Re: Centre freqencies and focusing variations

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Re: Centre freqencies and focusing variations
Posted by: Klaus Brebøl , E-mail: kbreboel@post3.tele.dk, on September 06, 1996 at 11:36:42:
In Reply to: Re: Centre freqencies and focusing variations posted by : Robert A. Day on September 04, 1996 at 20:30:39:
: : 1. I have worked with a variety of different PVDF and
: : co-polymer films from three different manufacturers
: : to produce high frequency, high resolution transducers.
: : Although the thickness of the material was adequate to
: : provide a resonance frequency of 100MHz the best we
: : could get was a centre frequency of 58MHz.
: : Even this frequency was only possible by using a
: : customised pulser producing a single cycle sine wave.
: : Care was taken to ensure optimum backing density using
: : araldite epoxy and still nothing approaching 100MHz
: : was possible. What would cause such a significant
: : drop in frequency?
: There are lots of possibilies. In order of ease of fixing
: the problem they are:
: - High frequencies are attenuated by water. Try heating
: the water to 40C. If the problem goes away then that's
: it.
: - Backing member to foil bondline is too thick. I would
: have to see the RF echo from a known reflector to tell.
: - The cable lengths between the pulser, receiver and
: transducer are too long. Need to get all very close to
: transducer for best performance.
: - The part is curved and is distorting the return.
: An important test of high frequency systems is to
: examine the RF signal obtained for artifacts that would
: indicate impedance mismatches and other problems both in
: the electronics and the transducer. Generally you should
: expect a clean single cycle return.
:
: : 2. Recently we have had numerous application where
: : contact focused probes are useful. Three varieties
: : have been manufacured by different companies; one uses
: : flat elements and plastic lenses,
: : another uses spherically shaped elements of uniform
: : thickness and the third uses thick flat
: : piezo-elements ground to a spherical shape on one side.
: : What differences on beam shape and frequency content
: : can be expected.
: I would expect the spherical bowl crystals to give the
: best result but I have never worked with the spherical
: on one side type and would really like to hear how you
: find them.
Piezoceramic ground to sperical shape on only one side
seems to the kind of topic that pops up from time to
time. I used to work for the danish company Ferroperm,
and sometimes did we manufacture small quantities for
experiments. There can be manufacturing problems due to
the variation in thickness, as the poling field gets
very high in the thin regions. But nothing that cannot
be solved. To me it seems as a another good idea, that
never took of.
Robert A. Day was interested in some results, which I
cannot offer directly, but I can suggest a reference.
Kobayashi, T.:
Focusing Ultrasonic Transducer with Broad Bandwith
of the Plano-Concave and Plano-Convex Shapes.
Proceedings of the 4th Symposium on Ultrasonic Electronics,
Tokyo 1983, Japanese Jour. of appl. Phys. Vol. 23
(1984), Suppl. 23-1,. pp122-124
According to my memory, more resent papers have been
published in IEEE trans. on sonics and ultrasonics, but
unfortunately do I not have any references.

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: : : 1. I have worked with a variety of different PVDF and : : : co-polymer films from three different manufacturers : : : to produce high frequency, high resolution transducers. : : : Although the thickness of the material was adequate to : : : provide a resonance frequency of 100MHz the best we : : : could get was a centre frequency of 58MHz. : : : Even this frequency was only possible by using a : : : customised pulser producing a single cycle sine wave. : : : Care was taken to ensure optimum backing density using : : : araldite epoxy and still nothing approaching 100MHz : : : was possible. What would cause such a significant : : : drop in frequency? : : There are lots of possibilies. In order of ease of fixing : : the problem they are: : : - High frequencies are attenuated by water. Try heating : : the water to 40C. If the problem goes away then that's : : it. : : - Backing member to foil bondline is too thick. I would : : have to see the RF echo from a known reflector to tell. : : - The cable lengths between the pulser, receiver and : : transducer are too long. Need to get all very close to : : transducer for best performance. : : - The part is curved and is distorting the return. : : An important test of high frequency systems is to : : examine the RF signal obtained for artifacts that would : : indicate impedance mismatches and other problems both in : : the electronics and the transducer. Generally you should : : expect a clean single cycle return. : : : : : 2. Recently we have had numerous application where : : : contact focused probes are useful. Three varieties : : : have been manufacured by different companies; one uses : : : flat elements and plastic lenses, : : : another uses spherically shaped elements of uniform : : : thickness and the third uses thick flat : : : piezo-elements ground to a spherical shape on one side. : : : What differences on beam shape and frequency content : : : can be expected. : : I would expect the spherical bowl crystals to give the : : best result but I have never worked with the spherical : : on one side type and would really like to hear how you : : find them. : Piezoceramic ground to sperical shape on only one side : seems to the kind of topic that pops up from time to : time. I used to work for the danish company Ferroperm, : and sometimes did we manufacture small quantities for : experiments. There can be manufacturing problems due to : the variation in thickness, as the poling field gets : very high in the thin regions. But nothing that cannot : be solved. To me it seems as a another good idea, that : never took of. : Robert A. Day was interested in some results, which I : cannot offer directly, but I can suggest a reference. : Kobayashi, T.: : Focusing Ultrasonic Transducer with Broad Bandwith : of the Plano-Concave and Plano-Convex Shapes. : Proceedings of the 4th Symposium on Ultrasonic Electronics, : Tokyo 1983, Japanese Jour. of appl. Phys. Vol. 23 : (1984), Suppl. 23-1,. pp122-124 : According to my memory, more resent papers have been : published in IEEE trans. on sonics and ultrasonics, but : unfortunately do I not have any references.

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