Phase and amplitude detector and method of determining errors

Details Phase and amplitude detector and method of determining errors Phase and amplitude detector and method of determining errors

2001-01-16

2002-05-28

Pascal, Robert (Department: 2817)

Amplifiers

Hum or noise or distortion bucking introduced into signal...

C455S126000

Reexamination Certificate

active

06396345

ABSTRACT:

FIELD OF THE INVENTION
This invention relates, in general, to a phase and amplitude detector and a method of determining errors, and is particularly, but not exclusively, applicable to the measurement of phase and amplitude errors for compensation purposes in the linearisation of power amplifiers.
BACKGROUND OF THE INVENTION
First and second generation cellular systems have historically used forms of signal modulation which are either constant envelope (e.g. Gaussian Minimum Shift Keying (GMSK) in the global system for mobile communication (GSM) or which result in relatively low levels of amplitude modulation. The linearity of the high power amplifiers used for such systems has therefore not been an important technical issue. Indeed, for constant envelope systems, it is standard practice to operate amplifiers either close to or actually in compression in order to maximise power efficiency. That is to say, the amplifiers are intentionally employed in a non-linear mode.
Third generation cellular systems, however, typically use linear spread-spectrum modulation schemes with a large amount of amplitude modulation on the signal envelope. When passed through a high power amplifier, the output is typically distorted in amplitude and phase by the inherent non-linearity of the amplifier. The amplitude and phase distortion effects are commonly referred to as AM—AM conversion and AM-PM conversion, respectively. Both distortion effects are principally a function of the amplitude envelope of the input signal and are insensitive to the input phase envelope.
In Code Division Multiple Access (CDMA) modulation schemes, quadrature amplitude modulation (QAM) and systems employing similar linear transmission mechanisms, a plurality of signals are simultaneously amplified and transmitted which cause the generation of a large amplitude component in the signal envelope. Unfortunately, when a large amplitude component is applied to a linear amplifier, its non-linear characteristics will tend to produce intermodulation products that reduce signal quality and can cause spectral spillage outside a particular licensed spectrum. Intermodulation products must, therefore, be controlled, but such control, as will be appreciated, should not be at the expense of reducing wanted signal strength.
Intermodulation products and associated distortion can be reduced by negative feedback of the distortion components, pre-distortion of the signal to be amplified to cancel the amplifier generated distortion, or by separating the distortion components from the amplifier output and feeding forward the distortion components to cancel the distortion of the amplifier output signal.
In a power amplifier, where linearisation is performed by correction as a function of signal envelope (either via feedback or via pre-distortion), there is a need for an accurate amplitude and phase comparator that can operate over the full dynamic range of the input signal. In addition, it is desirable for the detector to have a high processing speed to cope with wideband spread spectrum signals. In other words, whilst maintaining low cost and high efficiency design, power amplifiers require ancillary error detection circuitry that can identify and allow correction for non-linearity. Indeed, such correction circuitry is critically dependent upon an ability to measure accurately the phase and amplitude of both the input and output signals to the power amplifier, which signals generally (and, in the exemplary case of CDMA-based systems, inherently) have signal envelopes with associated large dynamic ranges (typically ~20 decibels). In fact, with this ancillary error detection circuitry, there is a requirement to measure small error components (typically of the order of a few tenths of a decibel) in amplitude and phase with respect to relatively large wanted signal excursions/envelopes.
Typical amplifier architectures incorporate a slow feedback loop to track out unit-to-unit variations, thermal drift and long-term component drift. The slow feedback loop eases amplifier set-up and allows a fast feedback or a pre-distortion mechanism to operate only on the amplifier induced, envelope-dependant distortion components. However, conventional phase and amplitude detectors of sufficient performance (associated with linearisation and specifically phase and amplitude error correction in a fast loop) have proven to be extremely difficult to set-up and to replicate on a commercial basis. In any event, it is desirable that a common detector mechanism is used to close both the fast error loop and the (somewhat auxiliary) slow feedback loop to ensure that both loops converge on a single phase/amplitude state.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention there is provided a detector operable to provide at least one error signal associated with at least one of a phase error term and an gain error term between a reference signal
R
and a feedback signal F, the detector characterised by: a vector generator responsive to the reference signal
R
, the vector generator producing a frame of reference vectors
R
1
-R
n
generated by a combination of the reference signal
R
with first
A
and second
P
offset vectors that provide an amplitude and phase displacement of the reference signal
R
; a signal combiner arranged to generate difference vectors E
1
-
E
n
by combining the frame of reference vectors
R
1
-R
n
and the feedback signal F, the difference vectors E
1
-
E
n
. expressing the phase (p) and the gain (a) error terms relative to the reference signal
R
and the first
A
and second
P
offset vectors; and an error signal detector responsive to the difference vectors
E
1
-
E
n
and arranged to provide a measure of the phase (p) and the gain (a) error terms required to support subsequent generation of the at least one error signal.
In a preferred embodiment, a combinatory circuitry coupled to the error signal detector is arranged to receive output signals from the error signal detector, the combinatory circuitry configured to isolate the phase error term and the gain error term in terms of the first
A
and second
P
offset vectors and the reference carrier vector
R
.
Preferably, the combinatory circuitry generates the at least one error signal through isolation of the phase error term from the gain error term, the at least one error term satisfying the general form:
X=P
1
−P
2
−P
3
+P
4
=−8
P
p
R
;
Y=P
1
+P
2
−P
3
−P
4
=−8
A
a
R
where a is the gain error term, p is the phase error term and P
n
are output amplitudes from the signal error detector for corresponding difference vectors E
1
-
E
n
.
In another aspect of the present invention there is provided a phase and amplitude comparator operable to provide signals relating to the difference in phase and amplitude between a reference signal
R
and a feedback signal F, wherein the comparator comprises vector generating means to produce four reference vectors
R
1
-R
n
which are related to the input reference vector signal
R
by the addition of further vectors ±
A
and ±
P
which are, respectively, in phase and in quadrature with
R
such that:
R
1
=
R
+
A
+
P
;
R
2
=
R
+
A
−
P
;
R
3
=
R
−
A
−
P
;
R
4
=
R
−
A
+
P
;
wherein the four reference vectors
R
1
-R
n
are added to four samples of the feedback signal F to produce four corresponding error vectors E
1
-E
4
whereby the vectors E
1
-E
4
can be used to generate phase (X) and amplitude (Y) comparative signals.
In another aspect of the present invention there is provided an amplifier circuit comprising: an input coupled to receive, in use, a reference signal
R
; phase and gain modulators coupled to the input; an amplifier coupled to the phase and gain modulators; a first directional coupler coupled to the input and arranged to sample the reference signal
R
; a second directional coupler coupled to the amplifier and arranged to sample an amplified ver

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