Abstract:
An apparatus for detecting the coincidence of two input signals comprises a first unit for deciding elevation between two input signals, a second unit for connecting a low level signal selected from the input signals to one output path for the low level signal and connecting a high level signal selected from the input signals to another output path for the high level signal as determined by the decision output, a third unit for producing a tolerance level for deciding one input signal level is coincident with another input signal level, a fourth unit for adding the low level signal form the output path to the tolerance level, and a fifth unit for comparing the adding output with the high level signal from the output path, and deciding one input signal level is coincident with another input signal level if a level of the addition output is higher than a level of the high level signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and a method for deciding whether one signal level is coincident with another signal level or not. 
     Detecting or deciding elevation or coincidence of two signal levels is required in various fields. For example, the diversity reception system in a mobile communication system uses plural reception systems having little correlation with each other and has to select one of the reception systems that has a higher signal level than the other reception system. In such a case, it is necessary to detect elevation of the signal levels to change the reception systems and also coincidence thereof to start processes corresponding to detection of the coincidence. 
     2. Description of the Related Art 
     It is possible to detect the coincidence of two signal levels by using electrical devices, e.g., an analog to digital (A/D) converter, a digital comparator, etc. Namely, the A/D converter converts an analog input signal to a digital value of n bits. The digital comparator compares the digital values from two A/D converters and decides whether one digital value is coincident with another digital value or not. 
     However, the conventional system as described above has to use two A/D converters for two analog inputs, thereby a circuit scale of the system becomes large. Recently, the most significant priority for design of an apparatus is the miniaturization of the apparatus, therefore parts and circuit scale of the apparatus must be small in size. However, the system as described above cannot satisfy the requirement of the miniaturization. 
     Further, the A/D converter has an adjusting circuit for adjusting its full-scale and a zero-point so that the circuit of the system is larger by adding the adjusting circuit. Furthermore, there is a problem that it is not easy to match the tolerance between the A/D converters so as not to decrease their detection accuracy by addition of the adjusting circuit. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide a detector for deciding the coincidence of two signal levels, which has a small scale circuit and no adjustment for the circuit, in order to solve the problems described above. 
     According to the present invention an apparatus for detecting the coincidence of two input signals comprises a first means for deciding the elevation between two input signals, a second means for connecting a low level signal selected from the input signals to one output path for the low level signal and connecting a high level signal selected from the input signals to another output path for the high level signal by a decision output from the first means, a third means for producing a tolerance level for deciding one input signal level is coincident with another input signal level, a fourth means for adding the low level signal from the output path of the second means to the tolerance level from the third means, and a fifth means for comparing the added output from the fourth means with the high level signal from the output path of the second means and deciding one input signal is coincident with another input signal if the level of the added output is higher than the level of the high level signal. 
     Further, according to the present invention a method for detecting the coincidence of two input signals comprises the steps of deciding the elevation difference between two input signals, adding a low level signal selected from the input signals by the decision to a tolerance level signal for deciding one input signal level is coincident with another input signal level, comparing the result of the addition with a high level signal selected from the input signals by the decision, and deciding one input signal level is coincident with another input signal level if the level of the result of addition is higher than the level of the high level signal by the comparison. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below with reference to the accompanying drawings. 
     FIG. 1 shows an example of a conventional detector that decides whether one input signal level is coincident with another input signal level or not. 
     FIG. 2 is a block diagram of a detector for deciding the coincidence of two input signal levels according to the present invention. 
     FIG. 3 is an explanatory view showing performance characteristics of the detector in FIG. 2 according to the present invention. 
     FIG. 4 is a preferred embodiment of a circuit of the detector according to the present invention. 
     FIG. 5(A) and FIG. 5(B) are explanatory views that show the operation of the coincident decision circuit schematically. 
     FIG. 6 is a timing diagram of the output signals in FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing the preferred embodiments according to the present invention, an example of the related art is provided with reference to accompanying the drawings (FIG. 1). 
     FIG. 1 shows an example of a conventional detector for deciding one input signal level is coincident with another input signal level. 
     In FIG. 1, two A/D converters 10, 12 convert analog input signals V A , V B  to digital values of n bits and then provide each digital value to a digital comparator 18. The digital comparator 18 decides whether one digital value is coincident with the other digital value or not. The conventional detector provides simplicity and high resolution, and also permits variable tolerance in the decision of the coincidence of two input signal levels by providing a simple way of comparing the digital values while excluding some of the lower bits of the digital values. 
     However, in the detector described above there are some problems in that the detector has to use two A/D converters 10, 12 so that the circuit scale of the detector becomes large, and each A/D converter 10, 12 has to use an adjusting circuit 14, 16 respectively to adjust its full-scale and a zero-point so that the circuit becomes even larger by adding the adjusting circuit 14, 16. Further, it is not easy to match the tolerance between the A/D converters 10, 12 by the adjusting circuit 14, 16 so as not to decrease in their detection accuracy. 
     FIG. 2 shows a block diagram of a detector for detecting the coincidence of two input signal levels according to the present invention. 
     In FIG. 2, a level comparator 21 compares one analog input signal V A  with another analog input signal V B , and decides the elevation of the two signal levels. A path switching circuit 22 has an interchangeable path switched by an output signal S 2  from the level comparator 21, through which each signal V A , V B  is connected to either of an output path P L  Or P H . The output path P L  terminates the lower level signal selected from the input signals V A , V B  and the output path P H  terminates the other higher level signal by the output signal S 2 . A tolerance generator 23 produces a reference voltage V REF  that corresponds to a tolerance level for deciding one input signal (V A  or V B ) level is coincident with another input signal (V B  or V A ) level. 
     A addition circuit 24 adds the reference voltage V REF  to the lower level signal from the output path P L . A coincidence decision circuit 25 compares the level of the addition output from the addition circuit 24 with the level of the high level signal from the output path P H , and if the output level from the addition circuit 24 is higher than the level of the high level signal from the output pass P H , that is V B  &gt;V A , V A  +V REF  &gt;V B  Or V A  &gt;V B , V B  +V REF  &gt;V A , the coincidence decision circuit 25 decides one input signal level is coincident with the other input signal level (V A  =V B ). 
     FIG. 3 is an explanatory view showing performance characteristics of the detector in FIG. 2. 
     In FIG. 3, it is assumed that one input signal V A  is a constant and another input signal V B  varies as shown by a broken solid line, and until a time t I  the signal level V A  is higher than the signal level V B  (V A  &gt;V B ) and after the time t 1  the signal level V A  is lower than the signal level V B  (V A  &lt;V B ). Until the time t 1 , the level comparator 21 decides V A  &gt;V B  and outputs a high level signal S 2  (H), and conversely, after the time t 1  the level comparator 21 decides V A  &lt;V B  and outputs a low level signal S 2  (L). The path switching circuit 22 is provided with the signal S 2 . If the signal S 2  is high (V A  &gt;V B ), the path switching circuit 22 connects the low level signal V B  to the output path P L  and connects the high level signal V A  to the output path P H . 
     Then the addition circuit 24 adds the reference voltage V REF  as a tolerance for the coincident decision to the signal V B  from the output path P L . The coincidence decision circuit 25 compares the level of an addition output signal (V B  +V REF ) from the addition circuit 24 with the level of the high level signal V A  from the output path P H , and decides whether the addition signal level is higher than the high level signal (V B  +V REF  &gt;V A ) or not. If V A  &gt;V B  and V B  +V REF  &gt;V B , the coincidence decision circuit 25 decides the signal V A  is coincident with the signal V B . Similarly, if V A  &lt;V B  and V A  +V REF  &gt;V B , the coincidence decision circuit 25 decides the signal V A  is coincident with the signal V B . 
     Therefore, until a time t 0  a decision of the coincidence decision circuit 25 is NO (V A  is not coincident with V B ) because V A  &gt;V B  and V B  +V REF  &lt;V A , and the coincidence decision circuit 25 outputs a low level signal S 1  (L). After the time to the decision is YES (V A  is coincident with V B ) because V A  &lt;V B  and V A  +V REF  &gt;V B , and the coincidence decision circuit 25 outputs a high level signal S 1  (H). 
     At the time t 1 , the signal V B  crosses the signal V A , and the signal S 2  from the level comparator 21 becomes low (V A  &lt;V B ). Thereby, the path switching circuit 22 connects the low level signal V A  to the output path P L  and connects the high level signal V B  to the output path P H  respectively. The addition circuit 24 outputs a signal V A  +V REF . The coincidence decision circuit 25 compares the signal V A  +V REF  with the signal V B  and decides whether V A  +V REF  &gt;V B  or not. From the time t 1  to time t 2  the decision is YES because V B  &gt;V A  and V A  +V REF  &gt;V B , and the coincidence decision circuit 25 outputs a high level signal S 1  (H). After the time t 2  the decision is NO since V B  &gt;V A  and V A  +V REF  &lt;V B , and the coincidence decision circuit 25 outputs a low level signal S l  (L). 
     Thus, the coincidence decision circuit 25 decides the signal V A  is coincident with the signal V B  during the time from t 0  to t 2 . The basis of the decision of the coincidence of the two signal levels is that the signal level from the output path P H  is smaller than the signal level from the addition circuit 26, and the reference voltage V REF  provides a tolerance of coincidence. This is generally written as follows V(P H )&gt;V(P L ), and V(P L )+V REF  &gt;V(P H ). It is possible to arrange the circuit of FIG. 1 by using a general use operational amplifier and comparator and to provide a small circuit with no adjustment. 
     FIG. 4 shows a preferred embodiment of a circuit according to the present invention. In FIG. 4, the level comparator 21 consists of an analog comparator using an operational amplifier A 1  and resistors R 1  and R 2 , and outputs a high level signal S 2  (H) when an input signal level V A  is higher than an input signal level V B  (V A  &gt;V B ) or outputs a low level signal S 2  (L) when V A  &lt;V B . The path switching circuit 22 consists of plural analog switches SW 1  -SW 4 . When the signal S 2  is high the switches SW 1 , SW 3  turn on and the switches SW 2 , SW 4  turn off. In that case (V A  &gt;V B ), the output path P L  is provided with the low level signal V B  and the output path P H  is provided with the high level signal V A . Conversely, when the signal S 2  is low (V A  &lt;V B ), the switches SW 2 , SW.sub. 4 turn on and the switches SW 1 , SW 3  turn off, and thereby the output path P L  is provided with the low level signal V A  and the output path P H  is provided with the high level signal V B . The analog switch consists of a CMOS transistor that is constructed by a parallel combination of a PMOS transistor and an NMOS transistor. 
     The tolerance generator 23 consists of a voltage divider comprised of resistors R 3  and R 4  and an operational amplifier A 2  as a voltage-follower circuit. The voltage divider produces the reference voltage V REF  by dividing a power supply voltage V a  and the amplifier A 2  outputs the reference voltage V REF  only through impedance conversion. Similarly, each the operational amplifiers A 3  and A 4  has an arrangement of a voltage-follower circuit with 100% negative-feedback of its output, and outputs the signal V A  or V B  from output path P H  and output path P L  respectively. The addition circuit 24 consists of resistors R 5  -R 8  and an operational amplifier A 5 , and adds the reference voltage V REF  to the output voltage V A  or V B  from the output path P L . 
     The coincidence decision circuit 25 consists of an analog comparator using resistors R 9 , R 10  and an operational amplifier A 6 . If the output signal level from the addition circuit 24 (V A , or V B  +V REF ) is higher than the output signal level from the output path P H  (V B  or V A ), the coincidence decision circuit 25 outputs a high level signal S 1  (H), or if the output signal level from the addition circuit 24 is lower than the output signal level from the output pass P H , the coincidence decision circuit 25 outputs a low level signal S 1  (L). 
     FIG. 5(A) and FIG. 5(B) are explanatory views that schematically show the operation of the coincidence decision circuit 25 as described above. FIG. 5(A) shows the case of V A  &gt;V B , therefore the coincidence decision circuit 25 outputs a low level signal (L) in a state 1 (V B  +V REF  &lt;V A ) or conversely outputs a high level signal (H) in a state 2 (V B  +V REF  &gt;V A ). FIG. 5(B) shows the case of V B  &gt;V A , and the coincidence decision circuit 25 outputs the low level signal (L) in a state 1 (V A  +V REF  &lt;V B ) and outputs the high level signal (H) in a state 2 (V A  +V REF  &gt;V B ). 
     FIG. 6 shows an example of timing diagrams of output signals in FIG. 4. In a section 1 the time base, V A  &gt;V B  and the output signal S 2  from the level comparator 21 becomes high (H), in response to the signal S 2  the path switching circuit 22 connects the input signal V A  to the output path P H  and the input signal V B  into the output path P L  respectively. The addition circuit 24 adds the signal V B  to the signal V REF  and outputs a signal V B  +V REF . Since V B  +V REF  &lt;V A  in the section 1, the output signal S 1  from the coincidence decision circuit 25 is low (L), and although V A  &gt;V B  in section 2, by increasing the signal V B  a level of the signal V B  +V REF  becomes higher than a level of the signal V A  (V B  +V REF  &gt;V A ) and then the output signal S 1  is high (H). When the signal V B  crosses the signal V A  (V A  &lt;V B ), the level comparator 21 exchanges the connection from the input signals V A , V B  to the output paths P L , P H , and thereby, the input signal V A  is connected to the output paths P L  and the input signal V B  is connected to the output paths P H . The addition circuit 24 outputs a signal V A  +V REF . The coincidence decision circuit 25 compares the signal V A  +V REF  with the signal V B . Therefore, the signal V A  becomes lower than the signal V B  in a section 3 (V A  &lt;V B ), and the output signal S 1  remains high (H) (V A  +V REF  &gt;V B ). Then the coincidence decision circuit 25 outputs a low level signal (L) in a section 4 because V A  &lt;V B  and V A  +V REF  &lt;V B . 
     As described above, the detector according to the present invention is able to vary the decision range of coincidence by adjusting the reference voltage V REF  precisely so that it is possible to decide the coincidence of two input signals V A , V B  under the condition that the input signal V A  is nearly equal to the input signal V B , by reducing the reference voltage V REF . Conversely, it is possible to decide the coincidence of two input signals V A , V B  under the condition that the signal V A  is quite different from the signal V B  by increasing the reference voltage V REF . Therefore, an extent of approximation for the decision can be set up optionally. To vary the reference voltage V REF  a variable resistor may be used instead of the resistor R 3  or R 4 . Further, by using an integrated circuit (IC) which has four operational amplifiers in a package, the detector can be remarkably reduced in terms of circuit scale to less than the conventional system described above and thereby the circuit in FIG. 4 can be constructed using only two small analog ICs. 
     As described above, the detector according to the present invention normally watches for elevation of two signal levels, and by an approximation of the difference between the two signal levels by comparison of the signal levels, decides whether one signal level is coincident with the other signal level when the extent of approximation is within the limit of a predetermined reference voltage. Therefore, the detector according to the present invention has a distinct advantage of being able to make a decision on coincidence by simple comparison. Further, the detector according to the present invention can be made cheaply by using a general use operational amplifier and a comparator, and the detector can be miniaturized by not adding an adjusting circuit.