Delay scheme and apparatus for focussing the transmission and reception of a summed ultrasonic beam

A beamformer includes an array having a plurality of channels of delay elements which are constructed of charge coupled devices (CCDs). Each channel has a first plurality of delay elements cells which perform beam focussing and a second plurality of delay elements which perform beam steering. In this manner beam steering delays and beam focussing delays may be calculated independently. Typically, the second plurality of delay elements have a resolution which is typically coarser than the resolution of the first plurality of delay element cells which function to focus the beam. A signal may be inserted into any one of the delay elements of the first plurality to provide an appropriate delay. The signal is then output from the last delay element of the first plurality to the second plurality. The signal is delivered from a selected delay element of the second plurality to a charge sum data bus, which combines the signals output from each of the channels. This combined signal may then be output to a common offset delay block, which in turn outputs the array signal having a common delay. A plurality of such arrays may be coherently combined. If the required delay exceeds the maximum delay provided by each channel, the output of each array may be offset by a maximum delay of a preceding array in the plurality of arrays in order to coherently combine all of the arrays. Desirably, a shift register having a single enable bit is dynamically controlled to select the appropriate delay by selecting a CCD delay cell.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present application is directed to a method and apparatus for 
processing signals, for example, ultrasonic signals, supplied to and/or 
received from a distributed antenna formed of a large number of antenna 
elements. More particularly, the present application is directed to 
processing delays necessary for coherently summing received ultrasonic 
signals and for generating a desired transmission beam wavefront focussed 
at a desired point along a desired focal path by developing a delay 
associated with each of the constituent elements of the antenna using a 
two stage charge transfer delay line associated with each antenna element. 
2. Background of the Invention 
Ultrasonic imaging typically utilizes a transducer array composed of a 
plurality of imaging elements which both transmit and receive ultrasound 
signals. Typically, a shaped wavefront is transmitted by the transducer 
array and focussed along a path of interest. Reflected energy, obtained 
from the objects of interest along the path, is monitored to resolve an 
image of the objects of interest. Currently, if high resolution is to be 
obtained when processing signals from the array, the ultrasound 
reflections from the object of interest received by each transducer 
element of the array must be coherently summed so that all reflected 
energy is added to form a single reflected signal. 
Resolution may be further enhanced if the wavefront generated by the array 
of transducer elements is closely focussed on the objects of interest. The 
more precise the focus of the coherent wavefront on an object of interest, 
the more accurate the possible imaging of the object. 
In order to accurately, coherently sum the ultrasound reflections received 
from each of the transducer elements or to focus the emitted beam, the 
signals from each of the elements must be delayed a desired amount. The 
accuracy of this delay affects the accuracy of the summing and the minimum 
resolvable detail of the ultrasonic imaging system. In order to accurately 
delay the reflection received by each imaging element, each imaging 
element should be associated with a delay path containing a large number 
of delay elements. Accurate delay is further useful in transmission since 
the more accurate the delays associated with individual antenna elements, 
the better the focus of the beam at a desired focal point. 
In order to accurately generate the coherent wavefront, the wavefront 
should be generated by each of a large number of antenna array elements to 
collectively generate a coherent wavefront of a desired shape. Where an 
area of interest is to be imaged, the coherent wavefront should be swept 
across the area of interest. Each delay element must exhibit the desired 
resolution and there must be a sufficient number of delay elements to 
provide the required delay. Thus, the higher the resolution desired, the 
greater the number of required delay elements for a particular delay. 
Also, the greater the number of delay elements, the greater the signal 
loss due to an increased number of transfers along the delay path. 
In the digital domain, a signal is digitized and the resulting digital data 
representing the input signal are stored in a semiconductor random access 
memory (RAM) with separate input and output data ports using different 
read and write clocks. The delay generated is equal to the delay between 
the generation of the read and write clocks. The delay generated may be 
accurately controlled by the use of precise clocks. 
In the analog domain, a signal may be applied to an analog delay line with 
multiple taps. The delay line may include several cascaded sections of 
constant inductor (L) and capacitor (C) elements. The delay is generated 
by selecting a signal from one of the multiple taps. The signal may be 
supplied to an acoustic material in which the propagation of the acoustic 
wave is slowed either through the bulk (BAW (bulk acoustic wave) device) 
or on the surface (SAW (surface acoustic wave) device) of the material. 
Delay is generated by selecting a signal from one of the multiple taps. 
A charge coupled device (CCD) with cascaded storage cells can also be used 
as the delay path. In such a case, the signal may be converted to a 
current (or charge) equivalent and applied to the channel of the CCD along 
which the applied input charge is transferred at a rate dependent on the 
charge transfer clock. The delay is varied by changing either the transfer 
clock rate or the tap from which the signal is selected. When operating in 
the analog delay regime, the signal may be injected into a selected tap 
and the end of signal output from the end of the delay line. 
Another method of analog delay is to inject the signal to the front end of 
the delay line and the output from a selected tap. The accuracy of 
generated delay is a function of the total delay value and the tap 
increment and accuracy. 
An exception to the above functional relationship is in the case of the CCD 
cell, in which the delay accuracy is determined by the accuracy of the 
transfer clock as well as the number of storage cells. Use of a CCD line 
for delay generally is known and is disclosed in, for example, Menig, "CCD 
Characterization and Application in an Ultrasonic Imagining System", MIT 
Thesis, May 1976, pp. 5-17, 23-29, and 32. 
The current delay schemes all have disadvantages associated with them. In 
particular, an undue number of delay elements are needed to provide 
sufficient delay while being able to differentiate between small 
differences in delay. The increased number of delay elements also reduces 
the signal strength, due to the increased number of transfers between 
delay elements, thereby also decreasing the signal to noise ratio. 
Therefore, the need still exists for a delay scheme and apparatus which 
provides more precise delays and less signal loss. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide a method and apparatus 
for transmitting and receiving ultrasonic waves having an improved 
available signal dynamic range. 
It is a further object of the present invention to provide a method and 
apparatus for transmitting and receiving ultrasonic waves which accurately 
compensates for time of arrival differences, thereby accurately coherently 
summing the incoming reflections. 
It is a further object of the present invention to provide a method and 
apparatus for transmitting and receiving ultrasonic waves which accurately 
focusses the transmitted ultrasonic pulses at a desired focal point by 
accurately delaying the emitted pulse supplied to each antenna element. 
It is another object of the present invention to provide a method and 
apparatus for forming fine resolution images accurately and repeatably. 
It is still another object of the present invention to perform the above 
mentioned objects while reducing the amount of noise injected into the 
delayed signals. 
SUMMARY OF THE INVENTION 
These and other objects will become more readily apparent from the detailed 
description given hereinafter and are provided by the method and apparatus 
for transmitting and receiving a summed ultrasonic beam of the present 
invention. The present invention utilizes charge transfer delay devices 
for delaying the signals transmitted and received by each element of a 
distributed antenna. Each of these delay devices is provided with focus 
delay cells and beam steering delay cells serially connected and accessed 
by the selection of both a desired input tap and a desired output tap. 
Such a dual selection allows the resolution of the delay elements at 
opposed ends of each delay device to differ, resulting in the ability to 
concatenate one set of delay elements having a first resolution with 
another set of delay elements having a second resolution. When these 
resolutions are different, the advantages of using relatively low 
resolution delay elements to provide a large delay range for beam steering 
and of using relatively high resolution delay elements to provide a more 
accurate delay for focussing are realized. Further, the corresponding 
disadvantages of using these delay elements of differing resolutions are 
eliminated. In particular, the low resolution provided by the low 
resolution delay elements is eliminated, and the increased number of 
elements and the signal loss resultant from using high resolution delay 
elements is avoided. 
An important feature of the present application allows the delays of each 
of the fine and coarse delay elements of a delay device associated with an 
individual antenna element to be independently and dynamically selectable 
so that each delay may be controlled in operation. For example, it is 
contemplated that the present invention may be used to dynamically vary 
the focus of the array during the reception period after the transmission 
of an ultreasonic pulse. 
Since the reflections received immediately after the transmission of the 
pulse are reflections from close objects, and since the reflections 
received later are from more distant objects, it is desirable to delay the 
reflected energy received by the end antenna elements longer immediately 
after a pulse is emitted and thereafter gradually reduce this delay as the 
desired focal point of the array moves away from the antenna to increase 
the array focal distance dynamically. 
To facilitate this dynamic delay adjustment the fine and coarse delay 
elements associated with each antenna element are controlled by the 
selection of an input and output tap of each combined fine and coarse 
delay device through the use of controllable shift registers. 
Preferably, according to one preferred embodiment, the high resolution 
delay elements are user selectable during the transmit mode and may be 
dynamically controlled during reception for the purpose of adaptively 
focussing the antenna to coherently sum the reflections at each reflected 
distance. The delay provided by the low resolution delay elements is 
preferably fixed for a scan line and is varied between scan lines to 
provide steering or translation of the beam across an area to be imaged. 
According to another aspect of the invention, the individual antenna 
elements and their associated delay devices which for the entire array may 
be assembled into sub-arrays which are easily combined to form a large 
total array. When the total array requires a delay larger than that 
provided by the delay devices associated with each of the antenna 
elements, the antenna array may be divided into a plurality of sub-arrays 
and an offset delay may be provided with each sub-array to uniformly delay 
the output of the sub-array as required. 
Since the offset delay is applied to the coherently summed output assembled 
from the outputs of the individual antenna elements of the sub-array,the 
total delay of delay elements associated with each of the antenna elements 
need only equal the maximum difference in delay within the sub-array. Any 
remaining delay may be provided by an offset delay associated with each of 
the sub-arrays. Since the coherently summed signal of each of the imaging 
elements of the sub-array is relatively larger in amplitude than the 
output of each of the individual delay devices, the relative degradation 
of the coherently summed sub-array output caused by noise is reduced. When 
the focus of the total array is curved, outputs of each channel of each 
sub-array may be delayed a different time, as desired, to produce a 
desired delay profile in the entire antenna array. 
Although the primary application of the present invention is in ultrasonic 
imaging, the array elements may be designed to operate at any frequency of 
interest The antenna array may be linear, convex, or concave and may 
consist, if desired, of an M.times.N two-dimensional array of antenna 
elements, each having a delay produced according to the teachings of the 
present application. 
It should be understood, however, that the detailed description and 
specific examples, while indicating the preferred embodiments of the 
invention, are given by way of illustration only, since various changes 
and modifications within the spirit and scope of the invention will become 
apparent to those skilled in the art from this detailed description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention is directed towards an ultrasonic distributed element 
array antenna and a delay device associated with each of the antenna array 
elements and providing necessary delay to both steer and focus the emitted 
beam as well as to coherently sum the reflection signals received by from 
each of the antenna elements. In particular, the present invention is 
directed to providing improved resolution by enhancing delay resolution 
while maintaining sufficient delay, not significantly increasing signal 
loss, and not significantly increasing the number of array elements. 
As noted above, conventional delay schemes either select an input tap or an 
output tap and provide a set level of delay depending upon the resolution 
of the delay elements used therein. In accordance with one embodiment of 
the present invention, by controllably selecting both the input and the 
output taps a channel having at least two controllable sections of delay 
elements may be provided. Each of these sections may have a different 
number of delay elements and a different resolution per delay element. 
Preferably, according to the teachings of the present application the 
relatively high resolution delay elements are used for beam focussing and 
are employed in sufficient number to produce the necessary focus delay. 
The resolution of the ultrasonic system of the present application is 
dependent on accurate focussing and thus sensing system resolution is 
improved by improved dynamic beam focus. A relatively lower resolution 
assembly of delay elements is used to steer the beam along a selected 
azimuth. The resolution of these elements is dependent on the angle 
between adjacent scan lines. Since the scan lines are preferable at 
regular intervals which may be selected in device design, the resolution 
for individual beam steering cells is selected to correspond to the 
desired scanning line interval. In this fashion, the lower resolution 
delay elements will be regularly shifted while scan line by scan line. 
If desired, the delay of the steering delay elements may be varied across 
the face of the array so that the delay of the steering elements at the 
ends of the array can be made greater and the delay of the steering 
elements at the center of the array are relatively smaller. 
Using a standard number of relatively low resolution delay elements in 
conjunction with a smaller number of high resolution delay elements, as 
shown generally in FIG. 1a, allows a channel to output a more accurate 
delay without significantly increasing the number of delay elements or the 
signal loss. 
By beam steering with one set of delay elements and beam focussing with 
another set of delay elements, delay development is considerably 
simplified as the combined delay does not have to be separately 
calculated. 
In another embodiment of the present invention, large arrays may be formed 
by combining two or more sub-arrays of antenna elements and associated 
delay devices as shown in FIG. 1a. When a delay larger than that provided 
by a single array is required in a large array, the array of antenna 
elements may be divided into a plurality of sub-arrays and an offset delay 
may be included in each sub-array to uniformly delay the output of each 
sub-array as needed to produce the desired delay for each of the antenna 
elements. 
Element Array Structure 
In the exemplary embodiment shown in FIG. 1a, an exemplary antenna element 
8 and its associated delay device are disclosed. The entire antenna array 
includes a plurality of antenna elements 8, one for each channel as well 
as an associated delay device for each channel, where the channels are of 
M number and are identified as channels CH1-CHM. Each of these channels 
CH1-CHM receives an input signal along an input line 16 and delivers an 
output signal along an output line 18. 
The input signal Iin is received in each channel by a first or focussing 
delay section 10 consisting of a plurality N of cascaded first CCD cells 
or delay elements A1-AN. The input signal is input to a selected one Ax of 
the cells A1-AN. 
The detailed structure and operation of the first delay section 10 is set 
forth below in connection with FIGS. 3a-3c. A shift enable signal S 
supplied along line 24, a first shift clock signal CK1 supplied along line 
26 and a first sampling clock signal CK2 supplied along line 27 control 
the operation of the first delay section 10. The final element AN of the 
first delay section delivers an output to a decimation filter 11. 
The decimation filter 11 is succeeded by a second delay section 12 
including a plurality P of second CCD cells or delay elements B1-BP. The 
decimation filter 11 delivers the signal from the final element AN of the 
first delay section to the first element B1 of the second delay section. A 
selected one Bx of the cells B1-BP delivers the output signal Iout to the 
output line 18. 
The detailed structure and operation of the second or beam steering delay 
section 12 is set forth below in connection with FIG. 4. A second sampling 
clock signal CK3 supplied along line 28 and a second shift clock signal 
CK4 supplied along line 30 control the operation of the second section 12. 
In the specific example shown in FIG. 1a, the sampling rate provided by the 
second clock signal CK3 of the second delay section 12 is slower than the 
sampling rate provided by the first sampling clock signal CK2 of the first 
delay section. As known in the art, the decimation filter 11 connects the 
two delay sections 10, 12 and reduces the sampling rate of the signals 
from the first section 10 to the second section 12. 
A charge sum data bus 40 receives the output signal Iout from each channel 
CH1-CHM along their respective output lines 18 and adds these output 
signals together. The output from the charge sum data bus 14 is supplied 
along an element array output line 20. 
Element Array Operation 
When the first or focus cells A1-AN have a finer resolution than that of 
second or beam steering cells B1-BP, the first section 10 provides a focus 
delay in addition to the principal amount of delay provided by the second 
section 12. Although not strictly necessary, advantageously 
p.multidot.P&gt;n.multidot.N, where n and p are the time resolution for the 
first and the second CCD delay sections 10, 12, and p&gt;n. In a specific 
example, p=1/(4F.sub.0) and n=1/(16F.sub.0). Typically, n.multidot.N=8T 
and p.multidot.P=64T where T=1/F.sub.0. 
The resolution of the sub-array shown in FIG. 1a is effectively the fine 
resolution of the first or focus cells A1-AN, rather than the coarse 
resolution provided by the second or beam steering cells B1-BP. With such 
a relationship between the first and second cells, the first cells A1-AN 
provide beam focus, while the second cells B1-BP provide beam steering, 
i.e., angular deviation from normal 22. 
The different functions provided by the different sections 10, 12 are 
illustrated in FIGS. 1b and 1c. FIG. 1b illustrates beam steering provided 
by the coarse second delay section 12 for a number of scan lines 6. The 
first delay section 10 adjusts the focus of the beam around a normal 9 to 
the steered beam 6, with the resulting beam indicated as 8 in FIG. 1c. 
All channels CH1-CHM operate in the same manner and have the same delay 
architecture. However, the signal injected into the first delay section 10 
is individually controlled for each channel CH1-CHM such that the 
injection point of CH1 may be different from that of CH2 or any other 
channel. Typically, the signal extracted from the second delay section 
will be the same for all of the channels CH1-CHM for a given scan line. In 
other words, while the delay in section 10 can be dynamically increased or 
decreased during signal receipt or transmission by shifting the control 
register left or right a selected number of cells during the receive or 
transmit time of a scan line, the beam steering delay in section 12 is not 
dynamically modified within a single scan line. 
First Or Focussing Delay Section Operation 
A detailed embodiment of the first delay section 10 which preferably 
implements the fine dynamic focussing delay is shown in FIG. 3a. FIG. 3a 
includes a first shift register 17, which includes a plurality N of first 
shift register elements R1-RN, and a corresponding plurality N of first 
switches S1-SN located between each shift register element R1-RN and its 
associated CCD delay cell A1-AN. The output of the first shift register 17 
is controlled by the first shift signal S supplied along the line 24 and 
the first sampling clock signal CK1 supplied along the line 26. The first 
shift register 17 may be any desired form of register suitable for 
providing a logic "1" to the desired switch Sx to activate the switch Sx. 
The input signal Iin for a respective one of the channels CH1-CHM is input 
along the input line 16 to the first switches S1-SN. The first switches 
S1-SN may be of any suitable construction and may be semiconductor 
switches or the like controllable by the shift register elements R1-RN as 
would be known to one of ordinary skill in the art. 
When activated in accordance with S and CK1, the first switches S1-SN send 
the input signal Iin to a corresponding one Ax of a plurality of first CCD 
cells A1-AN. Thus, the injection point or input tap, and hence the delay, 
is established for the first delay section 10. 
The injection cell Ax of the first or fine delay section 10 is selected by 
the content of the first shift register 17. For example, the pattern 
0001000 . . . 0! represents the selection of the fourth cell, i.e., 
Ax=A4. When a different delay is required, the logic "1" is shifted by an 
appropriate number of positions. 
The pattern may be shifted left or right by the signal from the first shift 
clock signal CK1 and shift enable signal S. Typically, this different 
delay is a decreasing value for the dynamic focus of deeper distance by 
shifting the logic "1" to the right by an appropriate number of positions, 
as can be seen, for example in FIG. 3a and 3b. Such shifting is shown, for 
example, in FIG. 3b, in which the first shift clock signal CK1 shifts the 
original signal in the first shift register elements R1-RN, one cell to 
the right when the first shift clock signal CK1 is synchronized to the 
acoustic propagation velocity of the media of interest, a continuous focus 
is attained. 
In FIG. 3c, the shift enable signal S is illustrated in the upper plot. The 
corresponding delay is shown in the lower plot. The first three S signals 
correspond to the configuration shown in FIG. 3a, while the last three S 
signals correspond to the configuration shown in FIG. 3b. 
Once the input signal Iin is input into the cell Ax, this signal travels to 
the last cell AN in accordance with the clock signal CK2 supplied along 
line 27. 
When the cascaded first cells A1-AN of the fine or focus delay section 10 
of each channel CH1-CHM each provide a resolution of .lambda./r.sub.1, 
where .lambda. is the input signal wavelength and r, is, the resolution 
factor of a first cell, each channel CH1-CHM has a maximum adjustable 
delay Dmax given by: 
EQU Dmax=N.times..lambda./r.sub.1 (1) 
The input signal (charge or current proportional to signal voltage 
amplitude) may be injected into any one of the N first cells A1-AN. The 
output is taken at the final first cell AN and supplied to the second 
delay section 12 via the decimation filter 11. 
In accordance with the first sampling clock signal CK2, the input signal 
Iin is supplied from a selected cell Ax of the first cells A1-AN to the 
final cell AN. The output appears at the Nth (final) cell AN after AN-Ax 
sampling clock cycles later, where Ax is the selected first cell location. 
When the cell Ax is the injection point, the focus delay D.sub.f before 
the signal appears at AN is: 
EQU D.sub.f =(AN-Ax).times..lambda./r.sub.1 (2) 
Thus, a desired focus delay can be achieved by appropriate selection of AX 
at .lambda./r.sub.1 intervals. 
Operation of Second Or Beam Steering Delay Section 
A detailed embodiment of the second or beam steering delay section 12 is 
shown in FIG. 4. The components of the second delay section 12 are similar 
to the first delay section 10. Rather than delivering an input signal Iin 
to a selected cell, an output signal is selected from one of the second 
cells B1-BP. The second delay section 12 does not use a shift enable 
signal as does the first delay section 10. 
FIG. 4 includes a second shift register 19, including a plurality P of 
second shift register elements R1-RP, and a corresponding plurality P of 
second switches S1-SP. Unlike the first shift register elements R1-RN, the 
second shift register elements R1-RP are not dynamically modified within a 
single scan line. In other words, the selected cell Bx from the second or 
coarse delay section 12 is preferably fixed for each scan line and is 
changed only between scan lines. An output of a selected cell Bx of the 
second cells B1-BP is selected by the second switches S1-SP. The signal 
from the decimation filter 11 travels from B1 to the selected cell Bx in 
accordance with the second sampling clock signal CK3 supplied along a line 
28. The selected output from the selected cell Bx is then supplied to the 
output line 18. 
When activated in accordance with the second shift clock signal CK4 
supplied by line 30, the second switches S1-SP send the output from the 
selected cell Bx to the output line 18. Thus, the extraction point or 
output tap is established. 
The extraction cell Bx of the second delay section 12 is selected by the 
content of the second shift register 19. For example, the pattern 0001000 
. . . 0! represents the selection of the fourth cell, i.e., Bx=B4, as 
shown in FIG. 4. 
The steering delay D.sub.S before the signal appears at the extraction 
point is: 
EQU D.sub.S =(BX-B1)(.lambda./r.sub.2) (3) 
where r.sub.2 is the resolution factor of a second cell. Thus, a desired 
beam steering delay can be realized by appropriate selection of Bx at 
.lambda./r.sub.2 intervals. 
Exemplary Construction of an Element Array 
In the configuration shown in FIG. 1a, the element array may contain 32 
channels (M=32). The first section 10 of each channel may contain 128 
cascaded CCD cells (N=128) . Each cell in the first section 10 may provide 
.lambda./16 resolution. Thus, from equation (1), each channel has a 
maximum adjustable delay of 8.lambda.. When, for example, cell A4 is the 
injection point, the delay before the signal appears at the last cell A128 
is given by equation (2) and equals 7.75.lambda.. When a different delay 
is required, the logic "1" is shifted by an appropriate number of 
positions, calculated in .lambda./16 increments. 
The second section 12 of each channel may contain 256 CCD cells (P=256), 
each providing a resolution of .lambda./4. Thus, the second section 12 
provides a steering delay of up to a 64.lambda. having a resolution of 
.lambda./4 to the focus delay of 8.lambda. of the first section 10. 
The total number of CCD cells is less than 13,000 for an element array 
having M=32 channels, which may be easily integrated into a single 
monolithic device. 
Combining Element Arrays 
As can be seen in FIGS. 2a-2d, when imaging or detecting requires more than 
M channels, a plurality J of M channel element arrays may serve as 
sub-arrays SA1-SAJ. These sub-arrays SA1-SAJ are combined to form a large 
array beamformer. 
In FIGS. 2a and 2b, an array having its focus normal to a central array 
axis 22 requires a maximum delay for the center sub-arrays and a minimum 
delay for the end sub-arrays. When the focal distance f equals the 
aperture length L (where L=SA1+SA2+SA3+. . . +SAJ , or, when M is the same 
for each sub-array, L=JM), corresponding to an f number equal to 1, the 
maximum delay Dmax is given by: 
EQU Dmax=0.118.times.L (4) 
From equation (1) Dmax also equals N.lambda./r. Thus, the section 10 delay 
may be used up to a maximum aperture length Lmax, given by: 
EQU Lmax=(N/0.118)(.lambda./r) (5) 
and focus to an f number equal to 1 to infinity, as shown in FIG. 2c. 
Thus, the maximum number Jmax of sub-arrays, where Jmax is an integer, that 
may be combined by simply physically connecting the outputs of the 
respective charge sum data buses is given by: 
EQU Jmax=Lmax/(M.multidot.d)! (6) 
where ! indicates truncation to an integer and d is the spacing between 
the element. 
FIG. 2a-2c shows an arrangement for interfacing J of the sub-arrays shown 
in FIG. 1a, making up a M X J channel complete array. If spacing between 
channels is d=0.5.lambda., then, for this condition and from equation (4), 
the maximum delay for the center sub-array is 0.12.times.JMd. If this 
maximum delay is less than the total delay N(.lambda./r.sub.1) provided by 
each channel, this delay may be provided by the J sub-arrays with no 
offset required. 
In the example shown in FIGS. 2a-2c, four sub-arrays (J=4), such as shown 
in FIG. 1, are interfaced, making up a complete array of 128 channels. 
From equation (4), if inter-element spacing is 0.5.lambda., the maximum 
delay is 0.12.times.128.times.0.5.lambda.=7.68.lambda.. From equation (5), 
the section 10 delay of 8.lambda. may be used for an aperture up to 
67.lambda.. In practice, assuming thirty-two channels, i.e., M=32, in each 
sub-array, from equation (6), four such sub-arrays may be connected 
without requiring any offset delay. 
The relationship between the required delay and the focal point is shown in 
FIG. 2d. This relationship may be used to choose the selected cell Ax of 
the first delay section 10. 
Element Array Offset 
When delays are to be larger than N.lambda./r, as required, for example, 
when an array is phase steered by angle .theta. as shown in FIGS. 5a-5c, 
or when using concave or convex arrays, the section 10 and 12 delays may 
not be sufficient without an inordinant number of delay elements. In 
particular, for the sub-arrays SA1-SAJ, delay profiles T1-T(J+1) are on 
respective ends thereof, such that the first sub-array SA1 has delay 
profiles T1 and T2, the second sub-array SA2 has delay profiles T2 and T3, 
etc., with the final sub-array SAJ having delay profiles TJ and T(J+1). 
When a delay which is larger than that which can be provided by a channel, 
for the particular example discussed above, 8.lambda., the delay provided 
by the first delay section 10 is not sufficient. This may be compensated 
for by providing an offset delay which is equal to the maximum delay of 
the preceding sub-array in addition to the variable delay profile across 
the sub-array of interest. 
In order to provide the desired offset to the entire sub-array, as can be 
seen in FIG. 6, a common offset delay block 40 is inserted at the output 
of the array shown in FIG. 1a. This offset delay block provides the same 
amount of additional delay for all channels and scan lines. While in one 
preferred embodiment the focus delay elements 10 and beam steering 
elements 12 are collectively used with an offset delay, the offset delay 
may be used with single resolution delay elements. 
As can be seen in FIG. 7, the common offset delay block 40 has a similar 
architecture to that of the second section 12 shown in FIG. 4. The common 
offset delay block 40 includes a third shift register 43, which has a 
plurality Q of third register elements R1-RQ, and a plurality Q of third 
switches S1-SQ. However, the charge capacity of each of the third cells 
O1-OQ in the common offset delay block 40 is greater than that of each of 
the first cells A1-AN in the first section 10 or the second cells B1-BP, 
the signal dynamic range of the third cells O1-OQ is greater than that of 
the first cells A1-AN, and there are more third cells in the offset delay 
block than in the first section 10, i.e., Q&gt;N. 
The offset delay block 40 inserts the combined signal all M channels from 
the summing data bus 14 into a first third cell O1. The inserted signal 
travels along the offset delay block 40 in accordance with a third 
sampling signal CK5 supplied along a line 42 until a selected cell Ox is 
reached. The selection is performed in accordance with a third sampling 
clock signal CK6 supplied along line 44. 
Similarly to the second delay section 12, the extraction cell Ox of the 
offset delay block is selected by the contact of the third shift register 
43. The offset delay Do is: 
EQU Do=(Ox-O1)(.lambda./r.sub.3) (7) 
where r.sub.3 is the resolution factor of the offset delay block 40. Thus, 
a desired offset delay can be realized by an appropriate selection of Ox 
at .lambda./r.sub.3 intervals. 
By varying the offset delay for each array and the variable first section 
10 for all channels within each array, all other steering angles and focal 
distances may be accommodated. Thus, a large array may be controlled for 
beamforming and steering using element array architecture including an 
offset serving as the sub-arrays of the large array. 
The charge capacity of the third cell in the common offset delay block 40 
is preferably at least equals that of the first cells in the first section 
10 by a number of channels M, in the specific example above, 32, in the 
element array. The signal dynamic range of the third cells in the common 
offset delay block 10 is at least equal to the square root of the number 
of channels in each sub-array, times more than that of the first cells in 
the first section 10. Preferably, there are forty-eight third cells, i.e., 
Q=48, in the offset delay block 40, providing 48.lambda. of delay with 
.lambda. resolution. Note however, that the higher resolution provided by 
the first section 10 is still maintained. 
FIG. 5b illustrates the differences in delay required from each sub-array 
when the focal length is at infinity and the large array is phase steered 
by angle .theta.. When the focal length is at infinity, only coarse and 
offset delay are required. When the focus is not at infinity, as shown in 
FIG. 5a, fine delay is also required to achieve the desired beam form. A 
detailed description of all these delay levels is shown in FIG. 5c and the 
key set forth therein. 
When the beam is steered along by the angle .theta. away from the center or 
normal 2.sub.-- of the aperture made up of the arrays SA1+SA2+SA3+ . . . 
+SAJ, the delay curve for SA1 is as illustrated by a region U1 and dotted 
area 50 in FIG. 5c. The dotted area 50 is realized using the first delay 
section 10 to implement dynamic focusing. The region U1 represents delay 
necessary for steering the beam. For the sub-array SA2, the delay curve is 
a combination of a dotted area 52, a shaded area 34 and a region U2. The 
delay represented by the shaded area 34 indicates that delay common to all 
array elements within the sub-array SA2. This common delay has a fixed 
value equal to a region bounded by the maximum width of the cross hatched 
region U1, which is the maximum delay steering delay of the previous 
sub-array SA1. Thus, this fixed offset delay may be inserted into the 
sub-array SA2 with a variable delay forming the region U2 for array 
elements and then summed to the output of the sub-array SA1. 
Similarly, for the sub-array SA3, an offset delay indicated by a shaded 
area 36 equals the maximum delay of the sub-array SA2, and a variable 
profile U3 will allow coherent summation to the previous sub-array 
combination, T1 to T3. All of the following sub-arrays may be treated in 
an analogous manner. For the final sub-array SAJ, an offset delay 
indicated by a shaded area 38 equals the maximum delay of the sub-array 
SA(J-1), and a variable profile UJ will allow coherent summation to the 
previous sub-array combination, T1 to TJ. Thus, the full array defined by 
the delay T1 to T(J+1) is coherently combined. 
Conclusion 
The present invention provides many advantages. These include allowing the 
processing of the signal in the analog domain while controlling the delay 
value precisely with fixed clocks, and eliminating the need for high speed 
analog-to-digital converters which have smaller dynamic range than the 
sampled data approach and consume high power. Further, use of a fixed 
clock eliminates any frequency modulation of the sample data within the 
CCD shift register. Synchronous clocking and a single frequency may be 
effectively employed to reduce clock noise and improve available dynamic 
range. The total number of delay elements required for large arrays may be 
reduced by using sub-aperture and offset delays. 
The use of a shift clock at 16F.sub.0, where F.sub.0 is the signal 
frequency, assures minimal time quantization errors in the beamformer, 
making this approach easier to implement than any form of time and/or 
amplitude interpolation scheme. The use of signal in either charge packet 
throughout the CCD delay shift register allows better signal sampling and 
efficient coherent summation of delayed signals. The use of the split CCD 
cells provides for signal attenuation and apodization. 
The invention being thus described, it would be obvious that the same may 
be varied in many ways. Such variations are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims.