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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for supplying the transmission
oscillator voltage for a piezoelectric sound source transducer.
2. Description of the Prior Art
It is known in the art to transmit ultrasonic waves generated by electrical
excitation with an ultrasonic source transducer, to receive the echo
signals with a sound pick-up transducer and to evaluate the electrical
echo signals thereby obtained, for example, for range finding. As a rule,
a transducer having a noticeable resonance step-up is employed in order to
increase the acoustic power and the sound source transducer consisting of
an oscillator having a low internal impedance is voltage-fed to its series
resonance frequency
f.sub.o =1/2.pi.(L.sub.m .multidot.C.sub.m).sup.1/2,
wherein L.sub.m and C.sub.m are mechanical inductance and the mechanical
capacitance of the equivalent circuit of the transducer.
Reception is executed with the assistance of a second, separate receiver
transducer whose parallel resonance is provided by
f.sub.p =1/2.pi.{L.sub.m .multidot.(C.sub.m .multidot.C.sub.o)/(C.sub.m
+C.sub.o)}.sup.1/2
where C.sub.o is the electrical parallel capacitance of the transducer in
its equivalent circuit. The receiver transducer is high-resistant for the
parallel resonant frequency f.sub.p and supplies a signal voltage from the
received echoes which can be evaluated with a correspondingly great
signal-to-noise ratio. Both transducers are matched in such a manner that
F.sub.o of the transmission transducer equals f.sub.p of the receiving
transducer. The fact that the frequency f.sub.o of a transducer is smaller
than the frequency f.sub.p forces the employment of two transducers which
are differently tuned, i.e. to different series resonant frequencies
f.sub.o (of the transmission transducer) and f'.sub.o (of the receiving
transducer).
Although it is at least conceviable, while accepting lower signal-to-noise
ratios and/or lower receiving power, to employ one and the same acoustic
transducer for transmitting and for receiving, particularly when the
receiving transducer also has a very poor oscillatory quality, i.e., a
very great band width (with a correspondingly small resonant step-up),
another possibility is to work with only a single transducer and, while
foregoing and effective electrical excitation and acoustic emission, to
excite the transducer as a transmitter in the parallel resonant frequency
f.sub.p.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a circuit for a
transmission sound source transducer which can be optimally excited at a
series resonant frequency f.sub.o and with which, alternately, receiving
can also be executed with the same, single transducer with a likewise
optimally high receiving efficiency and high signal-to-noise ratio.
The above object is achieved, according to the present invention, for a
circuit of the type generally set forth above, in that, for alternate
receiving in addition to transmission with a single sound source
transducer, the circuit contains an electrical series resonant circuit
having a capacitance and an inductance with which
f=1/2.pi.(LC).sup.1/2,
wherein the electrical series resonant circuit is matched to the series
resonant frequency s.sub.o of the sound source transducer and is connected
in parallel to the transducer, and which further contains a parallel
circuit consisting of two diodes connected anti-parallel, wherein the
diode parallel circuit is connected in parallel either to the inductance
or to the capacitance and wherein the receiver output is connected to the
capacitance or to the inductance.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention, its organization,
construction and operation will be best understood from the following
detailed description taken in conjunction with the accompanying drawing,
on which:
FIG. 1 is a schematic circuit diagram of a first variation of an exemplary
embodiment of the invention;
FIG. 2 is a schematic circuit diagram showing a corresponding, second
variation of an exemplary embodiment of the invention;
FIGS. 3 and 4 illustrate further devemopments of the invention having a
transformer output for the variations according to FIGS. 1 and 2; and
FIGS. 5 and 6 illustrate two variations for a decoupling of the transducer
and the oscillator from one another.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a sound source transducer which, according to the
invention, is to also be employed as a receiving transducer, in particular
with a high efficiency. An element 2, symbolically illustrated as a
swtich, operates at a switch to supply the excitation alternating voltage
of an oscillator 3 for the transmit mode to the sound source transducer 1
at predetermined transmit time intervals. When the switch 2 is closed,
therefore, the transmission clock or cadence can be determined. The switch
2 can be a clock modulation of the transmission oscillator 3, i.e. a
switching on and off of the oscillator. However, specific embodiments of
such a fundamental switch 2 which are advantageously designed within the
framework of the invention will be discussed below.
A series resonant circuit 4 comprising a capacitor C and an inductor L
which shall be described in terms of actual function in greater detail
below in connected parallel to the sound source transducer 1. With its
resonance
f=1/2.pi.(LC).sup.1/2
this series resonant circuit 4 is matched to the series resonant frequency
f.sub.o =1/2.pi.(C.sub.m L.sub.m).sup.1/2
of the transducer 1. So that the series resonant circuit 4 is not an
unnecessary short circuit 4 and parallel to the transducer 1 in the
transmit mode, i.e. given the switch 2 in a closed condition, according to
a further feature of the invention, a parallel connection 5 of a pair of
diodes D1 and D2 (as illustrated on the drawings) connected anti-parallel
to one another, is connected in parallel to either the inductor L (FIGS. 1
and 3) or to the capaictor C (FIGS. 2 and 4). The diodes D1 and D2
represent short circuits for the relatively high voltages occurring during
the transmit mode at the transducer 1 and, therefore, at the inductor L
and at the capacitor C (given non-existence of the diode parallel
circuit), short circuits for the respective half wave of the oscillator
alternating voltage which arrives at the diodes. This short circuit effect
of the diodes D1 and D2 leads to the fact that the series resonant circuit
4 is extremely greatly attenuated in the transmit mode, i.e. when the
switch 2 is closed, so that the series resonant circuit, given the high
excitation voltages of the oscillator 3 at the transducer 1, appears as a
resistor which is high-resistant relative to the transducer resistance
and, consequently, does not represent a significant shunt for the
electrical excitation of the transducer 1.
The conditions given ultrasonic reception with the transducer 1 are
completely different. Of necessity, the echo signals have the frequency
f.sub.o. An electrical EMF (of the frequency f.sub.o) is now generated as
a reception signal in the transducer 1 which now operates as a receiver
and with an internal resistance which continues to be low for the
frequency f.sub.o. This generated EMF, due to the switch 2 opened in the
pauses of the transmit mode, only occurs at the series resonant circuit 4
comprising the capacitor C and the inductor L. Given a relatively high
quality of the series resonant circuit 4, its resonant impedance in this
series resonance is small in comparison to the series resonant internal
resistance of the transducer 1. A particular advantage of employing the
series resonant circuit 4 lies in the resonance step-up of the reception
EMF of the transducer 1 at the capacitor C and at the inductor L. The
reception signal voltage occurring at the inductor L (FIGS. 1 and 3) or,
respectively at the capacitor C (FIGS. 2 and 4) is greater than the EMF of
the transducer by the factor
.beta.=.omega.L/R=1/.omega.CR
where R is the ohmic equivalent resistance of the transducer. As a rule,
this voltage always remains below a value of approximately 0.6 volts, in
particular below the threshold voltage of the diodes D1 and D2. In
comparison to such a reception voltage, the diodes D1 and D2 are
high-resistant and, therefore, are negligible as short circuits. A
parallel connection effect of the diodes D1 and D2 occurring at voltages
above 0.6 volts is without interest for the fact that, in this case, the
reception signal voltage is already so great that a limiting of the
reception signals does not represent a disadvantage. On the contrary, it
is even advantageous to keep signals which are too great away from the
following amplifiers
The signal output for the received echo signal of the transducer 1 is
referenced 6. In the embodiments of FIGS. 3 and 4, the inductor L of FIGS.
1 and 2 is designed as a transformer 7. With the transformer 7, the output
impedance of the circuit constructed in accordance with the present
invention can be simply dimensioned to that value which is desired at the
output terminal 6.
By dimensioning the capaictor C to be smaller than the value C.sub.o of the
electrical parallel capacitance of the sound source transducer or
dimensioning the capacitor C to the value C.sub.o .multidot.k.sup.2, where
k.sup.2 represents the electro-mechanical coupling factor of the
transducer and C.sub.o represents its electrical parallel capacitance, the
output voltage of the circuit constructed in accordance with the present
invention can be increased in that one selects the value C of the
capacitor of the series resonance circuit small, for example 1-30 times
smaller than the electrical parallel capacitance C.sub.o of the transducer
1, upon observation of the resonance condition
f.sub.o =1/2.pi.(LC).sup.1/2.
In the case where C=C.sub.o, the circuit of the present invention already
emits precisely as much voltage as a traditional, second receiving
transducer operated at a parallel resonant frequency f.sub.p. If, however,
the dimensioning mentioned above is even selected where C<C.sub.o, a
voltage increase approximately corresponding to the ratio C.sub.o /C
occurs at the capacitor C at the smaller capacitance thereof. This
possible increase of the output voltage by a factor .alpha. is limited by
the relative band width of the transducer to be achieved. For a large C
(C.apprxeq.C.sub.o), the band width is given only by the band width of the
transducer
f/f=1/Q.sub.m.
If, however, C.sub.o /C is selected greater than the reciprocal quadratic
coupling coefficient, an additional, noticeable loss of band width occurs.
The value
.alpha.=C.sub.o /C=1/k.sup.2
represents a meaningful limit for .alpha..
The equation
B=B.sub.m /1+k.sup.2 C.sub.o /C=1/1+.alpha.k.sup.2
holds true for the band width B. In the equation, k is the coupling
coefficient of the transducer 1 and B.sub.m is its natural band width. It
can be seen from the equation that the receiving band width decreases with
increasing .alpha.. There derives
.alpha.=1/k.sup.2
whereby a reduction of the band width (in comparison to that of the
transducer 1 in the series resonance) occurs by the factor 2. If the
reduction of the band width is without use-conditioned significance, then
a further limit for the voltage gain is provided by the quality of the
electrical series circuit
.alpha..sub.max =Q.sub.el =k.sup.2 Q.sub.m
where Q.sub.el is the oscillatory quality of the series resonant circuit
and Q.sub.m is the oscillatory quality of the transducer.
In conjunction with the switch 2, possibilities of its design which are
particularly advantageous within the framework of the present invention,
i.e. in conjunction with circuits constructed in accordance with the
present invention, were already pointed out above. The circuit element 2,
designated as a simple switch in principle is technically realized in such
a manner given arrangements in which the transducer 1 serves only as a
transmitter that one controls or, respectively, keys the generation and/or
emission of the oscillator alternating voltage of the oscillator 3 in one
of the many, known possible variations. As is readily apparent, a
separation between the transducer 1 and the oscillator 3, i.e. a
decoupling of the transducer 1 and the oscillator 3 from one another is
not required during the transmission pauses. In the present invention,
however, in which the transducer 1 also serves as a receiving transducer,
such a coupling between the transducer 1 and the oscillator 3 would be
disruptive during the receiving phase. This, particularly because of the
low internal resistance of the oscillator 3 and/or because of the high
noise output signal of the oscillator 3. According to the present
invention, therefore, it is of considerable advantage to undertake a
separation or, respectively, decoupling as symbolically indicated with the
switch 2 between the transducer 1 and the oscillator 3 during the
receiving phase.
FIGS. 5 and 6 respectively illustrate variations of this further
development of the invention for an embodiment of the invention according
to FIG. 1. These variations can likewise be realized with the same
advantages given circuits according to FIGS. 2-4.
In FIG. 5, the separation or, respectively, decoupling of the transducer 1
and the oscillator 3' achieved by the symbolic switch 2 in FIG. 1 is
realized with the assistance of a parallel connection 21 comprising a pair
of diodes 22 and 23 connected anti-parallel to one another. Like the
switch 2, this parallel connection 21 lies in series between the
transducer 1 and the oscillator 3'. The oscillator 3' is designed in such
a manner that, as indicated in FIG. 5, it supplies transmission pulses of
the alternating voltage with the frequency f.sub.o.
To be absolutely precise, the parallel connection 21 here only fulfills the
function of the decoupling, whereas the switching of the transmission
voltage occurs in the oscillator 3'. The diodes 22 and 23 have identical
threshold voltages of approximately 0.6 volts (like the diodes D1 and D2).
The diodes 22 and 23 are always high-resistant for the amplitude of the
receiving EMF generated in the transducer 1.
FIG. 6 illustrates a parallel connection 31 comprising two transistors 32
and 33 which are connected as a circuit designated as an emitter follower.
The bases of the transistors are connected in common and the emitters of
the transistors are connected in common. As FIG. 6 illustrates the
collector of the one transistor (for example the transistor 32) is
connected to ground and the collector of the other transistor (here the
transistor 33) is connected to an operating potential. The parallel
connection 31 comprising the transistors 32 and 33, as is also shown in
FIG. 6, in turn is connected in series between the transducer 1 and the
oscillator 3'. The transistors 32 and 33 in the circuit of FIG. 6
additionally see to a low excitation impedance, which is of further
significance for the principle of the present invention. This variation
according to FIG. 6 is to be preferred when the oscillator circuit 3'
available exhibits an internal resistance which is not sufficiently low
per se. An oscillator 3' with the function of transmitting clocked
transmission alternating voltages f.sub.0 is likewise provided in the
embodiment according to FIG. 6.
Although I have described my invention by reference to particular
illustrative embodiments thereof, many changes and modifications of the
invention may become apparent to those skilled in the art without
departing from the spirit and scope of the invention. I therefore intend
to include within the patent warranted hereon all such changes and
modifications as may reasonably and properly be included within the scope
of my contribution to the art.
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