JP4265459B2 - FSK modulation circuit and semiconductor integrated circuit - Google Patents

Description

  The present invention relates to an FSK modulation circuit that modulates a carrier wave by FSK (Frequency Shift Keying) in wireless data communication or the like. Furthermore, the present invention relates to a semiconductor integrated circuit for realizing such an FSK modulation circuit.

  Conventionally, the FSK modulation method is used in the field of wireless data communication such as remote keyless entry that locks and unlocks a door of an automobile without contact. In the case of remote keyless entry, binary FSK is used to switch the carrier frequency to one of two different frequencies according to the sign level ("high level" or "low level") of the data to be transmitted. Has been. The difference between these two different frequencies is called the FSK modulation width.

  Patent Document 1 below describes an FSK modulation circuit using a bipolar transistor. FIG. 5 shows a circuit diagram of a similar conventional FSK modulation circuit. This FSK modulation circuit is known as a voltage-controlled piezoelectric oscillation circuit, and is negative in order to continuously vibrate the piezoelectric vibrator X using the bipolar transistor Q1 and the capacitors C1 and C2. It is the structure which implement | achieves resistance. This FSK modulation circuit generates an output signal having a frequency substantially equal to the series resonance frequency of the piezoelectric vibrator X.

  In the FSK modulation circuit shown in FIG. 5, resistors R2, R3, and R4 are for determining the DC operating point of the bipolar transistor Q1. In addition, an inductor Ls and a variable capacitance diode Cv are connected in series with the piezoelectric vibrator X. In general, in a series resonance circuit including a piezoelectric vibrator, when a positive (inductive) reactance is connected in series, the series resonance frequency is apparently reduced, and a negative (capacitive) reactance is connected in series. If this is done, the series resonance frequency is apparently increased.

  That is, in this FSK modulation circuit, the apparent series resonance frequency is increased or decreased in the vicinity of the original series resonance frequency of the piezoelectric vibrator X by the positive reactance by the inductor Ls and the negative reactance by the variable capacitance diode Cv. Further, since the negative reactance amount can be adjusted by making the capacitance value of the variable capacitance diode Cv variable, the apparent increase / decrease amount of the series resonance frequency can be controlled from the outside.

  Input pulses are applied to both ends of the variable capacitance diode Cv via the resistor R1. Thereby, the voltage between the terminals of the variable capacitance diode Cv changes according to the voltage level of the input pulse. Here, the voltage level (high level or low level) of the input pulse corresponds to the code level of the data to be transmitted.

  The capacitance value of the variable capacitance diode Cv has a property of decreasing as the inter-terminal voltage increases. Therefore, when the input pulse becomes high level, the series resonance frequency apparently increases, while when the input pulse becomes low level, the series resonance frequency apparently decreases. In this way, it is possible to obtain an FSK modulation output signal in which the oscillation frequency shifts depending on the voltage level of the input pulse.

  The reason why the bipolar transistor is used as the amplifying element in the FSK modulation circuit shown in FIG. 5 is that it can operate up to a high frequency with low power consumption and is easily available as an individual component. According to a study, it has been found that even when a circuit is integrated using MOS transistors, the power consumption can be sufficiently reduced depending on the circuit configuration. The following Patent Document 2 describes a piezoelectric oscillation circuit using a MOS transistor as an example. By slightly modifying the piezoelectric oscillation circuit, it can be operated as an FSK modulation circuit. FIG. 6 shows a circuit diagram of a similar conventional piezoelectric oscillation circuit.

The piezoelectric oscillation circuit shown in FIG. 6 generates a negative resistance for continuously vibrating the piezoelectric vibrator X by the inverting amplifiers A1 to A3 and the capacitors Cg and Cd that are connected in series. These inverting amplifiers A1 to A3 are configured by complementarily combining P channel MOS transistors P1 to P3 and N channel MOS transistors N1 to N3, respectively. The resistor Rf determines the DC operating point of the inverting amplifiers A1 to A3. In FIG. 6, if only one stage of the inverting amplifier is used, it is the same as a generally well-known piezoelectric oscillation circuit having a CMOS configuration. However, by connecting three stages of inverting amplifiers, low power consumption is achieved. Negative resistance is obtained.
JP-A-6-85855 (first page, FIG. 2) JP 54-61447 A (first page, FIG. 3)

  By the way, with the rapid spread of wireless devices in recent years, interference with adjacent wireless frequencies has become a problem. In order to avoid this, it is necessary to eliminate the interference wave by narrowing the pass band of the filter of the receiving unit. In that case, it is required to narrow the frequency band of the FSK modulated output signal in consideration of frequency variation in order to keep the band of the radio wave to be transmitted within the pass band of the filter of the receiving unit.

  In the conventional FSK modulation circuit, the frequency band of the FSK modulation output signal is obtained by adding the manufacturing error width of the series resonance frequency of the piezoelectric vibrator, the FSK modulation width, and the frequency change width associated with the temperature dependence of the piezoelectric vibrator. It will be. Of these, if the FSK modulation width is narrowed, the S / N ratio (signal-to-noise ratio) of the demodulated signal deteriorates, and correct data cannot be restored from the received signal. Therefore, in order to narrow the frequency band of the FSK modulation output signal, it is conceivable to reduce the manufacturing error width of the series resonance frequency of the piezoelectric vibrator or to reduce the frequency change width associated with the temperature dependence of the piezoelectric vibrator. .

  However, in order to reduce the manufacturing error width of the series resonance frequency of the piezoelectric vibrator, it is necessary to incorporate strict fine adjustment of the frequency into the manufacturing process, or to perform non-defective product selection at the product shipping stage, which increases the manufacturing cost. Yield deterioration is inevitable. On the other hand, the frequency change width associated with the temperature dependence of the piezoelectric vibrator is uniquely determined according to the type of the piezoelectric vibrator. An increase in manufacturing cost due to the above is unavoidable and the circuit design difficulty is high.

  Therefore, in view of the above points, the present invention narrows the frequency band of the FSK modulation output signal that takes into account frequency variations while keeping the FSK modulation width constant without increasing the manufacturing cost and the yield. An object of the present invention is to provide an FSK modulation circuit capable of performing the above. Furthermore, an object of the present invention is to provide a semiconductor integrated circuit for realizing such an FSK modulation circuit.

  In order to solve the above-described problems, an FSK modulation circuit according to the present invention includes an oscillation circuit that drives an oscillator to perform an oscillation operation, and a variable capacitance unit that is connected in series with the oscillator and that can control the capacitance value. Storage means for storing information relating to the series resonance frequency of the vibrator, and a set of capacitance values among a plurality of capacitance values that the variable capacitance means can take based on the information stored in the storage means The oscillation of the oscillation circuit is selected by switching the capacitance value of the variable capacitance means to one of the selected capacitance values in accordance with the input pulse. Control means for shifting the frequency.

  Here, a piezoelectric vibrator may be used as the vibrator. Further, the variable capacitance means may include a plurality of capacitors having different capacitance values and a switch circuit connected in series to each of these capacitors, or may include a variable capacitance diode. . Furthermore, the storage means may store in advance information relating to the manufacturing error of the series resonance frequency of the vibrator.

  In addition, the semiconductor integrated circuit according to the present invention includes an oscillation circuit that performs an oscillation operation by driving a vibrator in which variable capacitance means whose capacitance value can be controlled is connected in series, and information on a series resonance frequency of the vibrator. Based on the information stored in the storage means and the storage means, a set of capacitance values is selected from a plurality of capacitance values that can be taken by the variable capacitance means, and according to the input pulse And a control means for shifting the oscillation frequency of the oscillation circuit by switching the capacitance value of the variable capacitance means to one of the selected one of the capacitance values. It has.

  According to the present invention, the capacitance value of the variable capacitance means is selected on the basis of the information related to the series resonance frequency of the vibrator stored in the storage means, without increasing the manufacturing cost and the yield. Thus, the frequency band of the FSK modulation output signal can be narrowed while taking into account frequency variations while keeping the FSK modulation width constant. Further, by manufacturing the main part of the FSK modulation circuit as a semiconductor integrated circuit, further cost reduction can be realized.

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. The same constituent elements are denoted by the same reference numerals, and the description thereof is omitted.
FIG. 1 is a diagram showing a configuration of an FSK modulation circuit according to an embodiment of the present invention. This FSK modulation circuit generates a negative resistance for continuously vibrating the piezoelectric vibrator X by the inverting amplifiers A1 to A3 and the capacitors Cg and Cd that are connected in series. These inverting amplifiers A1 to A3 are configured by complementarily combining P channel MOS transistors P1 to P3 and N channel MOS transistors N1 to N3, respectively. The resistor Rf determines the DC operating point of the inverting amplifiers A1 to A3.

  As the piezoelectric vibrator X, for example, a SAW (Surface Acoustic Wave) vibrator or an AT cut crystal vibrator can be used. The FSK modulation circuit shown in FIG. 1 generates an FSK modulation output signal having a frequency substantially equal to the series resonance frequency of the piezoelectric vibrator X. Here, the piezoelectric vibrator X has a property that the series resonance frequency is apparently lowered when inductive reactance is connected in series, and the series resonance frequency is apparently increased when capacitive reactance is connected in series. Have.

  An inductor Ls is connected to the piezoelectric vibrator X in series, and thereby the series resonance frequency is apparently lowered. Furthermore, the variable capacitance unit 1 is connected in series to the piezoelectric vibrator X and the inductor Ls, and thereby the series resonance frequency is apparently increased. As a result, the apparent series resonance frequency can be increased or decreased in the vicinity of the original series resonance frequency of the piezoelectric vibrator X. Furthermore, since the negative reactance amount can be adjusted by changing the capacitance value of the variable capacitance unit 1 in a plurality of stages, the amount of increase or decrease in the series resonance frequency can be controlled according to the voltage level of the input pulse. An FSK modulated output signal modulated based on the pulse is obtained. In order to control the capacitance value of the variable capacitance unit 1, a control unit 2 is provided.

  The storage unit 3 may be a semiconductor memory such as an EEPROM or a FLASH memory, or may be a potential switching means using a switch, a fuse, or the like. The storage unit 3 stores in advance information related to the manufacturing error of the series resonance frequency of the piezoelectric vibrator X. The capacitance value of the variable capacitance unit 1 is controlled according to the voltage level of the input pulse, and is controlled based on information stored in the storage unit 3. In FIG. 1, as an example, the manufacturing error is divided into three stages. When the series resonance frequency of the piezoelectric vibrator X is a low value, the LOW signal is set to a high level, and the series resonance frequency is an intermediate value. In some cases, the MID signal is at a high level, and when the series resonance frequency is a high value, the HIGH signal is at a high level.

  FIG. 2 shows a specific configuration example of the variable capacitance unit and the control unit shown in FIG. As shown in FIG. 2, the variable capacitance unit 1 includes capacitors Cs1 to Cs6 having different capacitance values, and transmission gates (analog switch circuits) Tg1 to Tg6 connected in series to the capacitors Cs1 to Cs6, respectively. , Inverters Iv1 to Iv6 for inverting control signals S1 to S6, respectively.

  Generally, in the N-value FSK modulation circuit, when the capacitance value of the variable capacitance unit is varied in K stages, (N × K) capacitors and (N × K) switch circuits are provided. . In this embodiment, N = 2 and K = 3, and six capacitors and six transmission gates are used. For example, in a binary FSK modulation circuit, the capacitance value can be varied only in two stages. By doing so, it is sufficient to prepare a maximum of four capacitors and four transmission gates, and conversely, if the capacitance value is made variable in four or more stages, the circuit becomes complicated. However, the frequency band of the FSK modulation output signal can be further narrowed while keeping the FSK modulation width constant.

  The control unit 2 includes AND circuits L1 to L6 and an inverter Iv7 that inverts an input pulse. Based on the LOW signal, MID signal, and HIGH signal supplied from the storage unit 3, the control unit 2 selects one set of capacitance values from the six types of capacitance values that the variable capacitance unit 1 can take. Then, according to the voltage level of the input pulse, the control signals S1 to S6 for switching the capacitance value of the variable capacitance unit 1 to any one of the selected one of the capacitance values. Is generated. The transmission gates Tg1 to Tg6 in the variable capacitance unit 1 are turned on when the respective control signals S1 to S6 become high level.

Next, the operation principle of the FSK modulation circuit according to this embodiment will be described. Here, in order to simplify the explanation, it is assumed that there is no frequency fluctuation accompanying the temperature dependence of the piezoelectric vibrator X.
FIG. 3 is a schematic diagram for explaining the operating principle of the FSK modulation circuit shown in FIG. 3A shows the manufacturing error range of the series resonance frequency of the piezoelectric vibrator X. FIG. In the case of a normal piezoelectric vibrator, the production error of the series resonance frequency is about ± 100 ppm for the standard product and about ± 50 ppm for the high-precision product.

  For example, in the case of a piezoelectric vibrator having a series resonance frequency reference value of 315.00 MHz, when a manufacturing error of ± 50 ppm occurs, as shown in FIG. The frequency will vary. The manufacturing error range of ± 15.75 kHz is divided into three equal parts, and the three sections are named LOW, MID, and HIGH in order from the lowest series resonance frequency. The FSK modulation circuit according to the present embodiment is characterized in that the amount of frequency shift is changed according to the division of the manufacturing error range thus determined. This will be described in detail below. In the following description, the FSK modulation width is 30 kHz.

  When the manufacturing error belongs to the LOW category, two kinds of frequency shifts of +25.5 kHz and −4.5 kHz are given to the original series resonance frequency of the piezoelectric vibrator. As a result, as shown in FIG. 3B, the frequency band of the FSK modulation output signal taking into account frequency variations is within a range of ± 20.25 kHz at the maximum with respect to the reference value.

  When the manufacturing error belongs to the MID category, two kinds of frequency shifts of +15 kHz and −15 kHz are given to the original series resonance frequency of the piezoelectric vibrator. As a result, as shown in FIG. 3C, the frequency band of the FSK modulated output signal taking frequency variations into consideration is within a range of ± 20.25 kHz at the maximum with respect to the reference value.

  When the manufacturing error belongs to the HIGH category, two types of frequency shifts of +4.5 kHz and −25.5 kHz are given to the original series resonance frequency of the piezoelectric vibrator. As a result, as shown in FIG. 3D, the frequency band of the FSK modulated output signal taking frequency variations into consideration is within a range of ± 20.25 kHz at the maximum with respect to the reference value.

  As described above, in any case, the frequency band of the FSK modulation output signal considering the variation in frequency is within a range of ± 20.25 kHz at the maximum with respect to the reference value, and the frequency bandwidth is 40 It is suppressed to 5 kHz. In the conventional FSK modulation circuit, the frequency bandwidth is 61.5 kHz, which is obtained by adding the FSK modulation width of 30 kHz to the manufacturing error range of ± 15.75 kHz. For example, the frequency band can be narrowed by 34% compared to the conventional FSK modulation circuit. In order to achieve such a frequency band reduction in the conventional FSK modulation circuit, the manufacturing error of the series resonance frequency of the piezoelectric vibrator must be reduced to ± 16.7 ppm, which is 1/3 of ± 50 ppm. . This is a level that is difficult to achieve with current manufacturing technology.

Next, a setting example of the capacitance value for giving the above frequency shift in the variable capacitance unit 1 shown in FIG. 2 will be described.
FIG. 4 shows an apparent impedance when an inductor Ls and a variable capacitor Cs are connected in series to a piezoelectric vibrator having a series resonance frequency of 315.00 MHz and the capacitance value of the variable capacitor Cs is changed. The absolute value is plotted. Note that when the FSK modulation circuit is assembled, the equivalent capacitance value Cc of the circuit is considered to be connected in series with the piezoelectric vibrator, and this also causes a change in the apparent series resonance frequency. Here, the equivalent capacitance value Cc is estimated to be about 2 pF, and is included in the calculation in advance.

  In FIG. 4, the frequency at which the absolute value of the impedance is a minimum value is substantially equal to the apparent series resonance frequency. Here, the inductance of the inductor Ls is 150 nH, and the capacitance value of the variable capacitor Cs is changed to 4.7 pF, 6.9 pF, 10 pF, 14 pF, 23 pF, and 45 pF. As a result, the amount of deviation from the original series resonance frequency of the piezoelectric vibrator is +25.5 kHz when Cs = 4.7 pF, +15 kHz when Cs = 6.9 pF, and +4 when Cs = 10 pF. 0.5 kHz, −4.5 kHz when Cs = 14 pF, −15 kHz when Cs = 23 pF, and −25.5 kHz when Cs = 45 pF. Therefore, by using these values, the capacitance values of the capacitors Cs1 to Cs6 are determined in the variable capacitance unit 1 shown in FIG. As a result, in the FSK modulation, the frequency band of the FSK modulated output signal as shown in FIG. 3 can be realized.

  Referring to FIG. 2 again, which of the capacitors Cs1 to Cs6 is used depends on the combination of the LOW signal, MID signal, HIGH signal supplied from the storage unit 3 (FIG. 1) and the input pulse. It is switched under the control of 2.

  When the LOW signal is at a high level, when the input pulse is at a high level, the control signal S1 is at a high level and the transmission gate Tg1 is turned on, so that the capacitor Cs1 = 4.7 pF is selected. As a result, the frequency shift of the FSK modulation output signal is +25.5 kHz. On the other hand, when the input pulse becomes low level, the control signal S4 becomes high level and the transmission gate Tg4 is turned on, so that the capacitor Cs4 = 14 pF is selected. As a result, the frequency shift of the FSK modulation output signal is −4.5 kHz. Therefore, when the LOW signal is at a high level, FSK modulation shown in FIG. 3B is performed.

  When the MID signal is at a high level, when the input pulse goes to a high level, the control signal S2 goes to a high level and the transmission gate Tg2 is turned on, so that the capacitor Cs2 = 6.9 pF is selected. As a result, the frequency shift of the FSK modulation output signal is +15 kHz. On the other hand, when the input pulse becomes low level, the control signal S5 becomes high level and the transmission gate Tg5 is turned on, so that the capacitor Cs5 = 23 pF is selected. As a result, the frequency shift of the FSK modulation output signal is −15 kHz. Therefore, when the MID signal is at a high level, FSK modulation shown in FIG. 3C is performed.

  When the HIGH signal is at a high level, when the input pulse goes to a high level, the control signal S3 goes to a high level and the transmission gate Tg3 is turned on, so that the capacitor Cs3 = 10 pF is selected. As a result, the frequency shift of the FSK modulation output signal is +4.5 kHz. On the other hand, when the input pulse becomes low level, the control signal S6 becomes high level and the transmission gate Tg6 is turned on, so that the capacitor Cs6 = 45 pF is selected. As a result, the frequency shift of the FSK modulation output signal becomes −25.5 kHz. Therefore, when the HIGH signal is at a high level, the FSK modulation shown in (d) of FIG. 3 is performed.

  As described above, by using the variable capacitance unit 1 and the control unit 2 shown in FIG. 2, an appropriate set of capacitance values can be selected corresponding to the manufacturing error of the series resonance frequency of the piezoelectric vibrator X. At the same time, the capacitance value of the variable capacitance unit 1 is switched to one of the selected one of the capacitance values in accordance with the voltage level of the input pulse.

  Referring to FIG. 1 again, at least the variable capacitance unit 1 excluding the capacitor, the control unit 2, the storage unit 3, and the inverting amplifiers A1 to A3 can be integrated in a semiconductor integrated circuit. Furthermore, one or more of the resistor Rf, the capacitors Cg and Cd, and the capacitor of the variable capacitance unit 1 may be integrated in the semiconductor integrated circuit. Depending on the design conditions, the inductor Ls may also be integrated in the semiconductor integrated circuit. It is possible to integrate them into

  In this embodiment, a MOS transistor is used as the amplifying element. However, as shown in FIG. 5, a bipolar transistor may be used as the amplifying element. In that case, in FIG. 5, the variable capacitance diode Cv is replaced with the variable capacitance section 1 (FIG. 2), and the control section 2 and the storage section 3 shown in FIG. 1 are added. Alternatively, the voltage applied to the variable capacitance diode Cv may be changed in a plurality of steps (for example, six steps) using the variable capacitance diode Cv as it is.

The variable capacitance unit 1 may be configured to output an arbitrary capacitance value in increments of 0.1 pF, for example, so that the selected capacitance value can be set separately from the outside. According to such a configuration, the versatility of the manufactured FSK modulation circuit can be enhanced.
Furthermore, in the present embodiment, the binary FSK modulation circuit has been described as an example for the sake of simplicity. However, even if the multi-level FSK modulation circuit is expanded by simultaneously adding a plurality of input pulses, It is obvious that it can be applied.

  The present invention can be used in an FSK modulation circuit that FSK modulates a carrier wave in wireless data communication or the like.

The figure which shows the structure of the FSK modulation circuit which concerns on one Embodiment of this invention. FIG. 2 is a circuit diagram illustrating a specific configuration example of a variable capacitance unit and a control unit illustrated in FIG. 1. FIG. 2 is a schematic diagram for explaining an operation principle of the FSK modulation circuit shown in FIG. 1. The figure which shows the absolute value of the impedance of the series resonance circuit containing a piezoelectric vibrator. The circuit diagram of the conventional FSK modulation circuit using a bipolar transistor. The circuit diagram of the conventional piezoelectric oscillation circuit using a MOS transistor.

Explanation of symbols

A1 to A3 inverting amplifier, P1 to P3 P channel MOS transistor, N1 to N3 N channel MOS transistor, Cg and Cd capacitor, Rf resistor, X piezoelectric vibrator, Ls inductor, 1 variable capacitance unit, 2 control unit, 3 storage Part, Cs1-Cs6 capacitor, Tg1-Tg6 transmission gate, Iv1-Iv7 inverter, L1-L6 AND circuit

Claims (6)

An oscillation circuit that performs an oscillation operation by driving a vibrator;
Variable capacitance means connected in series with the vibrator and capable of controlling a capacitance value;
Storage means for storing information relating to the series resonance frequency of the vibrator;
Based on the information stored in the storage means, a set of capacitance values is selected from among a plurality of capacitance values that can be taken by the variable capacitance means, and the variable capacitance means is selected according to an input pulse. Control means for shifting the oscillation frequency of the oscillation circuit by switching the capacitance value of the oscillation circuit to any one of the selected one of the capacitance values;
FSK modulation circuit comprising:   The FSK modulation circuit according to claim 1, wherein the vibrator includes a piezoelectric vibrator.   3. The FSK modulation circuit according to claim 1, wherein the variable capacitance means includes a plurality of capacitors having different capacitance values and a switch circuit connected to each of the plurality of capacitors in series.   The FSK modulation circuit according to claim 1, wherein the variable capacitance means includes a variable capacitance diode.   5. The FSK modulation circuit according to claim 1, wherein the storage unit stores in advance information relating to a manufacturing error of a series resonance frequency of the vibrator. An oscillation circuit that performs an oscillation operation by driving a vibrator in which variable capacitance means capable of controlling a capacitance value is connected in series;
Storage means for storing information relating to the series resonance frequency of the vibrator;
Based on the information stored in the storage means, a set of capacitance values is selected from among a plurality of capacitance values that can be taken by the variable capacitance means, and the variable capacitance means is selected according to an input pulse. Control means for shifting the oscillation frequency of the oscillation circuit by switching the capacitance value of the oscillation circuit to any one of the selected one of the capacitance values;
A semiconductor integrated circuit comprising:

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