U.S. patent application number 13/308346 was filed with the patent office on 2013-05-09 for method for tuning an oscillator frequency and device implementing the method.
This patent application is currently assigned to ST-ERICSSON SA. The applicant listed for this patent is Pierre Demaj, Philippe Thierion. Invention is credited to Pierre Demaj, Philippe Thierion.
Application Number | 20130115902 13/308346 |
Document ID | / |
Family ID | 45315661 |
Filed Date | 2013-05-09 |
United States Patent
Application |
20130115902 |
Kind Code |
A1 |
Demaj; Pierre ; et
al. |
May 9, 2013 |
METHOD FOR TUNING AN OSCILLATOR FREQUENCY AND DEVICE IMPLEMENTING
THE METHOD
Abstract
A method for tuning an oscillator frequency produced in a
radio-receiver chain is based on a slope analysis of a residual
difference between the oscillator frequency and a carrier frequency
of a radio-received signal. Depending on the slope value
calculated, a bandwidth of a low-pass time-filtering that is
effective for the frequency difference is increased or set back to
a default value. The method provides an improved trade-off between
tuning precision and dynamic behavior when the frequency difference
drifts rapidly.
Inventors: |
Demaj; Pierre; (Nice,
FR) ; Thierion; Philippe; (Nice, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Demaj; Pierre
Thierion; Philippe |
Nice
Nice |
|
FR
FR |
|
|
Assignee: |
ST-ERICSSON SA
Plan-les-Ouates
CH
|
Family ID: |
45315661 |
Appl. No.: |
13/308346 |
Filed: |
November 30, 2011 |
Current U.S.
Class: |
455/192.1 |
Current CPC
Class: |
H03J 7/02 20130101; H04B
1/28 20130101 |
Class at
Publication: |
455/192.1 |
International
Class: |
H04B 1/18 20060101
H04B001/18 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 2011 |
EP |
11306452.1 |
Claims
1. A method for tuning an oscillator frequency, produced by an
oscillator unit within a radio-receiver chain, in accordance with a
carrier frequency of a radio-received signal, wherein the
oscillator unit comprises a digitally-controlled crystal oscillator
adapted to produce an oscillator signal with said oscillator signal
having the oscillator frequency; and wherein the radio-receiver
chain also comprises an automatic frequency control module adapted
to analog-tune the digitally-controlled crystal oscillator so as to
reduce a frequency difference between the oscillator frequency and
the carrier frequency, wherein the automatic frequency control
module is further adapted to perform a low-pass time-filtering of
said frequency difference, the method comprising: (1) measuring
repeatedly the frequency difference; (2) averaging the successively
obtained frequency difference measurement results; and (3) when the
averaged measurement results obtained successively for the
frequency difference exhibit an absolute slope value higher than or
equal to a positive slope threshold, then increasing the bandwidth
of the low-pass time-filtering of the frequency difference.
2. The method according to claim 1, wherein the radio-receiver
chain further comprises a baseband frequency corrector module
adapted to digitally tune the oscillator unit so as to further
reduce the frequency difference between the oscillator frequency
and the carrier frequency, and wherein the low-pass time-filtering
of said frequency difference is a result of the from combined
operation of both the automatic frequency control module and the
baseband frequency corrector module.
3. The method according to claim 1 wherein in step (3), the
increasing of the bandwidth of the low-pass time-filtering of the
frequency difference is performed only when a count of positive
ones among variations between two averaged measurement results
obtained successively, or another count of negative ones among said
variations, is higher than or equal to a drift threshold, in
addition to the absolute slope value of said averaged measurement
results being higher than or equal to the slope threshold.
4. The method according to claim 1, wherein the slope value for the
averaged measurement results obtained successively for the
frequency difference, is calculated in step (3) over N averaged
measurement results as a difference between an Nth and a first one
of said N averaged measurement results, divided by N, with N being
an integer superior than or equal to two.
5. The method according to claim 1, wherein step (3) also comprises
resetting the bandwidth of the low-pass time-filtering of the
frequency difference down to a default bandwidth value, if the
absolute slope value of the averaged measurement results obtained
successively is less than the slope threshold.
6. The method according to claim 1, wherein in step (2), the
averaging of the measurement results for the frequency difference
is carried out with a fixed number of measurement results.
7. The method according to claim 1, wherein in step (1), the
frequency difference is measured at a frame rate of the
radio-received signal.
8. The method according to claim 1, wherein steps (1), (2), or (3)
are carried out starting from a series of successive measurements
of the frequency difference between the oscillator frequency and
the carrier frequency, and said series is reset when at least one
of the following conditions occurs: when one measurement of said
frequency difference appears to be invalid; or when the automatic
frequency control module has performed an analog tuning sequence
since the last measurement of the frequency difference.
9. An electronic device for a radio-receiver chain, wherein the
device comprises an oscillator unit adapted to produce an
oscillator signal, and said oscillator unit comprises: a
digitally-controlled crystal oscillator arranged to produce the
oscillator signal with said oscillator signal having a oscillator
frequency; and wherein the electronic device comprises: an
automatic frequency control module adapted to analog-tune the
digitally-controlled crystal oscillator so as to reduce a frequency
difference between the oscillator frequency and a carrier frequency
of a radio-received signal with a low-pass time-filtering of said
frequency difference; a measurement module adapted to repeatedly
measure repeatedly the frequency difference; an averaging module
adapted for averaging the successively obtained measurement results
of the frequency difference; and a bandwidth adaptor adapted for
increasing the bandwidth of the low-pass time-filtering of the
frequency difference, and arranged for being activated when the
averaged successively obtained measurement results for the
frequency difference exhibit an absolute slope value higher than or
equal to a positive slope threshold.
10. The device according to claim 9, further comprising a baseband
frequency corrector module adapted to digitally tune the oscillator
unit so as to further reduce the frequency difference between the
oscillator frequency and the carrier frequency, wherein the
low-pass time-filtering of said frequency difference results from a
combined operation of both the automatic frequency control module
and the baseband frequency corrector module.
11. The device according to claim 9, comprising an analog block and
a digital block, and wherein: the oscillator unit is comprised in
the analog block, and the measurement module for measuring the
frequency difference, the averaging module for averaging the
measurement results, and the bandwidth adaptor for increasing the
bandwidth of the low-pass time-filtering of the frequency
difference are comprised in the digital block.
12. A computer program product comprising a computer readable
medium, having thereon a computer program comprising program
instructions, the computer program being loadable into a
data-processing unit and adapted to cause execution of a method
according to claim 1 when the computer program is run by the
data-processing unit.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of European Application
Serial No. 11306452.1, filed Nov. 8, 2011, entitled METHOD FOR
TUNING AN OSCILLATOR FREQUENCY AND DEVICE IMPLEMENTING THE METHOD,
the specification of which is incorporated herein by reference in
its entirety.
TECHNICAL FIELD
[0002] The invention relates to a method for tuning an oscillator
frequency and an electronic device for a radio-receiver chain which
implements this method. It may be useful in particular when the
device is intended for a mobile user equipment which operates
according to the protocol "High Speed Downlink Packet Access"
(HSDPA) as specified in the release 5 of the WCDMA--3GPP standard
("Wideband-Code Division Multiple Access--3GPP").
BACKGROUND
[0003] An electronic device for a radio-receiver chain, for example
for a user's mobile communication equipment, may include an
oscillator unit which produces an oscillator frequency. The
oscillator unit may include a digitally-controlled crystal
oscillator, which is arranged to produce an oscillator signal
having an oscillator frequency. Such an electronic radio-receiver
device may also include an automatic frequency control (AFC)
module, which is arranged to analog-tune the digitally-controlled
crystal oscillator so as to reduce the frequency difference between
the oscillator frequency and a carrier frequency of a
radio-received signal.
[0004] Such a digitally-controlled crystal oscillator has the
advantage of having a low cost price, but the oscillator frequency
as produced directly by such a digitally-controlled crystal
oscillator may exhibit frequency-drift over time. Such drift may be
due to temperature variation of the oscillator, in particular when
the user equipment is transmitting data uplinks with a high
transmission rate. Indeed, such high transmission rates cause a
power amplifier of the mobile user equipment to rise in
temperature, which in turn causes the temperature of the
digitally-controlled crystal oscillator to increase. But, the
digitally-controlled crystal oscillator is usually devoid of any
temperature variation compensation system. So, the oscillator
frequency increases as a consequence of the rise of the oscillator
temperature. The amplitude of such frequency increase may be up to
20 Hz (Hertz). Then, received data from high transmission rate
radio-received signals are extremely sensitive to any mismatch
existing between the oscillator frequency and the carrier
frequency. However, the drift of the oscillator frequency which is
produced by the digitally-controlled crystal oscillator may also
have many causes other than temperature variations.
[0005] In addition, the carrier frequency of the radio-received
signal may also change in time, depending on the quality of the
radio-transmission between the mobile user equipment and a node B
from which the radio signal originates. As a consequence of the
variations of both the oscillator frequency produced by the
oscillator unit and the carrier frequency of the radio-received
signal, an apparent drift occurs for due to the difference between
these oscillator frequency and carrier frequency. But for obtaining
a symbol decoding with a minimum error rate by the radio-receiver
chain of the mobile user equipment, it is essential to reduce the
difference between the oscillator frequency and the carrier
frequency as much as possible.
[0006] To this purpose, it is also known that a radio-receiver
chain may additionally comprise a baseband frequency corrector
(BFC) module. Such baseband frequency corrector module may be
arranged for digitally tuning the oscillator unit, so that the
oscillator frequency matches the carrier frequency with residual
frequency difference further reduced compared to the efficiency of
the automatic frequency control module alone.
[0007] Implementation of the HSDPA protocol requires very fine
tuning of the oscillator frequency so as to match the carrier
frequency, in particular because of the very high downlink
transmission rate of the High Speed Dedicated Shared Channel
(HS-DSCH). Even a drift amplitude of a few Hertz difference between
the oscillator frequency and the carrier frequency has negative
impact on the error rate of the symbol decoding.
[0008] Also known is that the automatic frequency control module
when implemented without a baseband frequency corrector module, or
the combination of an automatic frequency control module with the
baseband frequency corrector module, operates as a low-pass
time-filtering of the frequency difference between the oscillator
frequency and the carrier frequency. This time-filtering creates a
trade-off between low residual frequency difference and efficiency
of the reduction of this frequency difference when drift occurs for
the frequency difference with a non-zero drift rate. Put another
way, a trade-off occurs between the static performance and the
dynamic behaviour for reducing the difference between the
oscillator frequency and the carrier frequency.
[0009] Then, what is needed is a technique and device that improves
the efficiency of controlling the oscillator frequency to better
match the carrier frequency, in particular in dynamic
conditions.
SUMMARY
[0010] To this end, one aspect of the invention proposes a method
for tuning an oscillator frequency within a radio-receiver chain
with a baseband frequency corrector module. An exemplary method
comprises the following steps: [0011] (1) measuring repeatedly the
frequency difference between the oscillator frequency produced by
the oscillator unit and the carrier frequency of the radio-received
signal; [0012] (2) averaging the measurement results which are
obtained successively for the frequency difference; and [0013] (3)
if the averaged measurement results that are obtained successively
for the frequency difference exhibit an absolute slope value higher
than or equal to a positive slope threshold, then increasing the
bandwidth of the low-pass time-filtering of the frequency
difference, as this time-filtering is being produced by the
automatic frequency control module alone or in combination with the
baseband frequency corrector module.
[0014] Thus, such an aspect of the invention proceeds by adapting
the bandwidth of the time-filtering effective for the frequency
difference based on the drift rate of this frequency difference. A
larger low-pass bandwidth is selected when the drift of the
frequency difference is observed to be more rapid. In particular, a
drift which is caused by a temperature increase of the oscillator
unit can be compensated efficiently in this way. In addition, the
average calculation performed in step (2) filters out
non-significant random drifts which are due to noise.
[0015] Embodiments of the invention are compatible with using a
low-cost digitally-controlled crystal oscillator. Preferably, they
are implemented when the baseband frequency corrector module is
present, for still higher efficiency.
[0016] Another advantage of embodiments of the invention results
from the possibility of implementation without modifying the design
of the radio-receiver chain, but only by adding a suitable means
for performing the additional steps (1) to (3). Furthermore, these
means for performing steps (1) to (3) may be software-type, which
leads to reduced additional costs.
[0017] According to one embodiment of the invention, the increase
of the bandwidth of the low-pass time-filtering of the frequency
difference may be performed in step (3) only if a further condition
is met. It may be performed if a count of positive ones among
variations between two averaged measurement results, which are
obtained successively, or another count of those of these
variations that are negative, is higher than or equal to a drift
threshold, in addition to the condition that the absolute slope
value of the averaged measurement results is to be higher than or
equal to the slope threshold.
[0018] In some embodiments of the invention, the slope value for
the averaged measurement results obtained successively for the
frequency difference may be calculated in step (3) over N averaged
measurement results as a difference between an Nth and a first one
of the N averaged measurement results, divided by N. Here N is an
integer superior than or equal to two. Such implementation of the
invention is easy, and it is rapid to execute during reception of
the radio signal.
[0019] Embodiments of the invention may be completed in case the
drift rate of the frequency difference decreases, so as to recover
the initial precision in the matching between the oscillator
frequency and the carrier frequency. To this purpose, step (3) may
also comprise resetting the bandwidth of the low-pass
time-filtering of the frequency difference down to a default
bandwidth value, if the absolute slope value of the averaged
measurement results obtained successively has become less than the
slope threshold.
[0020] In varying embodiments of the invention, the following may
be used, separately from one another or in combination of some of
them: [0021] in step (2), the averaging of the measurement results
for the frequency difference may be carried out with a fixed number
of measurement results; [0022] in step (1), the frequency
difference may be measured at a frame rate of the radio-received
signal; and [0023] steps (1) to (3) may be carried out starting
from a series of successive measurements of the frequency
difference between the oscillator frequency and the carrier
frequency, and [0024] this series may be reset when one or more of
the following conditions occur: [0025] when one measurement of the
frequency difference appears to be invalid; or [0026] when the
automatic frequency control module has performed an analog tuning
sequence since the last measurement of the frequency
difference.
[0027] The method according to embodiments of the invention is
particularly beneficial for a mobile user equipment which is
adapted to receive radio signals in accordance with the "High Speed
Downlink Packet Access" protocol. Indeed, the method provides
further reduced error rate in the symbol decoding, although the
downlink reception rate may be very high.
[0028] Other aspects of the invention propose an electronic device
for a radio-receiver chain, which includes an oscillator unit and
an automatic frequency control module, with the oscillator unit
comprising itself a digitally-controlled crystal oscillator.
According to these exemplary invention aspects, the device may
further comprise a measurement module, an averaging module and a
bandwidth adaptor adapted for performing steps (1) to (3) as
described earlier, possibly with at least one of the improvements
also described. In other exemplary embodiments, when the device
comprises an analog block and a digital block, the oscillator unit
is comprised in the analog block and the modules for performing the
steps (1) to (3) are comprised in the digital block. Optionally,
the device for a radio-receiver chain may still further include a
baseband frequency corrector module.
[0029] Still other aspects of the invention propose a computer
program product which comprises a computer readable medium, having
thereon a computer program comprising program instructions. The
computer program is loadable into a data-processing unit that is
adapted to cause execution of the method as described above when
the computer program is run by the data-processing unit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] For a more complete understanding, reference is now made to
the following description taken in conjunction with the
accompanying Drawings in which:
[0031] FIG. 1 illustrates a downlink transmission of a radio-signal
to a mobile user equipment implementing embodiments of the
invention.
[0032] FIG. 2 represents several modules of a radio-receiver chain
of the mobile user equipment of FIG. 1, which are involved by
embodiments of the invention.
[0033] FIG. 3 is a flowchart for an embodiment of the
invention.
DETAILED DESCRIPTION
[0034] Referring now to the drawings, wherein like reference
numbers are used herein to designate like elements throughout, the
various views and embodiments of a method for tuning an oscillator
frequency and device implementing the method are illustrated and
described, and other possible embodiments are described. The
figures are not necessarily drawn to scale, and in some instances
the drawings have been exaggerated and/or simplified in places for
illustrative purposes only. One of ordinary skill in the art will
appreciate the many possible applications and variations based on
the following examples of possible embodiments.
[0035] Embodiments of the invention are now described in detail,
and include using a baseband frequency corrector module. But, the
baseband frequency corrector module is optional so that other
invention embodiments may be implemented without the radio-receiver
chain including a baseband frequency corrector module.
[0036] According to FIG. 1, a mobile user equipment 10 can receive
a radio signal S from a node B with reference number 20. Such
transmission of signal S from the node B 20 to the mobile user
equipment 10 is called a downlink, as opposed to reverse
transmission from the mobile user equipment 10 to the node B 20,
which is called an uplink. The mobile user equipment 10 may be a
mobile phone terminal or a touchpad internet terminal for example.
The communication considered in this specification may implement
the UMTS standard, or the protocol HSDPA, although it is not
limited to this transmission system.
[0037] The radio signal S is comprised of a carrier with a defined
carrier frequency, and modulation of this carrier in accordance
with symbols to be transmitted. The carrier frequency of the signal
S as being radio-received by the mobile user equipment 10 may vary,
in particular, depending on radio transmission conditions existing
between the node B 20 and the user equipment 10. These transmission
conditions may include, in particular, a Doppler frequency shift
due to a movement of the mobile user equipment 10.
[0038] Embodiments of the invention relate to the operation of the
radio-receiver chain of the user equipment 10 when receiving the
signal S. This radio-receiver chain is partially represented in
FIG. 2. Obviously, it may comprise other modules in addition to
those indicated, for example filters, demapper, de-interleaver,
etc, but for the sake of clarity and because the operation of these
other modules is not modified by implementing embodiments of the
invention, they are not shown in FIG. 2. In a known manner, the
modules of the radio-receiver chain are distributed within an
analog block 11 and a digital block 12 of an electronic device.
[0039] The analog block 11 comprises an antenna 4 for sensing the
radio signal S, an analog amplifier 5, and a down-converter 6 to
produce in-phase sample I and quadrature sample Q. Also known is
the operation of the down-converter 6, which mixes the radio signal
S detected filtered and amplified with an oscillator signal FRQ
received from a local oscillator unit 1, internal to the
radio-receiver chain of the user equipment 10.
[0040] Internally to the analog block 11, the oscillator unit 1
comprises itself a crystal oscillator 2 and a controller 2a which
is noted CONTROL in FIG. 2 and connected to an analog control
terminal of the crystal oscillator 2. The crystal oscillator 2
produces the oscillator signal FRQ with an oscillator frequency
intended to match as much as possible the carrier frequency of the
radio signal S. The crystal oscillator 2 is of DCxO-type, standing
for digitally-controlled crystal oscillator. Such crystal
oscillator is not compensated for temperature variations, leading
to variations in the frequency of the signal produced. The
operation of the DCxO oscillator 2, in particular its oscillator
frequency, can be tuned using the controller 2a.
[0041] The digital block 12 comprises a baseband equalizer 7, noted
BB_EQUAL, a symbol decoder 9, noted DECOD, and an automatic
frequency control module 3, noted AFC. The I- and Q-samples are
transmitted to the baseband equalizer 7, possibly using an
analog-to-digital converter (not shown) intermediate between the
analog block 11 and the digital block 12. Then the I- and Q-samples
are further processed by the digital block 12, up to the decoder
9.
[0042] An input of the automatic frequency control module 3 is
connected to an output of the baseband equalizer 7 for receiving an
estimation of the difference existing between the oscillator
frequency and the carrier frequency. The automatic frequency
control module 3 is connected in turn to the controller 2a, for
tuning the DCxO oscillator 2 so as to reduce the frequency
difference between the oscillator signal FRQ and the received
signal S.
[0043] Such an arrangement of the radio-receiver chain of the
oscillator unit 1 produces a low-pass time-filtering of the
difference between the oscillator frequency and the carrier
frequency which is used for tuning the oscillator frequency in
accordance with the carrier frequency. Thus, variations of the
frequency difference, which exhibit a drift rate above a low-pass
bandwidth value, are poorly effective or not effective for the
tuning of the oscillator frequency.
[0044] In addition, there is a lower limit for the value of the
frequency difference, for the automatic frequency control module 3
to trigger an analog tuning of the DCxO oscillator 2. As a
consequence, even for slow variations of the carrier frequency, the
oscillator frequency as produced by the oscillator unit 1 has a
step-wise variation.
[0045] For smoothing this stepwise variation of the oscillator
frequency, the digital block 12 may also comprise a baseband
frequency corrector module 8, noted BFC. This baseband frequency
corrector module 8 is digitally operating, and is sensitive to
frequency difference values which are below the triggering limit of
the automatic frequency control module 3. It is usually connected
to a digital control terminal of oscillator unit 1 or of the
automatic frequency control module 3. In this way, the oscillator
frequency can be produced to match the carrier frequency with less
residual difference. But such residual difference may still impact
the error rate of the symbol decoding which is performed by the
decoder 9. Invention embodiments further improve the tuning of the
oscillator frequency according to the carrier frequency, in
particular when this latter is time-varying.
[0046] Although the baseband frequency corrector module 8 is
optional for implementing the invention, it is incorporated therein
because of producing finer frequency tuning When used, the baseband
frequency corrector module 8 participates to the time-filtering
which impacts the difference between the oscillator frequency and
the carrier frequency, which difference is effective for the
frequency tuning Thus, the bandwidth to be considered for this
time-filtering is the bandwidth resulting from the combined
operations of both the automatic frequency control module 3 and the
baseband frequency corrector module 8.
[0047] The digital block 12 may further comprise an additional
loop, for separating the in-phase sample I and quadrature sample Q
in an improved extent. To this purpose, the I- and Q-samples are
further mixed with another signal produced by a numerically
controlled oscillator 8a, noted NCO, using an additional mixer 8b
internal to the digital block 12. The frequency of the signal
produced by the numerically controlled oscillator 8a is digitally
controlled by the baseband frequency corrector module 8.
[0048] According to embodiments of the invention, the digital block
12 comprises an additional module 13, which may be
software-implemented. This additional module 13 cooperates with the
baseband frequency corrector module 8 for transmitting to the
automatic frequency control module 3 control signals for reducing
the residual difference between the oscillator frequency and the
carrier frequency. The frequency difference measurements used by
the additional module 13 may originate from the baseband frequency
corrector module 8, or from other modules within the digital block
12. The output of the additional module 13 is connected for further
controlling the DCxO oscillator 2, via the controller 2a and
possibly in combination with the control connection from the
automatic frequency control module 3. When using the additional
digital loop with the numerically controlled oscillator 8a and the
mixer 8b, the additional module 13 also participates in still
further improving the separation of the I- and Q-samples.
[0049] In FIG. 2, the additional module 13 introduced by the
present invention is noted SLOPE ANALYZER. Its operation is now
described in connection with FIG. 3 for an exemplary implementation
of the invention. It may be soft- or hard-ware implemented.
[0050] Referring now to FIG. 3, the method of the invention starts
with collecting a series of N averaged measurement results for the
difference between the oscillator frequency and the carrier
frequency. This frequency difference is measured or estimated
within the digital block 12 by carrying out the following steps:
[0051] ST0: resetting to a default bandwidth value of the frequency
difference low-pass bandwidth which is effective for tuning the
DCxO oscillator 2; [0052] ST1: collecting successive measured
values for the difference .DELTA.F between the oscillator frequency
and the carrier frequency; [0053] ST2: testing whether the number
Nb(.DELTA.F) of .DELTA.F values measured is sufficient for
performing an average calculation; [0054] ST3: calculation of the
average value <.DELTA.F> for the K last .DELTA.F values
measured, K being an integer higher than or equal to two; and
[0055] ST4: testing whether the number Nb(<.DELTA.F>) of
average values <.DELTA.F> is sufficient.
[0056] In step ST0, the default bandwidth value may be preferably
selected to ensure fine matching of the oscillator frequency with
the carrier frequency in stationary conditions.
[0057] In step ST1, the frequency difference .DELTA.F may be
measured continually during the reception of the radio signal S,
for example each time a new signal frame is processed.
[0058] In steps ST2 and ST3, the number K of the .DELTA.F values,
which are successively measured and collected for calculating the
next average value <.DELTA.F> may be five, for example,
corresponding to one value collected every 2 ms (millisecond) for
the frequency difference.
[0059] This sequence of collecting successive average values for
the frequency difference may be stopped and reset to step ST0 upon
each operation of the automatic frequency control module 3, which
changes the operation set point of the DCxO oscillator 2. It may
also be stopped and reset upon detecting that the measurement
result obtained for the current execution of step ST1 is not
valid.
[0060] After step ST4 has been performed successfully, N values are
available for the average frequency difference. These average
values are denoted <.DELTA.F>.sub.i, with the integer index i
varying from 1 to N, N possibly being equal to eight, for example.
Two average values <.DELTA.F>.sub.i which are calculated
successively may relate to respective sets of measured .DELTA.F
values which are separate but collected after one another.
Alternatively, each average value <.DELTA.F>.sub.i may be
calculated based on the K last .DELTA.F measurement values
collected, after a new .DELTA.F measurement has been achieved.
[0061] Then the successive variations of the average values
<.DELTA.F>.sub.i are calculated in step ST5, leading to N-1
signed variation values
<.DELTA.F>.sub.i<.DELTA.F>.sub.i-1, i varying from 2 to
N. Step ST6 consists thereafter in counting those among the
variation values <.DELTA.F>.sub.i<.DELTA.F>.sub.i-1
which are strictly positive, leading to count result Q.sub.+.
Similarly, Q- is the number of the variation values
<.DELTA.F>.sub.i<.DELTA.F>.sub.i-1 which are strictly
negative.
[0062] Steps ST7 to ST10 relate to possible increase of the
low-pass bandwidth for the frequency difference used for tuning the
DCxO oscillator 2, when the difference between the oscillator
frequency and the carrier frequency is observed to vary
rapidly.
[0063] Step ST7 sets a first condition for increasing the
bandwidth, based on the count results Q.sub.+ and Q-. One of these
latter count results needs to be higher than or equal to a first
threshold for the step sequence to be run further. This first
condition requires that a major number of variation values
<.DELTA.F>.sub.i<.DELTA.F>.sub.i-1 is of same algebraic
sign, meaning that the successive average values
<.DELTA.F>.sub.i exhibit a main tendency to increasing or
decreasing. The first threshold is called drift threshold and noted
TH_DRIFT, and may be equal to 0.75.times.N, for example. In case
this first condition is not met, the step sequence may be started
again from step ST0, with resetting of the low-pass bandwidth back
to its default value.
[0064] In alternative embodiments of the invention, the first
condition tested in step ST7 may relate to the absolute difference
between both count results Q.sub.+ and Q-. It may require that this
absolute difference |Q.sub.+-Q-| is higher than or equal to an
appropriate threshold value before proceeding further with step
ST8, otherwise the sequence is also started again with step
ST0.
[0065] If step ST7 is passed, then step ST8 comprises in
calculating a global variation from first to last average value:
<.DELTA.F>.sub.N-<.DELTA.F>.sub.1, and then calculating
the slope value
[<.DELTA.F>.sub.N-<.DELTA.F>.sub.1]/[(N-1).times.K].
[0066] Step ST9 sets a second condition for increasing the
bandwidth, based on the absolute value of the slope calculated in
step ST8. The absolute value for slope
[<.DELTA.F>.sub.N-<.DELTA.F>.sub.1]/[(N-1).times.K] is
to be higher than or equal to a second threshold which is strictly
positive, for passing step ST9. This second threshold is called
slope threshold and noted TH_SLOPE. For example, this slope
threshold TH_SLOPE may be set to 12.6 Hz/s (Hertz per second). If
the second condition of step ST9 is not met, the step sequence may
be started again from step ST0, with resetting of the low-pass
bandwidth back to its default value.
[0067] If the second condition of step ST9 is met, then the
bandwidth is increased in step ST10. In some implementations of the
invention, this increase may be produced by changing at least one
filter coefficient for the filter which is effective for the
frequency difference feedback. The bandwidth increase may be
stepwise with fixed increment, or with a varying increase magnitude
as a function of the calculated absolute slope value
|<.DELTA.F>.sub.N-<.DELTA.F>.sub.1|/[(N-1).times.K].
Optionally or because of a practical matter, a maximum limit may
exist for the bandwidth value. The bandwidth increase which is
controlled in step ST10 may be implemented by controlling the
baseband frequency corrector module 8. Thus, the dynamic
performance of the oscillator frequency control as a function of
the carrier frequency is improved. The operation of the
radio-receiver chain is then continued with the increased bandwidth
value, and the step sequence of FIG. 3 is repeated from step
ST1.
[0068] When the invention is implemented with a radio-receiver
chain devoid of a baseband frequency corrector module, the
additional module 13 may trigger the bandwidth increase by acting
directly onto the automatic frequency control module 3.
[0069] In the invention implementation just described, the
bandwidth value is tuned based directly on the slope value
calculated. In alternative implementations, the bandwidth tuning
may be performed based on a linear prediction of the evolution of
the frequency difference, the prediction being based on successive
calculated slope values.
[0070] In still further invention implementations, the bandwidth
value may be decreased with a process similar to that described
above for a bandwidth increase, when a slowdown is observed for the
drift of the difference between the oscillator frequency and the
carrier frequency.
[0071] It should be understood that the drawings and detailed
description herein are to be regarded in an illustrative rather
than a restrictive manner, and are not intended to be limiting to
the particular forms and examples disclosed. On the contrary,
included are any further modifications, changes, rearrangements,
substitutions, alternatives, design choices, and embodiments
apparent to those of ordinary skill in the art, without departing
from the spirit and scope hereof, as defined by the following
claims. Thus, it is intended that the following claims be
interpreted to embrace all such further modifications, changes,
rearrangements, substitutions, alternatives, design choices, and
embodiments.
* * * * *