Re: Centre freqencies and focusing variations

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Re: Centre freqencies and focusing variations
Posted by: Robert A. Day , E-mail: rockyd@netcom.com, on September 04, 1996 at 20:30:39:
In Reply to: Centre freqencies and focusing variations posted by : Ed Ginzel on September 02, 1996 at 21:51:17:
: 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.

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Re: Centre freqencies and focusing variations Klaus Brebøl 11:36:42 9/06/96 (size: 3258) (0)

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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.

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