U.S. patent application number 12/886684 was filed with the patent office on 2013-11-28 for rf transmitter and method of operation.
This patent application is currently assigned to MOTOROLA, INC.. The applicant listed for this patent is MOSHE BEN-AYUN, OVADIA GROSSMAN, MARK ROZENTAL, URI VALLACH. Invention is credited to MOSHE BEN-AYUN, OVADIA GROSSMAN, MARK ROZENTAL, URI VALLACH.
Application Number | 20130315344 12/886684 |
Document ID | / |
Family ID | 43427460 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130315344 |
Kind Code |
A9 |
BEN-AYUN; MOSHE ; et
al. |
November 28, 2013 |
RF TRANSMITTER AND METHOD OF OPERATION
Abstract
A linear RF transmitter (100) includes a forward path including
a baseband signal combiner (109) and an RF (radio frequency) power
amplifier (123), and a linearizing control loop from an output
(127) of the RF power amplifier to an input of the combiner (109).
A feedback control path (105, 107) of the loop delivers a baseband
error control signal to the combiner. The transmitter further
includes a test signal generator (102) to apply to the combiner in
a closed loop level training mode a test signal comprising a
voltage V.sub.in which increases with time in a non-linear manner
approaching an asymptotic limit such that in response an output
signal produced by the combiner is a voltage V.sub.e which is
substantially constant over a period of time.
Inventors: |
BEN-AYUN; MOSHE; (SHOHAM
CITY, IL) ; GROSSMAN; OVADIA; (TEL AVIV-YAFFO,
IL) ; ROZENTAL; MARK; (GEDERA, IL) ; VALLACH;
URI; (PETACH-TIKVA, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BEN-AYUN; MOSHE
GROSSMAN; OVADIA
ROZENTAL; MARK
VALLACH; URI |
SHOHAM CITY
TEL AVIV-YAFFO
GEDERA
PETACH-TIKVA |
|
IL
IL
IL
IL |
|
|
Assignee: |
MOTOROLA, INC.
Schaumburg
IL
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20110007841 A1 |
January 13, 2011 |
|
|
Family ID: |
43427460 |
Appl. No.: |
12/886684 |
Filed: |
September 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/US09/37676 |
Mar 19, 2009 |
|
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12886684 |
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Current U.S.
Class: |
375/297 |
Current CPC
Class: |
H03F 1/3294 20130101;
H04L 27/362 20130101; H03F 3/24 20130101; H03F 2200/451 20130101;
H03F 2200/408 20130101; H03F 2200/336 20130101; H03F 2200/207
20130101; H03F 2200/78 20130101; H03F 2200/168 20130101; H03F
1/3247 20130101; H03F 3/195 20130101 |
Class at
Publication: |
375/297 |
International
Class: |
H04L 25/49 20060101
H04L025/49 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2008 |
GB |
0805566.7 |
Claims
1. A linear RF transmitter including a forward path for receiving
and processing in an operational mode an input baseband signal and
for producing a modulated RF signal from the baseband signal, the
forward path including a baseband signal combiner and an RF (radio
frequency) power amplifier, a linearizing control loop including a
coupling from an output of the RF power amplifier to an input of
the combiner, wherein the control loop includes a feedback control
path operable to deliver a baseband error control signal to the
combiner, the transmitter further including a test signal generator
operable to apply to the combiner in a closed loop level training
mode a test signal comprising a voltage waveform V.sub.in which
increases with time in a non-linear manner approaching an
asymptotic limit such that in response an output signal produced by
the combiner is a voltage V.sub.e which is substantially constant
over a period of time.
2. A transmitter according to claim 1, wherein the voltage waveform
V.sub.in varies with time t according to an inverse exponential
relationship given by V in ( t ) = k .beta. A [ 1 - - t .tau. 1 ]
##EQU00010## where e represents an exponential, .tau.1 represents a
time constant of a filter first pole of the control loop, k
represents a constant and .beta.A represents a loop gain of the
control loop.
3. A transmitter according to claim 1 wherein the test signal
generator is operable to apply the test signal to a voltage level
which causes, in response, operation of the RF power amplifier to
reach a condition of compression and the voltage V.sub.e to rise
with time.
4. A transmitter according to claim 3 including a detector operable
to monitor the voltage V.sub.e to detect a change in the voltage
V.sub.e when the voltage V.sub.e rises with time after being
substantially constant with time.
5. A transmitter according to claim 4 wherein the detector
comprises a voltage comparator operable to compare the voltage
V.sub.e with a reference voltage.
6. A transmitter according to claim 4 [or claim 5] including a
baseband signal generator for producing the baseband signal,
wherein the detector is operable to provide to the baseband signal
generator a signal indicating the detected change in the voltage
V.sub.e and the baseband signal generator is operable to use the
signal provided to set a level of the baseband signal in an
operational mode of the transmitter.
7. (canceled)
8. A transmitter according to claim 1 [any one of the preceding
claims] including a low pass filter operable to provide a loop
filter first pole of the control loop and to receive a baseband
signal produced by the combiner, the voltage V.sub.e providing an
input to the low pass filter in the level training mode.
9-13. (canceled)
14. A transmitter according to claim 1 [any one of the preceding
claims] wherein the forward path includes: (i) an I channel for
receiving and processing an I component of the baseband signal, the
I channel including a first mixer for upconverting the I component
to produce an RF signal modulated by the I component, and (ii) a Q
channel for receiving and processing a Q component of the baseband
signal, the Q channel including a second mixer for upconverting the
Q component to produce an RF signal modulated by the Q component;
and the feedback control path includes: (i) an I feedback channel
for delivering an I component of the error control signal, the I
feedback channel including a third mixer for downconverting a
sampled RF signal produced by the RF power amplifier to produce the
I component of the error control signal; and (ii) a Q feedback
channel for delivering a Q component of the error control signal,
the Q feedback channel including a fourth mixer for downconverting
a sampled RF signal produced by the RF power amplifier to produce
the Q component of the error control signal.
15. A method of operation of a linear RF transmitter including an
RF power amplifier including applying a closed loop level training
mode which includes applying to a baseband signal combiner in a
forward path of a linearizing control loop of the transmitter a
test signal comprising a voltage waveform V.sub.in which increases
with time in a non-linear manner approaching an asymptotic limit,
developing in response a voltage V.sub.e as an output of the
combiner which is substantially constant over a period of time,
monitoring the voltage V.sub.e with time and detecting a rise in
the voltage V.sub.e following the period in which voltage V.sub.e
is substantially constant indicating that operation of the RF power
amplifier has reached a condition of compression.
16. A method according to claim 15 further including delivering, in
response to the detection, a signal to a digital signal processor
indicating that operation of the RF power amplifier has reached a
condition of compression.
17. A method according to claim 16 including setting by the digital
signal processor a baseband signal level using a time indicated by
the delivered signal.
18. A method according to claim 17 wherein the test signal is
produced by the digital signal processor and includes the digital
signal processor using the time indicated by the delivered signal
to identify a point in time on the test signal when the signal
level is at suitable level for selection.
19. A method according to claim 17 [or claim 18] which includes the
digital signal processor producing a baseband signal in an
operational mode at a level selected in the level training
mode.
20. A method according to claim 15 [any one of claims 15 to 19]
wherein the voltage waveform V.sub.in varies with time t according
to an inverse exponential relationship given by V in ( t ) = k
.beta. A [ 1 - - t .tau. 1 ] ##EQU00011## where e represents an
exponential, .tau.1 represents a time constant of a filter first
pole of the control loop, k represents a constant and .beta.A
represents a loop gain of the control loop.
21. A method according to [any one of the preceding claims 15 to
20] claim 15 which includes detecting a change in the voltage
waveform V.sub.e by a detector comprising a voltage comparator
which compares the voltage V.sub.e with a reference voltage.
22. A method according to claim 15 [any one of the preceding claims
15 to 21] which is operated using a low pass filter in the
linearizing control loop providing a loop filter first pole at a
frequency of at least 1 kiloHertz.
23. A method according to claim 22 wherein the loop filter first
pole has a time constant .tau.1 of less than about 100
microseconds.
24. A method according to claim 22 [or claim 23] including applying
the test signal for a duration which is greater than three times
the time constant .tau.1 of the first pole of the low pass
filter.
25. A method according to claim 24 including applying the test
signal for a duration of between about 100 microseconds and about
300 microseconds.
26. A method according to claim 15 [any one of claims 15 to 25]
including applying the test signal in the level training mode at
regular intervals between periods when the transmitter is operating
in an operational mode.
Description
TECHNICAL FIELD
[0001] The technical field relates to a radio frequency (RF)
transmitter and a method of operation of the transmitter. In
particular, the technical field relates to an RF transmitter
including a linearity control loop and a method of operation of
level training in the transmitter.
BACKGROUND
[0002] RF communication terminals normally employ an RF transmitter
to generate RF signals, a receiver to receive RF signals and an
antenna to send radiated signals produced by the transmitter
over-the-air to another terminal and to receive signals sent
over-the-air from another terminal for delivery to the receiver.
The transmitter normally includes an RF power amplifier, `RFPA`, to
amplify the RF signals before they are delivered to the antenna for
transmission.
[0003] It is desirable for the RF transmitter to be linear, i.e.
for the RFPA to produce a power amplification which is a linear
function of the power of the input signal provided to it, in order
to prevent distortion of the input signal and to minimize adjacent
channel interference. Many RF transmitters include at least one
control loop such as a Cartesian loop to provide linearization of
the RFPA of the transmitter.
[0004] The control loop may be operated in a training mode to set a
suitable strength level (of the baseband signal delivered along the
forward path) which in operation does not cause compression of the
RFPA.
[0005] The control loop may include a loop filter which may include
one or more filter stages. One purpose of the loop filter is to
constrain the bandwidth of the loop to ensure stability of the
loop. In filter analysis, it well known to define such a filter in
terms of the transfer function of the filter, especially parameters
known as the poles and zeros which are obtained from the transfer
function of the filter. For example, the transfer function H(s) of
an LTI filter used as a loop filter may be defined as
H(s)=Y(s)/X(s) where the terms Y(s) and X(s) are polynomial
expressions which can be factorised; the multiplying factors of the
factorised expressions can be written in the form, s-q.sub.i where
i is an integer, 1, 2, 3 . . . . The (possibly complex) numbers
q.sub.i are the roots of the polynomial. When s is set to the value
of any of these roots of the numerator polynomial term Y(s) which
results in the transfer function evaluating to zero, the root in
question is denoted as a `zero`. When s is set to the value of any
of the roots of the denominator polynomial term X(s) which results
in the transfer function approaching infinity, the root in question
is denoted as a `pole`. The concept of zeros and poles will of
course be familiar to those skilled in the art of designing
filters.
[0006] The loop filter used in a control loop such as a Cartesian
loop in a linear RF transmitter is normally designed to have a
first pole at a low frequency and a second pole and a zero at
higher frequencies. The precise positions in frequency of the first
pole, the second pole and the zero are selected according to the
properties of the RF signal that has to be transmitted by the
transmitter. In some transmitters, the selection of these positions
can lead to a serious problem during level training described
above. The loop filter may no longer be a perfect integrator and
this can cause difficulty in identifying during level training the
appropriate strength level needed to avoid compression of the
RFPA.
[0007] Thus, there exists a need for a linear RF transmitter, for
use in mobile communications, which addresses at least some of the
shortcomings of past and present techniques and/or procedures
employed for level training in such transmitters.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0008] The accompanying drawings, in which like reference numerals
refer to identical or functionally similar items throughout the
separate views which, together with the detailed description below,
are incorporated in and form part of this patent specification and
serve to further illustrate various embodiments of concepts that
include the claimed invention, and to explain various principles
and advantages of those embodiments.
[0009] In the accompanying drawings:
[0010] FIG. 1 is a block schematic diagram of an illustrative RF
transmitter.
[0011] FIG. 2 is a graph of a level plotted against time of an
input test signal used in the transmitter of FIG. 1, illustrating a
suitable waveform of the test signal.
[0012] FIG. 3 is graph of a level plotted against time of a
combined signal produced by a combiner of a forward path of the
transmitter of FIG. 1, the graph illustrating a suitable response
of the combined signal to application of the test signal
illustrated in FIG. 2.
[0013] FIG. 4 is a flow chart of an illustrative method of
operation of the RF transmitter of FIG. 1.
[0014] Skilled artisans will appreciate that items shown in the
drawings are illustrated for simplicity and clarity and have not
necessarily been drawn to scale. For example, the dimensions of
some of the items may be exaggerated relative to other items to
assist understanding of various embodiments. In addition, the
description and drawings do not necessarily require the order
illustrated. Apparatus and method components have been represented
where appropriate by conventional symbols in the drawings, showing
only those specific details that are pertinent to understanding the
various embodiments so as not to obscure the disclosure with
details that will be readily apparent to those of ordinary skill in
the art having the benefit of the description herein. Thus, it will
be appreciated that for simplicity and clarity of illustration,
common and well-understood items that are useful or necessary in a
commercially feasible embodiment may not be depicted in order to
facilitate a less obstructed view of these various embodiments.
DETAILED DESCRIPTION
[0015] Generally speaking, pursuant to the various embodiments to
be described, there is provided a linear RF transmitter including a
forward path for receiving and processing in an operational mode an
input baseband signal and for producing a modulated RF signal from
the baseband signal, the forward path including a baseband signal
combiner and an RF (radio frequency) power amplifier, a linearizing
control loop including a coupling from an output of the RF power
amplifier to an input of the combiner, wherein the control loop
includes a feedback control path operable to deliver a baseband
error control signal to the combiner, the transmitter further
including a test signal generator operable to apply to the combiner
in a closed loop level training mode a test signal comprising a
voltage V.sub.in which increases with time in a non-linear manner
approaching an asymptotic limit such that in response an output
signal produced by the combiner is a voltage V.sub.e which is
substantially constant over a period of time.
[0016] The voltage V.sub.in may vary with time t according to an
inverse exponential relationship. As demonstrated later, the
relationship may be defined as:
V in ( t ) = k .beta. A [ 1 - - t .tau. 1 ] ##EQU00001##
where e represents an exponential, .tau.1 represents a time
constant of the first filter pole of the control loop, k represents
a constant and .beta.A represents a loop gain of the control loop,
`A` being the gain of the loop in the forward path and `.beta.`
being the gain of the loop in the feedback path.
[0017] Applying the test signal in the non-linear form of V.sub.in
as defined above allows the voltage V.sub.e developed in response
at the output of the combiner, e.g. between the combiner and a loop
low pass filter, to be substantially constant over a period of
time.
[0018] Eventually, the test signal reaches a level at which the
RFPA of the transmitter is driven into a condition of compression.
At this level, the voltage V.sub.e begins to rise rapidly with
time. Detection of this level of the test signal allows the onset
of compression to be detected. By ensuring that the voltage V.sub.e
is substantially constant in the period of time prior to the onset
of compression in the level training mode, the onset of compression
in that mode may be detected more accurately than in the prior art,
thereby avoiding the possibility of a false detection of
compression of the RFPA.
[0019] Furthermore, applying the test signal in the non-linear form
of V.sub.in as defined above allows the voltage V.sub.e developed
in response to remain substantially constant even when the control
loop includes a low pass filter having a first filter pole at a
frequency which is substantially higher than zero Hertz, e.g. at
least 1 kHz. Such a control loop may suitably be employed in a
transmitter for transmitting a signal having a wideband modulation,
e.g. a modulation band wider than 25 kHz, such as a signal in
accordance with the TETRA 2 standard, the HPD (High Performance
Data) protocol, the HSD (High Speed Data) protocol or the WiMax
protocol.
[0020] Those skilled in the art will appreciate that these
recognized benefits and advantages and other advantages described
herein are merely illustrative and are not meant to be a complete
rendering of all of the benefits and advantages of the various
embodiments of the invention.
[0021] Referring now to the accompanying drawings, and in
particular to FIG. 1, there is shown a block schematic diagram of
an illustrative RF transmitter 100. The transmitter 100 includes a
Digital Signal Processor (DSP) 102 coupled to a Digital to Analog
Converter (DAC) 104 and a Digital to Analog Converter (DAC) 106.
The DSP 102 serves as a generator of a baseband signal in an
operational mode and a generator of a test signal in a training
mode. The DSP 102 is thus an illustration of the `test signal
generator` referred to herein. In an operational mode, the DSP 102
produces a baseband digital output `I` signal component which is
converted into analog form by the DAC 104. The DSP 102 also
produces a baseband digital output `Q` signal component which is
converted into analog form by the DAC 106.
[0022] An output signal produced by the DAC 104 is delivered as a
baseband I channel input signal to an I (In-phase) channel 101 of a
forward signal delivery path which extends eventually to an antenna
125. An output signal produced by the DAC 106 is delivered as a
baseband Q channel input signal to a Q (Quadrature phase) channel
103 of the forward path.
[0023] The I channel 101 includes a combiner 109 which may be a
differential summer. The combiner 109 is a baseband signal combiner
which receives as a first input signal the I channel input signal
from the DAC 104. The combiner 109 also receives as a second input
signal an error control signal from an I feedback control channel
105. The combiner 109 produces an output signal which is a
differential sum of (difference between) its first and second input
signals. The output signal produced by the combiner 109 is applied
in turn to an amplifier 111, a low pass filter 113, a further
amplifier 115 and a further low pass filter 117. The amplifiers 111
and 115 provide optional gain stages in the I channel 101. The low
pass filter 113 sets a first filter pole of a complete negative
feedback control loop which includes the I feedback control channel
105. The control loop is described in more detail later. The low
pass filter 117 sets a second filter pole and a zero of the
complete control loop.
[0024] The Q channel 103 includes a combiner 129 which may be a
differential summer 129. The combiner 129 is a baseband signal
combiner which receives as a first input signal the Q channel input
signal from the DAC 106. The combiner 129 also receives as a second
input signal an error control signal from a feedback control
channel 107. The combiner 129 produces an output signal which is a
differential sum of its first and second input signals. The output
signal produced by the combiner 129 is applied in turn to an
amplifier 131, a low pass filter 133, a further amplifier 135 and a
further low pass filter 137.
[0025] The amplifiers 131 and 135 provide optional gain stages in
the Q channel 103. The low pass filter 133 sets a first filter pole
of a complete negative feedback control loop which includes the
feedback control channel 107. The control loop is described in more
detail later. The low pass filter 137 sets a second filter pole and
a zero of the complete control loop. Either or both of the low pass
filters 137 and 117 may be programmable.
[0026] The first pole, the second pole and the zero of the control
loops that include the feedback control channels 105 and 107 may
have the same respective values in the two loops including,
respectively, the I channel 101 and the Q channel 103.
[0027] The first filter pole of each loop may be set at a frequency
of at least 1 kHz, e.g. at a frequency of about 3 kHz, to suit
wideband modulation operation of the transmitter 100. The same
first filter pole setting may be used for different applications of
the transmitter 100, e.g. for use of the transmitter 100
alternatively in a TETRA 1 system or terminal and in a TETRA 2
system or terminal as defined later.
[0028] The second filter pole and zero may be at frequency settings
which depend on the particular application and implementation of
the transmitter 100. The settings may be at different frequencies
for different applications of the transmitter 100. Illustratively,
the second pole may set at a frequency of about 25 kHz for use in a
TETRA 1 system and at a frequency of about 108 kHz for use in a
TETRA 2 system. Illustratively, the zero may be set to be at a
frequency of about 250 kHz for use in a TETRA 1 system and a
frequency of about 1 MHz for use in a TETRA 2 system.
[0029] A mixer 119 in the I channel 101 receives a baseband signal
produced by the low pass filter 117. The mixer 119 upconverts the
baseband signal to an RF carrier frequency by mixing the baseband
signal with a carrier frequency signal applied to the mixer 119.
The carrier frequency signal is produced by a local oscillator
141.
[0030] Similarly, a mixer 139 in the Q channel 103 receives an
output baseband signal produced by the low pass filter 137. The
mixer 139 converts the baseband signal to an RF carrier frequency
by mixing the baseband signal with a carrier frequency signal
applied to the mixer 139. The carrier frequency signal is a signal
produced by the local oscillator 141 and delivered to the mixer 139
via a phase shifter 143 which shifts the phase of the signal by
ninety degrees.
[0031] The mixer 119 and the mixer 139 respectively produce RF
output signals which are applied as input signals to a summer 121.
The summer 121 adds the input component signals applied to it to
produce a combined RF output signal which is applied to an RF power
amplifier (RFPA) 123 and is amplified by the RFPA 123. An amplified
output signal produced by RFPA 123 is delivered to the antenna 125
for sending by wireless communication to at least one remote
receiver (not shown).
[0032] The transmitter 100 may be part of a transceiver which
includes a receiver (not shown). In that case, an antenna switch or
duplexer or the like (not shown) may be located between the RFPA
123 and the antenna 125 to allow the RFPA 123 to be isolated from
the antenna 125 and the receiver when the antenna 125 and the
receiver are operating together in a receiving mode.
[0033] A coupler 127, e.g. a directional coupler, at an output of
the RFPA 123 serves as a sampler to sample the amplified RF output
signal produced by the RFPA 123. The sampled RF signal is applied
as a first input signal to a mixer 143 in the I feedback control
channel 105 and as a first input signal to a mixer 147 in the Q
feedback control channel 147. The mixer 143 also receives a second
input which is the signal produced by the local oscillator 141 via
a phase shifter 149. The mixer 147 also receives a second input
signal which is the signal produced by the local oscillator 141
delivered to the mixer 147 via the phase shifter 149 and a phase
shifter 145. The phase shifter 145 shifts the phase of the signal
applied to it by ninety degrees. The phase shifter 149 applies
adjusting shifts to the phase of the signal from the local
oscillator 141 when required during a phase training mode of the
transmitter 100 as described in more detail later.
[0034] The mixer 143 produces, by downconverting the frequency of
the RF signal applied to it, a baseband I error control signal
which is applied to the combiner 109. Similarly, the mixer 147
produces, by downconverting the frequency of the RF signal applied
to it, a baseband Q error signal which is applied to the combiner
129. As mentioned earlier, the error control signals applied
respectively to the combiner 109 and the combiner 129 are
differentially added to (subtracted from) the I channel input
signal and the Q channel input signal, respectively.
[0035] Thus, the linearizing control loop (Cartesian loop) of the
transmitter 100 comprises: the `I` loop extending from the combiner
109 via the I channel 101 to the coupler 127 and from the coupler
127 via the I feedback control channel 105 to the combiner 109; and
the `Q` loop extending from the combiner 129 via the Q channel 103
to the coupler 127 and from the coupler 127 via the Q feedback
control channel 107 to the combiner 129. The feedback control path
of the control loop comprises: the part of the `I` loop extending
from the coupler 127 via the I feedback control channel 105 to the
combiner 109; and the part of the `Q` loop extending from the
coupler 127 via the Q feedback control channel 107 to the combiner
129.
[0036] As will be apparent to those skilled in the art, the I
feedback control channel 105 and the Q feedback control channel 107
may each include one or more filters (not shown) and one or more
amplifiers (not shown).
[0037] The transmitter 100 has an operational mode in which the
transmitter 100 generates and sends RF signals in a usual manner.
The transmitter 100 also has at least one training mode including a
level training mode. The transmitter 100 may also include a phase
training mode. The training mode(s) may be applied at selected
times, e.g. at regular intervals between periods when the
operational mode is applied. The intervals employed for the
training mode(s) may be those defined in a protocol according to
which the transmitter is operating. For example, where the
transmitter 100 operates according to the TETRA 1 standard, the
intervals may correspond to time slots of the TETRA timing protocol
which are set aside for transmitter linearizing purposes, namely
slots known as the Common Linearisation Channel(CLCH).
[0038] During each operation of the level training mode, the
control loop comprising: (i) the I channel 101 of the forward path
and the I feedback control channel 105; and (ii) the Q channel 103
of the forward path and the feedback control channel 107; operates
in a closed loop mode. The DSP 102 generates a test signal which is
applied to either or both of the I channel 101 and the Q channel
103. The purpose of applying the test signal in the level training
mode is to detect a compression point or condition of the RFPA 123
when the operation of the RFPA 123 (in terms of its output power as
a function of its input power) becomes non-linear.
[0039] FIG. 2 is a graph 200 showing a plot 203 in which a level of
the input test signal when applied to the combiner 109 or the
combiner 129 or both is plotted against time. This input signal is
referred to herein as a signal V.sub.in. The signal V.sub.in
comprises a voltage waveform. The plot 203 which describes the
applied voltage waveform of the input test signal V.sub.in has a
shape which is curved rather than straight as indicated for
comparison purposes by a dashed straight line 201 representing a
conventional test signal. The plot 203 has an inverse exponential
form explained in more detail later. As the level of the test
signal V.sub.in in the plot 203 increases, the level approaches an
asymptotic limit indicated by a dashed horizontal line 205. The
level of the test signal V.sub.in reaches a practical maximum at a
time T. The shape of the plot 203 is selected so that a desired
combined signal V.sub.e developed in response at the output of the
combiner 109 and/or at the output of the combiner 129 (depending on
where V.sub.in is applied) is substantially constant with time.
[0040] FIG. 3 is a graph 300 of the combined signal V.sub.e plotted
against time showing a plot 303 which represents the desired form
of the combined signal V.sub.e. The combined signal V.sub.e is
another voltage waveform. The plot 303 has, until a time t1, a
substantially horizontal portion in which the level of the combined
signal V.sub.e is substantially constant with time. When the input
test signal V.sub.in reaches a particular level at the time t1, the
compression point of the RFPA 123 is reached causing the combined
signal V.sub.e indicated by the plot 303 to increase steeply with
time.
[0041] For comparison purposes, FIG. 3 also shows a dashed plot 301
which corresponds to the combined signal obtained using an input
test signal having the conventional straight line form indicated by
the line 201 in FIG. 2. Although the plot 301 has an increase in
gradient at the time t1, the plot 301 undesirably shows an increase
with time throughout the duration of the input test signal making
detection of the compression point inaccurate. Such detection can
be made in error before the RFPA 123 is driven into
compression.
[0042] The combined signal V.sub.e, as represented by the plot 303,
produced by the combiner 109 or by the combiner 129 or both, is
monitored by a detector to detect a point on the plot 303 when the
plot 303 begins to rise steeply at the time t1 indicating that the
RFPA 123 compression point has been reached. For example, as shown
in FIG. 1, the combined signal V.sub.e when produced by the
combiner 109 may be applied to a comparator 110 in which it is
compared with a reference signal. The level of the reference signal
may be selected to be equivalent to an instant in time when the
RFPA 123 reaches a specified point of compression on its power
transfer characteristic. The level selected will depend on the
particular implementation conditions employed. An illustrative
value of the level is for example about 90 mV for a transmitter of
a TETRA mobile terminal.
[0043] The comparator 110 produces an output indication signal when
the level of the combined signal V.sub.e reaches the level of the
reference signal. The output indication signal is applied to the
DSP 102. The DSP 102 correlates the instant of time when the output
indication signal is produced by the comparator 110 with a
particular level of the input test signal V.sub.in which was
provided by the DSP 102 and which caused the production of the
output indication signal. The DSP 102 is able therefore to detect a
corresponding level of the input test signal V.sub.in at which the
RFPA 123 is driven into compression. In response, the DSP 102 sets
a level (amplitude) for output baseband signals to be produced by
the DSP 102 in the next period of the operational mode. The level
is chosen to be a suitable margin below the level detected to cause
compression of the RFPA 123.
[0044] As known in the art, the transmitter 100 may have a phase
training mode which may conveniently be applied prior to the level
training mode, e.g. in the same training interval in which each
application of the level training mode is made. In the phase
training mode, the control loop including the feedback control
channels 105 and 107 is operated in an open loop mode to determine
in a known manner a correct phase for stable operation of the
control loop. The result of phase training is programmed to the
phase shifter 149 to apply a suitable phase shift. The phase shift
so set is applied in the next period of the operational mode with
the control loop operating in closed loop mode.
[0045] An illustrative analysis of the transmitter 100 which
supports the use of the input test signal V.sub.in having the form
of the plot 203 shown in FIG. 2 is as follows.
[0046] For ideal application of the level training mode, the first
filter pole of the control loop comprising the I feedback control
channel 105 and the I channel 101 of the forward path and/or the Q
feedback channel 107 and the Q channel 103 of the forward path is
at a frequency of zero so that the first pole acts properly as an
integrator. However, when the first filter pole of the control loop
is at a frequency significantly greater than zero, such as at an
illustrative frequency of at least 1 kHz, e.g. at a frequency of 3
kHz as produced by the low pass filters 113 and 133, the first pole
is no longer an integrator.
[0047] The combined signal V.sub.e provides (after amplification by
the amplifier 111 or the amplifier 133 as appropriate) an input to
the filter providing the first pole of the control loop, i.e. the
low pass filter 113 or the low pass filter 133 as appropriate. The
combined signal V.sub.e provides a suitable baseband signal to
monitor to detect compression of the RFPA 123. The combined signal
V.sub.e is related to the input test signal V.sub.in applied in the
appropriate forward channel, e.g. the I channel 101, by the
relationship:
V e = V in * 1 .beta. A ( Equation 1 ) ##EQU00002##
where .beta.A is the Cartesian feedback loop gain.
[0048] Where the first loop filter pole is a perfect integrator,
applying V.sub.in in the form of a straight line ramp, as indicated
by the line 201 in FIG. 2, will produce in response a form of the
combined signal V.sub.e which does not increase with time during
the linear operation of the RFPA 123, i.e. before the compression
point of the RFPA 123.
[0049] However, where the first pole is not a perfect integrator,
for example at a frequency of at least 1 kHz, such as a frequency
of about 3 kHz, then applying Vin in the form of a straight line
ramp, as indicated by the line 201 in FIG. 2, will produce a form
of Ve which is not constant. By assuming that the first loop filter
pole is dominant and the second loop filter pole and zero can be
neglected, it can be shown that V.sub.e(t) and time t have the
approximate relationship:
V e ( t ) .apprxeq. a .beta. A t ( 2 ) ##EQU00003##
In other words, where the first pole is not a perfect integrator
and V.sub.in is applied in the form of a straight line ramp,
V.sub.e(t) is rising as a function of time as illustrated by the
plot 301 shown in FIG. 3, where .alpha. is a constant (the angle of
the lower part of the plot 301 relative to the horizontal time
axis). It has been found that this form of V.sub.e(t) can lead to
errors in detection of the compression point of the RFPA 123.
[0050] In order to calculate V.sub.in as a function of time, i.e.
V.sub.in(t), so that V.sub.e(t) remains constant, the Laplace
transform V.sub.e(s) of V.sub.e(t) may be considered. The Laplace
transform V.sub.e(s) is given by:
V e ( s ) = k s = .tau. 1 .beta. A [ s + 1 .tau. 1 ] V in ( s ) (
Equation 3 ) ##EQU00004##
where V.sub.in(s) is the Laplace transform of V.sub.in(t).
Rearranging Equation 3 gives:
V in ( s ) = k .beta. A .tau. 1 [ 1 s + 1 .tau. 1 ] 1 s = G ( s ) 1
s where ( Equation 4 ) G ( s ) = k .beta. A .tau. 1 [ 1 s + 1 .tau.
1 ] ( Equation 5 ) ##EQU00005##
k is a constant and .beta.A and .tau.1 are as defined earlier. The
inverse Laplace transform of G(s), G(t), can be obtained as
follows. Using the property of Laplace transformation, if
V in ( s ) = G ( s ) 1 s ##EQU00006##
then:
V in ( t ) = .intg. 0 t G ( t ) t ( Equation 6 ) ##EQU00007##
Inverse Laplace transformation of G(s) defined in Equation 5 thus
allows G(t) to be obtained as follows:
G ( t ) = k .beta. A .tau. 1 - t .tau. 1 ( Equation 7 )
##EQU00008##
Thus, substituting the value of G(t) given in Equation 7 into
Equation 6 gives:
V in ( t ) = k .beta. A .tau. 1 .intg. 0 t - t .tau. 1 t = k .beta.
A [ 1 - - t .tau. 1 ] ( Equation 8 ) ##EQU00009##
Thus, the function defined by Equation 8 represents the form of the
voltage waveform required for the test signal V.sub.in(t) to
produce a constant value of the combined voltage V.sub.e until the
RFPA 123 is driven into compression.
[0051] The maximum value V.sub.--.sub.--.sub.max of the applied
waveform of the test signal V.sub.in(t) should be high enough to
drive the RFPA 123 into compression during the level training mode.
V.sub.in.sub.--.sub.max is given by:
V.sub.in.sub.--.sub.max=k.beta.A=.beta.V.sub.out.sub.--.sub.max
(Equation 9)
where V.sub.out.sub.--.sub.max is the maximum RMS level in Volts of
the signal produced by the RFPA 123. For a TETRA transmitter, the
output power needs to reach 6 Watts during level training as
specified by the TETRA standard. This is equivalent to an RMS value
of V.sub.out.sub.--.sub.max of about 17 Volts. In addition, in an
illustrative example, .beta.=-50 dB=1/316, k is 30 millivolts and A
is 67 dB. Using these values gives an illustrative maximum value
V.sub.in.sub.--.sub.max of the applied waveform V.sub.in(t) of
about 0.22 Volt. Thus, the maximum level of V.sub.in(t) when of the
form illustrated by plot 203 may be selected to be a suitable
value, e.g. in a range between about 0.1 Volt and about 1.0 Volt,
especially between about 0.1 Volt and 0.5 Volt.
[0052] A suitable value of the required duration of the applied
waveform V.sub.in(t) having the form of the plot 203 in FIG. 3 may
be a duration in the range of from about 100 microseconds to about
300 microseconds. An illustrative calculation of the duration is as
follows. It is known that for an exponential function having the
form defined by Equation 8, the exponential function reaches 95
percent of its final value (the asymptote 205) when the time has
reached a value t=3.tau.1. Thus, the duration may be set as a
suitable time longer (e.g. at least 105 percent longer) than
t=3.tau.1. As noted earlier, if for example the first filter pole
of the control loop has a frequency of 3 kHz, then t=3.tau.1 has a
value of about 159 microseconds. So, for the case where the first
pole has a frequency of 3 kHz, the duration of the waveform of the
test signal V.sub.in(t) may be a suitable time not less than about
165 microseconds. Typically, the duration may be selected to be a
time of between about 100 microseconds and about 300 microseconds,
such as a time of about 200 microseconds.
[0053] FIG. 4 is a flow chart of a method 400 which summarises
operation of the transmitter 100 in the embodiments described
above.
[0054] At 401 of the method 400, a normal operational mode of the
transmitter 100 is suspended and a closed loop training mode is
begun. This may be at the start of an interval set aside for the
training mode. For example where the transmitter 100 is a TETRA
transmitter, the interval may be a time frame set aside for such
training as defined by the TETRA standard.
[0055] At 403, a test signal generated by the DSP 102 is applied as
an I channel input signal to the combiner 109 and/or as a Q channel
input signal to the combiner 129. The test signal is a voltage
waveform V.sub.in having the inverse exponential form represented
by Equation 8 given earlier.
[0056] At 405 which is carried out in conjunction with 403, a
voltage V.sub.e developed at the output of the combiner 109 and/or
the combiner 129 as appropriate is monitored. The voltage V.sub.e
has, as a function of time, the form of the plot 303 shown in FIG.
3.
[0057] At 407, compression of the RFPA 123 is detected, e.g. by the
comparator 110, from an increase in V.sub.e, and an indication
signal is sent to the DSP 102.
[0058] In response to receiving the indication signal, the DSP 102
determines the voltage level of V.sub.in at which the RFPA 123 was
driven into compression and accordingly, at 409, sets a signal
level for baseband signals generated by the DSP 102 to avoid
compression of the RFPA 123. The DSP 102 may use the time indicated
by the delivered signal, i.e. the time of issue of the delivered
signal, to identify a point in time on the test signal comprising
the voltage waveform V.sub.in when the signal level is at suitable
level for selection.
[0059] Finally, the normal operational mode of the transmitter 100
is resumed at 411, and the DSP 102 uses the signal level set in the
level training mode for signals produced by the DSP 102 in the
normal operational mode.
[0060] The transmitter 100 and the method 400 are suitable for use
in: (i) in a narrow band communication terminal such as a terminal
operating according to the basic TETRA (TETRA 1) standard defined
by the European Telecommunications Standards Institute, in which
the transmitted signal will have a relatively narrow 25 kHz
modulation bandwidth; the first pole of the loop filter may be
selected accordingly to be at or near to a frequency of zero Hertz;
and also in: a communication terminal having a wider modulation
band, e.g. in a TETRA 2 terminal operating according to the TETRA 2
(TETRA Enhanced Data Services) standard defined by ETSI having a
modulation bandwidth of up to 150 kHz; the position of the loop
filter first pole in such a terminal is desirably shifted to a
higher frequency, e.g. a frequency of at least 1 kHz, e.g. about 3
kHz. The transmitter 100 may be a dual- or multi-mode transmitter
which operates in different selected modes (e.g. TETRA 1 and TETRA
2) in the same terminal.
[0061] In the foregoing specification, specific embodiments have
been described. However, one of ordinary skill in the art will
appreciate that various modifications and changes can be made
without departing from the scope of the invention as set forth in
the accompanying claims. Accordingly, the specification and
drawings are to be regarded in an illustrative rather than a
restrictive sense, and all such modifications are intended to be
included within the scope of present teachings. The benefits,
advantages, solutions to problems, and any element(s) that may
cause any benefit, advantage, or solution to occur or become more
pronounced are not to be construed as critical, required, or
essential features or elements of any or all the claims. The
invention is defined solely by the appended claims including any
amendments made during the pendency of this patent application and
all equivalents of those claims as issued.
[0062] Moreover in this document, relational terms such as `first`
and `second`, `top` and `bottom`, and the like, may be used solely
to distinguish one entity or action from another entity or action
without necessarily requiring or implying any actual such
relationship or order between such entities or actions. The terms
`comprises`, `comprising`, `has`, `having`, `includes`,
`including`, `contains`, `containing` or any other variation
thereof, are intended to cover a non-exclusive inclusion, such that
a process, method, article, or apparatus that comprises, has,
includes or contains a list of elements does not include only those
elements but may include other elements not expressly listed or
inherent to such process, method, article, or apparatus. An element
preceded by `comprises . . . a`, `has . . . a`, `includes . . . a`,
or `contains . . . a` does not, without more constraints, preclude
the existence of additional identical elements in the process,
method, article, or apparatus that comprises, has, includes,
contains the element. The terms `a` and `an` are defined as one or
more unless explicitly stated otherwise herein.
[0063] The terms `substantially`, `essentially`, `approximately`,
`about` or any other version thereof, are defined as being close to
as understood by one of ordinary skill in the art, and in one
non-limiting embodiment the term is defined to be within 10%, in
another embodiment within 5%, in another embodiment within 1% and
in another embodiment within 0.5%, of a stated value. It is to be
noted that these expressions may be used herein to indicate that a
value obtained in practice may be different from a nominal target
value because of the imprecise nature of implementing a circuit in
which the value applies.
[0064] The term `coupled` as used herein is defined as connected,
although not necessarily directly and not necessarily mechanically.
A device or structure that is `configured` in a certain way is
configured in at least that way, but may also be configured in ways
that are not listed.
[0065] It will be appreciated that some embodiments may be
comprised of one or more generic or specialized processors (or
"processing devices") such as microprocessors, digital signal
processors, customized processors and field programmable gate
arrays (FPGAs) and unique stored program instructions (including
both software and firmware) that control the one or more processors
to implement, in conjunction with certain non-processor circuits,
some, most, or all of the functions of the method and apparatus for
synchronization in a digital mobile communication system as
described herein. The non-processor circuits may include, but are
not limited to, a radio receiver, a radio transmitter, signal
drivers, clock circuits, power source circuits, and user input
devices. As such, these functions may be interpreted as steps of a
method to perform the synchronization in a digital mobile
communication system as described herein. Alternatively, some or
all functions could be implemented by a state machine that has no
stored program instructions, or in one or more application specific
integrated circuits (ASICs), in which each function or some
combinations of certain of the functions are implemented as custom
logic. Of course, a combination of the two approaches could be
used. Both the state machine and ASIC are considered herein as a
`processing device` for purposes of the foregoing discussion and
claim language.
[0066] Moreover, an embodiment including a memory can be
implemented as a computer-readable storage element having computer
readable code stored thereon for programming a computer (e.g.,
comprising a processing device) to perform a method as described
and claimed herein. Examples of such computer-readable storage
elements include, but are not limited to, a hard disk, a CD-ROM, an
optical storage device, a magnetic storage device, a ROM (Read Only
Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable
Programmable Read Only Memory), an EEPROM (Electrically Erasable
Programmable Read Only Memory) and a Flash memory.
[0067] Further, it is expected that one of ordinary skill,
notwithstanding possibly significant effort and many design choices
motivated by, for example, available time, current technology, and
economic considerations, when guided by the concepts and principles
disclosed herein will be readily capable of generating such
software instructions and programs and ICs with minimal
experimentation.
[0068] The Abstract of the Disclosure is provided to allow the
reader to quickly ascertain the nature of the technical disclosure.
It is submitted with the understanding that it will not be used to
interpret or limit the scope or meaning of the claims. In addition,
in the foregoing Detailed Description, it can be seen that various
features are grouped together in various embodiments for the
purpose of streamlining the disclosure. This method of disclosure
is not to be interpreted as reflecting an intention that the
claimed embodiments require more features than are expressly
recited in each claim. Rather, as the following claims reflect,
inventive subject matter lies in less than all features of a single
disclosed embodiment. Thus the following claims are hereby
incorporated into the Detailed Description, with each claim
standing on its own as a separately claimed subject matter.
[0069] In addition, in the foregoing Detailed Description, it can
be seen that various features are grouped together in various
embodiments for the purpose of streamlining the disclosure. This
method of disclosure is not to be interpreted as reflecting an
intention that the claimed embodiments require more features than
are expressly recited in each claim. Rather, as the following
claims reflect, inventive subject matter may lie in less than all
features of a single disclosed embodiment. Thus the following
claims are hereby incorporated into the Detailed Description, with
each claim standing on its own as a separately claimed subject
matter.
* * * * *