JP2015056728A - Oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oscillator that can precisely control an output frequency in response to a control voltage applied to a control voltage input end.SOLUTION: The oscillator includes: the control voltage input end and a ground end between which a control voltage for frequency regulation, which is a DC voltage, is supplied; an oscillation circuit that is connected between the input end and the ground end, and outputs a frequency regulated on the basis of the control voltage; a grounding line that connects the oscillation circuit and the ground end; and a voltage stabilization circuit that includes an operational amplifier disposed between the input end and the oscillation circuit, and suppresses a fluctuation in the control voltage due to a voltage drop of the grounding line. The voltage stabilization circuit is configured to add, to a voltage between the input end and a first position on the grounding line near the oscillation circuit, a voltage between the first position and a second position on the grounding line near the ground end.

Description

  The present invention relates to an oscillator to which a control voltage for frequency adjustment which is a DC voltage is supplied.

  The oscillator has a control terminal to which an analog DC control voltage is applied from the outside, and is configured to have an oscillation circuit having an EFC (Electronic Frequency Control) function in which the oscillation output frequency is controlled by the control voltage. It is known that

  FIG. 3 shows a block diagram of a crystal oscillator (OCXO: Oven Controlled Crystal Oscillator) 10 with a thermostatic bath. In the figure, reference numeral 3 denotes an oscillation circuit configured to perform the EFC. The oscillation circuit 3 includes a crystal resonator as a piezoelectric resonator constituting the oscillation unit. In the figure, 12 is a heater circuit. The oscillation circuit 3 and the heater circuit 12 are provided on the substrate. When the ambient temperature of the crystal resonator changes, the current supplied to the heater circuit 12 changes, and the ambient temperature of the crystal resonator is controlled to be constant. In the figure, reference numeral 13 denotes the control terminal (control voltage input terminal).

  For the purpose of reducing the manufacturing cost of the OCXO 10 and preventing the OCXO 10 from becoming large, the number of terminals provided for connecting the OCXO 10 to an external device is limited. In the figure, reference numeral 15 denotes one of the terminals, which is a ground terminal (ground end) connected to the ground 16 outside the OCXO 10. In the figure, 17 is a ground line, and 18 is an internal ground of the OCXO 10. The ground line 17 and the internal ground 18 are constituted by patterns provided on the substrate. For the above reason, the ground terminal 15 is provided in common for the heater circuit 12 and the oscillation circuit 3. That is, the oscillation circuit 3 and the ground terminal 15 are connected to each other by the ground line 17, and the heater circuit 12 is also connected to the ground terminal 15 through the ground line 17.

  Incidentally, in recent years, an oscillator may be required to be able to vary the oscillation frequency on the order of ± several ppm. When the oscillator is configured as the OCXO 10 as described above, the frequency temperature characteristic may be required to have a performance capable of outputting a frequency with an error of several ppb or less. However, a minute fluctuation occurs in the voltage level of the internal ground 18 due to the DC resistance of the ground line 17. As a result, the control voltage Vc input to the apparent oscillation circuit 3 varies. That is, the voltage drop of the ground line 17 causes the control voltage supplied to the oscillation circuit 3 to fluctuate, resulting in deterioration of the frequency temperature characteristics of the oscillator.

  This voltage drop will be described in more detail. Since the ground return current Ig flows from the oscillation circuit 3 and the heater circuit 12 to the direct current resistance Rg of the ground line 17, the voltage level of the internal ground 18 is increased by Ig × Rg = Vg. The control voltage Vc input to the circuit 3 apparently drops by Vg.

  The current Ig varies depending on the current supplied to the heater circuit 12. When the ambient temperature of the crystal unit is low, the current supplied to the heater circuit 12 increases and the current Ig also increases. If it becomes so, the said Vg will also become large. That is, the control voltage Vc input to the oscillation circuit 3 varies depending on the ambient temperature of the crystal resonator. As a result, the oscillation frequency of the oscillator deviates from the desired oscillation frequency.

  More specifically, when the variable range of the control voltage Vc is 0 to 3.3 V (center is 1.65 V) and the change in the output frequency of the OCXO 10 is ± 5 ppm, the control voltage Vc is When changing by 330 μV, the output frequency changes by 1 ppb. In the OCXO 10, if the DC resistance Rg is 10 mΩ and the current Ig changes from 150 mA to 500 mA by the operation of the heater circuit 12, the Vg changes from 1.5 mV to 5 mV. Thus, by changing Vg by 5 mV-1.5 mV = 3.5 mV, the output frequency varies by about 10.6 ppb even if the control voltage Vc applied to the control terminal 13 is constant.

  Under such circumstances, it has been difficult to construct an OCXO having frequency temperature characteristics capable of outputting a frequency with an error of several ppb or less as described above. Further, when the oscillator is configured not to include the heater circuit 12, a desired voltage cannot be accurately supplied to the oscillation circuit 3 by the DC resistance Rg, so that the oscillation frequency of the oscillator deviates from a desired value. There's a problem. Patent Documents 1 and 2 do not describe a technique that can solve such a problem. Patent Document 3 describes an oscillator configured to digitally add a digital temperature compensation value based on temperature information from a temperature sensor to a frequency control input digitally converted by an A / D converter for control. However, there is a problem that the configuration becomes complicated.

JP 7-263957 A JP 2012-156946 A US2006 / 0119437 A1

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an oscillator capable of controlling an output frequency with high accuracy according to a control voltage applied to a control voltage input terminal. is there.

The oscillator of the present invention has a control voltage input terminal and a ground terminal to which a control voltage for frequency adjustment, which is a DC voltage, is supplied,
An oscillating unit connected between the input end and the ground end, the output frequency of which is adjusted based on the control voltage;
A ground line connecting the oscillation unit and the ground end;
Including an operational amplifier provided between the input end and the oscillating unit, and a voltage stabilizing circuit for suppressing fluctuations in the control voltage due to a voltage drop in the ground line,
The voltage stabilizing circuit is in the vicinity of the ground end of the first position and the ground line to a voltage between the input terminal and a first position that is in the vicinity of the oscillation unit of the ground line. The input terminal, the first position, and the second position are connected to the operational amplifier so as to add a voltage between the second position and the second position.

  In the oscillator according to the present invention, a voltage stabilization circuit is provided between the control voltage input terminal and the oscillation circuit to suppress the fluctuation of the control voltage due to the voltage drop of the ground line. Thereby, the output frequency of the oscillator can be controlled with high accuracy.

It is a block diagram of OCXO which is embodiment of the crystal oscillator of this invention. It is the schematic of the frequency signal output circuit of the said OCXO. It is a block diagram of conventional OCXO.

  The OCXO 20 according to the embodiment of the present invention will be described with reference to FIG. In each part of the OCXO 20, parts that are configured in the same manner as the parts of the OCXO 10 described in the section of the background art are denoted by the same reference numerals as those of the parts of the OCXO 10, and description thereof is omitted. In the figure, reference numeral 21 denotes a power supply terminal of the OCXO 20 to which a power supply voltage Vcc is applied.

  FIG. 2 shows an example of a detailed configuration of the oscillation circuit 3. The oscillation circuit 3 includes a first crystal oscillator 31 and a second crystal oscillator 32, and a first oscillation circuit 31A and a second oscillation circuit 32A that oscillate these crystal oscillators. A frequency difference detection unit 33, a correction value calculation unit 34, an addition unit 35, a PLL circuit unit 36, a low-pass filter (LPF) 37, and a crystal voltage control are provided on the rear side of the first oscillation circuit 31A and the second oscillation circuit 32A. An oscillator (VCXO) 38 is connected. The oscillation circuit 3 includes an analog-digital converter (ADC) 39, and a control voltage Vc is supplied to the ADC 39 from the outside of the oscillation circuit 3.

  The PLL circuit unit 36 uses the oscillation output from the first oscillation circuit 31A as a clock signal, and corresponds to a phase difference between a pulse signal generated based on a frequency setting signal that is a digital value and a feedback pulse from the VCXO 38. The signal is converted to analog, and the analog signal is integrated and output to the low-pass filter 37. The output of the VCXO 38 is the oscillation output of the OCXO 20.

  The value corresponding to the frequency difference ΔF between the oscillation output f1 from the first oscillation circuit 31A and the oscillation output f2 from the second oscillation circuit 32A corresponds to the temperature of the atmosphere in which the crystal units 31 and 32 are placed. And it can be called a temperature detection value. For convenience of explanation, it is assumed that f1 and f2 also represent the oscillation frequencies of the first oscillation circuit 31A and the second oscillation circuit 32A, respectively. In this example, the frequency difference detection unit 33 extracts a value of {(f2-f1) / f1}-{(f2r-f1r) / f1r}, and this value is a temperature detection in which the value is proportional to the temperature. Corresponds to the value. f1r and f2r are the oscillation frequency of the first oscillation circuit 31A and the oscillation frequency of the second oscillation circuit 32A at a reference temperature, for example, 25 ° C., respectively.

  The correction value calculator 34 calculates a frequency correction value based on the relationship between the temperature detection value and a frequency correction value created in advance. The frequency correction value is a value for compensating for the fluctuation, that is, the temperature fluctuation of the clock signal when the temperature of the first crystal unit 31 fluctuates from the target temperature. The adder 35 receives the frequency correction value and the control voltage Vc converted from an analog value to a digital value by the ADC 39 and adds them together to set a frequency setting signal. The frequency setting signal is input from the adding unit 35 to the PLL circuit unit 36. By changing the control voltage Vc, the frequency setting signal output from the adding unit 35 to the PLL circuit unit 36 changes. As a result, the oscillation output frequency of the OCXO 20 changes.

  More specifically, the PLL circuit unit 36 includes a DDS (Direct Digital Synthesizer) circuit unit, a frequency divider, and a phase comparison unit. A reference clock is formed based on the sawtooth wave output from the DDS circuit unit, and the phase of the output signal obtained by dividing the output of the VCXO 38 and the reference clock is compared by the phase comparison unit. Is output. The output of the VCXO 38 is controlled by the output from the LPF 37. The DDS circuit unit uses a frequency signal output from the first oscillation circuit 31A as a reference clock, and receives a control voltage for outputting a sawtooth wave of a target frequency. However, since the frequency of the reference clock has a temperature characteristic, the control voltage input to the DDS circuit unit is added to the signal corresponding to the frequency correction value from the addition unit 35 in order to cancel the temperature characteristic. Has been. As described above, the OCXO 20 is also configured as a TCXO, and the dual temperature correspondence is performed by the action of the heater circuit 12 and the frequency correction by the correction value calculation unit 34, so that the output can be stabilized with high accuracy. It is configured as a device that can.

  The relationship between the temperature detection value and the frequency correction value is stored in a memory (not shown). For example, when (f2-f2r) / f2r = OSC2 and (f1-f1r) / f1r = OSC1, the relationship between (OSC2-OSC1) and temperature is obtained by actual measurement at the time of production of the crystal unit. A correction frequency curve that cancels the frequency variation with respect to the temperature is derived, and a ninth-order polynomial approximate expression coefficient is derived by the least square method. Then, polynomial approximation formula coefficients are stored in advance in the memory, and the correction value calculation unit 34 performs correction value calculation processing using these polynomial approximation formula coefficients.

  Furthermore, the temperature detection value is output from the frequency difference detection unit 33 to the heater control circuit 19, and based on this temperature detection value, the heater control circuit 19 is in the thermostatic chamber in which the crystal resonators 31 and 32 and the heater circuit 12 are placed. Power is supplied to the heater circuit 12 so that the atmosphere is maintained at a set temperature, and the heater circuit 12 generates heat in accordance with the supplied power. The greater the electric power supplied to the heater circuit 12, the greater the amount of heat generated by the heater circuit 12, and the greater the current flowing from the heater circuit 12 to the ground line 17 described above.

  Returning to FIG. 1, the description will be continued. A voltage stabilization circuit 4 is provided between the control terminal 13 and the oscillation circuit 3. The voltage stabilization circuit 4 suppresses fluctuations in the control voltage Vc input to the oscillation circuit 3 due to the voltage drop of the ground line 17 described above. The voltage stabilizing circuit 4 includes an inverting circuit 41 provided on the front side (control terminal 13 side) and an adder circuit 42 provided on the rear side (oscillation circuit 3 side). Lines 51 and 52 branched from the ground line 17 are connected to the voltage stabilization circuit 4. The line 51 connects the point P in the vicinity of the ground terminal 15 in the ground line 17 and the adder circuit 42. One end of the line 52 is connected to the point Q near the oscillation circuit 3 in the ground line 17, and the other end is branched and connected to the inverting circuit 41 and the adding circuit 42. That is, when viewed from the ground terminal 15, the point P is set at a position closer to the point Q. The heater circuit 12 is connected between these points PQ.

  The inverting circuit 41 includes a resistor Ra1, a resistor Ra2, and a first operational amplifier (operational amplifier) OP1. The resistor Ra1 is provided between the control terminal 13 and the negative input terminal of the operational amplifier OP1. The resistor Ra2 is a feedback resistor provided in a feedback path that connects the output terminal of the operational amplifier OP1 and the resistor Ra1 and the negative input terminal. The line 52 is connected to the + input terminal of the operational amplifier OP1. The resistors Ra1 and Ra2 of the inverting circuit 41 have the same resistance value. Thereby, the voltage output from the output terminal of the inverting circuit 41 becomes − (Vc−Vg).

  The adder circuit 42 includes resistors Rb1, Rb2, Rb3 and a second operational amplifier OP2. The line 52 is connected to the + input terminal of the operational amplifier OP2. The line 51 is connected to the negative input terminal of the operational amplifier OP2, and the line 51 is provided with a resistor Rb. The output terminal of the operational amplifier OP1 of the inverting circuit 41 is connected to the negative input terminal of the operational amplifier OP2 via a resistor Rb. A resistor Rb3 is provided in a feedback path connecting the output terminal of the operational amplifier OP2 and the negative input terminal of the operational amplifier OP2. The resistors Rb1, Rb2, and Rb3 are provided in parallel with each other at the − side input end. The output terminal of the operational amplifier OP2 is connected to the ADC 39 of the oscillation circuit 3 described above.

  From the line 51, a voltage (-Vg) obtained by subtracting the level variation Vg of the internal ground 18 due to the direct current resistance Rg of the ground line 17 is input to the resistor Rb1. The resistor Rb2 receives-(Vc-Vg), which is the output voltage of the inverting circuit 41. The adder circuit 42 adds these two input voltages, inverts the polarity, and outputs the result. The resistors Rb1, Rb2, and Rb3 have the same resistance value, and thereby − {− (Vc−Vg)} + Vg} = Vc is output from the adder circuit 42. That is, the voltage (−Vg) is canceled and only Vc remains and is output to the oscillation circuit 3.

  In this way, the control terminal 13, so that the voltage between the point Q and the point P being the second position is added to the voltage between the control terminal 13 and the point Q being the first position. Points P and Q are connected to the operational amplifiers OP 1 and OP 2 of the voltage stabilization circuit 4. That is, the voltage stabilizing circuit 4 has a role of preventing the influence of the fluctuation of the control voltage Vc due to the voltage drop between the points PQ of the ground line 17.

  When the ambient temperature of the crystal units 31 and 32 is relatively low, in order to raise the ambient temperature to the set temperature, the current (ground return current) flowing from the heater circuit 12 through the ground line 17 becomes large, and the ground line The voltage Vg of the ground line 17 due to the resistance component 17 increases. However, as described above, the voltage stabilization circuit 4 applies the control voltage Vc to the oscillation circuit 3 regardless of the increase in the voltage Vg, so that fluctuations in the oscillation output frequency of the OCXO 20 can be suppressed and applied to the control terminal 13. An oscillation frequency corresponding to the control voltage Vc is output from an output terminal (not shown).

  When the ambient temperature of the crystal resonators 31 and 32 is relatively high, in order to lower the ambient temperature to the set temperature, the current flowing from the heater circuit 12 through the ground line 17 becomes small, and the resistance component of the ground line 17 The voltage Vg of the ground line 17 is reduced. Also in this case, since the control voltage Vc is applied to the oscillation circuit 3 by the voltage stabilization circuit 4 as described above regardless of the increase in the voltage Vg, fluctuations in the oscillation output frequency of the OCXO 20 are suppressed.

  As described above, in the OCXO 20, since the fluctuation of the control voltage Vc supplied to the oscillation circuit 3 is suppressed, it is possible to suppress a decrease in the frequency temperature characteristic thereof. Although an example of the oscillator of the present invention has been described as the OCXO 20, the oscillator of the present invention may be configured as a configuration in which the constant temperature bath and the heater circuit 12 are not provided in the OCXO 20, that is, as a TCXO. In this case as well, fluctuations in the control voltage Vc input to the oscillation circuit 3 due to the influence of the voltage drop in the ground line 17 can be suppressed. Therefore, the oscillation frequency of the oscillator can be controlled with high accuracy.

  The oscillation circuit is not limited to the configuration of the oscillation circuit 3 described above, and may include a crystal resonator that is an oscillation unit and a varicap diode that is a variable capacitance element. As the control voltage Vc changes, the capacitance value of the varicap diode changes. Accordingly, an oscillation circuit having a configuration in which the resonance point of the oscillation circuit changes and the output frequency from the oscillation circuit changes can be used.

13 Control terminal 15 Ground terminal 17 Ground lines 31, 32 Crystal resonator 3 Oscillation circuit 4 Voltage stabilization circuit 41 Inversion circuit 42 Addition circuit

Claims (3)

A control voltage input terminal and a ground terminal supplied with a control voltage for frequency adjustment, which is a DC voltage,
An oscillation circuit connected between the input end and the ground end, the output frequency of which is adjusted based on the control voltage;
A grounding line connecting the oscillation circuit and the ground end;
Including an operational amplifier provided between the input terminal and the oscillation circuit, and a voltage stabilization circuit for suppressing fluctuations in control voltage due to a voltage drop in the ground line,
The voltage stabilization circuit is in the vicinity of the ground end of the first position and the ground line to a voltage between the input end and a first position of the ground line near the oscillation circuit. An oscillator characterized in that the input terminal, the first position, and the second position are connected to the operational amplifier so as to add a voltage between the second position and the second position. A heater circuit for making the temperature of the piezoelectric vibrator that is an oscillation part of the oscillation circuit constant,
The oscillator according to claim 1, wherein the heater circuit is connected to the ground end via the ground line.   The voltage stabilization circuit includes an inverting circuit including a first operational amplifier in which the control voltage input terminal is connected to a negative input terminal and the first position is connected to a positive input terminal; An adder circuit including a second operational amplifier in which an output terminal of the inverting circuit and the second position are connected to a negative input terminal, and the first position is connected to a positive input terminal; The oscillator according to claim 1, further comprising an oscillator.

Images