Abstract:
A local oscillation circuit comprising a crystal oscillating circuit for generating an output voltage having a constant frequency, and an interface part for converting the output voltage from the crystal oscillating circuit into a current signal, the current signal being used as a local oscillation signal to be mixed with the receiving signal from an antenna, whereby a receiving circuit, which can be made as a single semiconductor chip consuming little current, can be realized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a local oscillation circuit, without a phase locked loop (PLL) circuit, for providing an oscillating signal having a stable amplitude, and also relates to a receiving circuit, for mobile communication, including the local oscillation circuit. 
     2. Description of the Related Art 
     Receiving circuits include local oscillation circuits. In the prior art, local oscillation circuits include PLL circuits and oscillation circuits driven by the outputs of the PLL circuits. A PLL circuit and circuits other than an oscillator circuit may be integrated into one semiconductor chip. However, an oscillator circuit cannot be integrated into the semiconductor chip because an oscillator circuit includes many external parts such as variable capacitors, coils, capacitors, etc. Therefore, the prior-art local oscillation circuit has a problem in that it has a large number of circuit parts; the receiving circuit can not be made into one semiconductor chip; the size of the local oscillation circuit is large and, accordingly, the size of the receiving circuit is large. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned problems in the prior art, an object of the present invention is to provide a receiving circuit which can be made as a single semiconductor chip by employing a voltage-current converting interface part in a local oscillation circuit in the receiving circuit so as to reduce the number of parts in the local oscillation circuit part. 
     Another object of the present invention is to provide a receiving circuit which consumes little current. 
     Still another object of the present invention is to provide a receiving circuit in which the sensitivity with respect to an input signal from an antenna is independent of the power supply voltage. 
     To attain the above objects, there is provided, according to the present invention, a local oscillation circuit comprising a crystal oscillating circuit-for generating an output voltage having a constant frequency, and an interface part for converting the output voltage from the crystal oscillating circuit into a current signal, the current signal being used as a local oscillation signal to be mixed with the receiving signal from an antenna. 
     Since no PLL circuit is included in the local oscillation circuit, the number of parts in the local oscillation circuit can be made small in comparison with the local oscillation circuit including the PLL circuit. Further, since the voltage is converted into a current, the fluctuation of the amplitude of the oscillating voltage can be limited even when a low pass filter and a voltage controlled oscillator (VCO) are not employed. Therefore, the local oscillation circuit can be operated even when the power supply voltage is a low voltage, so that a local oscillation circuit consuming little current can be realized, resulting in a long life of a battery in the circuit or in a miniaturization of the battery. 
     Preferably, the interface part comprises a constant current source for converting the output voltage from the crystal oscillating circuit into a square wave signal having a frequency corresponding to the frequency of the output voltage, a filtering part for removing high frequency components in the square wave signal output from the constant current source, and a current interface part for converting a change in the voltage of the signal close to a sine wave output from the filtering part into a change in current. 
     By employing the constant current source which can provide a constant gain even when the power supply voltage is low, the fluctuation of the current due to variations of the manufacturing processes can be limited so that the local oscillation circuit can be incorporated into a receiving circuit to provide a stable input sensitivity independent of the power supply voltage. 
     Still preferably, the constant current source comprises a constant current source part connected to a power supply line, a load connected to the ground, and a switching part, connected between the constant current source and the load, which can be turned ON or OFF in response to the output voltage from the crystal oscillating circuit. By this construction, a square wave voltage having a desired amplitude can be obtained across the load. 
     Further preferably, the constant current source comprises a differential pair of transistors connected to the power supply line, and a power source for supplying a constant current to the differential pair of transistors. The constant current is independent of the temperature. The switching part is a switching transistor connected between one of the differential pair of transistors and the load. By this construction, in response to the voltage output, from the crystal oscillating circuit, to be input into the switching transistor, a current flows through one of the differential pair of transistors and the load. 
     Still further preferably, the current interface part comprises a first differential pair including a first transistor having an input to receive the output voltage from the filtering part and a-second transistor having an input to receive a reference voltage, a current supplying source for supplying a current to the first transistor and the second transistor in response to the output voltage from the filtering part, and a second differential pair including a pair of a third transistor and a fourth transistor for differentially passing a current from the current supplying source in response to the operation of the first differential pair. In this construction, a current flowing through the second pair is the local oscillation signal. 
     According to another aspect of the present invention, there is provided a receiving circuit comprising the above-mentioned local oscillation circuit, an antenna for receiving a signal, and a mixer circuit for mixing the output current from the interface part with a receiving signal from the antenna. 
     Preferably, the mixing circuit comprises a local oscillation interface circuit for conducting a current in response to an output current of the interface part, a mixing part for mixing a receiving signal from the antenna with a current flowing through the local oscillation interface circuit; and an output circuit for conducting a constant current through the mixing part. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and features of the present invention will be more apparent from the following description of the preferred embodiments when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram showing a receiving circuit according to an embodiment of the present invention; 
     FIG. 2 is a block diagram showing in detail the construction of an interface part in the receiving circuit shown in FIG. 1; 
     FIG. 3 is a circuit diagram showing an example of a constant current source in the interface part shown in FIG. 2; 
     FIG. 4 is a circuit diagram showing a practical example of the constant current source shown in FIG. 3; 
     FIG. 5 is a circuit diagram showing a practical circuit construction of a filter in the interface circuit shown in FIG. 2; 
     FIG. 6 is a circuit diagram showing a practical circuit construction of a current interface part in the interface part shown in FIG. 2; 
     FIG. 7 is a circuit diagram showing a practical construction of a mixing circuit shown in the receiving circuit shown in FIG. 1; 
     FIG. 8 is a block diagram showing an example of a prior-art receiving circuit; 
     FIG. 9 is a block diagram showing a construction of a prior-art local oscillation circuit in the receiving circuit shown in FIG. 8; 
     FIG. 10 is a circuit diagram of an inverter amplifier type which is an example of the crystal oscillating circuit shown in FIG. 9; and 
     FIG. 11 is a circuit diagram of an example of a Colpits oscillator circuit of an analog circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For better understanding of the present invention, a prior-art local oscillation circuit and the problems therein will first be described. 
     FIG. 8 is a block diagram showing an example of a prior-art receiving circuit. In the figure,  81  is an antenna for receiving a signal,  82  is an input signal amplifying part (LNA) for amplifying an input signal,  83  is a first band-pass filter for passing only necessary signals,  84  is a first local oscillation circuit (LO),  85  is a first mixer circuit for outputting a signal having a constant frequency which is a difference between the frequency of the signal passed through the first band pass filter  83  and the frequency of the signal output from the first local oscillation circuit  84 . 
     The frequency of the output signal from the first mixer circuit  85  is, for example, 130 MHz. From the first band pass filter  83 , signals having frequencies near 800 MHz are input to the first mixer circuit  85 . The frequencies of the signals are separated by 25 KHz for each channel. The first local oscillation circuit  84  gives a signal having a necessary frequency to the mixer circuit  85  so that the mixer circuit  85  outputs the signal having the above-mentioned constant frequency signal. 
     Reference numeral  86  represents a second band pass filter for passing a signal having the constant frequency of, for example, 130 MHz. Reference numeral  87  represents a second local oscillation circuit (LO). Reference numeral  88  represents a second mixer circuit. Reference numeral  89  represents a receive signal strength indicator (RSSI). 
     The second mixer circuit  88  outputs a signal having a constant frequency of, for example, 450 KHz. The frequency of the signal output from the second band pass filter  86  and input to the second mixer circuit  88  is, for example, 130 MHz. To obtain the constant frequency of 450 KHz at the output of the second mixer circuit  88 , it is sufficient for the second local oscillation circuit  87  to output a signal having a constant frequency of, for example, 129.55 MHz. 
     FIG. 9 is a block diagram showing an example of the construction of the prior-art first local oscillation circuit  84  or the prior-art second local oscillation circuit  87  shown in FIG.  8 . In FIG. 9, the prior art local oscillation circuit includes a PLL circuit part  94  which is driven by a crystal oscillator  91 , and an oscillating circuit part  98  which is driven by the output of the PLL circuit part  94 . 
     The PLL circuit part  94  includes a crystal oscillating circuit  92  for generating an oscillating signal based on the output signal from the crystal oscillator  91 , and a logic circuit  93  driven by the output of the crystal oscillating circuit  92 . The logic circuit  93  includes a prescaler which receives a signal from a voltage-controlled oscillator (VCO) circuit  96 , a phase comparing circuit for comparing a divided signal from the crystal oscillating circuit  92  with a divided signal of the crystal oscillating signal. The details of the PLL circuit part  94  are well known, and therefore the prescaler and the phase comparing circuit are not shown in the drawings. 
     The oscillating circuit part  98  includes a low-pass filter (LPF)  95  for making the output voltage of the logic circuit  93  constant, and the voltage controlled oscillator (VCO)  96  for outputting a signal having a frequency proportional to the output voltage from the low-pass filter  95 . 
     FIG. 10 is a circuit diagram of an inverter amplifier type which is an example of the crystal oscillating circuit  92  shown in FIG.  9 . This crystal oscillating circuit includes a first complimentary metal oxide semiconductor (CMOS) inverter INVL connected to the crystal oscillator  91 , a second CMOS inverter INV 2  connected in series with the first CMOS inverter INV 1 , and a resistor R connected in parallel to the first CMOS inverter INV 1 . 
     The operations of the circuits shown in FIGS. 8-10 are well known in the art so that the description thereof is omitted here. 
     The prior-art local oscillation circuit shown in FIG. 9 has two blocks, i.e., the PLL circuit  94  and the oscillating circuit  98 . The PLL circuit  94 , and the other circuit elements in the receiving circuit in FIG. 8, that is, the input amplifier circuit  82 , the first band pass filter  83 , the first mixer circuit  85 , the second band pass filter  86 , the second mixer circuit  88 , and the RSSI  89  shown in FIG. 8, can be formed as a single semiconductor chip. However, the oscillating circuit  98  is externally connected to the single semiconductor chip. This is because the VCO  96  in the oscillating circuit  98  includes a large number of external parts such as a variable capacitor, a coil, a capacitor, and so on, and the oscillating circuit  98  and the other circuits cannot be formed as a single semiconductor chip. Therefore, in the prior art, there are problems in that the number of the parts in the local oscillation circuit is large, the receiving circuit as a whole cannot be formed as a single semiconductor chip, the size of the local oscillation circuit is large, and the size of the receiving circuit as a whole is large. 
     In order to decrease the number of parts in the local oscillation circuit, it is possible to not employ the PLL circuit, but to directly use the frequency of the output signal from the crystal oscillating circuit  92  as the frequency of the local oscillation signal to be input into the mixer circuit  85  or  88 . If such a circuit design is employed, however, the fluctuation of the amplitude of the oscillating voltage caused by the fluctuation of the power supply voltage can not be eliminated so that the voltage from the local oscillation signal input to the mixer circuit fluctuates, causing a problem in that the gain of the mixer circuit fluctuates. 
     It is also possible to employ a Colpits oscillator circuit  92   a  of an analog circuit shown in FIG. 11 instead of the inverter-amplifier type as shown in FIG.  10 . In FIG. 11, the Colpits oscillator circuit  92   a  includes a transistor  110  and a current source  111  connected in series between a power supply line Vcc and the ground. In this circuit, because of the presence of the constant current source  111 , a constant current flows through the transistor. However, the current flowing through the transistor is so large that this power consumption becomes very large, disadvantageously resulting in that not only the life of a battery in the receiving circuit becomes short, but also the size of the transistor becomes as large as 100 times the size of an inverter amplifier type transistor. Further, in the Colpits oscillator, the value of a negative resistance which causes the oscillation to stop is very small. That is, the manufacturing margin is very small. 
     From another point of view, in the prior-art receiving circuit shown in FIG. 8, it is necessary to change the frequency of the signal output from the first local oscillation circuit  84 , however, it is not necessary to change the frequency of the output signal from the second local oscillation circuit  87 . Accordingly, the second local oscillation circuit  87  may not be constructed by a PLL circuit. 
     Now, an embodiment of the present invention will be described. 
     FIG. 1 is a block diagram showing a receiving circuit according to an embodiment of the present invention. In the figure, the same reference numerals as in FIG. 8 represent the same parts. The main differences between FIG.  1  and FIG. 8 reside in that, according to the invention, in place of the second local oscillation circuit  87  in FIG. 8, a local oscillation circuit  89   a  including a crystal oscillating circuit  10  and an interface part  20  is provided in FIG.  1 . Further, in place of the second mixer circuit  88  in FIG. 8, a mixer circuit  30  having a current interface is provided in FIG. 1. A reference numeral  11  represents an output terminal of the crystal oscillating circuit  10 ; a reference numeral  21  represents an input terminal of the interface part  20 ; and a reference numeral  24  represents an output terminal of the interface part  20 . 
     In operation, a crystal oscillating circuit  10  generates an output voltage having a fixed frequency. The interface part  20  converts a voltage change in the voltage output from the crystal oscillating circuit  10  into a current change. The mixer circuit  30  mixes the converted current output from the interface part  20  with the received signal from the antenna  81 . 
     FIG. 2 is a block diagram showing in detail the interface part  20  in the receiving circuit shown in FIG.  1 . In the figure, the interface part  20  includes a constant current source  25 , a filtering part  26 , and a current interface part  27 . Reference numeral  22  represents an output terminal of the constant current source  25 ; and a reference numeral  23  represents an output terminal of the filtering part  26 . 
     The constant current source  25  receives, at its input terminal  21 , the output voltage from the crystal oscillating circuit  10  and converts the output voltage into a square-wave signal having a frequency proportional to the frequency of the output signal. The filtering part  26  cuts high frequency components in the square-wave signal output from the constant current source  25  so as to output a signal close to a sine wave. The current interface part  27  converts the voltage change in the signal, which is the output of the filtering part  26  and which is close to the sine wave, into a change in current. 
     FIG. 3 is a circuit diagram showing an example of the circuit of the constant current source  25  shown in FIG.  2 . In the figure, the constant current source  25  includes a constant current source part  251  connected to the power supply line Vcc, a switching part  252  having a terminal connected to the constant current source part  251  and driven in response to the output voltage of the crystal oscillating circuit  10  (see FIG.  1 ), and a load  253  connected between another terminal of the switching part  252  and the ground. Across the load  253 , a square-wave voltage having a desired width and amplitude can be obtained. 
     FIG. 4 is a circuit diagram showing an example of the practical circuit of the constant current source  25  shown in FIG.  3 . In the figure, the constant current source  251  includes a differential pair of P-channel transistors  254  and  255 , and a band gap reference (BGR) circuit  258  which is a voltage source for outputting a constant reference voltage independent of the temperature. The output of the BGR circuit  258  is connected through the resistor  257  to the gate of an N-channel transistor  256 . The drain of the transistor  256  is connected to the gates of the transistors  254  and  255 . The source of the transistor  256  is connected to the ground. The switching part  252  shown in FIG. 2 is realized by an N channel switching transistor  252  connected between one transistor  254  of the differential pair of transistors and the load resistor  253 . 
     In operation of the circuit  25  shown in FIG. 4, since a constant voltage independent of the temperature is applied to the gate of the transistor  256 , a constant current always flows through the transistor  256 . Since the differential pair of the transistors  254  and  255  constitute a current mirror circuit, the current flowing through the transistor  256  is the same as the current flowing through the transistor  252  when the transistor  252  is in an ON state. As a result, a voltage with a constant amplitude independent from the temperature can be obtained across the load resistor  253 . It should be noted that the switching transistor  252  is in the ON state to provide the constant amplitude voltage across the load resistor  253  only when the voltage output from the crystal oscillating circuit  10  exceeds a predetermined level. From the above description, it will be apparent that a square-wave voltage, which is independent of the power supply voltage fluctuation and the frequency of which is proportional to the oscillating frequency, can be obtained across the load resistor  253 . 
     FIG. 5 is a circuit diagram showing a practical circuit construction of the filtering part  26  in the receiving circuit  20  shown in FIG.  2 . As is well known, the filtering part  26  includes a plurality of resistors  261  and a plurality of capacitors  262 , constituting a low-pass filter (LPF). When the square-wave signal output from the constant current source  25  shown in FIG. 4 is applied to an input terminal  22  of the filtering part  26 , a shaped signal having a wave form close to a sine wave is output from an output terminal  23  of the filtering part  26 . When this output signal is input into a mixer part  30  (see FIG.  1 ), the generation of harmonics of the square wave can be suppressed so that interference does not occur. 
     FIG. 6 is a circuit diagram showing a practical circuit construction of the current interface part  27  in the interface part  20  shown in FIG.  2 . In the figure, the current interface part  27  includes a first differential pair of transistors consisting of an N channel transistor  273  having a gate for receiving an output voltage from the output terminal  23  of the filtering circuit  26  and an N channel transistor  274  having a gate for receiving a reference voltage from a reference voltage source  277  which is formed by a voltage source such as the BGR circuit for outputting a constant reference voltage independent from the temperature, a current supplying source  276  for supplying a constant current to the first pair of transistors, and a second differential pair of transistors consisting of a pair of a P channel transistor  278  and a P channel transistor  279  which functions as a current mirror to differentially pass the current from the current supply source  276  in response to the operation of the first differential pair of transistors. P channel transistors  270 ,  271 , and  272  are load resistors constituting the current mirror. The source of the transistor  270  is connected to the power supply line Vcc; its drain is connected through the current source  275  to the ground; and its gate is connected to the drain. The source of the transistor  271  is connected to the power supply line Vcc; and its drain is connected to the drain of the N channel transistor  273 . The source of the transistor  272  is connected to the power supply line Vcc; and its drain is connected to the drain of the N channel transistor  274 . The gates of the transistors  270 ,  271 , and  272  are connected together. The gate and the drain of the transistor  270  are connected to each other. The source of the transistor  273  and the source of the transistor  274  are connected through the current source  276  to the ground. The source of the transistor  278  is connected to the drain of the transistor  272 . To the gates of the transistors  278  and  279  is connected a constant voltage obtained by dividing the power supply voltage by means of the resistors  280  and  281  connected in series between the power supply line Vcc and the ground. 
     The drains of the transistors  278  and  279  are connected to the output terminals  24  of this current interface part  27 . 
     In the operation of the circuit shown in FIG. 6, by means of the constant current source  275 , a constant current flows through the P channel transistor  270 . In response to the voltage of the signal output from the filtering part  26  to be applied to the input terminal  23 , the N channel transistor  273  is turned ON or OFF. In response to this change, the constant current flows through either one of the transistors  273  and  274  from the current source  276 . When the transistor  273  is in an OFF state, the current flows through the transistor  278 ; and when the transistor  274  is in an OFF state, the current flows through the transistor  279 . Thus, the voltage change at the input terminal  23  is converted into a current change at the output terminals  24 . This current change is input into the mixer circuit  30  shown in FIG.  1 . 
     FIG. 7 is a circuit diagram showing a practical circuit construction of the mixer circuit  30  in the receiving circuit shown in FIG.  1 . In the figure, the mixer circuit  30  includes a local oscillation interface circuit  301 , a mixing part  302 , and an output circuit part  303 . 
     The local oscillation interface circuit  301  includes a pair of N channel transistors  304  and  305 , and a pair of N channel transistors  306  and  307 . The drain and the gate of the transistor  304  are connected to one (LO) of the output terminals  24  of the interface part  20 . The source of the transistor  304  is connected to the ground. The gate of the transistor  305  is connected to the gate of the transistor  304 . The source of the transistor  305  is connected to the ground. The drain and the gate of the transistor  306  are connected to the other (XLO) of the output terminals  24  of the interface part  20 . The source of the transistor  306  is connected to the ground. The gate of the transistor  307  is connected to the gate of the transistor  306 . The source of the transistor  307  is connected to the ground. 
     The mixing part  302  includes a pair of N channel transistors  308  and  309 , a pair of transistors  310  and  311 , a reference voltage source  312 , formed by a BGR and so forth, for outputting a constant reference voltage independent from the temperature, and a pair of P channel load transistors  313  and  314 . 
     To the gates of the transistor  308  and the transistor  310 , a high frequency signal RXIN output from the local oscillation interface circuit  301  is applied. The sources of the transistors  308  and  309  are connected to the drain of the N channel transistor  305  in the local oscillation interface circuit  301 . To the gates of the transistors  309  and  311 , the reference voltage from the reference voltage source  312  is applied. The drains of the transistors  308  and  311  are connected through the load transistor  313  to the power supply line Vcc. The drains of the transistors  309  and  310  are connected through the load transistor  314  to the power supply line Vcc. 
     The output circuit part  303  includes a reference voltage source  316  formed by a BGR and so forth for outputting a constant reference voltage independent from the temperature, an N channel transistor  317 , a pair of N channel transistors  318  and  319 , and a load transistor  320 . 
     The constant voltage from the reference voltage source  316  is applied to the gate of the transistor  317 . The sources of the transistors  317  to  319  are connected through the constant current source  315  to the ground. The drain of the transistor  317  is connected to the drain and the gate of the load transistor  320 . The gates of the load transistors  313 ,  314 , and  320  are connected together. The sources of the transistors  313 ,  314  and  320  are connected to the power supply line Vcc. The drain and the gate of the transistor  318  are connected to the drains of the transistors  309  and  310  in the mixing circuit  302  and to one output terminal XOUT of this mixing circuit  30 . The drain and the gate of the transistor  319  is connected to the other output terminal OUT of this mixing circuit  30 . 
     In the operation of the circuit shown in FIG. 7, by means of the output circuit part  303 , constant currents always flow through the transistors  318  and  319  respectively. Each of the constant currents is the same as the current flowing through the transistor  317 . The currents flowing through the transistors  305  and  307  respectively are determined in response to the level of the output signals XLO and LO which are output from the interface part  20 . On the other hand, in response to the output signal RXIN from the second band pass filter  86 , the currents flowing through the transistors  308  and  310  and the currents flowing through the transistors  309  and  311  are determined. As a result, at the output terminal OUT connected to the drains of the transistors  308  and  311 , a mixed signal of the output signal RXIN of the second band pass filter  86  and the output signal LO of the interface part  20  can be obtained; and at the output terminal XOUT connected to the drains of the transistors  309  and  310 , a mixed signal of the output signal RXIN of the second band pass filter  86  and the output signal XLO of the interface part  20  can be obtained. 
     From the foregoing description, it will be apparent that, according to the present invention, the PLL circuit is not employed in the local oscillation circuit so that the number of parts in the local oscillation circuit can be reduced in comparison with the circuit employing a PLL circuit. As a result, a receiving circuit as a whole can be made of a single semiconductor chip. 
     Further, according to the present invention, since the interface between circuits has been changed from a voltage interface to a current interface, the fluctuation of the amplitude of the oscillating voltage due to the fluctuation of the power supply voltage can be suppressed. Therefore, a receiving circuit with a reduced power consumption can be realized so that the life time of the battery can be long or the battery can be miniaturized. 
     Still further, a constant current source circuit according to the present invention can provide a constant gain even when the power supply voltage is low so that the current does not fluctuate even when characteristics of parts in the receiving circuit may fluctuate due to variations of the manufacturing processes. Therefore, in the receiving circuit according to the present invention, the sensitivity with respect to the input signal from the antenna is stable and independent of the power supply voltage.