US 3702448 A
A sonic delay line wherein electric impulses are converted to acoustic impulses passed through glass and reconverted to electrical impulses, wherein the acoustic impedance of the transducer is matched to that of glass utilizing an alloy of approximately equal parts of bismuth and indium, by weight.
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Description (OCR text may contain errors)
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SEAR Cl-I R o-0 Boblett 1 Nov. 7, 1972  IMPEDANCEMATCHED ULTRASONIC  References Cited DELAY LINE WHEREIN ELECTRODES CONSIST 0F BISMUTH AND INDIUM UNITED STATES PATENTS I t E v B u L Krause  or 2 0 e 0s 8 3,252,722 5/1966 Allen ..333/30 3,517,345 6/1970 Krause ..333/30  Assignee: Ampex Corporation, Redwood City,
Cahf- Primary Examiner-Paul L. Gensler 22 Filed: Feb. 16, 1971 Attorney-Robe" Clay  Appl. No.: 115,219 7 v ABSTRACT I sonic delay line wherein electric impulses are con-  US. Cl ..333/30 R, 310/8, 333/32, verted to acoustic impulses passed through glass and 340/8 MM reconvened to electrical impulses, wherein the Cl -H03h 9/30 H04r17/00 7/38 acoustic impedance of the transducer is matched to  Flew Search "333/30 R1 32; 29/4731 that of glass utilizing an alloy of approximately equal parts of bismuth and indium, by weight.
5 Claims, 2 Drawing Figures PATENTED 7 1973 3. 702,448
I P s I l g N UT I OUTPUT INVENTOR.
EM/L V. BOBLETT F113- .2. W
ATTORNE V IMPEDANCE MATCHED ULTRASONIC DELAY LINE WHEREIN ELECTRODES CONSIST OF BISMUTH AND INDIUM SUMMARY OF THE INVENTION In the construction of bulk ultrasonic delay lines it is important to avoid reflections which result in unwanted echoes. In other words, although there may be a satisfactory delay of the main signal component, if a small portion of the signal is reflected back to the sender and again reflected back to the receiver, an echo will be produced which may be unacceptable in magnitude. In accordance with the present invention an acoustic matching system is employed in a delay line so that the third path signal (i.e., the signal which has been reflected first by the receiver and then again by the sender) is more than 60 dB down. Obviously the fifth and higher order path signals are even more greatly attenuated. I
In the past, such sonic delay lines have caused multiple echoes and an unacceptable degradation of the main signal. i
In general the objects of the present invention are achieved by employing a zero temperature coefficient glass together with an AC cut quartz crystal which matches the acoustic impedance of the glass with electrodes on the quartz crystals which consist of essentially equal parts of bismuth and indium. Thus'the glass, transducer and electrodes are all carefully matched in acoustic impedance so that there is a maximum transmission of the desired signal and a maximum attenuation of the reflected signal.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side view of an ultrasonic delay line embodying the present invention.
FIG. 2 is a perspective view in section on an enlarged scale of the delay line shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing by reference characters the delay element itself consists of a sheet of glass 3 which preferably has a zero acoustic temperature coefficient. Mounted on each side of the glass are quartz transducers 5 and 7 and these are preferably of the AC cut which operate in a shear mode. Although other quartz transducers might be used, these particular cuts give the best sonic impedance match with the glass. Mounted on each side of the glass are thin foils 9 and 13 of a bismuth-indium alloy. These thin foils serve as one of the electrodes on each of the quartz transducers and, by the selection of this particular alloy, make an almost perfect match between the glass and the quartz.
Mounted on the outside of each of the quartz plates is a relatively heavy block of metal 15 and 17. These metal blocks are also made of the same indium-bismuth alloy and serve as the outer electrodes for the quartz crystal and also, because of their composition and relatively large size, absorb any signal which might be reflected into them. It will be understood, of course, that the second path signal does not cause any problem if it can be effectively absorbed and prevented from becom min th'd th' l.Th' t li couldb mea n of t he lz lmd ll t rs l9 t l l 3 5nd b o k II while the output signal is coupled by means of connectors 21 to the foil 9 and block 15.
The various parts can be cemented together and it is not necessary that the cement be matched acoustically since it can be made so thin that it is acoustically invisible. In one practical embodiment of the invention, an epoxy resin was used to cement the parts together but the thickness of the resin was only from 5 to 10 microinches which is two to three orders of magnitude smaller than the acoustic wavelength.
In one practical embodiment of the invention a delay line having a l microsecond delay was made utilizing a glass layer 0.1 inches thick. The foil layer on each side of the glass was I mil in thickness while the crystals were 1.2 mils in thickness. The outer blocks were onesixteenth inch in thickness. This structure had an absorption less than 1.3 dB per inch at 20 MHz and the third path reflection signal was more than 60 dB down.
Various glasses and crystal configurations can be used without departing from the spirit of this invention but it is preferred that the glass have a zero acoustic temperature coefficient and that the crystals be quartz with an AC cut. However other glasses and other piezoelectric elements can be employed. The alloy itself, both for the foil and for the blocks can vary from 48 to 52 percent bismuth and 52 to 48 percent indium.
1. An ultrasonic delay line which includes a piece of glass as a sonic delay element with transducers on each side of the glass, said transducers consisting of piezoelectric crystals with electrodes thereon, wherein the improvement comprises electrodes of an alloy consisting of about equal parts of bismuth and indium, by weight.
2. The structure of claim 1 wherein the glass is temperature coefficient glass.
3. The structure of claim 1 wherein the electrodes between the crystals and the glass consist of a thin film of said bismuth-indium alloy while the outer electrodes consist of thick blocks of bismuth-indium alloy.
4. The structure of claim 1 wherein the crystals are quartz, have an AC cut and operate in a shear mode.
5. The structure of claim 1 wherein the electrodes are from 48 to 52 percent bismuth and 52 -48 percent indium, by weight.
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