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827 views
01:51 Sep-02-1996

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1208
Centre freqencies and focusing variations

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?

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.



 
00:30 Sep-04-1996

Robert A. Day

Engineering
Milky Way Jewels,
USA,
Joined Nov 1998
40
Re: Centre freqencies and focusing variations he 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 thespherical
on one side type and would really like to hear how you
find them.



 
01:00 Sep-05-1996

Splitt

R & D, -
Private,
Germany,
Joined Nov 1998
38
Re: Centre freqencies and focusing variations Centre frequencies and focusing variation

1. I fully understand your problems with the frequencies
of high frequency PVDF probes as we have made similar
experiences in the past.

The most important point are the measurement
conditions. As the sound attenuation in water is very
high and increases with the square of frequency, a
very short water path of only 1 to 2 mm is mandatory.
Otherwise the water will filter out the high frequency
content of the broad band spectrum and only the low
frequency end will pass resulting in 35 to 60 MHz for
a 100 MHz film.

The second point is a transmitter with a very short
rise time of only some nanoseconds to really excite
the high frequencies of the PVDF film. In addition
a short cable of some centimeters is recommended as
the capacity of the cable is in parallel to the low
capacity of the probe and builds a low path filter
together with the transmitter / receiver interface
electronics. The best solution is to directly connect
the probe to a miniaturized transmitter / pre-
amplifier modul without cable.

With the above described means you will get a centre
frequency of at least 80 MHz when using a 100 MHz PVDF
film.

2. In a first approximation I would guess that the
sound field structure is similair for all three cases
assuming that the effective crystal diameter,
frequency, bandwidth and the focus distance are
identical.

Of course, there can be severe differences in the
frequency content or pulse shape resp.. But this is
difficult to describe on a general basis as this
depends critically on the details of probe
construction as crystal material, backing, lens
material etc.. If you need good resolution and
reasonable sensitivity I would prefer the first
or the second design with a flat or curved element.
The third design with a grounded crystal may have
highest resolution but lower sensitivity.





 
01:36 Sep-06-1996
Klaus Brebøl
Re: Centre freqencies and focusing variations 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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