U.S. patent number 9,270,311 [Application Number 14/560,827] was granted by the patent office on 2016-02-23 for methods and systems for calibrating an analog filter.
This patent grant is currently assigned to Marvell World Trade Ltd.. The grantee listed for this patent is Marvell World Trade Ltd.. Invention is credited to Srinivas Pinagapany, Atul Salhotra, Sergey Timofeev.
United States Patent |
9,270,311 |
Pinagapany , et al. |
February 23, 2016 |
Methods and systems for calibrating an analog filter
Abstract
Devices and methods capable of addressing filter responses are
disclosed. For example, a method for compensating a first low-pass
filter and a second low-pass filter is disclosed. The method
includes injecting a reference tone f.sub.R and a cutoff tone
f.sub.C into the first low-pass filter, and measuring respective
filter responses of the reference tone f.sub.R and the cutoff tone
f.sub.C while changing capacitor codes that control a cutoff
frequency of the first low-pass filter until a first capacitor code
I.sub.CODE is determined that most accurately causes the first
low-pass filter to utilize a desired cutoff frequency f.sub.0,
performing a similar operation for the second low-pass filter until
a second capacitor code Q.sub.CODE is determined, and calibrating
for mismatch between the first low-pass filter and the second
low-pass filter.
Inventors: |
Pinagapany; Srinivas (San Jose,
CA), Timofeev; Sergey (Mountain View, CA), Salhotra;
Atul (Santa Clara, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Marvell World Trade Ltd. |
St. Michael |
N/A |
BB |
|
|
Assignee: |
Marvell World Trade Ltd. (St.
Michael, BB)
|
Family
ID: |
52232439 |
Appl.
No.: |
14/560,827 |
Filed: |
December 4, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150155898 A1 |
Jun 4, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61911740 |
Dec 4, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B
17/21 (20150115); H03H 11/1291 (20130101); H04B
17/0085 (20130101); H03H 7/0161 (20130101); H04B
1/123 (20130101); H04B 1/40 (20130101); H03H
2210/025 (20130101); H03H 2210/046 (20130101) |
Current International
Class: |
H04B
1/10 (20060101); H04B 17/00 (20150101); H04B
1/12 (20060101); H04B 1/40 (20150101) |
Field of
Search: |
;375/350,370,373,369 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report and Written Opinion issued Feb. 27,
2015 in PCT/US2014/068545 filed on Dec. 4, 2014. cited by
applicant.
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Primary Examiner: Ghulamali; Qutbuddin
Parent Case Text
INCORPORATION BY REFERENCE
This application claims the benefit of U.S. Provisional Application
No. 61/911,740 entitled "Analog Filter Calibration" filed on Dec.
4, 2013, the content of which is incorporated herein by reference
in its entirety.
Claims
What is claimed is:
1. A method for compensating for non-idealities in a filter circuit
that includes programmable filter circuitry including a first
low-pass filter and a second low-pass filter both having a common
desired cutoff frequency f0, the method comprising: for a first
desired bandwidth BW0 corresponding to the common desired cutoff
frequency f0, injecting a reference tone IR and a cutoff tone fC
into the first low-pass filter, and measuring respective filter
responses of the reference tone fR and the cutoff tone fC while
changing capacitor codes that control a cutoff frequency f0-I of
the first low-pass filter until a first capacitor code ICODE is
determined that causes the first low-pass filter to match the
desired cutoff frequency f0 as close as possible given an available
resolution of the capacitor codes; for the first desired bandwidth
BW0, injecting the reference tone fR and the cutoff tone fC into
the second low-pass filter, and measuring respective filter
responses of the reference tone fR and the cutoff tone fC while
changing capacitor codes that control a cutoff frequency f0-Q of
the second low-pass filter until a second capacitor code QCODE is
determined that causes the second low-pass filter to match the
desired cutoff frequency f0 as close as possible given the
available resolution of the capacitor codes; and further
calibrating for mismatch between the first low-pass filter and the
second low-pass filter for one or more additional bandwidths
greater than the first desired bandwidth BW0.
2. The method of claim 1, wherein the one or more additional
bandwidths include a second desired bandwidth BW.sub.1, where
BW.sub.1=N.times.BW.sub.0, where N is a positive integer greater
than 1.
3. The method of claim 2, wherein calibrating for mismatch between
the first low-pass filter and the second low-pass filter includes:
for a respective second cutoff frequency f.sub.1, where
f.sub.1=(N.times.f.sub.0)+.DELTA.f, where .DELTA.f is a cutoff
frequency offset for the second desired bandwidth BW.sub.1:
determining a capacitor code offsets .DELTA.I.sub.OFFSET and
.DELTA.Q.sub.OFFSET; adding the capacitor code offset
.DELTA.I.sub.OFFSET to the first capacitor code I.sub.CODE to
produce a first compensated capacitor code I.sub.C-CODE; and adding
the capacitor code offset .DELTA.Q.sub.OFFSET to the second
capacitor code Q.sub.CODE to produce a second compensated capacitor
code Q.sub.C-CODE, wherein the second cutoff frequency
f.sub.1=(N.times.f.sub.0)+.DELTA.f, where .DELTA.f is a cutoff
frequency offset for the second desired bandwidth BW.sub.1.
4. The method of claim 3, wherein BW.sub.0=20 MHz, BW.sub.1=40 MHz,
f.sub.0=8.75 MHz, f.sub.1=18.75 MHz, and .DELTA.f=1.25 MHz; or
wherein BW.sub.0=20 MHz, BW.sub.1=80 MHz, f.sub.0=8.75 MHz,
f.sub.1=38.75 MHz, and .DELTA.f=3.75 MHz.
5. The method of claim 3, wherein calibrating for mismatch between
the first low-pass filter and the second low-pass filter further
includes: determining a fractional capacitor code CI.sub.FRAC
corresponding to the first desired bandwidth BW.sub.0, the
fractional capacitor code CI.sub.FRAC being a value that lies
between two consecutive capacitor codes [I.sub.CODE, I.sub.CODE+1],
and that ideally corresponds to both a zero phase difference and a
zero power difference between the first low-pass filter and the
second low-pass filter; and using the fractional capacitor code
CI.sub.FRAC to determine the capacitor code offsets
.DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET.
6. The method of claim 5, wherein determining the fractional
capacitor code CI.sub.FRAC includes: interpolating a line using a
plurality of points with each point having a first dimension being
a combined I-Q capacitor code [I.sub.CODE, Q.sub.CODE], and a
second dimension being a respective measured phase offset between
the first low-pass filter and the second low-pass filter using a
respective combined I-Q capacitor code; and selecting a combined
I-Q capacitor code value that corresponds to a substantially zero
phase difference between the first low-pass filter and the second
low-pass filter.
7. The method of claim 5, wherein using the fractional capacitor
code C.sub.FRAC to determine the capacitor code offset
.DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET includes: rounding the
fractional capacitor code CI.sub.FRAC to a nearest integer to
produce the capacitor code offset .DELTA.I.sub.OFFSET and
.DELTA.Q.sub.OFFSET; adding the capacitor code offset
.DELTA.I.sub.OFFSET to the first capacitor code I.sub.CODE to
produce the first compensated capacitor code I.sub.C-CODE; and
adding the capacitor code offset .DELTA.Q.sub.OFFSET to the second
capacitor code Q.sub.CODE to produce the second compensated
capacitor code Q.sub.C-CODE.
8. The method of claim 7, wherein using the fractional capacitor
code CI.sub.FRAC to determine the capacitor code offsets
.DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET includes: rounding to
the nearest integer a scaled
value=[(1+.alpha..DELTA.fc)*.DELTA.C.sub.FRAC] to produce the
capacitor code offsets .DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET,
where .DELTA.C.sub.FRAC is a difference between the first capacitor
code CI.sub.FRAC and the second capacitor code Q.sub.CODE, a is a
scaling factor derived from empirical data, and .DELTA.fc is a
capacitor code difference corresponding to the cutoff frequency
offset .DELTA.f; adding the capacitor code offset
.DELTA.I.sub.OFFSET to the first capacitor code I.sub.CODE to
produce the first compensated capacitor code I.sub.C-CODE; and
adding the capacitor code offset .DELTA.Q.sub.OFFSET to the second
capacitor code Q.sub.CODE to produce the second compensated
capacitor code Q.sub.C-CODE.
9. The method of claim 8, further comprising: applying the first
compensated capacitor code I.sub.C-CODE to the first low-pass
filter; and applying the second compensated capacitor code
Q.sub.C-CODE to the second low-pass filter.
10. A wirelessly operating device that operates according to the
method of claim 1.
11. A device for compensating for non-idealities in a filter
circuit that includes programmable filter circuitry including a
first low-pass filter and a second low-pass filter both having a
common desired cutoff frequency f.sub.0 corresponding to a first
desired bandwidth BW.sub.0, the device comprising: code search
circuitry that controls the first low-pass filter and the second
low-pass filter; tone generation circuitry that injects a reference
tone f.sub.R and a cutoff tone f.sub.C into both the first low-pass
filter and the second low-pass filter, measurement circuitry that:
(1) measures respective filter responses of the reference tone
f.sub.R and the cutoff tone f.sub.C while the code search circuitry
changes capacitor codes that control a cutoff frequency f.sub.0-1
of the first low-pass filter until a first capacitor code
I.sub.CODE is determined that causes the first low-pass filter to
match the desired cutoff frequency f.sub.0 as close as possible
given an available resolution of the capacitor codes; and (2)
measures respective filter responses of the reference tone f.sub.R
and the cutoff tone f.sub.C while the code search circuitry changes
capacitor codes that control a cutoff frequency f.sub.0-Q of the
second low-pass filter until a second capacitor code Q.sub.CODE is
determined that causes the second low-pass filter to match the
desired cutoff frequency f.sub.0 as close as possible given the
available resolution of the capacitor codes; and calibration
circuitry configured to calibrate for mismatch between the first
low-pass filter and the second low-pass filter for one or more
additional bandwidths greater than a first desired bandwidth
BW.sub.0 of the desired cutoff frequency f.sub.0.
12. The device of claim 11, wherein each of the one or more
additional bandwidths include a second desired bandwidth BW.sub.1,
where BW.sub.1=N.times.BW.sub.0, where N is a positive integer
greater than 1.
13. The device of claim 12, wherein the calibration circuitry is
further configured to: for a respective second cutoff frequency
f.sub.1 for the second bandwidth BW.sub.1, determine a capacitor
code offsets .DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET; add the
capacitor code offset .DELTA.I.sub.OFFSET to the first capacitor
code I.sub.CODE to produce a first compensated capacitor code
I.sub.C-CODE; and add the capacitor code offset .DELTA.Q.sub.OFFSET
to the second capacitor code Q.sub.CODE to produce a second
compensated capacitor code Q.sub.C-CODE; wherein the second cutoff
frequency f.sub.1=(N.times.f.sub.0)+.DELTA.f, where Of is a cutoff
frequency offset for the second desired bandwidth BW.sub.1.
14. The device of claim 13, wherein the calibration circuitry is
further configured to calibrate for mismatch between the first
low-pass filter and the second low-pass filter by: determining a
fractional capacitor code CI.sub.FRAC corresponding to the first
desired bandwidth BW.sub.0, the fractional capacitor code
CI.sub.FRAC being a value that lies between two consecutive
capacitor codes [I.sub.CODE, I.sub.CODE+1], and that ideally
corresponds to both a zero phase difference and a zero power
difference between the first low-pass filter and the second
low-pass filter; and using the fractional capacitor code
CI.sub.FRAC to determine the capacitor code offset
.DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET.
15. The device of claim 14, wherein the calibration circuitry is
further configured to determining the fractional capacitor code
CI.sub.FRAC by: interpolating a line using a plurality of points
with each point having a first dimension being a combined I-Q
capacitor code [I.sub.CODE, Q.sub.CODE], and a second dimension
being a respective measured phase offset between the first low-pass
filter and the second low-pass filter using a respective combined
I-Q capacitor code; and selecting a combined I-Q capacitor code
value that corresponds to a substantially zero phase difference
between the first low-pass filter and the second low-pass
filter.
16. The device of claim 15, wherein the calibration circuitry is
further configured to use the fractional capacitor code C.sub.FRAC
to determine the capacitor code offset .DELTA..sub.OFFSET by:
rounding the fractional capacitor code C.sub.FRAC to a nearest
integer to produce the capacitor code offset .DELTA..sub.OFFSET;
adding the capacitor code offset .DELTA..sub.OFFSET to the first
capacitor code I.sub.CODE to produce the first compensated
capacitor code I.sub.C-CODE; and adding the capacitor code offset
.DELTA..sub.OFFSET to the second capacitor code Q.sub.CODE to
produce the second compensated capacitor code Q.sub.C-CODE.
17. The device of claim 15, wherein using the fractional capacitor
code CI.sub.FRAC to determine the capacitor code offsets
.DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET includes: rounding to
the nearest integer [(1+.alpha..DELTA.fc)*.DELTA.C.sub.FRAC] to
produce the capacitor code offsets .DELTA.I.sub.OFFSET and
.DELTA.Q.sub.OFFSET, where .DELTA.C.sub.FRAC is a difference
between the first capacitor code CI.sub.FRAC and the second
capacitor code Q.sub.CODE, a is a scaling factor derived from
empirical data, and .DELTA.fc is a capacitor code difference
corresponding to the cutoff frequency offset .DELTA.f; adding the
capacitor code offset .DELTA.Q.sub.OFFSET to the first capacitor
code I.sub.CODE to produce the first compensated capacitor code
I.sub.C-CODE; and adding the capacitor code offset
.DELTA.Q.sub.OFFSET to the second capacitor code Q.sub.CODE to
produce the second compensated capacitor code Q.sub.C-CODE.
18. The device of claim 11, wherein the device is configured to:
applies the first compensated capacitor code I.sub.C-CODE to the
first low-pass filter; and applies the second compensated capacitor
code Q.sub.C-CODE to the second low-pass filter.
19. A wirelessly operating device that incorporates the device of
claim 11.
Description
BACKGROUND
Wireless communication devices, such as cellular telephones,
contain sophisticated integrated electronics used to receive and
transmit wireless data. Unfortunately, the analog electronics of
such integrated electronics is subject to process variation from
one wafer to the next. This can result in characteristics of
various components--e.g., resistor values and capacitor
values--varying to the point that it may be impossible to use a
particular device without some form of individualized device
compensation. The issue of component variation can even extend to
devices within a single chip. Thus, even two identically-designed
devices in a single chip can and do exhibit substantial mismatch.
This problem tends to increase in severity as integrated circuit
geometries continue to shrink.
SUMMARY
Various aspects and embodiments of the invention are described in
further detail below.
In an embodiment, a method for compensating for non-idealities in a
filter circuit that includes programmable filter circuitry
including a first low-pass filter and a second low-pass filter both
having a common desired cutoff frequency f.sub.0 is disclosed. The
method includes, for a first desired bandwidth BW.sub.0
corresponding to the common desired cutoff frequency f.sub.0,
injecting a reference tone f.sub.R and a cutoff tone f.sub.C into
the first low-pass filter, and measuring respective filter
responses of the reference tone f.sub.R and the cutoff tone f.sub.C
while changing capacitor codes that control a cutoff frequency
f.sub.0-I of the first low-pass filter until a first capacitor code
I.sub.CODE is determined that most accurately causes the first
low-pass filter to utilize the desired cutoff frequency f.sub.0;
for the first desired bandwidth BW.sub.0, injecting the reference
tone f.sub.R and the cutoff tone f.sub.C into the second low-pass
filter, and measuring respective filter responses of the reference
tone f.sub.R and the cutoff tone f.sub.C while changing capacitor
codes that control a cutoff frequency f.sub.0-Q of the second
low-pass filter until a second capacitor code Q.sub.CODE is
determined that most accurately causes the second low-pass filter
to utilize the desired cutoff frequency f.sub.0; and further
calibrating for mismatch between the first low-pass filter and the
second low-pass filter for one or more additional bandwidths
greater than the first desired bandwidth BW.sub.0.
In another embodiment, a device for compensating for non-idealities
in a filter circuit that includes programmable filter circuitry
including a first low-pass filter and a second low-pass filter both
having a common desired cutoff frequency f.sub.0 corresponding to a
first desired bandwidth BW.sub.0 is disclosed. The device includes
code search circuitry that controls the first low-pass filter and
the second low-pass filter; tone generation circuitry that injects
a reference tone f.sub.R and a cutoff tone f.sub.C into both the
first low-pass filter and the second low-pass filter; measurement
circuitry that: (1) measures respective filter responses of the
reference tone f.sub.R and the cutoff tone f.sub.C while the code
search circuitry changes capacitor codes that control a cutoff
frequency f.sub.0-I of the first low-pass filter until a first
capacitor code I.sub.CODE is determined that most accurately causes
the first low-pass filter to utilize the desired cutoff frequency
f.sub.0; and (2) measures respective filter responses of the
reference tone f.sub.R and the cutoff tone f.sub.C while the code
search circuitry changes capacitor codes that control a cutoff
frequency f.sub.0-Q of the second low-pass filter until a second
capacitor code Q.sub.CODE is determined that most accurately causes
the second low-pass filter to utilize the desired cutoff frequency
f.sub.0; and calibration circuitry configured to calibrate for
mismatch between the first low-pass filter and the second low-pass
filter for one or more additional bandwidths greater than a first
desired bandwidth BW.sub.0 of the desired cutoff frequency
f.sub.0.
BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of this disclosure that are proposed as
examples will be described in detail with reference to the
following figures, wherein like numerals reference like
elements.
FIG. 1 is a block diagram of an example wireless communications
device capable of transmitting and receiving wireless signals.
FIG. 2 depicts a block diagram of the down-converter of FIG. 1.
FIG. 3 depicts the wireless communications device of FIG. 1
reconfigured so as to be capable of self-calibration.
FIG. 4 is a power response of an example low-pass filter used in
the wireless communications device of FIG. 1.
FIG. 5 depicts examples of phase mismatch that can occur between to
identically-designed low-pass filters as a function of capacitor
codes.
FIGS. 6A and 6B depict examples of how mismatch for low-pass
filters for a particular bandwidth becomes worse at higher
bandwidths.
FIG. 7 is a flowchart outlining a set of example operations for
providing compensating for mismatched low-pass filters.
DETAILED DESCRIPTION OF EMBODIMENTS
The disclosed methods and systems below may be described generally,
as well as in terms of specific examples and/or specific
embodiments. For instances where references are made to detailed
examples and/or embodiments, it is noted that any of the underlying
principles described are not to be limited to a single embodiment,
but may be expanded for use with any of the other methods and
systems described herein as will be understood by one of ordinary
skill in the art unless otherwise stated specifically.
One of the most significant disadvantages of modern
telecommunications equipment is that process variations for
integrated circuits will cause analog components to vary not just
between different wafers, but even for different devices on a
single chip. Thus, two identically-designed low-pass filters on a
single chip can be expected to have different cutoff frequencies.
These differences can be problematic. For example, modern
Orthogonal Frequency Division Modulation (OFDM) systems require a
pair of matched low-pass filters in their RF-to-baseband and
baseband-to-RF conversion circuitry, and even small amounts of
mismatch can cause an OFDM device to operate poorly and outside of
an industry specification.
To address these component variations, designers often incorporate
some form of calibration circuitry so that individual filters can
be adjusted to better conform with device specifications. Analog
low-pass filters, for example, may contain banks of capacitors that
can be programmably placed in and out of circuit such that a cutoff
frequency may be fine-tuned.
Unfortunately, because calibration processes cannot exactly match
every pair of low-pass filters due to practical circuit
limitations, filter mismatch will occur not just under the
conditions for which the calibration took place, but will likely be
worse for other conditions that the filters must address. For
example, assuming that two digital filters are calibrated using a
bandwidth of 20 MHz, amplitude and phase variations between the two
filters will increase for bandwidths of 40 MHz, and increase more
for bandwidths of 80 MHz. Part of these increasing variations is
caused by non-ideal components within analog filters, and part is
due to the fact that the analog filters will need to be
reprogrammed to address different cutoff frequencies as a function
of bandwidth. By way of example, a analog low-pass filter for an
OFDM communication system operating for a bandwidth of 20 MHz will
require an 8.75 MHz cutoff frequency while an 18.75 MHz cutoff
frequency will be needed for a 40 MHz bandwidth, and a 38.75 MHz
cutoff frequency will be needed for an 80 MHz bandwidth.
FIG. 1 is a block diagram of an example wireless communications
device 100 capable of transmitting and receiving wireless signals.
As shown in FIG. 1, the wireless communications device 100 includes
a receive antenna 102, a down-converter 104, a first (I Channel)
Analog-To-Digital Converter (I-ADC) 112, a second (Q Channel)
Analog-To-Digital Converter (Q-ADC) 114, a transmit antenna 122, an
up-converter 124, a first (I Channel) Digital-To-Analog Converter
(I-DAC) 132, a second (Q Channel) Digital-To-Analog Converter
(Q-DAC) 134, and a processor 150. As the operations of the various
components 102-150 of FIG. 1 are well understood, a detailed
description of their operation under normal communications will be
omitted.
FIG. 2 depicts a block diagram of the down-converter 104 of FIG. 1.
As shown in FIG. 2, the down-converter 104 includes a low-noise
amplifier (LNA) 210, a first mixer 220, an I-baseband filter 230, a
second mixer 222, a Q-baseband filter 232, a local oscillator (LO)
240 capable of producing a local oscillation signal
cos(.omega..sub.LO t), where .omega..sub.LO is the local
oscillation frequency, and a phase shift device 242 capable of
shifting the local oscillation signal cos(.omega..sub.LO t) by
-.pi./2 radians. As with FIG. 1, because the operations of the
various components 210-232 are well understood, a detailed
description of their operation under normal communications will be
omitted. However, it is to be appreciated that, because wireless
communication devices are often limited to only transmitting or
receiving at any given point in time, most if not all of the
various components 210-232 can be used for the up-converter 124 of
FIG. 1 without detriment. Such an arrangement has a further
advantage in that only a single pair of low-pass filters will need
to be calibrated.
FIG. 3 depicts the wireless communications device 100 of FIG. 1
reconfigured so as to be capable of self-calibration. Also shown in
FIG. 3, functional components of the processor 150 dedicated to
filter calibration are displayed. Such functional components
include tone generation circuitry 152, code search circuitry 154,
power/phase measurement circuitry 156 and calibration circuitry
158. In various embodiments, the embedded circuitries 152-158 may
individually be made from dedicated logic, may exists as
software/firmware routines located in a tangible, non-transitory
memory and operated upon by one or more processors, or exist as
combinations of software/firmware processors and dedicated
logic.
In operation, each of the I-baseband (low-pass) filter 230 and the
Q-baseband (low-pass) filter 232 are calibrated such that each
will, to a practical extent possible, have a common desired cutoff
frequency f.sub.0 corresponding to a first desired bandwidth
BW.sub.0. While there is no limitation as to the particular
bandwidths or cutoff frequencies that may be used, for the purposes
of explanation the first desired bandwidth BW.sub.0 is 20 MHz, and
the corresponding desired cutoff frequency f.sub.0 is 8.75 MHz.
Similarly, while there is no limitation as to the types of low-pass
filters that may be used, for the purposes of explanation and
practical example, the I-baseband filter 230 and Q-baseband filter
230 are both fifth-order Chebyshev Type-1 filters using
switch-capacitor technology.
Initial calibration starts with the tone generation circuitry 152
(via the I-DAC 132 and the Q-DAC 134) injecting both a reference
tone f.sub.R and a cutoff tone f.sub.C into each of the I-baseband
filter 230 and the Q-baseband filter 232. The I-baseband filter 230
and the Q-baseband filter 232, in turn, provide a respective output
response consistent with their respective non-ideal cutoff
frequencies, f.sub.0-I and f.sub.0-Q, while the power/phase
measurement circuitry 156 (via the I-ADC 112 and the Q-ADC 114)
measures the respective filter responses.
During this time, the code search circuitry 154 will vary separate
digital control codes ("capacitor codes" or "cap codes") to the
I-baseband filter 230 and the Q-baseband filter 232 until the
respective non-ideal cutoff frequencies, f.sub.0-I and f.sub.0-Q,
match the ideal cutoff frequency f.sub.0 as close as possible given
the available resolution of the capacitor codes. For example,
assuming that the I-baseband filter 230 and the Q-baseband filter
232 each have a capacitor code resolution of 8 bits, the code
search circuitry 154 can provide any number of search algorithms to
provide capacitor codes within a range of [-128 to 127] until
respective particular capacitor codes are selected that most
accurately causes the baseband filters {230, 232} to utilize the
desired cutoff frequency f.sub.0. These selected capacitor codes
will be referred to below as the first capacitor code I.sub.CODE
and the second capacitor code Q.sub.CODE.
FIG. 4 is a power response 400 of an example low-pass filter
useable in the wireless communications device of FIG. 1 and useful
to explain how the reference tone f.sub.R and the cutoff tone
f.sub.C may be used to select an appropriate capacitor code and
utilize an appropriate cutoff frequency. As shown in FIG. 4, the
power response 400 is atypical of a fifth-order Type-1 Chebyshev
filter. The reference tone f.sub.R, which is well within the
pass-band region, is assigned a value of 1.25 MHz, and the cutoff
tone f.sub.C is assigned a value of 10 MHz. The power ratio of the
responses for the reference tone f.sub.R and the cutoff tone
f.sub.C will vary as a function of the cutoff frequency f.sub.0 so
as become larger as the cutoff frequency f.sub.0 decreases, and
become smaller as the cutoff frequency f.sub.C increases. The power
ratio for an ideal cutoff frequency f.sub.0 of 8.75 MHz can be
precisely determined, and a capacitor code can be adjusted until
the power response 400 best reflects a known, predictable power
ratio for the filter responses of the reference tone f.sub.R and
the cutoff tone f.sub.C.
Returning to FIG. 3, once the appropriate capacitor codes
{I.sub.CODE, Q.sub.CODE} are selected, the calibration circuitry
158 performs further calculations so as to better calibrate the
I-baseband filter 230 and the Q-baseband filter 232 to compensate
for filter mismatch for one or more additional bandwidths greater
than bandwidth BW.sub.0.
Typically, the one or more additional bandwidths will be a multiple
of BW.sub.0. For example, in various embodiments, a second desired
bandwidth BW.sub.1 will equal N.times.BW.sub.0, where N is a
positive integer greater than 1.
While bandwidths may be multiples of one another, respective cutoff
frequencies for such larger bandwidths will not be multiples of one
another. For instance, assuming BW.sub.0=20 MHz and f.sub.0=8.75
MHz, a second bandwidth BW.sub.1 of 40 MHz will use a respective
cutoff frequency f.sub.1 of 18.75 MHz, which represents a "cutoff
frequency offset" .DELTA.f of 1.25 MHz (18.75 MHz-(2*8.75 MHz)=1.25
MHz). Similarly, again assuming BW.sub.0=20 MHz and f.sub.0=8.75
MHz, a second bandwidth BW.sub.1 of 80 MHz will use a respective
cutoff frequency f.sub.1 of 38.75 MHz, which represents a cutoff
frequency offset .DELTA.f of 3.75 MHz (38.75 MHz MHz-(4*8.75
MHz)=3.75 MHz).
Although employing a cutoff frequency offset can be highly
advantageous, such offsets are problematic in that the offsets may
cause mismatch between a pair of low-pass filters at BW.sub.1 to
increase to a point where the increased mismatch causes a wireless
device to fall outside of performance specifications. Accordingly,
the calibration circuitry 158 is configured to, for a respective
second cutoff frequency f.sub.1 for a second/higher bandwidth
BW.sub.1, determine a capacitor code offsets .DELTA.I.sub.OFFSET
and .DELTA.Q.sub.OFFSET commensurate with the frequency offset
.DELTA.f, add the capacitor code offset .DELTA.I.sub.OFFSET to the
first capacitor code I.sub.CODE to produce a first compensated
capacitor code I.sub.C-CODE, and add the capacitor code offset
.DELTA.Q.sub.OFFSET to the second capacitor code Q.sub.CODE to
produce a second compensated capacitor code Q.sub.C-CODE.
However, the capacitor code offsets must not just reflect the
frequency offset .DELTA.f, but must also take into consideration a
"fractional capacitor code" CI.sub.FRAC corresponding to the first
desired bandwidth BW.sub.0, the fractional capacitor code
CI.sub.FRAC being a value that lies between two consecutive
capacitor codes [I.sub.CODE, I.sub.CODE+1] on I rail, keeping Qcode
unchanged, and that ideally corresponds to both a zero phase
difference and a zero power difference between a first low-pass
filter and a second low-pass filter.
FIG. 5 depicts a chart 500 showing examples of phase mismatch that
can occur between two identically-designed low-pass filters as a
function of capacitor codes and capacitor code offsets
.DELTA.I.sub.OFFSET/.DELTA.Q.sub.OFFSET to be used for other
bandwidths. As shown in FIG. 5, five example responses are provided
representing different capacitor code offsets
.DELTA.I.sub.OFFSET/.DELTA.Q.sub.OFFSET, with the center (dotted)
line representing a capacitor code offset
.DELTA.I.sub.OFFSET/.DELTA.Q.sub.OFFSET=0. The X-axis is a
dimension being a combined I-Q capacitor code [I.sub.CODE,
Q.sub.CODE], and the Y-axis is a second dimension representing
respective measured phase offsets between a first low-pass filter
and a second low-pass filter as a function of the respective
combined I-Q capacitor codes. The point 502 at which the dotted
line displays zero phase mismatch occurs about half-way between I-Q
capacitor code [71,6D] (signed hexadecimal notation representing a
difference of 4) and I-Q capacitor code [70,6D] (signed hexadecimal
notation representing a difference of 3).
The fractional capacitor code CI.sub.FRAC will be a real,
non-integer, number, and as such is incompatible with programmable
filter circuitry that relies on discrete switches to
program/calibrate. As such, the capacitor code offset
.DELTA.I.sub.OFFSET/.DELTA.Q.sub.OFFSET may be determined by
rounding the fractional capacitor code CI.sub.FRAC to a nearest
integer, adding the capacitor code offset .DELTA.I.sub.OFFSET to
the first capacitor code I.sub.CODE to produce the first
compensated capacitor code I.sub.C-CODE, and adding the capacitor
code offset .DELTA.Q.sub.OFFSET to the second capacitor code
Q.sub.CODE to produce the second compensated capacitor code
Q.sub.C-CODE.
In various embodiments, the capacitor code offsets
.DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET are calculated by
rounding to the nearest integer the formula [(1+.alpha.
.DELTA.fc)*.DELTA.C.sub.FRAC], where .DELTA.C.sub.FRAC is a
difference between the fractional first capacitor code CI.sub.FRAC
and the second capacitor code Q.sub.CODE, .alpha. is a scaling
factor derived from empirical data, and .DELTA.fc is a capacitor
code difference corresponding to the cutoff frequency offsets
.DELTA.I.sub.OFFSET and .DELTA.Q.sub.OFFSET. If .DELTA.fc=0, then
the capacitor code offset calculation is reduced to rounding to the
nearest integer the formula [.DELTA.C.sub.FRAC]. However, assuming
.DELTA.fc.noteq.0, scaling factor .alpha. must be factored.
While a scaling factor .alpha. may be determined in a number of
ways, in a number of embodiments a scaling factor .alpha. is
determined based on empirical data. FIGS. 6A and 6B depict examples
of how mismatch for low-pass filters for a particular bandwidth
becomes worse at higher bandwidths. While FIGS. 6A and 6B are
exemplary, conceptually they are based on real-world experience so
as to demonstrate that filter mismatch will increase as a function
of .DELTA.fc and the magnitude of BW.sub.1. An appropriate scaling
factor .alpha. will reflect desired compensation for different
.DELTA.fc and different magnitudes of BW.sub.1.
Again returning to FIG. 3, once the calibration circuitry 158 has
determined the first compensated capacitor code I.sub.C-CODE and
the second compensated capacitor code Q.sub.C-CODE, the processor
150 applies the first compensated capacitor code I.sub.C-CODE to
the first/I-baseband (low-pass) filter 230, and applies the second
compensated capacitor code Q.sub.C-CODE to the second/Q-baseband
(low-pass) filter 232, where after the baseband filters 230 and 232
may be used for higher bandwidths.
FIG. 7 is a flowchart outlining a set of example operations for
providing compensating for mismatched low-pass filters, such as the
I-baseband filter 230 and Q-baseband filter 232 discussed above and
with respect to FIGS. 1-6. Such operations compensate for
non-idealities in a filter circuit that includes programmable
filter circuitry including a first low-pass filter and a second
low-pass filter both having a common desired cutoff frequency
f.sub.0. It is to be appreciated to those skilled in the art in
light of this disclosure that, while the various functions of FIG.
7 are shown according to a particular order for ease of
explanation, that certain functions may be performed in different
orders or in parallel.
At S702, for a first desired bandwidth BW.sub.0 corresponding to
the common desired cutoff frequency f.sub.0, a reference tone
f.sub.R and a cutoff tone f.sub.C are injected into both the first
low-pass filter and the second low-pass filter using, for example,
separate DACs under the control of some form of tone generation
circuitry.
At S704, the responses of the first low-pass filter and the second
low-pass filter are digitized using respective ADCs so as to
measure power responses of the reference tone f.sub.R and cutoff
tone f.sub.C. During this time, a capacitor code that controls a
cutoff frequency f.sub.0-I of the first low-pass filter is varied
until a first capacitor code I.sub.CODE is determined that most
accurately causes the first low-pass filter to utilize the desired
cutoff frequency f.sub.0. Similarly, a capacitor code that controls
the second low-pass filter is varied until a second capacitor code
Q.sub.CODE is determined that most accurately causes the second
low-pass filter to utilize the desired cutoff frequency
f.sub.0.
At S708, a fractional capacitor code CI.sub.FRAC is determined
again noting that a fractional capacitor code CI.sub.FRAC is a
non-integer value that lies between two consecutive capacitor codes
[I.sub.CODE, I.sub.CODE+1], and that ideally corresponds to both a
zero phase difference and a zero power difference between the first
low-pass filter and the second low-pass filter. While the
particular methodology may vary from embodiment to embodiment, one
approach to determining the fractional capacitor code C.sub.FRA may
be had by interpolating a line using a plurality of points with
each point having (See, FIG. 5) a first dimension being a combined
I-Q capacitor code [I.sub.CODE, Q.sub.CODE], and a second dimension
being a respective measured phase offset between the first low-pass
filter and the second low-pass filter using a respective combined
I-Q capacitor code, then selecting a combined I-Q capacitor code
value that corresponds to a substantially zero phase difference
between the first low-pass filter and the second low-pass
filter.
At S710, a scaling factor .alpha. is derived, for example, from
empirical data. At S712, capacitor code offsets .DELTA.I.sub.OFFSET
and .DELTA.Q.sub.OFFSET are determined by rounding to the nearest
integer a scaled value=[(1+.alpha. .DELTA.fc)*.DELTA.C.sub.FRAC],
where .DELTA.C.sub.FRAC is a difference between the fractional
first capacitor code .DELTA.C.sub.FRAC and the second capacitor
code Q.sub.CODE, .alpha. is the scaling factor derived at S710,
CI.sub.FRAC is the fractional capacitor code derived at S708, and
.DELTA.fc is a capacitor code difference corresponding to the
cutoff frequency offset .DELTA.f determined at S706.
At S714, a first compensated capacitor code I.sub.C-CODE is
calculated by adding the capacitor code offset .DELTA.I.sub.OFFSET
to the first capacitor code I.sub.CODE. Similarly, a second
compensated capacitor code Q.sub.C-CODE is calculated by adding the
capacitor code offset .DELTA.Q.sub.OFFSET to the second capacitor
code Q.sub.CODE. At S716, an operating bandwidth is changed from
BW0 to BW1, the first compensated capacitor code I.sub.C-CODE is
applied to the first/I low-pass filter, and the second compensated
capacitor code Q.sub.C-CODE is applied to the second/Q low-pass
filter.
While the invention has been described in conjunction with the
specific embodiments thereof that are proposed as examples, it is
evident that many alternatives, modifications, and variations will
be apparent to those skilled in the art. Accordingly, embodiments
of the invention as set forth herein are intended to be
illustrative, not limiting. There are changes that may be made
without departing from the scope of the invention.
* * * * *