sharply attenuated (e.g.,
on the order of 50 dB). Although such limitations may be readily
overcome for traditional forms of modulation, such as FM, they are
difficult to achieve using more contemporary, digitally based
modulation formats, such as M-ary modulation.
[0003] Attenuating sidebands sufficiently to meet industry or
regulatory-based standards using such modulation techniques
requires very linear signal processing systems and components.
Although relatively linear components can be obtained at a
reasonable cost for the relatively low bandwidths (baseband) of
telephone networks, linearizing components such as power amplifiers
at RF frequencies can be prohibitively expensive.
[0004] A fundamental difficulty in linearizing an RF power
amplifier is the fact that it is an inherently non-linear device,
and generates unwanted intermodulation distortion products (IMDs).
IMDs manifest themselves as spurious signals in the amplified RF
output signal, separate and distinct from the RF input signal. A
further manifestation of IMD is spectral regrowth or spreading of a
compact spectrum into spectral regions that were not occupied by
the RF input signal. This distortion causes the phase-amplitude of
the amplified output signal to depart from the phase-amplitude of
the input signal, and may be considered as an incidental (and
undesired) amplifier-sourced modulation of the RF input signal.
[0005] A straightforward way to implement a linear RF power
amplifier is to build it as a large, high power device, but operate
the amplifier at a only a low power level (namely, at a small
percentage of its rated output power), where the RF amplifier's
transfer function is relatively linear. An obvious drawback to this
approach is the overkill penalty--a costly and large sized RF
device. Other prior art techniques which overcome this penalty
include feedback correction techniques, feedforward correction, and
pre-distortion correction.
[0006] Feedback correction techniques include polar envelope
correction (such as described in U.S. Pat. No. 5,742,201), and
Cartesian feedback, where the distortion component at the output of
the RF amplifier is used to directly modulate the input signal to
the amplifier in real time. Feedback techniques possess the
advantage of self-convergence, as do negative feedback techniques
in other fields of design. However, systems which employ negative
feedback remain stable over a limited bandwidth, which prevents
their application in wide-bandwidth environments, such as
multi-carrier or W-CDMA. Feedforward and predistortion correction,
however, are not limited in this regard.
[0007] In the feedforward approach, error (distortion) present in
the RF amplifier's output signal is extracted, amplified to the
proper level, and then reinjected with equal amplitude but opposite
phase into the output path of the amplifier, so that (ideally) the
RF amplifier's distortion is effectively canceled.
[0008] With predistortion correction, a signal is modulated onto
the RF input signal path upstream of the RF amplifier. The ideal
predistortion signal has a characteristic, which is the inverse of
the distortion expected at the output of the high power RF
amplifier, so that when subjected to the distorting transfer
function of the RF amplifier, it effectively cancels the distortion
behavior.
[0009] Either predistortion or feedforward may be made adaptive by
extracting an error signal component in the output of the RF
amplifier and then adjusting the control signal(s), in accordance
with the extracted error behavior of the RF amplifier, so as to
effectively continuously minimize distortion in the amplifier's
output.
[0010] One of the conventional mechanisms for extracting the error
signal component is to inject a pilot (tone) signal into the signal
flow path through the amplifier and measure the amplifier's
response. A fundamental drawback to the use of a pilot tone is the
need for dedicated pilot generation circuitry and the difficulty of
placing the pilot tone within the signal bandwidth of the
amplifier. Other approaches employ a high intercept receiver to
detect low level distortion in the presence of high power carriers,
which adds substantial complexity and cost.
SUMMARY OF THE INVENTION
[0011] In accordance with the present invention, RF power amplifier
distortion is accurately measured, even in the presence of
multi-frequency input signals, by using a swept local oscillator to
tune respective RF input and output receivers. The power detected
by the tuned input receiver is compared with a power reference to
determine the presence of carrier at the amplifier's input.
Whenever the power detected by the input receiver exceeds the power
of the reference--indicating the presence of carrier energy within
the tuned receiver's bandwidth--a similar signal path through the
output tuned receiver may be controllably blanked with a high
isolation switch. As a result, as the output receiver is swept
across the bandwidth of the amplifier output signal, only
distortion energy will be detected by the output receiver. The
distortion energy detected by the output receiver may be digitized
and processed to control pre-distortion correction circuitry
upstream of the RF amplifier, or gain/phase adjustment circuitry in
the error path of a feedforward error correction loop.
[0012] Pursuant to a first, dual (input-output) receiver-based
embodiment of the present invention, an adaptive predistortion
circuit is installed upstream of an RF power amplifier that has a
relatively "low" carrier-to-interference distortion ratio (C/I)
output signal. By relatively a low C/I ratio output signal is meant
one in which the RF carrier level is effectively indistinguishable
from that of intermodulation products, such as for the case of
mixed modulation multicarrier signals and multicarrier signals
having different power levels. (In contrast, a relatively `high`
C/I output signal, such as that produced at the output of a highly
linear RF amplifier with equal power carriers, is one in which the
level of the RF carrier is readily distinguishable from that of the
IMDs.)
[0013] In the first, predistortion embodiment of the invention, the
RF input signal to be amplified is coupled through a directional
coupler to an input mixer and an IF bandpass filter used as part of
a swept input receiver, which detects the presence of carrier
energy at the input to the RF amplifier. Whenever the carrier
energy detected by the input receiver exceeds a predefined
threshold, a controllably swept output receiver coupled through a
directional coupler to the output of the RF amplifier is blanked by
a threshold detector. The output of the threshold detector is
monitored by a digital signal processor (DSP) controller to keep
track of where (in the swept spectrum) carrier energy is
located.
[0014] A common sweep frequency for each of the input and output
receivers is derived from the same local oscillator, that is
controlled by a digital sweep-control signal generated by the DSP.
The output of the swept oscillator is split and fed to respective
mixers of the input and output receivers. The IF output of the
input mixer is filtered by a slightly wider bandpass filter and
coupled to a carrier energy detector, whose output is monitored by
a threshold detector. The output of the threshold detector is
coupled to a blanking detector input of the DSP and to control
ports of isolation switches in the output receiver.
[0015] During controlled variation (e.g., sweep) of the drive
frequency for the input and output receivers, as long as the output
of the carrier energy detector does not exceed a prescribed
threshold associated with an RF carrier signal, the signal flow
path through the output receiver is considered to be representative
of amplifier distortion, and is therefore detected as an error
signal by the DSP. In response to this error signal the DSP
adaptively adjusts the parameters of a predistortion unit in order
to compensate for the distortion.
[0016] However, if the detected carrier energy exceeds the
prescribed carrier-associated threshold, the output of the
threshold detector changes state, providing both a blanking signal
to the DSP and a control signal to blank (interrupt) the signal
flow path through the output receiver. In this manner, the DSP's
adjustment of the parameters of the predistortion unit will remain
independent of the presence of an RF carrier. Moreover, such
carrier-based selective blanking of the distortion measurement
receiver circuitry prevents saturation of the output receiver's IF
amplifier, and allows the use of lower third order intercept (IP3)
components.
[0017] In accordance with a second, predistortion embodiment of the
invention for use with an RF power amplifier having a high C/I
ratio, the circuit architecture of the controllably blanked
distortion energy measurement subsection is simplified. In
particular, the input receiver mixer is eliminated, leaving only
the output receiver mixer, which downconverts the output of the RF
power amplifier. To allow for carrier threshold-based blanking, the
downconverted output receiver's mixer is split into two paths: one
to a wider band carrier--threshold detector, the other to a
narrower band distortion detector, via isolation switches.
[0018] As in the first embodiment, the output of the carrier
detector is compared with a threshold during the frequency sweep of
the local oscillator. Whenever the detector output exceeds the
threshold--indicating the presence of carrier energy within the
carrier detector's bandwidth--a blanking signal is generated, so as
to interrupt the signal path to the distortion detector.
[0019] In addition to applying the invention to measure distortion
for adjusting the parameters of a predistortion unit upstream of
the RF amplifier, the invention may be employed in an RF power
amplifier distortion measurement and correction scheme, in which a
DSP-controlled adaptive predistortion adjustment circuit is
installed in a feed-forward cancellation amplifier path downstream
of the RF amplifier. Again, either a low or a high C/I ratio
version of the controllably blanked distortion energy measurement
subsection described above may be employed, depending upon the
amplifier's characteristics.
[0020] Pursuant to a third embodiment of the invention, a
DSP-controlled, adaptive gain/phase adjustment circuit is installed
in the error path of a feed-forward amplifier, which utilizes a
relatively low C/I ratio main RF power amplifier. The RF input port
to the main RF power amplifier is coupled to a first RF signal loop
that includes an upstream gain/phase adjustment circuit, such as a
vector modulator. The RF input port is further fed through a
directional coupler to a second RF signal flow path via a delay
line to a first port of an RF carrier cancellation combiner of a
feed-forward error extraction and reinjection loop. A portion of
the amplified signal output of the RF amplifier is extracted and
coupled to a second port of the carrier cancellation combiner. The
carrier cancellation combiner serves to cancel a time-aligned RF
carrier component in the second RF signal flow path from the output
of the RF amplifier and provides an RF error signal representative
of the distortion or IMDs.
[0021] The RF error signal produced by the RF cancellation combiner
is coupled to a DSP-controlled gain/phase adjustment circuit for
the feed-forward error correction and reinjection loop. The output
of this gain/phase adjustment circuit is amplified in a
feed-forward RF error amplifier and reinjected into the output path
of the main RF amplifier. In order to monitor and adaptively
control the gain and phase of the feed-forward error path, its
associated control processor is supplied with amplifier distortion
signals by way of a controllably blanked distortion energy
measurement subsection, configured and operating in the same manner
as the first embodiment, described above.
[0022] In accordance with a fourth embodiment of the invention for
a relatively high C/I ratio amplifier, the dual receiver-containing
controllably blanked distortion energy measurement subsection of
the low C/I amplifier of the third embodiment is replaced by the
reduced complexity single receiver-based, controllably blanked
distortion energy measurement subsection of the second
embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 diagrammatically illustrates an RF power amplifier
distortion measurement and pre-distortion correction scheme in
accordance with a first, dual receiver embodiment of the
invention;
[0024] FIG. 2 diagrammatically illustrates an RF power amplifier
distortion measurement and pre-distortion correction scheme of a
second, single receiver embodiment of the invention;
[0025] FIG. 3 diagrammatically illustrates an RF power amplifier
distortion measurement and post-distortion correction scheme of a
third, dual receiver embodiment of the invention; and
[0026] FIG. 4 diagrammatically illustrates an RF power amplifier
distortion measurement and post-distortion correction scheme of a
fourth, single receiver embodiment of the invention.
DETAILED DESCRIPTION
[0027] Before describing in detail the new and improved RF power
amplifier distortion measurement and correction mechanism in
accordance with the present invention, it should be observed that
the invention resides primarily in a prescribed arrangement of
conventional RF communication circuits, associated digital signal
processing components and attendant supervisory control circuitry,
that controls the operation of such circuits and components. As a
result, the configuration of such circuits components, and the
manner in which they interface with other communication system
equipment have, for the most part, been illustrated in the drawings
by readily understandable block diagrams, which show only those
details that are pertinent to the present invention, so as not to
obscure the disclosure with details which will be readily apparent
to those skilled in the art having the benefit of the description
herein. Thus, the block diagram illustrations are primarily
intended to show the major components of an RF amplifier distortion
measurement and correction system in a convenient functional
grouping, whereby the present invention may be more readily
understood.
[0028] FIG. 1 diagrammatically illustrates a non-limiting example
of a first embodiment of an RF power amplifier distortion
measurement and correction scheme in accordance with the present
invention, in which an adaptive predistortion circuit is installed
upstream of an RF amplifier 10 having a relatively "low" carrier to
intermod ratio (C/I). As pointed out above, by relatively low C/I
ratio RF amplifier is meant one in which the RF carrier level is
effectively indistinguishable from that of intermodulation
products. As a non-limiting example, a low C/I ratio may describe
amplifiers with intermodulation products higher than -50 dBC.
[0029] As shown in FIG. 1, an RF input signal RF.sub.in to be
amplified is coupled to an input port 11 of a signal input path to
the RF power amplifier 10, the distortion characteristic of which
is to be measured by a controllably blanked distortion energy
detector subsection 100. In order to monitor the RF input signal
for the presence of carrier energy, the RF input port 11 is coupled
through a first directional coupler 13 to a first input 21 of a
mixer 22 within a controllably tuned or swept input receiver 20,
and to a digitally controlled predistortion unit 14 installed in
the signal input path to the RF power amplifier 10.
[0030] The predistortion unit 14, which is operative to dynamically
adjust the amplitude and phase of the RF input signal to the RF
amplifier 10, may contain a vector modulator driven by a complex
polynomial work function, and is coupled to receive a set of
weighting coefficients w.sub.o, w.sub.1, w.sub.2. . . , w.sub.n,
supplied over a multi-link 15 by a performance monitoring and
parameter updating digital signal processor (DSP) 16. The DSP
executes one or more error minimization algorithms (e.g., power or
least mean) for adjusting the distortion generated by the
predistortion unit 14. The output of the RF power amplifier 10 is
coupled to an RF output port RF.sub.out and through a second
directional coupler 17 to a first input 31 of a mixer 32 within a
controllably tuned or swept output receiver 30. The output of the
directional coupler 17 is representative of the amplified original
RF input signal and any intermodulation (spectral regrowth)
distortion products (IMDs) introduced by the RF amplifier 10.
[0031] Each of the input and output receivers 20, 30 is controlled
by a digital sweep-control signal generated by the DSP 16. For this
purpose, digital sweep-control signal lines 17 are coupled to a
digital-to-analog converter (DAC) 41, which produces an analog
output sweep voltage that is filtered in a low pass filter 43 and
coupled to a voltage controlled oscillator (VCO) 45. The output of
the VCO 45 is coupled to an input port 51 of a Wilkinson splitter
50. Wilkinson splitter 50 has a first output port 52, which is
coupled through a buffer amplifier 55 to a second input 23 of mixer
22, and a second output port 53, which is coupled through a buffer
amplifier 57 to a second input 33 of mixer 32. The IF output 25 of
mixer 22 is filtered by a wider band bandpass filter 61 and coupled
through a buffer amplifier 63 to a carrier power detector 65, shown
as a diode, the cathode of which is capacitor-coupled to
ground.
[0032] The carrier power detector 65 has its output coupled to a
threshold detector 67, the output of which is coupled to a blanking
detector input 18 of the DSP 16, and to respective control ports
71, 81 of a pair of controlled high isolation switches 70 and 80 in
the output receiver 30. In the absence of the output of carrier
power detector 65 exceeding a prescribed threshold associated with
an RF carrier signal, the output of the threshold detector 67 is at
a first logic state. However, if the carrier power detector 65
detects power in excess of the prescribed threshold, the output of
the threshold detector 67 changes to a second logic state. This
change in state of the blanking signal input 18 to the DSP 16 is
employed to controllably blank the output receiver 30, through
which RF amplifier distortion is measured.
[0033] For this purpose, the IF output 35 of mixer 32 is coupled to
a first input port 72 of switch 70, a second input port 73 of which
is impedance-terminated, as shown. Switch 70 has an output port 74
coupled through a narrower band bandpass filter 75 to a first input
port 82 of switch 80, a second input port 83 of which is
impedance-terminated, as shown. Switch 80 has an output port 84
coupled through an IF buffer amplifier 85 to a (distortion) power
detector 91, shown as a diode whose cathode is capacitor-coupled to
ground, and which serves to measure the distortion power within the
output receiver bandwidth generated by RF amplifier 10.
[0034] The distortion power detector 91 has its output coupled
through a lowpass filter 93 to an analog-to-digital converter (ADC)
95, the digitized output of which is coupled over link 97 to a
distortion detection input 19 of the DSP 16. As described above,
this digitized output of the distortion power detector is
integrated and processed by the DSP 16 using one or more error
minimization algorithms for controlling the variable attenuator and
phase shift components in the predistortion unit 14.
[0035] In accordance with the operation of the controllably blanked
distortion energy measurement subsection 100, the signal path
through the output receiver 30 is normally coupled through switches
70 and 80 to the distortion power detector 91, the output of which
is sampled, digitized and coupled to the distortion input 19 of the
DSP 16, as described above.
[0036] As the DSP 16 sweeps the control voltage input to the VCO
45, the tuning frequency for each of the input and output receivers
20 and 30 is swept in common. During this frequency sweep, the
power detected by the carrier power detector 65 of the input
receiver 20 is applied to threshold detector 67, whose threshold
differentiates between carriers and distortion. As long as the
threshold of the threshold detector 67 is not exceeded, it is
inferred that the output of receiver 30 is distortion power
produced in the RF power amplifier 10. This distortion power is
digitized and coupled to the processor 16 and integrated over an
entire sweep for controlling the predistortion correction circuitry
14, as described above.
[0037] However, whenever the output of the carrier power detector
65 exceeds the threshold of threshold detector 67 --indicating that
the output receiver is tuned near carrier energy--the output of the
threshold detector 67 changes to its second logic state, as
described above. This causes the signal paths through switches 70
and 80 to be interrupted, effectively blanking the output receiver
30, so that the distortion correction operation performed by DSP 16
is not effected by the carrier. This carrier-based selective
blanking of the distortion measurement receiver circuitry prevents
saturation of the output receiver's IF amplifier 85, and allows the
use of lower IP3 components. The bandwidth of the input receiver
20, which is dictated by the bandpass filter 61, may be made
slightly wider than the bandwidth of the output receiver 30 to
provide a guardband, as appropriate, for the switching
operation.
[0038] FIG. 2 diagrammatically illustrates a non-limiting example
of an RF power amplifier distortion measurement and correction
scheme in accordance with a second embodiment of the present
invention, in which an adaptive predistortion circuit is installed
upstream of an RF amplifier having a relatively "high" C/I ratio.
As pointed out above, by relatively high C/I ratio RF amplifier is
meant a highly linear RF amplifier in which the level of the RF
carrier is readily distinguishable from that of intermods. As a
non-limiting example, a high C/I ratio may fall in a range on the
order of -65 to -70 dBC.
[0039] The use of a high C/I RF amplifier allows the output mixer
to be used by both the carrier detector and the distortion
detector, without introducing errors in distinguishing between a
carrier signal and distortion. This simplifies the circuit design
of a controllably blanked distortion energy measurement subsection
200 by eliminating the input mixer and its associated
amplifier.
[0040] More particularly, as in the embodiment of FIG. 1, the RF
input signal RF.sub.in to be amplified is coupled to an input port
11 of a signal path for the RF power amplifier 10. However, as
there is no input receiver, the signal path provided by the first
directional coupler of FIG. 1 is absent in FIG. 2. Instead, the RF
input port is coupled directly to the digitally controlled
predistortion unit 14. The output of the RF power amplifier 10 is
coupled to an RF output port RF.sub.out, and through a directional
coupler 17 to the first input 31 of the mixer 32 within the
controllably swept output receiver 30 of the reduced complexity
controllably blanked distortion energy measurement subsection
200.
[0041] As in the first embodiment, the output receiver 30 is
controlled by a digital sweep signal generated by the DSP 16 and
coupled to DAC 41. The analog sweep voltage produced by the DAC 41
is filtered in low pass filter 43 and coupled to VCO 45. The output
of VCO 45 is coupled through buffer amplifier 57 to the second
input 33 of mixer 32. The output 35 of mixer 32 is coupled to input
port 51 of Wilkinson splitter 50. Wilkinson splitter 50 has its
first output port 52 filtered by wider band bandpass filter 61 and
coupled through a buffer amplifier 63 to carrier power detector 65.
As in the embodiment of FIG. 1, the carrier power detector 65 has
its output coupled to threshold detector 67, the output of which is
coupled to DSP 16 and to respective control ports 71, 81 of
controlled high isolation switches 70 and 80 in the output
receiver.
[0042] The second output port 53 of the Wilkinson splitter 50 is
coupled to the first input port 72 of switch 70, the second input
port 73 of which is impedance-terminated, as shown. Switch 70 has
output port 74 coupled through a narrower band bandpass filter 75
to the first input port 82 of switch 80, the second input port 83
of which is impedance-terminated, as shown. Switch 80 has its
output port 84 coupled through the IF buffer amplifier 85 to the
distortion power detector 91, to measure the (distortion) energy in
the output of the RF amplifier 10. The distortion power detector 91
has its output coupled through lowpass filter 93 to an
analog-to-digital converter (ADC) 95, whose digitized output of
coupled over link 97 to DSP 16.
[0043] The operation of the high C/I embodiment of FIG. 2 is
similar to that of FIG. 1 described above, except that there is no
input receiver. Namely, the signal path through the output receiver
30 is normally coupled through switches 70 and 80 to detector 91,
the output of which is sampled, digitized and coupled to DSP 16. As
the DSP 16 controllably varies the control voltage input to the VCO
45, the tuning frequency for the output receiver 30 is swept.
During this sweep, the output of the carrier power detector 65 is
compared in the threshold detector 67 with a threshold which
differentiates between carriers and intermodulation distortion.
[0044] Whenever the output of the carrier power detector 65 exceeds
the exceeds the threshold of threshold detector 67--indicating that
the receiver is tuned on carrier power from the RF amplifier 10,
the blanking detector-coupled output of the threshold detector 67
changes to its second logic state, so that the signal paths through
switches 70 and 80 are interrupted, effectively blanking the output
receiver 30. As a consequence, the energy detected by the
distortion power detector 91 for the output receiver 30 during the
sweep is RF power amplifier distortion energy that is exclusive of
carriers. This distortion power is digitized and coupled to the
distortion detection input 19 of the processor 16 for controlling
the predistortion correction circuitry 14, as described above.
[0045] FIG. 3 diagrammatically illustrates a non-limiting example
of a third embodiment of an RF power amplifier distortion
measurement and correction scheme in accordance with the present
invention, in which a DSP-controlled gain/phase adjustment circuit
is installed in a feed-forward error path, coupled downstream of an
RF amplifier 10 having a relatively "low" C/I ratio, and whose
distortion characteristic is to be measured.
[0046] For this purpose, the RF input port 11 is coupled through an
input buffer amplifier 301 and directional coupler 303 to a first
RF signal flow path 311 of a carrier cancellation loop 310, that
includes a gain/phase adjustment circuit 313 coupled upstream of
the RF amplifier 10. The gain/phase adjustment circuit 313 may
comprise a vector modulator formed of respective RF amplitude and
phase adjustment circuits 315 and 316. As shown by the broken line
inputs to circuits 315 and 316, gain/phase adjustment circuit 313
may be controlled by amplitude and phase adjustment signals sourced
from the DSP 16.
[0047] The input port 11 is further coupled to a second RF signal
flow path 312 containing a delay circuit 314, the output of which
is coupled to a first port 321 of an RF carrier cancellation
combiner 320 (for example, a Wilkinson splitter/combiner, as a
non-limiting example) of a feed-forward error extraction and
reinjection loop 330. The delay circuit 315 is operative to provide
a delay corresponding to the insertion delay imparted to the first
RF signal flow path 311 by the gain/phase adjustment circuit 313
and RF amplifier 10.
[0048] A portion of the amplified signal output of the RF amplifier
10 is extracted via a directional coupler 317 and coupled to a
second port 322 of the carrier cancellation combiner 320. As in the
first two embodiments, the output of the directional coupler 317 is
representative of the amplified original RF input signal and any
IMDs introduced by the RF amplifier 10. The carrier cancellation
combiner 320 is operative to cancel the delayed (time aligned) RF
carrier component supplied by the second RF signal flow path 312
from the output of the RF amplifier 10, and thus provide an RF
error signal representative of the IMDs.
[0049] The RF error signal produced by the RF cancellation combiner
320 is coupled to a DSP-controlled gain/phase adjustment circuit
350 of the feed-forward distortion cancellation loop 330. The
gain/phase adjustment circuit 350 is shown as comprising a vector
modulator having respective amplitude and phase control units 351
and 353. The amplitude and phase control units 351 and 353 have
respective control inputs 352 and 354 to which amplitude and phase
adjustment signals (G.sub.2,.PHI..sub.2) are sourced from the DSP
16.
[0050] To control the operation of the gain/phase adjustment
circuit 350, a digital gain control signal from the DSP 16 is
converted into analog format by a DAC 355. The output of DAC 355 is
filtered in a low pass filter 356 and coupled to gain control input
352 of gain control circuit 351. A digital phase control signal
from the DSP 16 is converted into analog format by a DAC 357, the
output of which is filtered in a low pass filter 358 and coupled to
gain control input 354 of phase control circuit 353.
[0051] The output of the gain/phase adjustment circuit 350 is
amplified in a feed-forward RF error amplifier 360, and then
reinjected into the output path 319 of the RF amplifier 10 by way
of a reinjection directional coupler 363. The directional coupler
363 is installed downstream of a delay unit 365, to which the
output path 319 of RF amplifier 10 is coupled. The delay unit 365
provides an effective delay corresponding to the insertion delay of
the components in an RF error signal flow path 331 through the RF
cancellation combiner 320, gain/phase adjustment circuit 350 and
feed-forward RF error amplifier 360 of the feed-forward error in
feed-forward error correction and reinjection loop 330.
[0052] In order to monitor and adaptively control the parameters of
the gain/phase adjustment circuit 350, the digital signal processor
16 is coupled through the controllably blanked switching components
of the RF output-monitoring receiver circuitry 30 within the
controllably blanked distortion energy measurement subsection 100,
which is configured and operates in the same manner as described
above with reference to the embodiment of FIG. 1. In particular, a
portion of the composite signal in the RF amplifier's signal output
path 319 is extracted via a directional coupler 367 installed
downstream of reinjection directional coupler 363. This extracted
signal is coupled to a first input 31 of mixer 32 of the output
receiver 30.
[0053] The difference in operation of the embodiments of FIG. 3 and
FIG. 1 relates to the use of the measured distortion energy by the
DSP 16. In the embodiment of FIG. 1, the measured distortion energy
is used to controllably adjust the parameters of the predistortion
unit 14 in the signal input flow path to the RF amplifier 10. In
the embodiment of FIG. 3, the measured distortion energy is used to
controllably adjust the parameters of the gain/phase adjustment
circuit 350 in the RF error signal flow path 331 to feed-forward RF
error amplifier 360 of the feed-forward error correction and
reinjection loop 330.
[0054] FIG. 4 diagrammatically illustrates a non-limiting example
of an RF power amplifier distortion measurement and correction
scheme in accordance with a fourth embodiment of the present
invention, which is effectively a combination of the error
measurement mechanism of FIG. 2 and a feed-forward cancellation
amplifier architecture of FIG. 3 for an RF amplifier 10 having a
relatively "high" C/I ratio.
[0055] For this purpose, the preamplification signal processing
loop 310 and the feed-forward error extraction and reinjection loop
330 of the high C/I embodiment of FIG. 4 are configured and operate
in the same manner as those in the low C/I embodiment of FIG. 3.
The high C/I embodiment of FIG. 4 differs from the low C/I
embodiment of FIG. 3, by replacing controllably blanked distortion
energy measurement subsection 100 with the reduced complexity
controllably blanked distortion energy measurement subsection 200
of the embodiment of FIG. 2.
[0056] As will be appreciated from the foregoing description of the
invention, RF power amplifier distortion is readily accurately
measured, even in the presence of multi-frequency input signals, by
using a swept local oscillator to tune a receiver, whose output is
compared with a threshold associated with the carrier. Whenever the
power detected by the input receiver exceeds the
threshold--indicating that the input receiver is tuned on a
carrier--the signal path through the output receiver is blanked. As
a consequence, as the output receiver is swept, its output is
inferred to contain only distortion energy, which is digitized and
processed to control pre-distortion correction circuitry or
post-distortion circuitry of a feed-forward error correction loop.
Advantageously, because frequency information is available during
the oscillator sweep, selected samples may be emphasized, so that
predistortion weights may be chosen to achieve optimum mask
performance, rather than merely minimizing the total distortion
energy over the operating bandwidth. The invention overcomes the
relatively slow processing and high dynamic range requirements
associated with spectrum analyzer techniques, which use a single
output receiver to accurately measure the level of both carriers
and distortion.
[0057] While we have shown and described several embodiments in
accordance with the present invention, it is to be understood that
the same is not limited thereto but is susceptible to numerous
changes and modifications as are known to a person skilled in the
art, and we therefore do not wish to be limited to the details
shown and described herein, but intend to cover all changes and
modifications as are obvious to one of ordinary skill in the
art.
* * * * *