Re: Centre freqencies and focusing variations

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Re: Centre freqencies and focusing variations
Posted by: Splitt , E-mail: Sp@kk-a-col.mhs.compuserve.com, on September 05, 1996 at 11:00:30:
In Reply to: Centre freqencies and focusing variations posted by : Ed Ginzel on September 02, 1996 at 21:51:17:
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.

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Comments:
: 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. : :

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