Abstract:
A transmitter circuit and method for an ultrasonic thickness measurement system having an ultrasonic transducer and a cable with known cable impedance, the cable connecting at least the transmitter circuit to the ultrasonic transducer, the transmitter circuit comprising: an electrical pulse voltage signal emitter for producing an electrical pulse voltage signal to be propagated over the cable to the ultrasonic transducer, the electrical pulse voltage signal having a pulse nominal voltage value; a matched impedance circuit for matching a cable impedance of the cable at the transmitter circuit for an electrical parasitic reflection signal of the electrical pulse voltage signal, the electrical parasitic reflection signal being caused by an impedance mismatch between the cable and the ultrasonic transducer, a parasitic nominal voltage value of the electrical parasitic reflection signal being at most the pulse nominal voltage value.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to non-destructive testing using ultrasonic techniques. More particularly, the invention is applicable to inspection techniques using an ultrasonic transducer linked by a cable to the transmitter-receiver or transmitter circuit, where the transmitter sends unipolar or bipolar pulse wave voltage signals to the ultrasonic transducer through the cable. 
       BACKGROUND OF THE ART 
       [0002]    The ultrasonic transducer converts the voltage pulse to mechanical movement mainly oriented perpendicularly in the direction of the emission surface of the transducer. This mechanical movement produces the ultrasonic wave in the transmission medium. There are many types of transmission mediums, including liquids, gels, plastic wedges, etc. For thickness measurement applications, the part under test is located perpendicular to the main direction of propagation of the ultrasonic wave to make sure that the wave strikes the surface of the part under test perpendicularly. Ultrasonic mirrors can be used to change the direction of the ultrasonic wave and make sure that the ultrasonic wave strikes the surface of the part under test perpendicularly. 
         [0003]    When the ultrasonic wave strikes the surface of the part under test, a portion of the ultrasonic wave is reflected, like a sound echo, and a portion is transmitted into the material of the part under test. The same transducer or a different transducer can be used to receive the reflected wave and convert it into a signal voltage called front wall echo. The front wall echo signal is amplified and conditioned by the receiver circuit. In the mean time, the transmitted ultrasonic wave in the material of the part under test strikes the opposite surface of the part under test and a portion of the ultrasonic wave is reflected and comes back to the transducer used to receive the ultrasonic echoes. The transducer converts this second ultrasonic wave to a signal voltage called back wall echo. The electronic equipment receiving both the front wall echo and the back wall echo detects them and then determines the time interval between both echoes. This time interval corresponds to the additional time the ultrasonic wave has taken to propagate back and forth in the part under test. Usually, the speed of sound in the part under test is known. For thickness measurement applications, the thickness is equal to the speed of sound in the material multiplied by the time interval between the 2 echoes divided by 2. 
         [0004]    This technique of thickness measurement can be used to measure the thickness of each layer for a multilayer part, based on each time interval found between the echoes. 
         [0005]    In some inspection applications, the transducer and the transmitter-receiver or transmitter circuit are connected by a long cable because the location at which the transducer should be located is too small. Tube inspection from the inside is a good example of a testing environment where a long coaxial cable is required to connect the transmitter-receiver or transmitter circuit to the transducer because the tube inside diameter can only allow for the transducer to be inserted therein. 
         [0006]    The ultrasonic transducer has complex impedance which greatly varies with the frequency. The emitted pulse is sent from the transmitter circuit to the transducer over the coaxial cable and a large portion of the pulse signal is reflected back in the coaxial cable towards the transmitter circuit because the transducer impedance does not match the coaxial cable impedance. The reflected pulse signal goes back to the transmitter-receiver or transmitter circuit which also has no matched impedance with the coaxial cable impedance. This impedance mismatch causes another reflection which creates another pulse at the transmitter circuit going back towards the transducer. The second transmitted pulse is delayed in time relative to the first pulse emitted by the transmitter circuit. The second transmitted pulse is smaller in amplitude, but it is important enough to cause a second ultrasonic wave in the material under test. This second wave produces another front wall echo. This second front wall echo is a parasitic signal that might get superposed to the back wall echo when the thickness of the layer under test produces a time interval close to the time interval between the first and second transmit pulses. In that case, the wall thickness is not measured with a fair level of confidence because the back wall echo cannot be precisely identified. 
       SUMMARY 
       [0007]    In an ultrasonic thickness measurement system using a cable between the transmitter circuit and the transducer, the wall thickness measurement range can be expanded to thinner material thickness values using a matched impedance circuit which contributes to reduce reflections in the cable. 
         [0008]    According to one broad aspect of the present invention, there is provided a transmitter circuit for an ultrasonic thickness measurement system having an ultrasonic transducer, a receiver circuit and a cable with known cable impedance, the cable connecting at least the transmitter circuit to the ultrasonic transducer. The transmitter circuit comprises an electrical pulse voltage signal emitter for producing an electrical pulse voltage signal to be sent over the cable to the ultrasonic transducer, the electrical pulse voltage signal having a pulse nominal voltage value; a matched impedance circuit in the transmitter circuit, the matched impedance circuit matching the cable impedance of the cable at the transmitter circuit for a electrical parasitic reflection signal of the electrical pulse voltage signal, the electrical parasitic reflection signal being caused by an impedance mismatch between the cable and the ultrasonic transducer, a parasitic nominal voltage value of the electrical parasitic reflection signal being at most the pulse nominal voltage value. 
         [0009]    In one embodiment, a shape of the electrical parasitic reflection signal has at least one of a positive component and an inverted component. 
         [0010]    In one embodiment, the receiver circuit and the transmitter circuit are provided by a single transmission-reception circuit and wherein the cable connects the transmitter-reception circuit to the ultrasonic transducer. 
         [0011]    According to another broad aspect of the present invention, there is provided a method for reducing an impact of an electrical parasitic reflection signal in an ultrasonic measurement system having a transmitter circuit, a transmission ultrasonic transducer and a transmission cable with known cable impedance, the transmission cable connecting at least the transmitter circuit to the transmission ultrasonic transducer. The method comprises emitting an electrical pulse voltage signal at the transmitter circuit; sending the electrical pulse voltage signal having a pulse nominal voltage value over the transmission cable to the transmission ultrasonic transducer in a transmission medium; receiving, at the transmitter circuit, an electrical parasitic reflection signal caused by an impedance mismatch between the transmission cable and the transmission ultrasonic transducer, a parasitic nominal voltage value of the electrical parasitic reflection signal being at most the pulse nominal voltage value; matching an impedance of the transmission cable at the transmitter circuit for the electrical parasitic reflection signal. 
         [0012]    In one embodiment, the method further comprises receiving a front wall voltage signal and at least one second wall voltage signal at a receiver circuit; determining a thickness for a material using a speed of sound in the material and a time interval between the receiving the front wall voltage signal and receiving the at least one second wall voltage signal at the receiver circuit. 
         [0013]    In one embodiment, the front wall voltage signal and the at least one second wall voltage signal are provided by: transforming the electrical pulse voltage signal into an ultrasonic wave traveling in the transmission medium at the ultrasonic transmission transducer; receiving a front wall ultrasonic echo wave and at least a second wall ultrasonic echo wall from the material at a receiving ultrasonic transducer; transforming the front wall ultrasonic echo wave into the front wall voltage signal at the receiving ultrasonic transducer and transforming the at least the second wall ultrasonic echo wall into the at least one second wall voltage signal at the receiving ultrasonic transducer, wherein the front wall voltage signal has a front wall nominal value and the at least the second wall voltage signal has a second wall nominal value; sending the front wall voltage signal and the at least one second wall voltage signal over a reception cable to the receiver circuit. 
         [0014]    According to another broad aspect of the present invention, there is provided a method for measuring a thickness of a material, comprising: emitting an electrical pulse voltage signal at a transmitter circuit; sending the electrical pulse voltage signal having a pulse nominal voltage value over a transmission cable to a transmission ultrasonic transducer in a transmission medium; receiving, at the transmitter circuit, an electrical parasitic reflection signal caused by an impedance mismatch between the transmission cable and the transmission ultrasonic transducer, a parasitic nominal voltage value of the electrical parasitic reflection signal being at most the pulse nominal voltage value; matching an impedance of the transmission cable at the transmitter circuit for the electrical parasitic reflection signal; transforming the electrical pulse voltage signal into an ultrasonic wave traveling in the transmission medium at the ultrasonic transmission transducer; receiving a front wall ultrasonic echo wave and at least a second wall ultrasonic echo wall from the material at a receiving ultrasonic transducer; transforming the front wall ultrasonic echo wave into a front wall voltage signal at the receiving ultrasonic transducer and transforming the at least the second wall ultrasonic echo wall into at least one second wall voltage signal at the receiving ultrasonic transducer, wherein the front wall voltage signal has a front wall nominal value and the at least the second wall voltage signal has a second wall nominal value; sending the front wall voltage signal and the at least one second wall voltage signal over a reception cable to a receiver circuit; receiving the front wall voltage signal and the at least one second wall voltage signal at the receiver circuit; determining the thickness for the material using a speed of sound in the material and a time interval between the receiving the front wall voltage signal and receiving the at least one second wall voltage signal at the receiver circuit. 
         [0015]    In one embodiment, the transmission ultrasonic transducer and the receiving ultrasonic transducer are provided by a single transmission-reception ultrasonic transducer and wherein the transmission cable and the reception cable are provided by a single transmission-reception cable. 
         [0016]    In one embodiment, the pulse nominal voltage value is between −50 V and −400 V. 
         [0017]    In one embodiment, the front wall nominal value and the second wall nominal value is between −1 V and 1 V. 
         [0018]    In one embodiment, the method further comprises changing a direction of the ultrasonic wave using an ultrasonic mirror to allow the ultrasonic wave to strike the material perpendicularly. 
         [0019]    According to another broad aspect of the present invention, there is provided a transmitter circuit for an ultrasonic thickness measurement system having an ultrasonic transducer and a cable with known cable impedance, the cable connecting at least the transmitter circuit to the ultrasonic transducer, the transmitter circuit comprising: an electrical pulse voltage signal emitter for producing an electrical pulse voltage signal to be propagated over the cable to the ultrasonic transducer, the electrical pulse voltage signal having a pulse nominal voltage value; a matched impedance circuit for matching a cable impedance of the cable at the transmitter circuit for an electrical parasitic reflection signal of the electrical pulse voltage signal, the electrical parasitic reflection signal being caused by an impedance mismatch between the cable and the ultrasonic transducer, a parasitic nominal voltage value of the electrical parasitic reflection signal being at most the pulse nominal voltage value. 
         [0020]    In one embodiment, a shape of the electrical parasitic reflection signal has at least one of a positive component and a negative component. 
         [0021]    In one embodiment, the cable impedance of the cable is one of 50, 75, 93 and 95 Ohms. 
         [0022]    In one embodiment, the pulse nominal voltage value is between −50 V and −400 V. 
         [0023]    In one embodiment, the matched impedance circuit is at least one resistance provided in the transmitter circuit to impact a circuit impedance of the transmitter circuit for the electrical parasitic reflection signal, the circuit impedance matching the cable impedance. 
         [0024]    In one embodiment, the matched impedance circuit further includes at least one diode, the at least one diode controlling an effect of the at least one resistance on the circuit impedance. 
         [0025]    In one embodiment, the matched impedance circuit is one resistance in series with a diode in which current circulates during transmission of the electrical pulse voltage signal. 
         [0026]    In one embodiment, the matched impedance circuit is one resistance in series with a diode in which current circulates after the transmission of the electrical pulse voltage signal has ended. 
         [0027]    According to another broad aspect of the present invention, there is provided a method for reducing an impact of an electrical parasitic reflection signal in an ultrasonic measurement system having a transmitter circuit, an ultrasonic transducer and a cable with known cable impedance, the cable connecting at least the transmitter circuit to the ultrasonic transducer, comprising: emitting an electrical pulse voltage signal at the transmitter circuit; propagating the electrical pulse voltage signal having a pulse nominal voltage value over the cable to the ultrasonic transducer; receiving, at the transmitter circuit, an electrical parasitic reflection signal caused by an impedance mismatch between the cable and the ultrasonic transducer, a parasitic nominal voltage value of the electrical parasitic reflection signal being at most the pulse nominal voltage value; matching an impedance of the cable at the transmitter circuit for the electrical parasitic reflection signal. 
         [0028]    In one embodiment, a shape of the electrical parasitic reflection signal has at least one of a positive component and a negative component. 
         [0029]    In one embodiment, the cable impedance of the cable is one of 50, 75, 93 and 95 Ohms. 
         [0030]    In one embodiment, the pulse nominal voltage value is between −50 V and −400 V. 
         [0031]    In one embodiment, the matching comprises providing at least one resistance in the transmitter circuit to impact a circuit impedance of the transmitter circuit for the electrical parasitic reflection signal, the circuit impedance matching the cable impedance. 
         [0032]    In one embodiment, the matching further comprises providing at least one diode, the at least one diode controlling an effect of the at least one resistance on the circuit impedance. 
         [0033]    In one embodiment, matching comprises providing one resistance in series with a diode in which current circulates during transmission of the electrical pulse voltage signal. 
         [0034]    In one embodiment, the matching comprises providing one resistance in series with a diode in which current circulates after the transmission of the electrical pulse voltage signal has ended. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration an example embodiment thereof and in which: 
           [0036]      FIG. 1  is an example of an ultrasonic wall thickness measurement system using a cable between the transducer and the electronics; 
           [0037]      FIG. 2A  (prior art) is an example of an ultrasonic signal after the first stage of amplification obtained on thick material;  FIG. 2B  (prior art) is an example of an ultrasonic signal after the first stage of amplification obtained on thin material;  FIG. 2C  is an example of an ultrasonic signal after the first stage of amplification obtained on the thin material of  FIG. 2B  with the impedance matching circuit present; 
           [0038]      FIG. 3A  (prior art) is an example conventional electronic circuit design for a unipolar transmitter coupled with a MOSFET as the input of the amplifier receiver;  FIG. 3B  is an example impedance matching circuit for a unipolar transmitter coupled directly to the amplifier receiver; 
           [0039]      FIG. 4A  (prior art) is an example conventional electronic circuit design for a unipolar transmitter coupled with a bridge of diodes as the input of the amplifier receiver; and  FIG. 4B  is an example impedance matching circuit for a unipolar transmitter coupled with a bridge of diodes as input of the amplifier receiver; and 
           [0040]      FIG. 5A  (prior art) and  FIG. 5B  (prior art) show example signals at the output of the transmitter circuit ( FIG. 5A ) and at the input of the transducer ( FIG. 5B ),  FIG. 5C  and  FIG. 5D  show example signals in the test environment of  FIG. 5A  and  FIG. 5B  with the matched impedance circuit being provided in the transmitter circuit, at the output of the transmitter circuit ( FIG. 5C ) and at the input of the transducer ( FIG. 5D ). 
       
    
    
       [0041]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION 
       [0042]    The invention consists in an impedance-matching circuit of the transmitter-receiver or transmitter circuit which contributes to reduce the impact of the reflection of the pulse signal on the transducer back towards the transmitter-receiver or transmitter circuit. By matching the impedance of the transmitter-receiver circuit with the impedance of the coaxial cable, the pulse energy is mainly transferred in the circuit and there is no strong reflection back towards the transducer. 
         [0043]      FIG. 1  shows a diagram of some elements of an example tube wall thickness testing system. The transmit-receiver circuit  101  is used to generate the pulse voltage signal sent to the transducer  104  and to amplify the echo signals received from the transducer  104 . In this example, the tube to inspect  108  is 25 m long. The coaxial cable  102  is used to connect the transmitter-receiver circuit  101  to the transducer  104 . The coaxial cable  102  is long, for example, 30 m long. The nominal impedance of the coaxial cable  102  is, for example, 50 Ω. 
         [0044]    An ultrasonic wave  107  is emitted by the transducer  104  when the pulse voltage transmitted by the transmitter-receiver circuit over the cable  102  arrives on the transducer  104 . There is a short time interval between the time the pulse is emitted from the transmitter-receiver circuit  101 , passes through the coaxial cable  102  and arrives at the transducer  104 . This time interval depends on the electrical signal propagation speed in the cable and on the cable length. For this example, the electrical signal propagation speed is 2×10 8  m/s and the coaxial cable length is 30 m. The time interval is equal to the coaxial cable length divided by the electrical signal propagation speed in the coaxial cable. A time interval of 150 ns (30 m/(2×10 8  m/s)) is obtained. 
         [0045]    There is a rotating mirror  106  used to change the direction of the ultrasonic wave with respect to the tube. The rotation of the mirror  106  controls the angular position of the ultrasonic wave with the circumference of the tube  108 . The rotation of the mirror is done by a turbine. The stator  103  of the turbine does not turn and the rotor  105  of the turbine turns. In this example, the tube is filled with water  109 . 
         [0046]    The impedance of the transducer  104  does not match the 50Ω of the coaxial cable  102  and this mismatch produces a reflection of the transmitted pulse in the coaxial cable  102 . This reflection runs in the coaxial cable at the same speed as previously and then arrives at the transmitter-receiver with an additional time interval of 150 ns. The impedance matching circuit contributes to avoid another reflection of the signal back from the transmitter-receiver circuit to the transducer  104 . 
         [0047]      FIG. 2A  (prior art) shows an example of a signal obtained at the output of the receiver circuit. The horizontal axis is the time and the vertical axis is the signal voltage. The front wall echo  201  is the dominant signal with few associated oscillations. There is a second front wall echo  202  delayed by about 300 ns relative to the first front wall echo  201 . This time delay between the first and second front wall echoes is due to the reflection of the pulse signal in the coaxial cable, which corresponds in this example to 150 ns for each way. The back wall echo  203  is about 800 ns later than the front wall echo  201 . This delay depends on the thickness of the material and on the ultrasound wave speed in the part under test. In this example, the sound velocity is 5000 m/s and the wall thickness is 2 mm (5000 m/s×800 ns/2). 
         [0048]    In  FIG. 2B  (prior art), the wall thickness of the material is 0.75 mm. In that case, the front wall echo  204  does not really change, but the back wall echo moves to the left because the wall is thinner. The back wall echo is moved to about 300 ns (0.75 mm×2/5000 m/s) from the front wall which corresponds to the position of second front wall echo. The result is that the second front wall echo and the back wall echo are superposed  205 . In this example, it is not possible to locate the back wall echo accurately. 
         [0049]    In  FIG. 2C , the transmitter-receiver circuit has been modified with a matched impedance circuit as per the present invention. The other components are unchanged and equivalent to those of  FIG. 2B . The front wall echo  206  does not really change, but the back wall echo signal  207  can now be located accurately because the second front wall echo is strongly reduced. 
         [0050]      FIG. 5A  (prior art) and  FIG. 5B  (prior art) show example signals at the output of the transmitter circuit ( FIG. 5A ) and at the input of the transducer ( FIG. 5B ). The initial pulse  501  emitted by the transmitter circuit is propagated to the transducer at the electrical signal propagation speed in the coaxial cable. After the delay of propagation in the cable, the first pulse  503  arrives at the transducer. The first pulse  503  causes the transducer to create a first ultrasonic wave in the material, for which a front wall echo and a back wall echo will be received. The complex impedance of the transducer also produces a reflection in the cable which may have positive and negative components. The once-reflected signal is propagated back in the cable and arrives at the transmitter. The once-reflected signal is again reflected at the transmitter circuit due to the unmatched impedance between the cable and the transmitter circuit. The twice-reflected signal  502  is deformed (when compared to the initial pulse  501 ) due to the unmatched impedance. In this example, the positive component sees low impedance whereas the negative portion of the reflected signal sees high impedance. The twice-reflected signal  502  travels towards the transducer. The twice-reflected signal which arrives at the transducer  504  is a second electrical pulse which, from the transducer&#39;s perspective, appears to come from the transmitter. The second pulse  504  causes the transducer to create a second ultrasonic wave in the material, for which a front wall echo and a back wall echo will be received. If the front wall echo of the second pulse  504  superposes in time with the back wall echo of first pulse  503 , it causes an ambiguity. 
         [0051]      FIG. 5C  and  FIG. 5D  show example signals in the test environment of  FIG. 5A  and  FIG. 5B  with a matched impedance circuit as per the present invention being provided in the transmitter circuit, at the output of the transmitter circuit ( FIG. 5C ) and at the input of the transducer ( FIG. 5D ). The initial pulse  505  emitted by the transmitter is sent to the transducer over the coaxial cable. After the transmission delay the first pulse signal  507  arrives at the transducer. There is a signal reflection at the transducer and then the once-reflected signal goes back to the transmitter circuit. The once-reflected signal arrives at the transmitter circuit after the propagation delay. The once-reflected signal is mainly absorbed by the impedance matching circuit and only a small twice-reflected signal  506  is reflected back towards the transducer. The twice-reflected signal  506  is not fully eliminated due to the remaining small residual mismatch caused by the tolerances of the coaxial cable and the imperfection of the circuit. The small twice-reflected signal  506  arrives at the transducer after the propagation delay and is considered to be a second pulse  508  emitted by the transmitter circuit from the transducer&#39;s perspective. The second pulse  508  causes the transducer to create a second ultrasonic wave in the material, for which a front wall echo and a back wall echo will be received. However, the amplitude of the second pulse  508  is so small that the front wall echo of the second pulse  508  will be negligible with respect to the amplitude of the back wall echo of the first pulse  507 . 
         [0052]      FIG. 3A  (prior art) shows an example conventional electronic circuit design for an ultrasonic transmitter-receiver. The transmitter circuit includes DC high voltage supply  303 , capacitor C 1 , resistor R 1 , diode D 1 , MOSFET driver  304 , MOSFET Q 1  and diode D 2 . The receiver circuit includes resistor R 4 , MOSFET Q 2 , MOSFET driver  306 , diode D 4 , diode D 5 , resistor R 5 , operational amplifier U 1  and other processing stages  305 . The coaxial connector P 1  and the coaxial cable (not shown) are shared by both the transmitter and receiver circuits. 
         [0053]    The DC high voltage supply  303  provides a positive DC voltage to charge the capacitor C 1  by the resistor R 1 , the capacitor C 1  and the diode D 1  path. The MOSFET driver  304  circuit provides a positive pulse wave shape signal when a pulse is triggered and the MOSFET Q 1  is ON. Otherwise, the MOSFET driver  304  provides a voltage close to 0 V. The MOSFET Q 1  is OFF when the MOSFET driver  304  is at about 0 V. When the MOSFET Q 1  is ON a negative pulse voltage is present at coaxial connector P 1  and some current flows through the transducer by D 2 , coaxial connector P 1  and the coaxial cable. MOSFET Q 2  controlled by MOSFET driver  306  is an interrupter that is OFF during the transmission of the signal pulse. MOSFET Q 2  is ON after the transmission of the signal pulse for acquiring the echo signals by the receiver circuit. The behavior of Q 2  is the inverse of Q 1 . When the MOSFET driver  304  goes back to 0 V, the MOSFET Q 1  stops to conduct and the capacitor C 1  starts recharging by the R 1 , C 1 , D 1  path. 
         [0054]    After finishing the transmission of the signal pulse, if there is a signal reflected from the transducer to the circuit, the impedance seen by the signal is not matched to the coaxial cable impedance. More precisely, if a relatively large positive signal reflection comes back from the transducer, the impedance seen by the signal is the sum of the impedance of diode D 2  and diode D 1 . This impedance is typically lower than 5Ω which creates major reflection in the cable. If a relatively large negative signal reflection comes back from the transducer, the impedance seen by the signal is the sum of resistor R 4 , the impedance of the MOSFET Q 2  and the impedance of the diode D 5 . Indeed, MOSFET Q 2  in series with R 4 , saturates when a large current passes from its drain to its source and then its impedance is relatively high. The sum of the impedance is much more than the impedance required to avoid reflection. 
         [0055]    Other stages of amplification and signal processing  305  can be carried out on the signal. 
         [0056]      FIG. 3B  shows an example transmitter-receiver circuit where some components have been added or modified when compared to that of  FIG. 3A  to get matched impedance when a pulse signal is reflected on the transducer back towards the transmitter circuit. The added components in the transmitter circuit are: resistor R 2  in series with diode D 1 , path R 3 -D 3  in parallel to path D 1 -R 2 -D 2  which includes resistor R 3  in series with diode D 3 . In the receiver circuit, MOSFET Q 2  and its MOSFET driver  306  are replaced by a short circuit. 
         [0057]    The DC high voltage supply  303  provides a positive DC voltage to charge the capacitor C 1  by the resistor R 1 , the capacitor C 1 , the resistor R 2  and the diode D 1  path. The MOSFET driver  304  circuit provides a positive pulse wave shape signal when a pulse is triggered and the MOSFET Q 1  is ON. Otherwise, the MOSFET driver  304  provides a voltage close to 0 V. The MOSFET Q 1  is OFF when the MOSFET driver  304  is at about 0 V. When the MOSFET Q 1  goes ON, the measure point  301  drops close to 0 V and the measure point  302  drops in negative voltage due to the charge of the capacitor C 1 . Some current flows through the transducer by D 2 , coaxial connector P 1  and the coaxial cable. Some current also flows in R 3 , D 3 , R 4  and D 5 . When the MOSFET driver  304  goes back to 0 V, the MOSFET Q 1  stops to conduct and the capacitor C 1  starts recharging by the R 1 , C 1 , R 2  and D 1  path. 
         [0058]    After finishing transmission of the signal pulse, if there is a signal reflected from the transducer to the circuit, the circuit absorbs the reflection because the circuit has the same impedance as the coaxial cable. 
         [0059]    More precisely, if a relatively large positive signal reflection comes back from the transducer, the impedance seen by the signal is resistor R 4  in parallel with resistor R 2  in parallel with R 1 , assuming the voltage drop in diode D 1 , diode D 2 , diode D 4  and capacitor C 1  are negligible. To avoid reflection, R 4 //R 2 //R 1  should be equal to the coaxial cable impedance. If the coaxial cable impedance is 50Ω, R 4 //R 2 //R 1  should equal to 50Ω. 
         [0060]    If a relatively large negative signal reflection comes back from the transducer, the impedance seen by the signal is resistor R 4  in parallel with R 3 , assuming the voltage drop in the diode D 3  and the diode D 5  are negligible. To avoid reflection R 4 //R 3  should be equal to the coaxial cable impedance. If the coaxial cable impedance is 50Ω, R 4 //R 3  should equal to 50Ω. 
         [0061]    For example, if a 50Ω coaxial cable impedance is used and R 4  equals 100Ω and R 3  equals 100Ω, a 50Ω impedance (R 4 //R 3 =50Ω) is obtained which absorbs the negative signal reflection from the transducer. In the same manner if R 4  is equal to 100Ω and R 2  is set at 110Ω and R 1  at 1100Ω, a 50Ω impedance (R 4 //R 2 //R 1 ) is obtained which absorbs a positive signal reflection from the transducer. 
         [0062]      FIG. 4A  (prior art) shows an example of another conventional electronic circuit design for an ultrasonic transmitter-receiver. The transmitter circuit is identical to that of  FIG. 3A . The receiver circuit is different from that of  FIG. 3A . Resistor R 4 , MOSFET Q 2  and MOSFET driver  306  are omitted. Resistor R 6 , resistor R 7 , resistor R 8 , diode D 6 , diode D 7 , diode D 8  and diode D 9  are added. 
         [0063]    In transmission mode, the circuit of  FIG. 4A  behaves like the circuit of  FIG. 3A . In receiver mode, if a relatively large positive signal reflection comes back from the transducer, the impedance seen by the signal is typically lower than 5Ω which creates major reflection in the cable. If a relatively large negative signal reflection comes back from the transducer, the impedance seen by the signal is much more than the impedance required to avoid reflection. Indeed, R 6  is typically greater than 200Ω. 
         [0064]    Other stages of amplification and signal processing  405  can be carried out on the signal. 
         [0065]      FIG. 4B  shows an example transmitter-receiver circuit where some components have been added or modified when compared to that of  FIG. 4A  to get matched impedance when a pulse signal is reflected on the transducer back towards the transmitter circuit. The added components in the transmitter circuit of  FIG. 4B  are: resistor R 2  in series with diode D 1 , path R 3 -D 3  in parallel to path R 2 -D 2  which includes resistor R 3  in series with diode D 3 . The receiver circuit of  FIG. 4B  is identical to that of  FIG. 4A . 
         [0066]    The DC high voltage supply  403  providing a positive DC voltage to charge capacitor C 1  by resistor R 1 , capacitor C 1 , resistor R 2  and diode D 1  path. The MOSFET driver  404  circuit provides a positive pulse wave shape signal when a pulse is triggered and the MOSFET Q 1  is ON. Otherwise, the MOSFET driver  404  provides a voltage close to 0 V. The MOSFET Q 1  is OFF when the MOSFET driver  404  is at about 0 V. When Q 1  goes ON, the measure point  401  drops close to 0 V and the measure point  402  drops in negative voltage due to the charge of the capacitor C 1 . Some current flows through the transducer by D 2 , coaxial connector P 1  and the coaxial cable. Some current also flows in R 3 , D 3 , R 6  and D 6 . When the MOSFET driver  404  goes back to 0 V, the MOSFET Q 1  stops to conduct and the capacitor C 1  starts recharging by the R 1 , C 1 , R 2  and D 1  path. 
         [0067]    After finishing the transmission of the signal pulse, if there is a signal reflected from the transducer to the circuit, the circuit absorbs the reflection because the circuit has the same impedance as the coaxial cable. 
         [0068]    More precisely, if a relatively large positive signal reflection comes back from the transducer, the impedance seen by the signal is resistor R 7  in parallel with R 2  in parallel with R 1 , assuming the voltage drop in diode D 8 , diode D 2 , diode D 1  and capacitor C 1  are negligible. To avoid reflection R 7 //R 2 //R 1  should be equal to the coaxial cable impedance. 
         [0069]    If a relatively large negative signal reflection comes back from the transducer, the impedance seen by the signal is resistor R 6  in parallel with R 3 , assuming that the voltage drop in diode D 3  and diode D 6  is negligible. To avoid reflection, R 6 //R 3  should be equal to the coaxial cable impedance. 
         [0070]    For example, if a 50Ω coaxial cable impedance is used and R 6  equal 300Ω and R 3  equals 60Ω, a 50Ω impedance (R 6 //R 3 =50Ω) is obtained, which absorbs a negative signal reflection from the transducer. Similarly, if R 7 =R 6 =300Ω and R 2  is 63Ω and R 1  is 1260Ω, a 50Ω impedance (R 7 //R 2 //R 1 ) is obtained, which absorbs a positive signal reflection from the transducer. 
         [0071]    It is important to note that the impedance matching circuit matches the cable impedance of the cable at the transmitter circuit for an electrical parasitic reflection signal of the electrical pulse voltage signal. The electrical parasitic reflection signal is caused by an impedance mismatch between the cable and the ultrasonic transducer. The parasitic nominal voltage value is at most equal to the pulse nominal voltage value. The impedance matching circuit does not match the impedance of the cable at the receiver circuit such as to prevent reflecting the transducer emitted pulse. The transducer emitted pulses are low voltage signals which, even when reflected, only cause low voltage parasitic signals which can be readily ignored during signal processing. 
         [0072]    The electrical parasitic reflection signal caused by a reflection of the electrical pulse voltage signal emitted by the transmitter has a nominal value which can reach the nominal value of the original electrical pulse voltage signal and its impact is much more important on the acquired signals. It can cause a significant ambiguity. Its shape can have a positive component and an inverted component. 
         [0073]    Example nominal voltage values for the electrical pulse voltage signal emitted by the transmitter are between −50 V and −400 V and example nominal voltage values for the signals emitted by the transducer are smaller than +/−1 V. 
         [0074]    As will be readily understood, it is not necessary for the impedance matching circuit to fully eliminate the reflection of the pulse. A significant reduction of its nominal voltage value will allow discriminating it in the results. 
         [0075]    As will be readily understood, the length of the cable between the transmitter circuit and the transducer at which the impedance matching circuit becomes relevant is related to the thickness of the material to be tested. If the length of a return trip of the signal in the cable corresponds to the length of a return trip of the ultrasonic wave in the material to be tested, an ambiguity will be created in the acquired voltage signal using conventional systems. 
         [0076]    Vc is the electrical signal propagation speed in a cable used for the ultrasonic testing system, namely 2×10 8  m/s +/−20%. Let us consider it is 2×10 8  m/s. Vp is the speed of sound in a material under test, typically between 4000 and 6000 m/s. Let us consider it is 5000 m/s. Let us assume that the minimal thickness of the material to be tested is Emin=0.5×10 −3 m and that the maximum thickness of the material to be tested is Emax=3.0×10 −3 m. Lc is the length of the cable. 
         [0077]    The time required for a return trip of the ultrasonic wave in the material to be tested is Tp=2×E/Vp. The time required for a return trip of the electrical signal in the cable is Tc=2×Lc/Vc. The ambiguity is present in the acquired voltage signal using conventional systems when Tp=Tc, namely when 2×E/Vp=2×Lc/Vc which yields Lc=E×Vc/Vp. At Emin, Lc is (0.5×10 −3  m)×(2×10 8  m/s)/(5000 m/s)=20 m. At Emax, Lc is (3.0×10 −3  m)×(2×10 8  m/s)/(5000 m/s)=120 m. 
         [0078]    In this example, the impedance matching circuit is useful when the cable length is between 20 m and 120 m, namely when the cable has a length which is 40 000 times ((2×10 8  m/s)/(5000 m/s) longer than the thickness of the material to be tested. Outside of this range of cable lengths, the impedance matching circuit may not be required but still would not cause any negative impact on the acquired signals. 
         [0079]    As will be readily understood, the impedance matching circuit can be designed to match any cable impedance. Standard cable impedances available in today&#39;s market include 50, 75, 93 and 95 ohms Cables with other cable impedances could be used in the present system without departing from the invention. The matched cable impedance could be the nominal cable impedance or a tested actual cable impedance. 
         [0080]    As will be readily understood, other impedance matching circuits which are adapted to match the cable impedance of the cable at the transmitter circuit for an electrical parasitic reflection signal of the electrical pulse voltage signal are equivalents to the example circuits detailed herein. In particular, one skilled in the art will appreciate that the diodes and MOSFET elements could be replaced by semiconductor rectifiers, switches and interrupters in general. One skilled in the art will select a set of appropriate circuit components to match the impedance and may elect to use two or more resistors or diodes in series or in parallel in place of the illustrated single resistor or diode. Moreover, the DC high voltage supply may provide a negative DC voltage in an inverted impedance matching circuit. 
         [0081]    As will be further understood, the transmitter circuit could be designed to emit a positive or negative electrical pulse voltage signal. Alternatively, the transmitter circuit could be designed to emit a bipolar pulse, namely a pulse with consecutive positive and negative components, in any order. 
         [0082]    The embodiments described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the appended claims.