US3811067A

US3811067A - Ultrasonic imaging tube

Info

Publication number: US3811067A
Application number: US00258635A
Authority: US
Inventors: W Turner
Original assignee: Automation Industries Inc
Current assignee: VITRO Corp A CORP OF DE
Priority date: 1970-10-08
Filing date: 1972-06-01
Publication date: 1974-05-14
Anticipated expiration: 1991-05-14

Abstract

An improved ultrasonic imaging tube having a piezoelectric face plate or conversion plate, a controlling screen adjacent thereto and a collector electrode back of the screen for receiving current generated by the conversion plate. The collector current is utilized to control a video tube synchronized with the ultrasonic tube, and is shunted in feedback fashion to the screen to control terminal admittance thereof and thereby increase the signal to noise ratio of the system as well as to raise the effective signal level.

Description

United States Patent [19] Turner May 14, 1974 ULTRASONIC IMAGING TUBE [75] Inventor: William R. Turner, Silver Spring,

[73] Assignee: Automation Industries, Inc., New

York, NY.

[22] Filed: June 1, 1972 [21] Appl. No.: 258,635

Related US. Application Data [52] US. Cl 315/11, 315/12, 340/5 MP [51] Int. Cl. H01j 31/48 [58] Field of Search 315/12, 11,9; 313/65 R, 313/65 A, 68; 340/5 MP [56] References Cited UNITED STATES PATENTS 3,213,675 10/1965 Goldman 340;3l5/5 MP;13 R 3,236,944 2/1966 Jacobs 340/5 MP 3,718,032 2/1973 Gray 340/5 MP X 2,668,190 2/1964 Sziklai 313/67 UX "71' l '{vfy'v 2 5 2,775,719 12/1956 Hansen .l. 313/65 A X 2,899,580 8/1959 Drunctl ct ul.. 313/89 2,903,617 9/1959 Turner 315/12 2,957,340 10/1960 Rocha 73/675 R 3,197,661 7/1965 Sinclnir.... 313/68 R 3,290,674 12/1966 Calhoun... 313/64 X 3,577,171 5/1971 Turner 315/12 X Primary Examiner--Le1and A. Sebastian Assistant Examiner-P. A. Nelson Attorney, Agent, or Firm-Jones and Lockwood 5 7] ABSTRACT An improved ultrasonic imaging tube having a piezoelectric face plate or conversion plate, a controlling screen adjacent thereto and a collector electrode back of the screen for receiving current generated by the conversion plate. The collector current is utilized to control a video tube synchronized with the ultrasonic tube, and is shunted in feedback fashion to the screen to control terminal admittance thereof and thereby in crease the signal to noise ratio of the system as well as to raise the effective signal level.

4 Claims, 5 Drawing Figures C game ULTRASONIC IMAGING TUBE: This is a division of my application Ser. No. 79,135, now abandoned filed Oct. 8-, 1970 which, in turn is a division of my-parent application Ser. No. 756,866-filed 1 Sept. 3, 1968, now U.S. Pat. No. 3,577,171 havingcissued on May 4, 1971.

This invention pertains to an improved ultrasonic tube which is equipped with a piezoelectric face plate or conversion plate capable of converting ultrasonic to electric signals and vice versa. The electric signals picked up by the ultrasonic tube can be displayed on a cathode ray tube acting as a video tube.

Heretofore, the signal current output from the'high conductance ultrasonic converter tube has been obtained directly from a screen closely spaced to-theconversion plate. This screen, when swept by a cathode ray beam, establishes the electric field at the conversion plate surface necessary for the high conductance path between the electron beam landing point and the screen. However, the capacitance of the screen to ground sets a lower limit on the termination-admittance, and hence, the signal level, achievable" for a given bandwidth; the capacitance between the conversion plate surface and the screen introduces an extraneous signal current that must be removed by signal processing; and the small percentage of the primary electron beam intercepted by the screen producesa signicant amount of tube noise.

The present invention avoids the defects of priordevices by the use of a high transparency screen and'a separate collector electrode which draws the signal current therethrough. The electrode is so positionedat a distance from the screen that the capacity coupled signal and primary noise are eliminated; Moreover,'th'e* signal on the collector electrode'is amplified in a circuit that balances out electrode capacitance.an'disthenap plied to the screen in feedback to producea potential which increases the electric field at the conversion plate. The effective termination admittance is thus re duced by the factor of current multiplication, increasing the degree of conversion plate element control and' raising the output signal level.

Although the invention disclosed herein might be ap-- plied to various types of ultrasonic-electronic convert ers, optimum performance would'be attained by utilizing it in the high-conductance type converter disclosed:

in prior United States Patent to W. R. Turner, U.S. Pat.

No. 2,903,617 of Sept. 8, 1959, or in'the article entitled. Ultrasonic Imaging by W. R. Turner published in'Ul trasonics for October-December 1965', pp. 182-187. Also, it could be readily adapted to the Transceiver ing in its operation in reciprocal modes.

The principal object of the'invention is toiprovide an improved ultrasonic-electronic video system havinga:

higher signal conversion efficiency and reduced noise factor.

A further object of the invention is to provide'an im aging tube equipped with a collector electrode'in'addb tion to the customary screen.

A still further object isto make an'output connection from the aforesaid collector electrode to an amplifier, to feed said amplified signal to the'video phase of a-system, and to shunt said signal to the screen to lower the admittance margin of the imaging tube.

Still another object of the invention is to modify the geometry of an image tube so as to permit the use of a separate collector electrode independently of the screen adjoining the piezoelectric face of the tube, in receiving mode of its operation.

With the above and related objects in view, the invention is described below in conjunction with the drawings, in which:

FIG. 1 is a partial diagrammatic showing of a high conductance image. tube preceding this invention.

FIG. 2 is'a'partial. analog circuit depicting properties of the tube-in terms of electrical components, and including a coupling networkutilized to balance out capacitive components thereof.

FIG. 3"is a partial cross-section of the image tube of this invention.

FIG. 4-is a diagrammatic circuit showingithe several components and novel coupling employed in the outputiphase of the. subject'invention.

FIG. 5* is a diagrammatic representation of a video projection system to display the signal from the output of the tube of the invention.

Referring to fragmentary enlargement'FlG. 1, the highconductance image tube disclosed'in applicants U.S. Pat. No; 3,600,936, referred to above consists of an-envelope,.cathode ray gunand sweep plates (as in FIG. 3) and employs a piezoelectric face plate or conversion plate sealed to'the envelope. The plate is covered by a grounded, thin, protective membrane generally designated-8wh'ich is in contact with a suitable ultrasonic transmitting: medium such as water. The other side of the plate.1*(within the envelope) carries arr-insulated target surface 3 which-h'as' a high secondary emission yield'(greater than unity). Mounted in proximity of target' surface-3;.isa closely spaced but highly transparentv wire capture screen'4; While the conversion'plate liscontinuous physically, it functions in cooperation with screen 4 and cathode ray beam or electron beam 2 as though' it was composed of a plurality of discrete-transducer elements, the area 5 of which is-determined by the area of the impinging beam.

The'impact of cathode ray beam 2 upon target surface 3';.landing"area 5'; gives rise to a space charge 6 which serves asa. coupling medium between landing area SFand screen4i W-hen'the space charge is in equilibrium, the electrons therein are'at zero velocity and are' free to move either to landing area 5 or toward screen 4; acting asacapture electrode. The direction of" electron motion may be determined by minute changes of electric field inthe vicinity of zero velocity position 'and random fluctuation in emissionvelocity. If

an alternating potential should appearon landing area tions ofanimagingtube 'in' receiving and transmittingmod'es, respectively. Thus, theelectron beam2 sweeping over target'surface '3'forms a conductive (or resistive) link from the beam landing area to the screen 4 which provides for alternating current flow between them in opposite directions depending on operational mode.

FIG. 2 depicts, in dotted rectangle a, an analog of three discrete transducer elements (landing areas) 5 of plate 1, which may be impinged by cathode ray beam 2 in a predetermined, desired sequence. Each such transducer comprises the following equivalent electrical'components: a capacitance (C being the electrical capacitance of the element; a conductance 11 (0,) being the radiation conductance of the element when the front face of the conversion plate is coupled to an ultrasonic transmission medium 9 such as water; and a current generator 12 (1]) being the electrical analog of ultrasonic energy entering the element through the transmission medium. The result of such excitation is an alternating potential on the landing area. Alternately, an internal excitation of the element by an alternating current impressed upon the landing area will cause current to flow into the radiation admittance, and hence a conversion into ultrasonic energy that is projected into the transmission medium.

The relation between the equilibrium electron current and the potentials defining the electric field in front of the emission surface is given by,

(I) where A is the space charge area, L, the spacing from the emission surface to the capture electrode, L,., the distance from the emission surface to the plane of minimum electron velocity, V,, the capture electrode potential, V, the emission surface potential, and V the average secondary emission velocity in electron volts.

Alternately, the relation between the current and the potential difference from the beam landing point to the capture electrode can be defined as a small signal conductance.

(2) in this expression, L,., being ignored in relation to L.

Thus, the action of the electron beam can be represented as a conductive (or resistive) link from the beam landing point to the screen. This provides a bilateral path for alternating current flow between the beam landing point and the screen.

A negative capacitance network 16 is employed with the above high conductance image tube to improve performance. This is a class of network described in detail by H. W. Bode in his book Network Analysis and Feedback Amplifier Design." This network exhibits at its terminals a negative capacitance 17 (C,) in parallel with a termination conductance 18 (G). The negative capacitance must be sustained over a frequency bandwidth sufficient to include the important sidebands of the electrical signal. In cancels the stray capacitances l4 and terminating on the screen, and under certain circumstances can also act through the beam conductance 13 to cancel the capacitance 10 of the beam landing point element only.

A necessary condition for cancellation of the element capacitance is,

(3) where B is the susceptance of the element capacitance 10 (C,). Since B is a small quantity, G, must also be limited in value if the required value of G, is to be realizable. 6,, however. is limited in minimum value by a network realizability relation derived by Bode and given approximately as,

G, z C, 4B

(4) where C is the total shunt capacitance to be cancelled at the image tube terminal, and B is the bandwidth of the network.

As a consequence of these relationships, a number of compromises have been necessary in selecting the exact geometry for the high conductance image tube of the prior art.

The beam conductance can be increased by reducing the spacing between the screen and the conversion plate surface. This, however, increases the capacitative coupling 14 between the conversion plate and the screen through which extraneous signal current flows from the remaining ultrasonically excited plate areas, and also increases the capacitance to ground for the screen, further limiting the minimum value of the termination conductance 8 (6,). Alternately, the beam conductance can be increased by increasing the primary beam current 2. Higher beam currents result in an increased spreading of the electron beam through space charge repulsion, and hence a lessening of electronic resolution. More important, however, is the noise associated with the small proportion of primary beam current intercepted by the screen 4. Since this beam noise current is a significant component of total tube noise, as beam current is increased, this resulting noise source can cancel the effectiveness of any signal current improvement.

The invention herein discloses a method of signal extraction and feedback that significantly changes the limitations on tube geometry, termination network character, and operating parameters so that improved image tube performance is possible. The fundamental departure from my prior imaging systems resides in the use of a separate collector electrode for signal current collection.

As will be seen in FIG. 3, the new imaging tube employs the usual conversion plate 1, having a grounded membrane 8 for contact with an ultrasonic conductive medium, and a secondary emission surface 3 impacted by cathode ray beam 2. The construction of the capture screen 4 is essentially unchanged from the prior high conductivity tube, except that its spacing from the conversion plate surface may be decreased. The current output or collector electrode 20 is located behind the screen. The electrode 20 is generally annular in configuration and may be formed as a conductive coating on the side wall of the image tube.

Signal current diversion to the electrode 20 is possible because the secondarily emitted electrons approach the screen 4 with a velocity corresponding to the potential difference between the landing point 5 and the screen 4. Thus, the high transparency of screen 4 directly captures only those electrons incident upon screen wires and a large fraction of the electrons are propelled beyond the screen and are returned to it only because the screen is set at a high bias potential with respect to all nearby grounded surfaces.

The change in electrostatic fields within the image tube to effect signal current diversion to the new electrode 20 is effected by reducing the DC bias potential on screen 4 to an intermediate value, and applying a higher potential to the electrode 20. The potential of screen 4 need only be sufficiently high with respect to grounded sections of the tube wall to cause it to act as a control and to assure that all secondarily emitted electrons 7 will pass through the screen rather than terminate upon grounded areas adjacent to the emission surface. The bias potential on the electrode 20 must then be sufficient to divert the electrons to electrode 20 before axial velocity has carried the electrons beyond the electrode 20 to other portions of the tube.

It should be noted that the diversion of signal current from the screen to a separate output electrode has no effect on the screens control of emission at the space charge layer. Current released from the space charge region is wholly controlled by the potential difference between landing point 5 and screen 4. This will not be affected by bias changes on the screen because the insulated surface of the conversion plate will automatically adjust itself with reference to the screen to the potential difference given by,

v. v, 1., L/ W 1 x 5.68 x V volts sated. The block diagram shows a circuit for current 1 amplification with capacitance compensation, and includes the feedback connection described next below.

Current multiplication with capacitance compensation is obtained using a differential voltage amplifier 22. The terminal 21 of collector electrode of the image tube is connected directly to one terminal of the amplifier, with precautions to limit wiring capacitance. Capacitance 23 (C,,) is the total capacitance associated with the electrode, and conductance 24 (G is the total conductance. The amplifier is selected for high input impedance, low shunt input capacitance, and low equivalent input noise. lts output is fed back to screen 4 from junction 30 viaterminal 19.

The signal for the inverse phase to the amplifier is a voltage derived from a current transformer 25 shunted by capacitance 26 (C and conductance 27 (G Assuming a unity ratio transformer, the ratio of current is given by the equation.

where Y, is the total termination admittance at the screen terminal 19 and A is amplifier gain. The second expression results if the product Y,A is made large compared to (G +jwC A further simplification is possible if the ratio (F /WC,

is made equal to h/WC f/ ll z c/ li K Two signal outputs A and B are indicated in the block diagram. Output A is through the negative capacitance network 16 connected to the junction 30. An output signal at this point would contain capacitatively coupled signal and primary beam noise but these would be reduced in their effect by the signal current multiplication. The second output B is from tap 28 on the feedback line. The signals from outputs A or B can be displayed by passingthe output through appropriate video processing circuits 34 for display on a video tube 36,. as shown in FIG. 5.

The subject of feedback system stability has been treated extensively in the literature. A potential source of instability for this application is capacitative coupling between screen 4and the output electrode 20. An electrostatic shield 29 (FIG. 3) for the lead from screen 4 to terminal 19 is shown as being indicative of methods by which this coupling can be reduced. Other forms of electrostatic shielding or neutralization techniques might be used.

The alternating potential applied to the screen is given by,

e KKi Y,

(7) where K is the fraction of the original secondary signal current passing through the screen and'becoming 1' The effective termination admittance at the screen is,

il al rt t/ Thus, the effective termination is the actual admittance of the screen to ground including stray capacitance and the effect of the termination network, divided by the net current multiplication. The small fraction of i terminating on the screen is ignored in this analysis as being insignificant in relation to 1' Similarly, the current arising from capacitative coupling to the screen, and the noise current intercepted from the primary beam are regarded as insignificant in comparison to i if a large value is used for G, and A.

New limits, of course, will exist on how large K can be made, how large G, can become, and how small C,, can be made, etc. Nevertheless, within these limits a major signal-to-noise improvement will be possible.

Having described the invention with reference to a preferred embodiment, it is to be understood that modiflcations may be made for practicing the invention without departing from the spirit and scope thereof.

I claim:

1. An ultrasonic imaging tube system comprising an envelope, a cathode ray gun, a cathode ray beam generated by said gun, a conversion plate sealed to the envelope, an electric transparent screen mounted within the envelope in close proximity to the conversion plate, a collector electrode mounted within the envelope between the screen and the gun, the beam generated by said gun passing through the screen and impacting on the plate whereby an AC potential applied to the screen causes generation of ultrasonic signals by the plate, an amplifier, the input of the amplifier being electrically connected to the collector electrode, and a feedback connection from the output of said amplifier to the screen, wherein the feedback to the screen lowers the effective ac termination admittance of the screen.

2. A device according to claim 1 including output terminals in series with the feedback connection to the screen to transmit amplified signals from the collector electrode to a video projection system.

3. A device according to claim 2 in which the amplifier input comprises a capacitance conductance network for balancing the capacitance of the collector electrode, and in which the amplifier output is coupled to a negative capacitor conductance network which serves to balance out the capacitance of the screen to ground.

4. A device according to claim 3 wherein the negative-capacitance conductance network is in shunt with the feedback and output terminals, and a further set of output terminals across said last network.