U.S. patent number 3,611,401 [Application Number 04/762,030] was granted by the patent office on 1971-10-05 for beam steering system for phased array antenna.
This patent grant is currently assigned to General Electric Company. Invention is credited to Terrence E. Connolly.
United States Patent |
3,611,401 |
Connolly |
October 5, 1971 |
BEAM STEERING SYSTEM FOR PHASED ARRAY ANTENNA
Abstract
A beam-steering system for phased array antennas is disclosed,
wherein the phases of the signals fed to the individual elements of
the array are varied by phase shifters in selectable discrete steps
to obtain directivity or steering of the array for transmitting
and/or receiving. Special circuitry monitors the roundoff phasing
errors of the phase shifters, and alters the settings of certain of
the phase shifters at certain times in order to compensate for and
eliminate the effects of the accumulated roundoff phasing errors.
Thus, improvements in beam-steering accuracy and other
characteristics are obtained, and/or the system is simplified and
reduced in cost because fewer (and coarser) steps of selectable
phase shifts can be employed.
Inventors: |
Connolly; Terrence E.
(Baldwinsville, NY) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
25063928 |
Appl.
No.: |
04/762,030 |
Filed: |
September 24, 1968 |
Current U.S.
Class: |
342/377; 342/372;
367/138 |
Current CPC
Class: |
H01Q
3/34 (20130101) |
Current International
Class: |
H01Q
3/30 (20060101); H01Q 3/34 (20060101); H01q
003/26 () |
Field of
Search: |
;343/778,853,854,1SA |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Claims
I claim:
1. A phased array antenna system having a plurality of radiation
elements, a plurality of phase shifters respectively associated
with said radiation element, antenna-steering means including a
source of successive steering command signals each indicative of
phase shift values providing a desired antenna radiation
characteristic and further including phase shifter control means
connected to said phase shifters and to said command signal source
to receive therefrom said steering command signals, said phase
shifter control means being responsive to said steering command
signals to set said phase shifters at phase shift values
approximating said desired values of phase shifts whereby roundoff
errors occur for the settings of at least some of said phase
shifters, monitoring means connected to said antenna steering means
for deriving therefrom a roundoff error signal indicative of the
value of phase shifter setting error, and compensating means
connected with said monitoring means to receive said roundoff error
signal therefrom and to recirculate the error signal to said
antenna steering means for addition thereby to the next successive
steering command signal thus accumulating roundoff errors for
providing an adjusted phase setting of at least one of said phase
shifters to at least partially compensate for said accumulated
roundoff errors.
2. A system as claimed in claim 1, in which the amplitude
characteristics of said radiation elements are given a nonuniform
distribution, and including storage means for storing the amplitude
distribution of said radiation elements, and weighting means
connected with said storage means and with said monitoring means,
said weighting means being adapted to apply to said accumulated
roundoff errors a weighting factor based on the relative amplitude
characteristics of adjacent radiation elements.
3. A phased array antenna system having a plurality of radiation
elements, a plurality of phase shifters respectively associated
with said radiation elements, each of said phase shifters being
adapted to shift phase in a limited number of discrete step-values,
antenna-steering means including a source of digital steering
command signals for providing successive sets of multibit digital
signals each comprising a number of bits greater than said number
of phase shifter step-values and being indicative of desired
step-values for said phase shifters to obtain desired radiation
characteristics of said array of radiation elements, said
antenna-steering means further including control means responsive
to a number of the more significant bits of each set of multibit
digital signals of respectively setting said phase shifters to
discrete phase-shift settings adjacent to said desired phase-shift
values whereby roundoff errors occur in the settings of the phase
shifters, monitoring means responsive to a number of the less
significant bits of each set of multibit digital signals thereby to
obtain a measure of the value of said roundoff errors and to output
a roundoff error signal, and compensating means connected to
receive said roundoff error signal from said monitoring means and
to recirculate the error signal to said antenna-steering means for
addition thereby to the next successive steering command signal for
providing an adjusted phase setting of at least on e said phase
shifters to at least partially compensate for said roundoff
errors.
4. A system as claimed in claim 3, in which the amplitude
characteristics of said radiation elements are given a nonuniform
distribution, and including storage means for storing the amplitude
distribution of said radiation elements, and weighting means
connected with said storage means and with said monitoring means,
said weighting means being adapted to apply to said accumulated
roundoff errors a weighting factor based on the relative amplitude
characteristic of adjacent radiation elements.
5. A system as claimed in claim 4, in which said amplitude
distribution is characterized by the greatest amplitude being at
the center of said array of radiation elements and tapering to
minimum amplitudes at the outer edges of said array, said source of
digital-steering command signals being adapted to provide said
successive sets of digital signals first for the center elements of
the array and thence for elements successively outwardly from the
center, whereby said weighting factor is successively based on
ratios of lesser amplitude characteristics.
6. A system as claimed in claim 3, in which said control means,
said monitoring means and said compensating means comprise, in
combination, an adder having first and second inputs with said
first input connected to receive said multibit digital signals from
said source of digital steering command signals, a phase signal
channel connected to the output of said adder and adapted to
receive only the relatively more significant bits of said digital
signals outputted thereby and further adapted to control said phase
shifter step values in accordance with these relatively more
significant bits, a roundoff error channel connected to the output
of said adder to receive therefrom only the relatively less
significant bits of said digital signals and connected to output a
roundoff error signal to said second input of said adder thereby to
temporarily circulate and accumulate said roundoff errors until the
value thereof is sufficiently great to affect the value of said
relatively more significant bits of the digital signals outputted
by the adder to thus provide said adjusted phase setting.
7. A system as claimed in claim 6, in which the amplitude
characteristics of said radiation elements are given a nonuniform
distribution, and including a recirculating shift register loaded
with the amplitude characteristics of successive radiation
elements, delay means interposed in the recirculating path between
the input and output of said recirculating shift register, a
divider having inputs respectively connected to the input and
output of said recirculating shift register so as to provide a
weighting factor based on relative amplitudes of said radiation
elements, a multiplier interposed in the roundoff error
recirculating path between said roundoff error channel and said
second input of the adder, and means connecting said weighting
factor output of the divider to an input of said multiplier,
whereby said roundoff errors are altered by said weighting factor.
Description
The invention herein described was made in the course of or under a
contract or subcontract thereunder, with the United States
Army.
BACKGROUND OF THE INVENTION
The invention is in the field of phased array antennas, for
transmitting and/or receiving radiation, which employ a plurality
of signal radiation elements in relatively fixed positions. As used
herein, "radiation elements" means array elements for transmitting
and/or receiving radiation of desired frequencies such as are used
for radar, sonar, and other applications.
Phase shifters associated with the various radiation elements
permit the beam shape and directivity of the array to be varied. A
linear (straight line) array varies the directivity in a single
plane, and a planar array can vary the directivity in two planes.
The elements of the array may comprise dipoles, microwave horns, or
other suitable radiators for high frequencies such as are used for
radar, or piezoelectric transducers such as are used for sonar. The
directivity and other desirable characteristics of the array such
as low side lobe energy can be further improved by properly
controlling the properly amplitude response of the various
elements, in addition to the phase control.
One type of array provides a fixed tapered amplitude response along
each row of radiation elements, the amplitude being greatest at the
center and tapering to lesser amplitudes toward the end elements.
Phase control means vary the phase shifts of the phase shifters
associated with the elements, so as to aim or steer the array beam
pattern in a desired direction or to cause the beam to scan over an
angle of space. Various mechanical and electrical types of phase
shifters have been employed, and have been controlled by electrical
or mechanical means for varying the phase shift. The amounts of
phase shift required at the different phase shifters for causing
the array to form a desired beam shape and aim it in a given
direction can be determined mathematically. When in operation, the
desired beam shape and direction or scanning pattern is determined
by a human operator or by electrical circuitry, and logic circuits
then set the phases of the elements so that the array provides the
desired beam direction or scanning.
Continuous control of each phase shifter over its phase-shifting
range can provide the most precise control of the beam shape and
directivity, but is complicated and costly. Therefore, it is
advantageous to control the phase shifters in discrete steps of
phase shift. For example, a prior system employs 48 radiation
elements per row, and each of the phase shifters is controllable in
six phase-shift steps of 180.degree., 90.degree., 45.degree.,
221/2.degree., 111/4.degree., and 55/8.degree.. The desired phase
shift is determined for each element for every desired beam shape
and position, and an adjacent step value of phase shift is then
assigned. That is, the desired value of each phase shift is
"rounded off" to an available value. This arrangement of discrete
phase shifting, though simpler and less expensive than continuously
variable phase control, results in less-than-perfect beam shape and
control over its directivity, because of the roundoff errors of
phase shifts. The use of a greater number of smaller steps of phase
shifts improves the performance, whereas the use of a smaller
number of larger steps of phase shifts reduces the cost and
complexity. Therefore, a compromise is made between performance on
the one hand and cost and complexity on the other hand. Thus, a
need has existed for eliminating or reducing this compromise.
SUMMARY OF THE INVENTION
Objects of the invention are to provide an improved phased array
antenna system; to provide such a system with improved beam
directivity and/or simplified phase-shift devices and control
circuitry; and to substantially reduce or eliminate the need for
compromise between performance and complexity.
The invention comprises, briefly and in a preferred embodiment, an
improvement in a phased-array system having a plurality of
radiation elements each having a phase shifter associated therewith
and adapted to shift phase in a limited number of discrete steps,
and means for controlling the phase shifters to cause the beam or
radiation pattern of the array to have a desired shape and to aim,
steer, or scan in a desired manner. In accordance with an important
feature of the invention, circuitry is provided for altering the
phase shifts of certain of the phase shifters, at certain times, to
compensate for the roundoff errors of the other phase shifters.
In a preferred embodiment of the invention the phase shifter
settings are repetitively determined in rapid sequence by logic
circuits, and the circuitry of the invention functions to
continuously determine (monitor) the accumulated total roundoff
error of the phase shifter settings which have been determined.
Whenever the accumulated error exceeds a certain value as compared
with an adjacent step-range of the next successive phase shifter to
be set, the circuitry causes this phase shifter to be set at a
step-value adjacent to that which it normally would be given, thus
tending to compensate for and eliminate the effects of the
accumulated errors. This procedure of monitoring, comparing, and
compensating occurs repetitively during operation of the
system.
In a further and more sophisticated preferred embodiment of the
invention, for use with systems in which signal gain or amplitude
is tailored or "weighted" across the array, circuitry is provided
to alter the roundoff errors in the monitor circuit by a weighting
factor based on the ratio of amplitude of the preceding array
element to that of the element for which the phase shifter is
currently being set.
The invention permits the use of simplified phase shifters, in some
cases having only half of the number of phase-shift steps as in
prior art arrangements (three instead of six steps of phase shift,
for example), with no degradation of beam pointing accuracy. This
not only reduces the cost and complexity of the phase shifters, but
also lowers the cost and complexity of the digital logic circuitry
which drives the phase shifters. The reduced complexity improves
the reliability of the system. The importance of these improvements
will be more fully appreciated if it is realized that a typical
array antenna employs thousands of phase shifters and associated
digital drive circuits.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is an electrical diagram of a preferred embodiment of the
invention, and
FIG. 2 is an electrical diagram of specific phase-shift control
circuitry employed in a preferred embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the invention embodied in a radar system of the type
comprising a linear array antenna utilizing discrete-step phase
shifters to achieve beam forming and steering for transmitting and
receiving. The antenna, designated generally by reference numeral
11, comprises a plurality of radiation elements 12 (also
individually designated R1, R2...RN) arranged in linear array and
grouped into two subarray units symmetrically disposed with respect
to the center of the array. A plurality of branch feed lines 13 are
respectively connected to feed signals to and from the radiating
elements 12 via discrete-step phase shifters 14 (also individually
designated P1, P2...PN) the phases of which are controlled by
signals applied thereto from a steering command signal source 15
over connections 17 for one subarray and 19 for the other
subarray.
Each of the branch feed lines 13 of the left-hand subarray connects
via a cross guide coupler 21 to a common series feed line 23, and
the branch feed lines 13 of the right-hand subarray similarly
connect to a common series feed line 25 via cross guide couplers
21. The cross guide couplers 21 are individually designated C1,
C2...CN. Terminations T1 and T2 are provided at ends of the series
feed lines and of the branch lines, respectively, for impedance
matching purposes.
Further details of the series feed configuration are disclosed in
Kinsey U.S. Pat. No. 3,258,774; further details of a preferred
embodiment of the phase shifters 14 are disclosed in Hair U.S. Pat.
No. 3,290,622; and further details of a preferred embodiment of the
cross guide couplers 21 are disclosed in Kinsey U.S. Pat. No.
3,230,483; these patents being assigned to the same assignee as the
present invention. The two subarray units connect through a hybrid
junction 27 to a transmitter 29 and to a receiver 30, both of which
may be of conventional construction and operation. The hybrid
junction 27, which may be of conventional construction, is shown as
the four port type providing two 3 -db. terminals 28 respectively
connected to ends of the series feed lines 23 and 25 and also
providing sum and difference terminals 31 for connection to the
transmitter 29 and receiver 30. The arrangement may be commutated
so as to electrically reverse the sum and difference outputs at
terminals 31 in synchronism with the alternate transmit and receive
functions of the system. A timer 33 synchronizes the functioning of
the transmitter 29, receiver 30, and steering command signal source
15.
The arrangement thus far described with reference to FIG. 1 may
comprise the arrangement described in more detail in Connolly U.S.
Pat. No. 3,380,053, assigned to the same assignee as the present
invention.
In accordance with a preferred embodiment of the present invention,
an error compensation circuit 36 is interconnected with the
steering command signal source 15 and functions to reduce or
eliminate the effects of the accumulated roundoff errors of the
settings of the incremental phase shifters 14.
The functioning of the invention will now be described generally
with respect to FIG. 1, and then will be described in greater
detail with respect to the specific embodiment shown in FIG. 2.
The antenna 11 is capable of providing various desired radiation
beam widths and directions in a horizontal plane (assuming linear
array 11 is positioned in a horizontal plane), and a stacked array
of these linear arrays provides a planar array which can control
the beam shape and direction both horizontally and vertically. This
beam pattern is controlled by setting the phase shifters 14 to
various values of phase shift, in well-known manner. The phase
distribution needed across the array to achieve a desired beam
pattern may be determined in various known ways, such as from
Fourier-transform theory. Also, it is known practice to provide a
tapered amplitude distribution across the array to reduce undesired
side lobes.
The phase shifters could be properly set by means of a memory
device which stores all desired phase-shifter settings for all
desired beam shapes and directions. For a planar array having
thousands of phase shifters, and even for a linear array having 40
or more phase shifters, this would require a very large memory and
has the further disadvantage of being inflexible as to any desired
beam shapes or directions other than stored in the memory.
Therefore, numerous prior systems have employed an arrangement of
logic circuitry which repetitively computes the required phase
shifts, for the phase shifters 14, in synchronism with the radar or
sonar transmitted pulse repetition frequency. The required phase
shifts are computed by the circuitry from a suitable formula stored
in memory and the phase shifters 14 are set accordingly, in a short
time (a few microseconds, for example) immediately before or after
the transmission of each radar or sonar pulse, or at both times if
different phase settings are desired for receiving than for
transmitting. This rapid repetitive setting of phase shifters need
not cause any loss in system operational time, because the same
short time periods are required for other functions such as
operation of the transmit-receive switch. As will be apparent
subsequently from the description of FIG. 2, the logic circuitry
computation of the required phase shifts need not be limited to the
short time intervals of setting the phase shifters.
It is desirable for each of the phase shifters 14 to be adjustable
in discrete step-values of phase shift, for example as disclosed in
the aforesaid U.S. Pat. No. 3,290,622 (Hair) and 3,380,053
(Connolly), so that they can be set to their various phase-shift
values by means of digital or binary-type pulses provided by the
steering command signal source 15. Examples of phase shifters that
have been employed, are four-step phase shifters having selectable
phase-shift values of 180.degree., 90.degree., 45.degree. and
221/2.degree., and six-step phase shifters having selectable
phase-shift values of 180.degree., 90.degree., 45.degree.,
221/2.degree., 111/4.degree. and 55/8.degree.. Since the
phase-shift values computed by the logic circuits in the steering
command signal source 15 must be rounded off to an adjacent value
available at the respective phase shifters 14, it is evident that
in order to increase the accuracy of beam shaping, pointing, and
scanning, it is necessary to increase the number of step-values of
phase shift in the phase shifters 14. This not only increases the
complexity of the phase shifters 14 and the circuitry of the
steering command signal source 15, but also considerably increases
the wiring complexity as will be evident when it is realized that,
for six-step phase shifters 14, each of the wires shown (in
connections 17 and 19) connected to individual phase shifters 14
must consist of a group of six wires, in addition to the common or
"ground" connections to the phase shifters.
The error compensation circuit 36, in accordance with the
invention, improves the accuracy of the antenna array without the
necessity of increasing the number of step-values of the phase
shifters. Alternatively, for a given required accuracy, the
invention permits a reduction in the number of step-values required
of the phase shifters. The error compensation circuit 36 comprises,
in a preferred embodiment which will be described with respect to
FIG. 2, a monitor circuit for accumulating the roundoff errors of
the phase shifters as their settings are determined by the steering
command signal source 15. The value of the accumulated errors is
temporarily stored, and whenever this accumulated error exceeds a
certain value as compared with an adjacent step range of the next
phase shifter to be set, correction circuitry causes this phase
shifter to be set to its adjacent step value so as to partly or
completely compensate for and eliminate the effects of the
accumulated roundoff error. This procedure occurs repetitively,
whereby each time the phase shifters have all been set the effect
of accumulated roundoff error is minimal or nonexistent.
Now referring to FIg. 2, the timer 33 and the Steering Command
Signal Source 15 are the same as shown in FIG. 1. The Steering
Command Signal Source 15 is of conventional design, and is shown as
providing a 12-bit output 41 of which each sequential group of 12
bits represents a phase shifter setting desired for one of the
phase shifters 14. The 12-bit command signal 41 is fed into an
adder 42, which adds thereto an eight-bit recirculating roundoff
error signal as will be described. The 12-bit output of the adder
42 becomes split, the four most significant bits thereof being fed
into a four-bit phase signal channel 44, and the eight least
significant bits, which constitute the roundoff error, being fed
into a roundoff error channel 45. The output of the four-bit phase
signal channel 44 is connected via line 46 (which actually consists
of four conductors although a single line is shown for convenience)
to the input of a commutating switch 47 which functions to
alternately commutate the phase signals to a left subarray shift
register 48 and a right subarray shift register 49. The commutating
switch 47 is electronic, and its commutating rate is controlled
from the timer 33 via a time divider 51 so that it will commutate
the phase control signals properly to the left and right subarray
shift registers 48 and 49. An inverter 52 is provided in the
connection to one of the subarray shift registers, to provide
proper phase distribution across the array.
In the example shown in FIG. 2, each subarray may consist of 21
radiation elements and associated four-step phase shifters and
therefore each of the subarray shift registers 48 and 49 has a
capacity of 84 bits, i.e. four bits for each of the 21 phase
shifters. The subarray shift registers 48 and 49 are sequentially
loaded, in series, with the phase shift control bits, and at
appropriate times such as just prior to or following the main pulse
of radar or sonar radiation, a phase shifter loading trigger
circuit 53 (which may be under control of the timer 33) gates on a
plurality of gated drivers 56 and 57, respectively connected in the
left and right subarrays which set the phase shifters 14 in
accordance with a particular distribution pattern as has been
determined by the steering command signal source 15.
As thus far described with respect to FIG. 2, the settings of some
of the phase shifters will be erroneous, due to roundoff error,
since the phase shifters are set by only the four most significant
bits of the 12-bit steering command signal. In accordance with the
invention, the eight roundoff error bits are fed to the roundoff
error channel 45, from which the output signals 61 thereof are fed
through a delay network 62, having a delay time equal to one word
of the steering command signal, to a multiplier 63 in which the
roundoff error signal is multiplied by a weighting factor that is
derived from the amplitude distribution across the array elements,
as will be described. The output signals 64 from the multiplier 63
are applied to an input of the adder 42, where they are added to
the 12 -bit steering command input signals 41. Thus, the eight-bit
roundoff error signal is repetitively added to the incoming 12-bit
steering command signal in the adder 42, and whenever the
accumulated roundoff error becomes sufficiently great, it will
affect the four-bit most significant part of the signal that is
applied through channel 44 to the phase shifter control circuits
thus altering the phase setting of various phase shifters in a
manner to correct for accumulated roundoff error.
The multiplier 63 is not necessary to the operation of the
invention if the amplitude distribution is uniform across the
array, or if compensation for a weighted amplitude distribution is
not required.
In order to factor in the effects of a weighted amplitude
distribution across the array when correcting for roundoff
phase-shift error, in accordance with the invention, a shift
register 66 is provided for storing the amplitude distribution
pattern of the array. A signal delay network 67 is provided in a
feedback loop around the shift register 66, and has a delay period
equal to one word of phase shifter information. The shift register
66 is actuated, and the amplitude distribution therein is
circulated therethrough via the delay device 67, at a rate
determined by the timer 33. The input and output of the shift
register 66 are applied to a divider circuit 68, which divides the
output of the shift register 66 by the delayed input thereto, thus
generating at the output 69 thereof a signal based on the ratio of
the amplitude of the preceding array element to that of the array
element for which the phase shift is currently being determined. In
a preferred type of weighted amplitude distribution, the amplitude
characteristic will be relatively greatest at the center of array,
and will taper toward lesser values at the ends of the array.
The amplitude weighting signals are applied via connection 69 to
the multiplier 63, where they are multiplied by the successive
roundoff error signals, so that the phase control improvement
achieved by the invention will be all the more accurate due to the
phase correction being thus based on the relative amplitudes of the
preceding elements in which the roundoff errors have occurred.
As has been pointed out above, the invention repetitively monitors,
and corrects for, the accumulated roundoff errors and thus permits
considerable simplification of the system by permitting the use of
phase shifters having relatively fewer steps of phase shift, or,
alternatively, improves the accuracy of beam shaping and steering
without the necessity of employing more complicated and more costly
phase shifters and associated circuitry for providing a greater
number of finer steps of phase shift as has been necessary in
accordance with the prior art.
ALthough the preferred embodiment, as shown, uses parallel format,
sequential word arithmetic binary logic circuitry, the same
function can be performed with serial format, sequential word logic
or completely parallel arithmetic circuitry, as well as analog or
hybrid analog and digital computational circuitry. Also, while the
invention has been described as applied to a series-feed array
system, the invention can also be advantageously used in arrays
employing other feed arrangements such as corporate feed, space
feed, etc.
While preferred embodiments of the invention have been shown and
described, various other embodiments and modifications thereof will
become apparent to persons skilled in the art, and will fall within
the scope of invention as defined in the following claims.
* * * * *