U.S. patent application number 12/825193 was filed with the patent office on 2011-12-29 for method and system for propagation time measurement and calibration using mutual coupling in a radio frequency transmit/receive system.
Invention is credited to Eric N. Boe, John Fraschilla, William L. Lewis.
Application Number | 20110319034 12/825193 |
Document ID | / |
Family ID | 44671923 |
Filed Date | 2011-12-29 |
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United States Patent
Application |
20110319034 |
Kind Code |
A1 |
Boe; Eric N. ; et
al. |
December 29, 2011 |
METHOD AND SYSTEM FOR PROPAGATION TIME MEASUREMENT AND CALIBRATION
USING MUTUAL COUPLING IN A RADIO FREQUENCY TRANSMIT/RECEIVE
SYSTEM
Abstract
A method and system use the mutual coupling property of multiple
antenna elements for measuring differences in propagation time
among various signal paths involving antenna elements in a radio
frequency transmit/receive system. The method and system alleviate
the need for external test equipment by using the same hardware
used in standard operation of the transmit/receive system for
performing propagation time measurement through the generation,
mutual coupling, and acquisition of a specially selected reference
signal. In an embodiment involving calibration of various signal
paths to realize matched propagation times, the signal energy
returned through these various paths during standard system
operation arrives for acquisition more closely coincident in time,
increasing the instantaneous bandwidth of the system.
Inventors: |
Boe; Eric N.; (Long Beach,
CA) ; Lewis; William L.; (Redondo Beach, CA) ;
Fraschilla; John; (Marina Del Rey, CA) |
Family ID: |
44671923 |
Appl. No.: |
12/825193 |
Filed: |
June 28, 2010 |
Current U.S.
Class: |
455/67.14 |
Current CPC
Class: |
G01S 7/4017
20130101 |
Class at
Publication: |
455/67.14 |
International
Class: |
H04W 24/00 20090101
H04W024/00 |
Claims
1. A method for measuring aspects of propagation times among
various transmitting or receiving signal paths in a radio frequency
transmit/receive system, comprising: generating a radio frequency
reference signal; transmitting the reference signal through a
transmit side comprising at least one transmitting signal path
comprising an antenna radiating element, which transmitting signal
path and antenna radiating element are also usable for transmitting
an operational signal; receiving the reference signal through a
receive side comprising at least one receiving signal path
comprising an antenna receiving element, which receiving signal
path and antenna receiving element are also usable for receiving an
operational signal; propagating the reference signal through a
mutual coupling field from at least one said antenna radiating
element to at least one said antenna receiving element; wherein at
least one of the transmit side or the receive side comprises a
plurality of said transmitting signal paths or said receiving
signal paths, respectively, as subject signal paths; and processing
the received reference signal to determine at least one
characteristic of signal propagation time in relation to at least
one of the subject signal paths.
2. The method of claim 1 wherein: the receive side comprises the
plurality of receiving signal paths as the subject signal paths;
propagating the reference signal comprises propagating the
reference signal from at least one antenna radiating element to at
least two antenna receiving elements; and at least two antenna
receiving elements are disposed substantially along an
equipotential surface of a mutual coupling field of at least one
antenna radiating element.
3. The method of claim 1 wherein: the transmit side comprises the
plurality of transmitting signal paths as the subject signal paths;
propagating the reference signal comprises propagating the
reference signal from at least two antenna radiating elements to at
least one antenna receiving element; and an antenna receiving
element is disposed at a point of substantially equipotential
mutual coupling with respect to respective mutual coupling fields
of at least two antenna radiating elements.
4. The method of claim 1, wherein said processing comprises
counting elapsed time.
5. The method of claim 1, wherein said processing comprises
ascertaining at least one component related to at least one
heterodyned product of at least one reference signal instance.
6. The method of claim 1, wherein the at least one characteristic
comprises the absolute propagation time of a signal through at
least one of the subject signal paths.
7. The method of claim 1, wherein the at least one characteristic
comprises the relative difference in signal propagation time
between at least two of the subject signal paths.
8. The method of claim 1, further comprising mixing a reference
signal instance propagated through at least one of the subject
signal paths with at least a reference signal instance fed from a
generator of the reference signal.
9. The method of claim 1, further comprising mixing a reference
signal instance propagated through at least one of the subject
signal paths with at least one reference signal instance propagated
through another of the subject signal paths.
10. The method of claim 1, wherein the reference signal comprises
pulse compression modulation.
11. The method of claim 1, further comprising adjusting propagation
time of a signal through at least one of the subject signal paths
based on a determination of at least one characteristic of
propagation time of a signal in relation to at least one of the
subject signal paths.
12. The method of claim 11, wherein adjusting propagation time
further comprises making one or more additional adjustments to the
propagation time of a signal through at least one of the subject
signal paths subsequent to a first adjustment and based on an
additional one or more determinations of at least one
characteristic of propagation time of a signal in relation to at
least one of the subject signal paths made subsequent to a first
determination.
13. The method of claim 11, wherein the adjusting propagation time
causes an instance of a signal propagating through one of the
subject signal paths to complete its propagation through the
system's signal path for said instance at substantially the same
time as a signal instance propagating through another of the
subject signal paths completes its propagation through the system's
signal path for such other instance.
14. A radio frequency transmit/receive system, comprising: an
exciter capable of producing a radio frequency reference signal; a
transmit side comprising at least one transmitting signal path
comprising an antenna radiating element, which transmitting signal
path and antenna radiating element are usable for transmitting
operational signals; a receive side comprising at least one
receiving signal path comprising an antenna receiving element,
which receiving signal path and antenna receiving element are
usable for receiving operational signals; wherein at least one of
the transmit side or the receive side comprises a plurality of said
transmitting signal paths or said receiving signal paths,
respectively, as subject signal paths, and wherein at least one
antenna receiving element is disposed in proximity to at least one
antenna radiating element permitting mutual coupling; a signal
receiver capable of acquiring a signal; and analysis circuitry
configured to determine based on a reference signal received from
mutual coupling at least one characteristic of signal propagation
time in relation to at least one of the subject signal paths.
15. The system of claim 14, wherein the receive side comprises the
plurality of receiving signal paths as the subject signal paths,
and at least two antenna receiving elements are disposed
substantially along an equipotential surface of a mutual coupling
field of at least one antenna radiating element.
16. The system of claim 14, wherein the transmit side comprises the
plurality of transmitting signal paths as the subject signal paths,
and an antenna receiving element is disposed at a point of
substantially equipotential mutual coupling with respect to
respective mutual coupling fields of at least two antenna radiating
elements.
17. The system of claim 14, wherein the analysis circuitry is
configured to determine absolute propagation time through at least
one of the subject signal paths.
18. The system of claim 14, wherein the analysis circuitry is
configured to determine relative difference in propagation times
between at least two of the subject signal paths.
19. The system of claim 14, further comprising a signal mixer
disposed to mix a reference signal instance propagated through at
least one of the subject signal paths with at least a reference
signal instance propagated through another at least one of the
subject signal paths.
20. The system of claim 14, further comprising a signal mixer
disposed to mix a reference signal instance propagated through at
least one of the subject signal paths with at least a reference
signal instance fed from the exciter.
21. The system of claim 14, further comprising a propagation time
adjustment control in communication with the analysis circuitry and
also in communication with at least one propagation time adjustment
mechanism disposed in a subject signal path, and capable of
adjusting a propagation time of a signal through at least one
subject signal path.
Description
BACKGROUND OF THE INVENTION
[0001] Emerging applications for radio frequency transmit/receive
systems are demanding increased instantaneous bandwidth. Such
applications include high-data-rate communications,
low-probability-of-detection communications, simultaneous occupancy
of radio frequency spectrum by multiple users, high-resolution
imaging radar, low-probability-of-detection radar, wide-bandwidth
electronic signals surveillance, and wide-bandwidth jamming of
signals. In all these applications, it is essential for energy from
the entire desired spectrum bandwidth to arrive at the receiver. In
systems featuring multiple antenna elements associated with
multiple signal paths in parallel, maximal energy transfer requires
that the signals propagating through these multiple paths arrive at
the receiver simultaneously, or as near simultaneously as possible.
This does not necessarily occur naturally in a system, because
various signal paths associated with various antenna elements may
feature different signal propagations times, due to diverse causes
such as component manufacturing tolerances and differences in
signal path length, leading to offsets in signal energy arrival
times.
[0002] Prior approaches to the problem of multiple signal paths
causing signal returns scattered in time have used combinations of
strategies such as matching various hardware units in propagation
time during manufacture, selecting for use in a particular system
those hardware units that as manufactured happen to feature the
most similar propagation times, and propagation time measurement
among various signal paths in a particular system using external
measuring equipment followed by manual calibration adjustments
using propagation time adjustment mechanisms in various of the
system's signal paths. Where propagation time matching or
calibration is not employed, the system may simply be used with its
bandwidth or field of view reduced to the limits permitted by the
propagation time mismatches occurring among various uncalibrated
signal paths associated with various antenna elements. This last
approach is particularly common for systems using the current
generation of electronically scanned array or "ESA" antennas, which
use phase and amplitude adjustment for beam formation and beam
steering.
[0003] Prior approaches that involve propagation time measurement
and calibration have required dedicated external equipment.
Moreover, propagation time measurement and calibration using these
approaches have required additional circuitry that bypasses the
normal operating signal paths and feeds signals specially between
different portions of the system for measurement. Such special
bypassing and feeding leads to measurements for only a portion of
the system signal paths, rather than for the entire lengths of the
relevant operational signal paths, and so may leave unmeasured and
uncalibrated those propagation time mismatches arising in other
portions of the signal paths.
SUMMARY OF THE INVENTION
[0004] A method and system that uses the same hardware and the same
signal paths both for regular operation and for propagation time
measurement and calibration, and that can measure and calibrate
propagation time for the entire lengths of parallel signal paths
within a system, would improve on the prior art and would be
beneficial. The present invention achieves such goals by leveraging
two characteristics often found in systems using ESA antennas, and
other systems as well. First, these systems often possess the
ability simultaneously to transmit signals from their transmitting
side and antenna radiating elements while receiving signals through
their antenna receiving elements and receiving side. Second, these
systems display the phenomenon known as "mutual coupling" among
multiple antenna elements, wherein a portion of the signal being
transmitted from a radiating element is coupled directly back into
a receiving element. This phenomenon has often been considered a
nuisance, but the present invention turns the mutual coupling
phenomenon to its advantage for determining and calibrating
propagation time among parallel signal paths and so achieving
improved instantaneous bandwidth performance, without the need for
dedicated external measurement equipment.
[0005] The present invention is directed to a method and system for
a radio frequency transmit system that uses a specially selected
reference signal and takes advantage of the mutual coupling
property of multiple antenna elements to determine, without the
need for dedicated external test equipment, the difference in
signal propagation times through various parallel signal paths
under test, here called "subject signal paths." It does so using
mutual coupling either between a common antenna radiating element
transmitting to multiple antenna receiving elements or between
multiple antenna radiating elements transmitting to a common
antenna receiving element. Certain embodiments of the invention
include calibration of the system by adjusting the propagation time
through one or more of the subject signal paths to match the
propagation time through other of the subject signal paths, thus
allowing signals passing through the various parallel subject
signal paths to arrive for acquisition at the signal receiver more
closely coincident in time, for maximum bandwidth energy transfer
and increased instantaneous bandwidth.
[0006] According to one embodiment, the present invention is
directed to a method for measuring propagation times among various
receiving signal paths in a radio frequency transmit/receive
system. In this method, a radio frequency reference signal is
generated, that signal is transmitted through a transmit side
comprising a transmitting signal path comprising an antenna
radiating element, and that transmitting signal path and antenna
radiating element are also usable for transmitting an operational
signal. That signal is also received through a receive side
comprising a receiving signal path comprising an antenna receiving
element, and that receiving signal path and antenna receiving
element are also usable for receiving an operational signal. That
signal is also propagated through a mutual coupling field from at
least one antenna radiating element to at least one antenna
receiving element. At least one of the transmit side or the receive
side comprises a plurality of transmitting signal paths or
receiving signal paths, respectively, and these signal paths are
the subject signal paths under analysis. Also in this method, the
received reference signal is processed to determine at least one
characteristic of signal propagation time for at least one of the
subject signal paths.
[0007] According to one embodiment where the plurality of subject
signal paths is on the receive side, the reference signal is
propagated from at least one antenna radiating element to at least
two antenna receiving elements each disposed substantially along an
equipotential surface of the mutual coupling field. According to
another embodiment where the plurality of subject signal paths is
on the transmit side, the reference signal is propagated from at
least two antenna radiating elements to at least one antenna
receiving element that is disposed at a point of substantially
equipotential mutual coupling with respect to the respective mutual
coupling fields of at least two of those radiating elements.
[0008] According to one embodiment, the processing being undertaken
includes the counting of elapsed time; according to another
embodiment, the processing being undertaken includes the
ascertaining of one or more components related to at least one
heterodyned product of one or more instances of the reference
signal.
[0009] According to one embodiment, the characteristic of signal
propagation time being measured is the absolute propagation time of
a signal through the subject signal path. According to another
embodiment, the characteristic being measured is relative
difference in signal propagation time between two of the subject
signal paths.
[0010] According to one embodiment, the method includes mixing a
reference signal instance propagated through a subject signal path
with a reference signal instance that is fed from the exciter.
According to another embodiment, the method includes mixing a
reference signal instance propagated through one of the subject
signal paths with a reference signal instance propagated through
another of the subject signal paths.
[0011] According to one embodiment, the reference signal includes
pulse compression modulation.
[0012] According to other embodiments, the method includes
adjusting propagation time of a signal through one or more subject
signal paths based on a determination of at least one
characteristic of signal propagation time of a signal in relation
to at least one subject signal path. According to one of those
embodiments, the process further includes making one or more
additional adjustments to the propagation time of a signal through
at least one of the subject signal paths subsequent to the first
adjustment and based on additional one or more determinations of at
least one characteristic of propagation time of a signal in
relation to at least one of the subject signal paths made
subsequent to a first determination. According to another one of
those embodiments, the propagation time is adjusted to cause an
instance of a signal propagating through one of the subject signal
paths to complete its propagation through the system's signal path
for such instance at substantially the same time as a signal
instance propagating through another of the subject signal paths
completes its propagation through the system's signal path for such
other signal instance.
[0013] According to one embodiment, the present invention is
directed to a radio frequency transmit/receive system comprising an
exciter capable of producing a radio frequency reference signal, a
transmit side comprising at least one transmitting signal path
comprising an antenna radiating element, which transmitting signal
path and antenna radiating element are usable for transmitting
operational signals, a receive side comprising at least one
receiving signal path comprising an antenna receiving element,
which receiving signal path and antenna receiving element are
usable for receiving operational signals, wherein at least one of
the transmit side or the receive side comprises a plurality of said
transmitting signal paths or said receiving signal paths,
respectively, as subject signal paths, and wherein at least one
antenna receiving element is disposed in proximity to at least one
antenna radiating element permitting mutual coupling, a signal
receiver capable of acquiring a signal, and analysis circuitry
configured to determine based on a reference signal received from
mutual coupling at least one characteristic of signal propagation
time in relation to at least one of the subject signal paths.
[0014] According to one embodiment where the plurality of subject
signal paths is on the receive side, at least two of the antenna
receiving elements are disposed substantially along an
equipotential surface of a mutual coupling field of at least one
antenna radiating element. According to another embodiment where
the plurality of subject signal paths is on the transmit side, an
antenna receiving element is disposed at a point of substantially
equipotential mutual coupling with respect to respective mutual
coupling fields at least two antenna radiating elements.
[0015] According to one embodiment, the analysis circuitry is
configured to determine absolute propagation time through at least
one of the subject signal paths. According to another embodiment,
the analysis circuitry is configured to determine relative
difference in propagation times between at least two of subject
signal paths.
[0016] According to one embodiment, the system includes a signal
mixer disposed to mix a reference signal instance propagated
through a subject signal path with a reference signal instance
propagated through another subject signal path. According to
another embodiment, the system includes a signal mixer disposed to
mix a reference signal instance propagated through at least one of
the subject signal paths with a reference signal instance fed from
the exciter.
[0017] According to another embodiment, the system includes a
propagation time adjustment control in communication with the
analysis circuitry and also in communication with at least one
propagation time adjustment mechanism disposed in a subject signal
path, and capable of adjusting propagation time of a signal through
at least one subject signal path.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic view of a radio frequency
transmit/receive system according to the present invention that
features a common transmitting signal path and multiple receiving
signal paths.
[0019] FIG. 2 is a schematic view of a radio frequency
transmit/receive system according to the present invention that
features multiple transmitting signal paths and a common receiving
signal path.
[0020] FIG. 3 is a flow chart outlining a method according to the
present invention for measuring the propagation times of various
receive-side subject signal paths in a radio frequency
transmit/receive system by counting absolute elapsed time while a
reference signal instance propagates through the system signal
path.
[0021] FIG. 4 is a flow chart outlining a method according to the
present invention for measuring the propagation times of various
receive-side subject signal paths in a radio frequency
transmit/receive system by mixing a local oscillator reference
signal instance with a reference signal instance propagated through
the system signal path.
[0022] FIG. 5 is a flow chart outlining a method according to the
present invention for measuring the relative difference in
propagation times between various receive-side subject signal paths
in a radio frequency transmit/receive system by mixing reference
signal instances propagated through various subject signal
paths.
[0023] FIG. 6 is a flow chart outlining a method according to the
present invention for measuring propagation time differences among
various subject signal paths and calibrating those paths to reduce
those propagation time differences in a radio frequency
transmit/receive system where the direction of those differences is
originally known.
[0024] FIG. 7 is a flow chart outlining a method according to the
present invention for measuring propagation time differences among
various subject signal paths and calibrating those paths to reduce
those propagation time differences in a radio frequency
transmit/receive system where the direction of those differences is
not originally known.
[0025] FIG. 8 is a flow chart outlining a method according to the
present invention for measuring the propagation times of various
transmit-side subject signal paths in a radio frequency
transmit/receive system by counting absolute elapsed time while a
reference signal instance propagates through the system's full
signal path.
[0026] FIG. 9 is a flow chart outlining a method according to the
present invention for measuring the propagation times of various
transmit-side subject signal paths in a radio frequency
transmit/receive system by mixing a local oscillator reference
signal instance with a reference signal instance propagated through
the system signal path.
[0027] FIG. 10 is a flow chart outlining a method according to the
present invention for measuring the relative difference in
propagation times between various transmit-side subject signal
paths in a radio frequency transmit/receive system by mixing
reference signal instances propagated through various subject
signal paths.
DETAILED DESCRIPTION
[0028] Embodiments in accord with the present invention are
directed to a method and system for a radio frequency
transmit/receive system that employs a selected reference signal
and takes advantage of the mutual coupling property of multiple
antenna elements to determine aspects of propagation time of a
signal through multiple parallel signal paths, here called "subject
signal paths," that are also traversed by an operational signal
between its generation and its reception during normal operation of
the system. The generation of the reference signal and the
acquisition of either the reference signal or signals derived from
mixing instances of the reference signal are performed by the same
exciter and receiver as are used for normal operational, permitting
the aspects of signal path propagation time to be determined
without the need for dedicated external measurement equipment.
[0029] Embodiments in accord with the present invention display
reciprocity, that is, the methods of measurement and compensation
are similar or identical whether the subject signal paths are found
on the transmit side of a system with a common receiving signal
path, or whether the subject signal paths are found on the receive
side of a system with a common transmitting signal path. In certain
embodiments, the method and system permit the calibration of one or
more of the plurality of subject signal paths so that the
propagation time through each of the plurality of subject signal
paths is more nearly matched.
[0030] In certain embodiments of the present invention, the aspect
of propagation time being determined is the absolute propagation
time of a signal through a subject signal path, usually determined
as part of the absolute propagation time, or a proxy for that
absolute propagation time, of a signal through the full signal path
of the system, including that subject signal path. The difference
in propagation times can then be computed by measuring the
difference between two or more absolute propagation time
measurements. In certain other embodiments, the aspect of
propagation time being determined is directly the relative
difference in propagation time between two subject signal
paths.
[0031] Considering first the embodiment of the present invention
featuring a common transmitting signal path and multiple receiving
subject signal paths, as depicted in FIG. 1, the transmit side of
the transmit/receive system comprises an exciter 102 connected to
transmitting signal path 104 including any electronics 106 in that
signal path and antenna radiating element 108. The system also
includes first receive-side subject signal path 112 comprising
antenna receiving element 110 and any electronics 114 in that
subject signal path, connected to a signal receiver 122. The system
further comprises at least a second receive-side subject signal
path 118 comprising antenna receiving element 116 and any
electronics 120 in that signal path, also connected to receiver
122. Both antenna receiving elements are disposed in the mutual
coupling field 140 of the antenna radiating element. Although two
receive-side subject signal paths are depicted in FIG. 1,
additional subject signal paths beyond two may be attached to the
receiver and have their signal propagation time measured and/or
calibrated, with the antenna receiving element for each such
subject signal path also within the mutual coupling field 140.
[0032] In embodiments comprising additional optional components,
propagation time adjustment mechanisms 124, 126 may be disposed in
one or more of the receive-side subject signal paths. The system
may also feature an adjustment control 138 connected to exciter 102
and receiver 122, and connected to the propagation time adjustment
mechanisms 124 and 126 to control them. Further, the multiple
subject signal paths may each feed a mixer 128. The multiple
subject signal paths may also share a common receive-side signal
path 130 including any electronics 132 in that common signal path.
Moreover, the subject signal paths 112, 118 or common signal path
130 may feed a mixer 136 that is also fed by a local oscillator
feed line 134 from the exciter.
[0033] The terms "attached," connected," "feed" or "fed" as used
here may include any form of the two referenced items being in
communication with each other, such as direct or indirect
electrical, electronic, optical, RF, or waveguide connection, or
any other form of attachment or association that promotes signal or
data communication. It should also be noted that the exciter,
mixers, receiver, adjustment control, and other electronics may be
implemented in and operate in the analog and/or digital domain,
using specialized electrical and/or optical circuitry, mechanical
members, special purpose computers, ASICs and/or firmware, general
purpose computers programmed with special purpose software, or any
other media usable for signal or data conduction and
processing.
[0034] According to one embodiment of a method for implementing the
present invention in a radio frequency transmit/receive system
having multiple receive-side subject signal paths, as depicted in
FIG. 3, a first receive-side subject signal path 112 is connected
(302) to the signal receiver or otherwise enabled to receive a
signal. A reference signal with appropriate properties that enable
observation of small differences in time is generated (304) by the
system exciter 102. This signal is propagated (306) through the
transmitting signal path 104, any signal path electronics 106, and
antenna radiating element 108. The majority of the signal is
radiated into space, but a small portion is mutually coupled (308)
from radiating element 108 through mutual coupling field 140 into
antenna receiving elements 110 and 116. The mutually coupled signal
propagates (310) through first subject signal path 112, including
through receiving element 110 and any electronics 114 of that
subject signal path, and into signal receiver 122 where it is
acquired (312). In this embodiment, the absolute propagation time
through the system is measured. Upon the exciter emitting the
reference signal, the system begins measuring (314) elapsed time,
such as by counting the pulses of the system clock, and ceases
(316) measuring or counting when receiver 122 acquires the
reference signal. The total elapsed time measurement, such as the
accumulated clock pulse count, is retained (318) as a measure of
signal propagation time from exciter to receiver through the first
subject signal path.
[0035] The first subject signal path is disconnected (320) from the
signal receiver or otherwise disabled from receiving a signal, and
a second subject signal path 118 is connected (322) to the signal
receiver or otherwise enabled to receive a signal. Another instance
of the reference signal is emitted (324) from the exciter and
another measurement of elapsed time begins (326). The reference
signal propagates (328) through the transmitting signal path, and
propagates (330) across the mutual coupling field. The mutually
coupled signal propagates (332) through second subject signal path
118, including receiving element 116 and any electronics 120 of
that subject signal path, and into signal receiver 122 where it is
acquired and the measurement of elapsed time is terminated (336),
and the elapsed time measurement for the second subject signal path
is retained (338). The difference in propagation time can be
determined by comparing (340) the elapsed propagation time through
the system with each of the respective subject signal paths
connected. In the case of counting system clock pulses, this
comparison is as simple as subtracting one pulse count from the
other pulse count. Since the propagation time through the rest of
the total signal path other than the subject signal paths is the
same in each instance, any difference in the elapsed total
propagation times corresponds (342) to the difference in the
propagation times between the two subject signal paths. As many
additional subject signal paths as desired may be connected and the
actions of transmitting, receiving, and elapsed time measuring may
be repeated (344) for each, to determine their propagation times
and relative propagation time differences. It should be noted that
the above need not necessarily be performed in the exact order
discussed here.
[0036] In any propagation time measurement method, it is necessary
for the exciter to be able to generate a reference signal waveform
and the receiver to be able to receive such a waveform that renders
the salient characteristics of propagation time as observable
quantities. Optimally, a waveform should be selected that lies
within the transmit and receive capability of the system hardware
without exceeding or straining the optimal capabilities of the
exciter and receiver subsystems. Many waveforms capable of yielding
suitable resolution regarding propagation time are known to those
skilled in the art, but the optimal type of signal depends on the
measurement method being employed. Direct counting of elapsed time
as exemplified by the embodiment just discussed requires a highly
resolvable signal, such as a single, very short pulse of high
frequency. The duration of such a pulse is related to the desired
resolution and precision of measurement and compensation.
Measurement and compensation corresponding to distance resolution
of about a foot would require a pulse duration of approximately a
nanosecond. Moreover, a typical propagation time calibration
requirement in systems where supplemental phase alignment circuitry
is available might be to within one wavelength of the system's
center frequency, which would correspond, for example, to 100
picoseconds at 10 GHz. In systems where beam formation is
implemented without such phase-shift supplementation, the pulse
duration for a system using such a center frequency would need to
be even more brief, on the order of 1 picosecond. Such ultra-brief
pulse durations may severely tax the abilities of the exciter and
receiver or even be impossible to achieve. It may also be difficult
for the receiver to detect such an ultra-brief pulse amidst noise
or interference. For systems using exciters and receivers of
limited capability, these difficulties may render the approach of
directly counting elapsed time less than optimal.
[0037] Fortunately, longer-period reference signals that are easier
to transmit and receive may be also used, in combination with pulse
compression techniques that are known to those skilled in the art.
Such signals may for example feature pulse compression modulation
such as frequency modulated chirp, stepped frequencies, random
code, or multi-chip Barker code, with the parameters of such
signals selected to yield the desired time measurement
resolution.
[0038] An additional advantage of using a pulse compression
modulation such as a very fast-ramping linear frequency-modulated
chirp waveform, beyond its usability with pulse compression
techniques, is that mixing two instances of such a chirp having
different time phase produces heterodyne frequencies that directly
correspond to the time offset between the two signal instances.
Specifically, dividing the mix-product frequency offset by the
frequency ramp rate yields the difference in time between the two
instances. This allows an embodiment mixing time-offset instances
of such a chirp signal to determine a time difference between them
simply by observing the resultant signal frequencies rather than by
measuring elapsed time.
[0039] In an embodiment using the mixing of a chirp-type reference
signal and observation of the heterodyned results, as depicted in
FIG. 4, the first receive-side subject signal path 112 is connected
(402) to the signal receiver 122 or otherwise enabled to receive a
signal. When the reference signal is emitted (404) by the exciter
102, it is propagated (408) to the transmitting signal path 104 and
also propagated (406) through a local oscillator feed line 134 to a
signal mixer 136 in the receiving signal path. The reference signal
propagates (408) through the transmitting signal path, propagates
(410) across the mutual coupling field 140, and propagates (412)
through the first subject signal path 112 into the other input of
mixer 136, where the propagated reference signal instance and the
local oscillator feed signal instance are mixed (414). The
heterodyned signal from the mixer is acquired (416) by the receiver
122, and its frequency profile is analyzed (418) to determine the
difference in arrival time of the respective signal instances at
the mixer. That frequency profile correlates (420) with signal
propagation time through the system including the first subject
signal path.
[0040] The first receive-side subject signal path 112 is
disconnected (422) from signal receiver 122 or otherwise disabled
from receiving a signal, and the second receive-side subject signal
path 118 is connected (424) to signal receiver 122 or otherwise
enabled to receive a signal. When the reference signal is emitted
(426) by the exciter 102, it is propagated (430) through the
transmitting signal path 104 and also propagated (428) through
local oscillator feed line 134 to signal mixer 136 in the receiving
signal path. The reference signal propagates (432) across the
mutual coupling field 140, and propagates (434) through the second
subject signal path 118 into the other input of mixer 136, where
the propagated reference signal instance and the local oscillator
feed signal instance are mixed (436). The heterodyned signal is
acquired (438) by the receiver 122, and its frequency profile is
analyzed (440) to determine the difference in arrival time of the
respective signal instances at the mixer. That frequency profile
correlates (442) with signal propagation time through the system
including the second subject signal path.
[0041] The difference in propagation time between the two subject
signal paths can be determined by comparing (444) the elapsed
propagation time through the system with each of the respective
subject signal paths connected. Since the propagation time through
the rest of the total signal path other than the subject signal
paths is the same in each instance, any difference in the
determined total system signal propagation times corresponds (446)
to the difference in the propagation times between the two subject
signal paths. As many additional subject signal paths as desired
may be connected and the actions of transmitting, receiving, and
elapsed time measuring may be repeated (448) for each to determine
their propagation times and relative propagation time differences.
It should be noted that the above need not necessarily be performed
in the exact order discussed here.
[0042] In another embodiment that uses the mixed and heterodyned
signal approach, instead of making a separate measurement to
determine the signal propagation time for each subject signal path,
the difference in propagation times between two different
receive-side subject signal paths is determined directly, from a
single measurement. This embodiment does not require a local
oscillator feed line. Instead, as depicted in FIG. 5, two
receive-side subject signal paths 112, 118 are connected (502) to
receive a signal simultaneously, each subject signal path feeding
an input of mixer 128. In this embodiment, the reference signal is
emitted (504) from the exciter, propagates (506) through the
transmitting signal path, and propagates (508) across the mutual
coupling field 140. The mutually coupled signal propagates (510)
through both the first subject signal path 112 and the second
subject signal path 118, and the signal instances flowing through
those two subject signal paths are mixed (512) at mixer 128. The
heterodyned signal from the mixer is acquired (514) by the receiver
122, and the heterodyne frequency products of the mix of reference
signal instances as received through the two subject signal paths
are analyzed (516) to determine the difference in arrival time at
the mixer of the signal instances fed through the two subject
signal paths respectively, which difference corresponds (518) to
the relative difference in propagation time between the two subject
signal paths. As many additional subject signal paths pairs as
desired may be connected and the actions of transmitting,
receiving, mixing and analyzing may be repeated (520) for each such
pair to determine the relative propagation time differences between
such pair. It should be noted that the above need not necessarily
be performed in the exact order discussed here. In contrast to the
information derived in the previously discussed approaches
involving absolute propagation time through the system, since this
embodiment's approach involves only relative difference in
propagation time, the information that cannot be gleaned from a
single examination of the mixed signal in this approach is which of
the subject signal paths features the shorter signal propagation
time and which subject signal path features the longer one;
additional action, such as the trial adjustment of subject signal
path propagation time discussed below, is necessary to yield that
information.
[0043] In another embodiment of the present invention involving
multiple receive-side subject signal paths, the system comprises
propagation time adjustment control circuitry 138 connected to
exciter 102, receiver 122, and propagation time adjustment
mechanisms 124, 126 that are deposed in one or more of the subject
signal paths. In this embodiment, the differences in propagation
times between two or more subject signal paths may be used by the
propagation time adjustment control circuitry to adjust the
propagation time adjustment mechanisms. All subject signal paths
may feature such a propagation time adjustment mechanism, or a
subject signal path featuring such a propagation time adjustment
mechanism may be adjusted to match the propagation time of a
subject signal path side that does not feature such a propagation
time adjustment mechanism.
[0044] In an embodiment involving propagation time calibration and
that determines the absolute propagation time through a subject
signal path, as depicted in FIG. 6, the adjustment starts with
measuring and determining (602) the relative propagation time
difference between a pair of subject signal paths based on their
respective absolute propagation times. If the adjustment control
circuitry decides (604) that the time difference is sufficiently
near zero, that is, if it is small enough to yield a level of
calibration that is desired or practical, the calibration process
terminates (606). If the difference is not sufficiently small, the
adjustment control circuitry adjusts (608) the appropriate
propagation time adjustment mechanism in at least one of the
subject signal paths to alter the propagation time in that subject
signal path or those subject signal paths in a direction that will
reduce the difference. The process then repeats, performing again
the time difference determination (602) to see if the adjustment
has brought the system within optimal parameters. If it has not,
the adjustment and measurement are repeated. The process may also
include a decision (610) whether the most recent adjustment has
caused the system to overshoot a zero time difference and begin to
suffer a propagation time difference in the other direction, in
which case before making the next adjustment (608) the direction of
adjustment is reversed (612), either by reversing the direction of
adjustment to the propagation time adjustment mechanism in the
currently addressed subject signal path, or by switching to address
in the same adjustment direction the propagation time adjustment
mechanism in the other subject signal path. It should be noted that
the above need not necessarily be performed in the exact order
discussed here.
[0045] In an embodiment involving propagation time calibration in
which an absolute elapsed time is counted, or in which the
difference between each respective subject signal path is
determined by mixing signals separately from respective subject
signal paths with a local oscillator feed line, the system can
determine immediately which subject signal path has the shorter
signal propagation time and which subject signal path has the
longer one, and the appropriate propagation time adjustment
mechanism may be adjusted accordingly. However, in the approach
where the signals propagated through various subject signal paths
are mixed with each other, only the size of the relative time
difference can be determined by analyzing the frequency profile of
the mixed signal, and not which of the subject signal paths
features the shorter signal propagation time and which subject
signal path features the longer one. Therefore, it can not be known
in advance in which direction a propagation time adjustment
mechanism should be adjusted. In this situation, as depicted in
FIG. 7, the system begins as in the prior embodiment, by
determining (702) the propagation time difference, deciding (704)
whether that difference is sufficiently small, and finishing (706)
if it is. If it is not sufficiently small, a small trial adjustment
(708) is made in either direction arbitrarily to a propagation time
adjustment mechanism, the measurement process is repeated (710),
and the system decides (712) whether the trial adjustment was in
the correct direction and made the time difference smaller, or
whether it was in the incorrect direction and made the time
difference larger. If the trial adjustment decreased the
propagation time difference, further iterative measurement and
adjustment (714) is performed in that same direction to reduce the
time difference toward zero. If, however, the trial adjustment
increased the propagation time difference, the direction of the
adjustment is reversed (716) before the system begins the further
iterative measurement and adjustment process (714). It should be
noted that the above need not necessarily be performed in the exact
order discussed here.
[0046] The result of the trial adjustment also yields information
regarding which of the two subject signal paths currently possesses
the shorter propagation time and which possesses the longer
propagation time. This is so because the system will then have the
information on which subject signal path was adjusted, in which
direction it was adjusted, and whether that adjustment led to a
smaller or larger difference in the propagation times between the
two. This information regarding which subject signal path has the
longer propagation time is useful for example in a situation where
the propagation time difference is not being nulled to zero, but is
instead being set to some non-zero point. This may be the case for
example where the antenna element of each subject signal path is
not located along the same equipotential surface of the mutual
coupling field, as discussed below, and so a compensating factor
must be applied to cause the signal instances through the
respective subject signal paths to arrive at the receiver more
closely coincident in time.
[0047] Any of the adjustment processes described herein for a
particular pair of subject signal paths may be repeated for
additional pairs of subject signal paths that feature propagation
time adjustment mechanisms, in order to match the propagation time
among all the subject signal paths. One way to achieve this is to
set one subject signal path as the standard for comparison, pair
that standard path with each of the other subject signal paths in
turn, and measure and calibrate each of those other subject signal
paths to match its propagation time to the propagation time of the
standard path.
[0048] Regarding disposition of the plurality of antenna receiving
elements with respect to the mutual coupling field of the antenna
radiating element, perhaps the simplest arrangement is to dispose
each of the receiving elements along an equipotential surface in
the mutual coupling field, so that the phase, amplitude, and time
delay of the signal from the radiating element is the same for each
of the receiving elements across the frequency range of interest.
However, if the transfer function of the radiating element is
known, the receiving elements could be disposed within any location
in the mutual coupling field, and the transfer function applied to
the analysis of the relative difference in propagation times to
compensate for any contribution to the difference from disparate
placement of the receiving elements in the mutual coupling
field.
[0049] The present invention is equally useful for systems
featuring a plurality of subject signal paths on the transmit side
of the system as for those with a plurality of subject signal paths
on the receive side of the system. Considering next an embodiment
of the present invention featuring multiple transmit-side subject
signal paths and a common receive path, as depicted in FIG. 2, the
transmit side of the transmit/receive system comprises an exciter
202, which feeds a first transmit-side subject signal path 204
comprising any electronics 206 in that signal path and antenna
radiating element 208. Exciter 202 also feeds at least a second
transmit-side subject signal path 210 comprising any electronics
212 in that signal path and antenna radiating element 214. The
system also comprises receiving signal path 216, including antenna
receiving element 218 and any electronics 220 in that signal path,
and further comprises a signal receiver 222. The antenna receiving
element 218 is disposed in both the mutual coupling field 238 of
antenna radiating element 208 and the mutual coupling field 240 of
antenna radiating element 214. Although two transmit-side subject
signal paths are depicted in FIG. 2, additional subject signal
paths beyond two may be fed by the exciter and have their signal
propagation time measured and/or calibrated, with the common
antenna receiving element also within the mutual coupling field of
the antenna radiating elements associated with such additional
subject signal paths.
[0050] In embodiments comprising additional optional components,
propagation time adjustment mechanisms 228, 230 may be disposed in
one or more of the transmit-side subject signal paths. The system
may also feature an adjustment control 232 connected to exciter 202
and receiver 222, and connected to the propagation time adjustment
mechanisms 228 and 230 to control them. The multiple subject signal
paths may also share a common transmit-side signal path 234,
including any electronics 236 in that signal path. The receive-side
signal path 216 may feed a mixer 226 that is also fed by a local
oscillator feed line 224 from the exciter.
[0051] The terms "attached," connected," "feed" or "fed" as used
here may include any form of the two referenced items being in
communication with each other, such as direct or indirect
electrical, electronic, optical, RF, or waveguide connection, or
any other form of attachment or association that promotes signal or
data communication. It should also be noted that the exciter,
mixers, receiver, adjustment control, and other electronics may be
implemented in and operate in the analog and/or digital domain,
using specialized electrical and/or optical circuitry, mechanical
members, special purpose computers, ASICs and/or firmware, general
purpose computers programmed with special purpose software, or any
other media usable for signal or data conduction and
processing.
[0052] An embodiment of a method for implementing the present
invention in a radio frequency transmit/receive system having
multiple transmit-side subject signal paths is depicted in FIG. 8.
This method is conceptually similar to the method depicted in FIG.
3 involving a system with multiple receive-side subject signal
paths, and FIG. 8 is sketched in brief to show the parallels to and
any differences from the method in FIG. 3. According to this
embodiment, a first transmit-side subject signal path 204 is
connected (802) to the exciter 202 or otherwise energized to
propagate a signal, a reference signal with appropriate properties
that enable observation of small differences in time is generated
(804) by the system exciter, and the system begins (806) counting
elapsed time. The signal propagates (808) through the system signal
path of the connected subject signal path 204 including any
electronics 206 and its antenna radiating element 208, across the
mutual coupling field 238 into antenna receiving element 218,
through the receive side signal path 216 and into the signal
receiver 222, where it is acquired (810), and the system ceases
(812) counting and retains the counted elapsed time.
[0053] The first transmit-side subject signal path 204 is
disconnected or de-energized and second transmitting signal path
210 is connected or energized (814). The generation and propagation
of the signal through the system and the counting of elapsed time
is repeated (816), with the reference signal from the exciter
propagating through the second transmitting signal path 210,
including any electronics 212 and its antenna radiating element
214, propagating across the mutual coupling field 240 to receiving
element 218, and propagating through the receive side signal path
216 and into the signal receiver 222, where the signal is acquired
and the total elapsed time count is stopped and retained (818). As
with a system involving receive-side subject signal paths, the
difference in propagation time can be determined by comparing (820)
the elapsed propagation time through the system with each of the
respective subject signal paths connected, with any difference in
the elapsed total propagation times corresponding (822) to the
difference in the propagation times between the two subject signal
paths. As many additional transmit-side subject signal paths as
desired may be connected and the transmitting, receiving, and
elapsed time measuring may be repeated (824) for each. It should be
noted that the above need not necessarily be performed in the exact
order discussed here.
[0054] As with the embodiments involving receive-side subject
signal paths, other embodiments using mixed and heterodyned signals
may be applied to systems with multiple transmit-side subject
signal paths. One such embodiment is depicted in FIG. 9. The
process used with this embodiment is conceptually similar to the
receive-side process depicted in FIG. 4; FIG. 9 is sketched in
brief to show the parallels to and any differences from the method
depicted in FIG. 4. In this embodiment, the first transmit-side
subject signal path 204 is connected (902) to the exciter 202 or
otherwise enabled to transmit a signal. The reference signal is
emitted (904) by the exciter 202, and it is propagated (906) both
through the first subject signal path 204, across mutual coupling
field 238, and through receive side signal path 216 to one input of
signal mixer 226, and also through local oscillator feed line 224
to the other input of mixer 226. The signal instances are (908)
mixed in the mixer, the heterodyned result acquired by receiver
222, and its frequency profile analyzed to determine the difference
in arrival time of the respective signals at the mixer, which
corresponds (910) to the signal propagation time through the total
system with the first subject signal path connected. The first
transmitting subject signal path 204 is disconnected or
de-energized and second transmitting subject signal path 210 is
connected or energized (912), the signal generation, propagation to
the mixer of the signal instances propagated through the system and
also from the exciter, heterodyning of the signal instances, and
acquiring and analysis of the heterodyned signal is repeated (914),
and the frequency profile corresponds (916) to the signal
propagation time through the total system with the second subject
signal path connected. Comparing (918) the propagation time through
the system with each of the respective subject signal paths
connected corresponds (920) to the difference in propagation time
between the two subject signal paths. It should be noted that the
above need not necessarily be performed in the exact order
discussed here.
[0055] In another embodiment that uses the mixed and heterodyned
signal approach in connection with a system having multiple
transmit-side subject signal paths, the difference in propagation
times between two different subject signal paths is determined
directly from a single measurement. The process used with this
embodiment is depicted in FIG. 10, and is conceptually similar to
the receive-side process depicted in FIG. 5; FIG. 10 is sketched in
brief to show the parallels to and any differences from the method
depicted in FIG. 5. This embodiment does not require a local
oscillator feed line or receive-side mixer. Instead, both of the
transmit-side subject signal paths 204 and 210 are connected (1002)
to transmit the reference signal. The reference signal is generated
(1004) by exciter 202 and propagates (1006) through both first
subject signal path 204 and second subject signal path 210,
resulting in the antenna radiating elements 208 and 214 both
mutually coupling (1008) the reference signal into the receiving
element 218. A mixer circuit is not required to mix the instances
of the reference signal radiated by the multiple radiating elements
in this embodiment, because the mixing occurs in the mutual
coupling field. The mixed and heterodyned reference signal
propagates (1010) through the receive side signal path 216, is
acquired by receiver 222, and its frequency products are analyzed
to determine the difference in arrival time at the receiving
element 218 of the signals fed through the two transmitting subject
signal paths. That frequency profile corresponds (1012) to the
difference in signal propagation times between the two
transmit-side subject signal paths. As with the similar embodiment
involving receive-side subject signal paths, since this
embodiment's approach involves only relative differences in
propagation time, the information that cannot be gleaned from a
single observation of the mixed signal in this approach is which of
the subject signal paths features the shorter signal propagation
time and which subject signal path features the longer one, and
that determination requires further action, for example as
described herein involving trial propagation time adjustments. It
should be noted that the above need not necessarily be performed in
the exact order discussed here.
[0056] In another embodiment of the present invention involving a
radio frequency transmit/receive system having multiple
transmit-side subject signal paths, a propagation time adjustment
mechanism 228, 230 may be disposed in one or more of the
transmit-side subject signal paths. The system may also feature an
adjustment control system 232 connected to exciter 202 and receiver
222, and connected to the propagation time adjustment mechanisms
228, 230 to control them. The considerations, configurations, and
methods for using the measured relative difference in propagation
time to calibrate transmit-side subject signal paths are the same
as those discussed above in relation to calibrating receive-side
subject signal paths, and will not be repeated here.
[0057] Regarding disposition of the plurality of antenna radiating
elements and their mutual coupling fields with respect to the
common antenna receiving element, perhaps the simplest arrangement
is to dispose each of the each of the radiating elements so that
the receiving element is at an equipotential point in each of the
mutual coupling fields, such that the phase, amplitude, and time
delay of the signals from each of the radiating elements is the
same for the receiving element across the frequency range of
interest. However, if the transfer functions of the radiating
elements are known, the receiving element could be disposed within
any location in the respective mutual coupling fields, and the
transfer functions applied to the analysis of the relative
difference in propagation times to compensate for any contribution
to the difference from the receiving element's various positioning
with respect to the mutual coupling fields of the various antenna
radiating elements.
[0058] Notably, the mixers in any of the embodiments discussed
herein or in other embodiments need not be mechanisms separate
from, the receiver; the receiver may acquire the plurality of
signals individually and mix them either in the analog domain or
digitally as part of its operation. The system may further perform
in either the analog or digital domain any necessary or desirable
signal processing or signal analysis regarding an acquired signal,
for example generation and processing of a cross-correlation
function for Barker code analysis. Such signal processing may be
performed by the receiver, the adjustment control, or any other
circuitry, module, electronics, or computational element within the
system.
[0059] One benefit of the embodiments according to the present
invention is that the measurement and calibration process requires
no significant reconfiguration of or addition to the operational
transmit/receive hardware, only the transmission of a reference
signal instead of an operational signal and perhaps a slight,
temporary reconfiguration of the signal flows through the system.
Further, the calibration process is very brief and occupies only a
very small portion of the operational duty cycle of the system.
Calibration can thus be performed periodically during normal
operations, such as once every few seconds. Calibration can also be
performed on demand, such as when the performance of the system is
detected to be shifting or degrading, or upon manual command of a
system operator.
[0060] The embodiments in accord with the present invention do not
require a transmit side or a receive side to be limited to only a
single signal path, and a system may practice the present invention
with pluralities of subject signal paths on both the transmit side
and the receive side, for instance through a multi-step measurement
and calibration process. In such a multiple-plurality system, for
example one transmit-side subject signal path can be energized and
multiple receive-side subject signal paths may be measured and
calibrated from it, and separately one or more calibrated
receive-side subject signal paths may be energized and used to
measure and calibrate multiple transmit-side subject signal
paths.
[0061] Although limited embodiments of the present invention have
been specifically described and illustrated, many modifications,
combinations, and variations will be apparent to those skilled in
the art. Accordingly, it is to be understood that a radio frequency
transmit/receive system constructed and a measurement and
calibration method practiced according to the principles of this
invention may be embodied other than as specifically described
herein. The invention is also defined in the following claims.
* * * * *