Abstract:
A method of tuning an input signal (S) is described. The method comprises the steps of amplifying the input signal (S) by a gain element ( 14 ) having a variable impedance ( 22 ) associated therewith and thereby generating an output signal (C), comparing the output signal (C) with a reference voltage (V), tuning the variable impedance ( 22 ) such that the impedance is increased if the amplitude of the output signal (C) is smaller than the reference voltage (V), and that the impedance is decreased if the amplitude of the output signal (C) is greater than the reference voltage (V).

Description:
The present invention hereby claims priority under 35 U.S.C. §119 on European patent application number 01130607.3 filed Dec. 24, 2001, the entire contents of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to a method and an apparatus for tuning an input signal. 
   BACKGROUND OF THE INVENTION 
   U.S. Pat. No. 5,923,164 describes a signal tuning apparatus including a gain element, a variable impedance and a control device. The gain element is an operational amplifier having an input for receiving an input signal to be tuned, and an output with an output signal. A feedback path extends from the output to the input. The variable impedance is provided in the feedback path for varying the gain of the operational amplifier. The variable impedance includes a plurality of resistors and a plurality of switches. The resistors are connected in series in the feedback path. Each of the resistors has a switch in parallel for switching the resistor into or out of the feedback path. The output signal of the operational amplifier is connected to the control device as an input signal. Based on this input signal, the control device generates a plurality of switching signals which are provided to the switches of the variable impedance. Furthermore, the control device receives a minimum reference voltage and a maximum reference voltage which represent a bandwidth for the output signal of the operational amplifier. 
   In operation, the signal tuning apparatus of U.S. Pat. No. 5,923,164 compares the output signal of the operational amplifier with the minimum and maximum reference voltage. If the output signal is smaller than the minimum reference voltage or greater than the maximum reference voltage, the gain of the operational amplifier is increased or decreased by switching appropriate resistors into or out of the feedback path of the operational amplifier. If the output signal is greater than the minimum reference voltage and smaller than the maximum reference voltage, then the gain of the operational amplifier is not changed. Thus, the output signal of the operational amplifier is held within the bandwidth defined by the minimum and maximum reference voltage. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a method and an apparatus for tuning an input signal which is more accurate and requires less efforts. 
   According to an embodiment of the present invention, an input signal is tuned in dependence of a single reference voltage. Therefore, the input signal does not vary between two voltage levels but only toggles around the single reference voltage. As a result, the input signal more accurately remains at the voltage level of the reference voltage. 
   Furthermore, as only a single reference voltage is present, only a single comparator is necessary to perform the tuning the input signal. 
   The invention will be better understood from the following description taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a signal tuning apparatus according to the invention, 
       FIG. 1   a  is a schematic diagram of a control device comprised in the tuning apparatus of  FIG. 1 , 
       FIG. 2  is a schematic diagram of a magnetostrictive displacement transducer incorporating the signal tuning apparatus of  FIG. 1 , and 
       FIG. 3  is a schematic diagram of a signal to be tuned by the signal tuning apparatus of FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a signal tuning apparatus  12  including a gain element  14 , a variable impedance  22  and a control device  30 . 
   The gain element  14  is an operational amplifier provided with at least one input  16  and an output  18 . The input  16  of the operational amplifier receives an input signal S which is the signal to be tuned. At the output  18  of the operational amplifier, an output signal C is available which is the tuned signal. 
   The output  18  of the gain element  14  is connected with its input  16  via a feedback path  20  which includes the variable impedance  22 . 
   The variable impedance  22  comprises n+1 resistors  26  and n+1 switches  24 . The resistors  26  are indicated by the abbreviations R 0 , R 1 , R 2 , . . . , R n  and the switches  24  are indicated by the abbreviations S 0 , S 1 , S 2 , . . . , S n , with n being any number greater than zero. The resistors  26  are connected in series in the feedback path  20 . Each of the resistors  26  is provided with one of the switches  24  in parallel. If one of the switches  24  is closed, the corresponding resistor  26  is switched out of the feedback path  20 . However, if one of the switches  24  is open, the respective resistor  26  is effective within the feedback path  20 . 
   The output signal C is provided as an input signal to the control device  30 . Furthermore, the control device  30  receives a reference voltage V and a clocking signal CLOCK as other input signals. The control device  30  generates n+1 binary switching signals  32  which are indicated by the abbreviations O 0 , O 1 , O 2 , . . . , O n . Every one of the switching signals  32  is dedicated to one of the switches  24 . Depending on the binary state of the switching signal  32 , the corresponding switch  24  is open or closed. 
   The resistors  26  increase in resistance from R 0  to R n  in a binary increasing fashion. Thus, R 1  has a resistance value of 2 1  times R 0 , R 2  has a resistance value of 2 2  times R 0 , and so on. 
   In  FIG. 1 , as an example, a number of eight resistors  26 , i.e. R 0  to R 7 , is provided with a resulting number of eight switches  24 , i.e. S 0  to S 7 , and eight switching signals  32 , i.e. O 0  to O 7 . The eight switching signals  32  may be seen as a digital value between the binary number “0000 0000” and the binary number “1111 1111”, which correspond to the decimal numbers “0” and “256”. Thus, any resistance value between R 0  and 256 times R 0  may be generated depending on the digital value of the switching signals  32 . 
   In operation, the control device  30  compares the output signal C, i.e. the tuned signal, with the reference voltage V. This comparison is performed at every clocking signal CLOCK. 
   If the output signal C is greater than the reference voltage V, then the digital value of the switching signals  32  is decreased by one, i.e. by the binary number “0000 0001”. If the output signal C is not greater than the reference voltage V, then the digital value of the switching signals  32  is increased by one, i.e. by the binary number “0000 0001”. 
   Thus, if the output signal C is greater than the reference voltage V, then the resistance of the feedback path  20  is decreased thereby decreasing the gain of the gain element  14 , and if the output signal C is not greater than the reference voltage V, then the resistance of the feedback path  20  is increased thereby increasing the gain of the gain element  14 . 
   As a result, such decrease or increase of the gain of the gain element  14  is performed at every clocking signal CLOCK. If the output signal C is equal to the reference voltage V, then it is possible that the output signal C is alternatively decreased and increased with every clocking signal CLOCK. 
     FIG. 1   a  shows the control device  30  of  FIG. 1  in more detail. The control device  30  comprises a voltage evaluating element  42  and a signal generating circuit  34 . The voltage evaluating element  42  comprises a comparator  36  which is supplied with the output signal C of the gain element  14  at a first input  38  and with the reference voltage V at a second input  40 . The comparator  36  generates an output signal which is supplied to the signal generating circuit  34 . The signal generating circuit  34  generates the eight switching signals  32 , i.e. O 0  to O 7 , which are supplied to the eight switches  24 , i.e. S 0  to S 7 . The generation of the eight switching signals  32  is performed in dependence of the output signal generated by the comparator  36  and supplied to the signal generating circuit  34 . 
     FIG. 2  shows a magnetostrictive linear displacement transducer  58  incorporating the signal tuning apparatus  12  of FIG.  1 . The transducer  58  includes housing members  60 , a waveguide  64 , a coil  71  and a magnet  72 . 
   The waveguide  64  is made of magnetostrictive material and is tubular in shape. The coil  71  is located proximate to one of the two ends of the waveguide  64 . The other one of the two ends of the waveguide  64  is provided with damping material  66 . In addition, the end of the waveguide  64  which is located proximate to the coil  71  may also comprise damping material  66 . 
   The coil  71  surrounds the waveguide  64  without being in contact with it. The magnet  72  is disposed linearly along the waveguide  64  and is connected to an object  73  such that the position of the magnet  72  along the waveguide  64  corresponds to the position of the object  73 . 
   A conductive wire  68  is connected to a pulse generator  70 . The conductive wire  68  extends through the interior of the entire length of the waveguide  64  and returns to the pulse generator  70  on the exterior of the waveguide  64 . The waveguide  64 , the conductive wire  68 , the coil  71  and the magnet  72  are contained in an outer tube  74 . 
   As an alternative, it is possible that the waveguide  64  and the conductive wire  68  are combined into a single wire. 
   The coil  71  is connected with the signal tuning apparatus  12  wherein the signal received by the signal tuning apparatus  12  is the input signal S to be tuned. The signal tuning apparatus  12  is connected to a displacement determination device  76  wherein the output signal C provided by the signal tuning apparatus  12  is the tuned signal. Furthermore, the displacement determination device  76  is connected to the pulse generator  70 . 
   In operation, the pulse generator  70  periodically generates a single excitation pulse  78  on the conductive wire  68  exactly every e.g. 2 milliseconds. The excitation pulse  78  passes through the conductive wire  68  and combines with the magnetic field of the magnet  72 . Thus, a torsion wave  80  is created within the waveguide  64  which propagates away from the magnet  72  and back to the coil  71 . When the torsion wave  80  reaches the coil  71 , it is converted into the signal S. 
   The production of the excitation pulse  78  is also transmitted to the displacement determination device  76 . 
   The signal S from the coil  71  is passed to the signal tuning apparatus  12  where it is tuned to the reference voltage V as described in connection with FIG.  1 . Then, the displacement determination device  76  receives the tuned signal C and measures the interval of time between the production of the excitation pulse  78  and the receipt of the tuned signal C. Using the interval of time and the known speed of the torsion wave  80  within the waveguide  64 , the displacement determination device  76  is able to determine the position of the object  73 . 
     FIG. 3  shows the input signal S to be tuned by the signal tuning apparatus  12  of FIG.  2 . Furthermore,  FIG. 3  shows the excitation pulse  78  and the reference voltage V. All signals and pulses shown in  FIG. 3  are depicted over the time t. 
   As described in connection with  FIG. 2 , the excitation pulse  78  is generated and transmitted through the waveguide  64 . On its way along the waveguide  64 , the excitation pulse  78  passes through the coil  71 . There, the excitation pulse  78  creates some disturbances of the signal S which are depicted by the reference numeral  90  in FIG.  3 . These disturbances are taken into consideration neither by the signal tuning apparatus  12  nor by the displacement determination device  76 . 
   As described in connection with  FIG. 2 , the torsion wave  80  is created and is converted into the signal S. This leads to a deflection of the signal S which is depicted by the reference numeral  91  in FIG.  3 . This deflection is recognised by the signal tuning apparatus  12  and the displacement detection device  76  as described below. 
   The signal tuning apparatus  12  receives the signal S and performs the tuning of the signal S as described in connection with FIG.  1 . However, this tuning of the signal S is not performed at once but at a later point in time as described below. 
   The displacement detection device  76  receives the tuned signal C and performs the evaluation of the position of the object  73  as described in connection with FIG.  2 . 
   For that purpose, in a first step, the displacement detection device  76  detects that point in time when the signal S becomes greater than a detection voltage D. This point in time is depicted with the reference numeral  94  in FIG.  3 . The detection voltage D serves to recognise the deflection of the signal S created by the torsion wave  80  as described in connection with FIG.  2 . 
   As shown in  FIG. 3 , the detection voltage D is smaller than the reference voltage V but greater than a zero voltage Z. The zero voltage Z is that voltage which is present as the input signal S when no excitation pulses  78  or other signals are passed through the conductive wire  68 . 
   Then, in a second step, the displacement determination device  76  detects that point in time when the signal S becomes equal to the zero voltage Z. In  FIG. 3 , this point in time is depicted with the reference numeral  95 . It shall be emphasised that the described detection is performed subsequently after the signal S becomes greater than the detection voltage D so that the point in time  95  is that point in time when the signal S becomes equal to the zero voltage Z for the first time after the point in time  94 . 
   The displacement determination device  76  generates a response pulse  79  depending on the point in time  95 . As shown in  FIG. 3 , the falling edge of the response pulse  79  corresponds to the point in time  95 . The interval of time between the excitation pulse  78  and the response pulse  79  is then used by the displacement determination device  76  to evaluate the position of the object  73  as described in connection with FIG.  2 . 
   The signal tuning apparatus  12  receives the signal S. Similar to the description above, the signal tuning apparatus  12  detects the point in time  94 . Subsequently, the signal tuning apparatus  12  checks whether the signal S becomes greater than the reference voltage V. In the case of the signal S as shown in  FIG. 3 , the signal tuning apparatus  12  recognises that the signal S does not become greater than the reference voltage V. Therefore, the signal tuning apparatus  12  increases the gain of the gain element  14  by “+1”, i.e. by R 0  as described in connection with FIG.  1 . 
   As already mentioned, the tuning of the signal S is not performed at once but at a later point in time. As shown in  FIG. 3 , the tuning of the signal S, i.e. the increase of the gain of the gain element  14  by “+1” is performed in a point in time which is depicted with the reference numeral  97 . This point in time  97  is located after the response pulse  79  and prior to the next excitation pulse  78 ′. 
   In particular, the tuning of the signal S is not performed between the excitation pulse  78  and the response pulse  79 , i.e. during the time interval which is used by the displacement detection device  76  to evaluate the position of the object  73 . Therefore, this evaluation of the position of the object  73  cannot be negatively influenced by the tuning of the signal S. As well, the tuning of the signal S is not performed shortly before the next excitation signal  78 ′. Therefore, the tuning of the signal S cannot have any negative influence on the next excitation pulse  78 ′ and the resulting torsion wave  80 . 
   Instead, the tuning of the signal S is performed at the point in time  97  such that any resulting change of the gain of the gain element  14  does not have any impact on the evaluation of the position of the object  73 . In particular, the point in time  97  is selected such that any oscillation of the signal S due to the change of the gain of the gain element  14  has finished prior to the next excitation signal  78 ′. 
   As described in connection with  FIG. 2 , the excitation pulse  78  is generated exactly every e.g. 2 milliseconds. The interval of time between the excitation pulse  78  and the response pulse  79  is less than 2 milliseconds. Therefore, due to the fixed periodic generation of the excitation pulse  78 , it is possible to select a fixed point in time  79  for the tuning of the signal S, which is outside the interval of time between the excitation pulse  78  and the response pulse  79 . In particular, it is possible to define the point in time  97  as a fixed time interval prior to the excitation pulse  78 . 
   As a result, the clocking signal CLOCK as described in connection with  FIG. 1  may be selected with the identical periodic repetition as the excitation pulse  78 , but with a fixed displacement with respect to the excitation pulse  78  which is identical to the aforementioned fixed time interval prior to the excitation pulse  78 .