JP2001257531A - Crystal oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crystal oscillator which operates with the high stability of a frequency even just after the supply of power and is excellent in phase noise characteristic in a floor area. SOLUTION: With the output of OCXO2 as reference, a PLL control system detects a phase difference with the output of VCXO1 by means of a phase comparator 3, fetches its DC component by means of LPF5 and inputs it to VCXO7 as controlled voltage via a comparison switching unit 6. On starting a power source, the VCXO controlled voltage at the time of PLL control stored in a memory part 8 applied to the VCXO7 via the unit 6. With the outside of this VCXO7 as that of this oscillator, after the output frequency of the OCXO 2 is stabilized, the unit 6 is switched so as to constitute a PLL loop and the precision of the VCXO7 is pulled into the frequency precision of the OCXO2 to fetch the output of this VCXO7 as that of this oscillator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly stable crystal oscillator, and more particularly, to obtain frequency stability and phase noise characteristics by obtaining an output from a voltage controlled crystal oscillation circuit phase-locked to the output of a crystal oscillation circuit with a thermostat. A crystal oscillator that can be improved.

[0002]

2. Description of the Related Art Conventionally, a crystal oscillator having excellent frequency stability has been widely used as a frequency reference for mobile radio communication, a time reference for a high-speed transmission line, and the like. There are several factors that degrade the frequency stability of the crystal oscillator, but one of the main causes is a change in the resonance frequency due to the temperature of the crystal unit of the oscillation source. The frequency characteristics of the crystal unit vary greatly depending on the cut-out angle from the crystal material. For example, even in the case of an AT cut with a small frequency change, the frequency characteristics are approximately ± 20 to +/− 20 ° C. ± 30pp
A change of about m is inevitable. On the other hand, for example, a mobile radio mobile phone terminal requires a frequency stability of about ± 2 to ± 5 ppm, so that the oscillation frequency was controlled in accordance with the temperature change, and the frequency change of the crystal resonator was compensated within the above value. Temperature compensated crystal oscillators (TCXO) are widely used.
However, a crystal oscillator serving as a frequency reference for a radio base station or a measuring instrument has higher frequency stability, for example, ± 1 ×
^{10 - 7 ~ ± 5 × 10} - high stability of about ¹⁰ has been requested, can not be realized in such a frequency stability is the temperature compensated crystal oscillator (TCXO). For this reason, the crystal oscillator and the oscillation circuit having the factor of the frequency change with respect to the temperature change are confined in a constant temperature bath and maintained at a constant temperature so as not to be affected by the ambient temperature change. Means for achieving stability are taken, and are generally referred to as "high-stability crystal oscillators with a thermostat".

FIG. 4A is a schematic structural view showing an example of a conventional high-stability crystal oscillator with a thermostatic oven.
It comprises an oscillation circuit 22, an output circuit 23, a thermostat 24, a heater 25, a heater control unit 26, and a temperature detection unit 27. In the figure, when the power is turned on, the quartz oscillator 2
1. The oscillation circuit 22 operates to output a signal of an initial frequency from the output circuit 23, and the heater 25 heats the inside of the thermostat 24. The temperature in the tank is detected by a temperature detecting section 27, and the heater control section 2 based on the detected information.
The operation of the heater 25 is controlled by the control signal from the controller 6. As a result, the tank temperature is maintained at a predetermined temperature, and the output circuit 23 outputs an output signal stable at a predetermined frequency. The temperature of the thermostat of the high-stability crystal oscillator with a thermostat as described above is generally set according to the frequency temperature characteristics of the crystal unit. For example, taking a commonly used AT-cut quartz resonator as an example, a change in frequency of the quartz resonator is represented by a cubic curve with respect to a change in temperature.
Since the inflection point temperature has the least frequency change with respect to temperature change, the temperature of the incubator temperature is the inflection point temperature,
That is, it is general that the temperature is set to a constant value between 70 and 90 ° C.

As is well known, the thickness of a base plate of a crystal unit becomes thin in inverse proportion to the oscillation frequency. For example, in the case of a crystal unit having an oscillation frequency of 100 MHz by AT cut, the thickness becomes extremely thin, several tens of μm. . In addition, crystal oscillators
Residual stresses of various stresses are accumulated in the manufacturing process, and a holder for holding the crystal unit and a package for packing the same are also subjected to the stress. Such various stresses are gradually released when the crystal oscillator is heated in a thermostat and the ambient temperature rises,
The stress is applied to the vibrator, so that the vibrator for high-frequency oscillation, which is extremely thin, is deformed even by a slight stress, and the oscillation frequency fluctuates greatly, thereby deteriorating the frequency rising characteristics and aging characteristics. For this reason, the thickness of the quartz crystal plate cannot be reduced to a certain thickness or less, and the maximum resonance frequency of the vibrator is about 50 MHz, and strict stability is required in the frequency-temperature characteristics. If so, it will be even lower. Therefore, in order to satisfy a strict standard required for an oscillator for a mobile radio communication base station or the like at a high frequency such as 100 MHz, the thickness is thicker and the frequency variation is small even if some stress is generated. Oscillate a resonator with a low resonance frequency,
A method of multiplying the oscillation output up to 100 MHz has been adopted.

[0005]

Generally, the output frequency of a temperature-compensated crystal oscillator (TCXO) used in a system in which the required level of frequency stability is relatively low is as short as several milliseconds to several tens of milliseconds. , And the frequency change during this period is also small, so that the system can start operating shortly after power-on. However,
Used in systems that require high frequency stability, a high-stability crystal oscillator with a constant-temperature bath whose bath temperature is set to the inflection point temperature is a stable oscillation frequency when the temperature of the bath reaches the set temperature after the power is turned on. This requires a certain period of time as shown in FIG. 4B, during which time a large frequency fluctuation occurs due to thermal shock and temperature characteristics of the crystal resonator. This frequency variation reaches several tens of ppm even when an AT-cut crystal resonator is used, and the time required for the frequency to stabilize usually requires a long time of several minutes to several tens of minutes. Therefore, in a system using the above-described crystal oscillator, it is necessary to wait for the operation to start until the output frequency becomes constant. Further, if a means for quickly reaching a predetermined temperature by using a special vibrator having a good frequency rising characteristic is used, there is a problem that the cost increases and the power consumption increases.

As described above, a crystal oscillator that oscillates a crystal resonator having a low resonance frequency and multiplies the oscillation frequency for output has little variation in characteristics such as frequency temperature characteristics and frequency aging. In recent years, it has been known that the phase noise characteristic which is regarded as important in a transmission source of a high frequency clock signal or the like deteriorates according to the equation (1) according to the multiplication order. D = 20log10N (dB) (1) Where, N: Multiplied order, D: Degradation amount of phase noise From the equation (1), for example, when the oscillation output of a crystal resonator having a resonance frequency of 5 MHz is multiplied to 100 MHz, N = 2.
0, the phase noise deterioration amount becomes D = 26 dB, and FIG.
It deteriorates as shown in the example of (c).

It is generally known that the phase noise characteristic of a crystal oscillator is greatly influenced by the Q value of an oscillation loop including a crystal oscillator and the oscillator excitation level. In general, particularly for a crystal oscillator for a high-stability oscillator, it is common to apply a convex process with a large curvature to the surface of the oscillator in order to increase the Q value. It has an exciting current level characteristic that changes greatly in a following curve. Therefore, as is well known, the excitation level of the oscillator in the highly stable crystal oscillator is set to a considerably low excitation level in order to suppress a frequency change due to a level change, and is set in a floor area (a noise frequency area of about 100 kHz or more). Generally, the noise level is higher than that of a normal crystal oscillator. Therefore, by multiplying the oscillation output having such a large noise level, the phase noise level contained in the multiplied signal is further increased. There is a problem that the phase noise characteristic in the floor area is significantly deteriorated. To solve this problem, the use of an SC-cut quartz resonator having a flat excitation current level characteristic over a wide current range, for example, increases the manufacturing cost of the quartz resonator and complicates the oscillator circuit, resulting in an expensive oscillator. There was a problem of becoming. The present invention has been made to solve the above problems,
An object of the present invention is to provide a crystal oscillator that operates with high frequency stability immediately after power-on and has excellent phase noise characteristics in the floor region.

[0008]

In order to solve the above-mentioned problems, according to the first aspect of the present invention, there is provided a crystal oscillation circuit with a thermostatic chamber whose temperature is controlled by a thermostatic chamber and a voltage control type whose oscillation frequency is controlled by a control voltage. A crystal oscillation circuit, wherein the output of the voltage-controlled crystal oscillation circuit is phase-synchronized with the crystal oscillator circuit output with the constant temperature bath by a phase synchronization circuit,
A crystal oscillator that obtains an output from the phase-locked voltage-controlled crystal oscillation circuit, wherein the voltage-controlled crystal oscillation circuit is an oscillation circuit formed of a crystal resonator having a flat resonator excitation level characteristic. It is characterized by.

According to a second aspect of the present invention, there is provided a crystal oscillation circuit with a thermostatic chamber whose temperature is controlled by a thermostat and a voltage-controlled crystal oscillation circuit whose oscillation frequency is controlled by a control voltage. The output of the crystal oscillation circuit is a crystal oscillator that is phase-locked by the phase synchronization circuit to the output of the crystal oscillation circuit with the thermostatic chamber and obtains an output from the voltage-controlled crystal oscillation circuit that is phase-locked and controlled. From the voltage-controlled crystal oscillation circuit to which the stored predetermined control voltage is applied, and when the control voltage detected from the phase-locked loop falls below a predetermined threshold, An output is obtained from the voltage-controlled crystal oscillation circuit that is phase-locked by a circuit.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on an embodiment shown in the drawings. FIG. 1 is a schematic diagram showing an embodiment of a crystal oscillator according to the first aspect of the present invention. As shown in the figure, this oscillator comprises a crystal oscillator circuit (hereinafter, OCXO) 1 with a thermostat, frequency dividers 2 and 4 for converting a high-frequency signal into a phase comparison frequency, and an input signal. A phase comparator 3 that outputs a voltage corresponding to the phase difference or the frequency difference, and a low-pass filter (hereinafter, LP) that removes an AC component superimposed on the signal and outputs a DC component
F) 5 and a voltage-controlled crystal oscillation circuit (hereinafter, VCXO) 6 with a temperature compensation circuit. In the above configuration, in this oscillator, an output which is phase-locked (PLL) -controlled to the oscillation output of the OCXO1 is extracted from the VCXO6 as follows. That is, a general quartz crystal oscillator which has almost no oscillation output of the OCXO1, which is a highly stable crystal oscillator having high frequency accuracy, and has very little excitation current level characteristics, so that even if the excitation level is raised, deterioration of the frequency characteristics is extremely small. The oscillation output of the VCXO 6 constituted by
The signal is converted into a phase comparison frequency at 4 and the phase is compared at the phase comparator 3. Then, the phase difference information based on the oscillation output of the OCXO1 is converted into a voltage, the AC component is removed in the LPF 5, and then input to the VCXO 6 as a control voltage. The output signal of the VCXO 6 is controlled by the control voltage so as to be drawn into the output frequency accuracy of the OCXO 1.

As shown in FIG. 3A, the output signal obtained as described above can obtain an output having a much lower phase noise level than the output of the conventional high-stability oscillation circuit. That is, the phase noise characteristic of the phase-locked oscillator output changes with the cut-off frequency of the low-pass filter used in the phase-locked loop as a boundary, and the noise level in a frequency region lower than this cut-off frequency is the phase-locked oscillator output. A noise level appears. On the other hand, it is known that a noise level in a frequency region higher than the cutoff frequency has characteristics of a synchronized oscillator output. Therefore, the phase noise characteristic of this oscillator is OCXO1 in the low frequency region.
The noise level of the output is equivalent to that of the conventional multiplied output, and the noise level of the VCXO6 is obtained in a high frequency region, that is, the characteristic is significantly improved compared to the conventional multiplied output. Will be.

FIG. 2 is a schematic diagram showing a second embodiment of the crystal oscillator according to the second aspect of the present invention.
As shown in FIG. 1, this oscillator comprises a crystal oscillator circuit with a thermostat (hereinafter, OCXO) 11, frequency dividers 12 and 14 for converting a high-frequency signal into a phase comparison frequency, and an input signal. A phase comparator 13 that outputs a voltage corresponding to a phase difference or a frequency difference, and a low-pass filter (hereinafter, LPF) 1 that removes an AC component superimposed on a signal and outputs a DC component
5, a voltage controlled crystal oscillation circuit (hereinafter, VCXO) 16 with a temperature compensation circuit, a comparison switch 17 including a comparison unit (not shown) and a switching unit (not shown), and a predetermined predetermined voltage. The memory unit 18 stores data, and the voltage generator 19 outputs a voltage of the data stored in the memory unit 18.

Here, the phase locked loop (PLL) control circuit is configured as follows. That is, the oscillation output of the VCXO 16 and the oscillation output of the OCXO 11 are compared by the phase comparator 13 to convert the phase difference information into a voltage.
Input to the VCXO 16 via the
Control is performed so that the output frequency of the XO 16 is drawn to the output frequency accuracy of the OCXO 11. The memory 18 has an OCX
The VCXO control voltage when the PLL operation is performed between the OCXO 11 and the VCXO 16 in a state where the temperature of the thermostatic chamber O11 is stabilized at a predetermined temperature, that is, the output voltage value of the LPF 15 is previously converted to digital and stored. Is what it is. When the power is turned on, the switching unit of the comparison switch 17 is connected to the VCXO 16 to the voltage generator 19, and in this state, the comparison unit of the comparison switch 17 sets a predetermined threshold value and the LPF 15 And the output voltage of the L
When the output voltage of the PF 15 falls below the threshold value, the connection of the switching unit of the comparison switch 17 is switched to the VCXO 16
Is connected to the LPF 15.

When the power of the present oscillator is turned on, the stored predetermined VCXO 16 control voltage value is read from the memory 18 and the control voltage of the value read from the voltage generator 19 is compared with the comparison switch. 17 to the VCXO 16. VCXO16 controlled by this control voltage
Outputs an oscillation frequency compensated to an accuracy of several ppm or less. Until the temperature of the oven of the OCXO 11 stabilizes at a predetermined temperature and the output frequency of the OCXO 11 exceeds the frequency accuracy of the VCXO 16, the VCX
The oscillation output of O16 is output as the output of the present oscillator.

During this time, the OCXO 11 oscillates with large frequency fluctuations. This output and the output of the VCXO 16 are converted into phase comparison frequencies by frequency dividers 12 and 14 and input to the phase comparator 13. The phase comparator 13
Compares the two output signals, outputs an output signal corresponding to the phase difference to the LPF 15, removes an AC component in the signal from the output of the phase comparator 13 by the LPF 15, and outputs the signal to the comparison unit of the comparison switch 17. Output. In the comparison section of the comparison switch 17, a predetermined voltage set in advance based on the output voltage of the LPF 15 and a VCXO 16 control voltage in a state where the PLL control is operated to pull the output frequency of the VCXO 16 to the output frequency accuracy of the OCXO 11 And when the output voltage of the LPF 15 becomes equal to or less than the threshold, the switching connection of the comparison switch 17
Switching is performed so that XO 16 is connected to LPF 15.

By the operation of the switching unit, the OCXO
A PLL control loop for synchronizing the output of the VCXO 16 with reference to 11 is formed. The OCXO 11, frequency dividers 12, 14, phase comparator 13, LPF 15, and VCX
The operation of the PLL control circuit composed of O16 is the same as the operation of the oscillator of FIG. As a result, the VCXO 16 phase-locked to the OCXO 11 output
Is taken out as the output of this oscillator, and FIG.
As shown in (b), a highly accurate oscillation output of ± 1 ppm or less can be obtained immediately after the power is turned on. In the configuration of FIG.
As a matter of course, it is necessary to take measures such as disposing the CXO 16 so as not to be affected by the temperature change of the constant temperature bath of the OCXO 11 and the peripheral devices, and heat insulation.

[0017]

As described above, in the conventional crystal oscillator with a constant temperature oven, a large frequency fluctuation of several tens ppm occurs for several minutes to several tens minutes after the power is turned on, and the phase noise level is a general crystal. According to the present invention, the crystal oscillator has a disadvantage that it is larger than the oscillator, but the cost is low and the frequency change after power-on is improved within ± 1 ppm, or the phase noise level in the floor region is extremely low. High quality crystal oscillator can be provided.
Furthermore, for example, when this oscillator was used as a measuring instrument frequency reference, the measuring instrument could not be used at all until the frequency became stable after the power was turned on. For example, it is possible to greatly increase the speed of startup of a system that uses an oscillator for a frequency reference or a time reference.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of a crystal oscillator according to the first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram showing one embodiment of a crystal oscillator according to the invention of claim 2;

FIG. 3A shows a phase noise characteristic (C / C) of the oscillator of FIG. 1;
FIG. 3B is a characteristic diagram illustrating an example of N characteristic), and FIG. 3B is a diagram illustrating an example of a frequency change after power is turned on in the crystal oscillator of FIG. 2.

FIG. 4 (a) is a schematic diagram illustrating an example of a conventional crystal oscillator with a thermostat, FIG. 4 (b) is a diagram illustrating an example of a frequency change after power-on in the crystal oscillator of FIG. )
Is a phase noise characteristic diagram (C /
FIG. 9 is a diagram showing an example of (N characteristic).

[Explanation of symbols]

1. ・ Crystal oscillator circuit (OCXO) with thermostat,
4. Divider, 3 Phase comparator, 5 Low-pass filter (LPF), 6 Temperature-compensated voltage controlled crystal oscillator (VCXO), 11 Crystal oscillator with thermostat ( OCXO), 12, 14, ... frequency divider, 13
..Phase comparator, 15..Low-pass filter (LPF), 1
6. Temperature-compensated voltage-controlled crystal oscillation circuit (VCX
O), 17..comparison switch, 18..memory part,
19 ··· Voltage generator, 21 ·· Crystal oscillator, 22 ···
Oscillation circuit, 23 ... output circuit, 24 ... constant temperature bath,
25 heater, 26 heater control unit, 2
7 ... Temperature detector

Claims (2)

[Claims] 1. A crystal oscillation circuit with a thermostat controlled by a thermostat and a voltage-controlled crystal oscillation circuit whose oscillation frequency is controlled by a control voltage, wherein the output of the voltage-controlled crystal oscillation circuit is a phase-locked loop. A phase-locked crystal oscillator circuit, and obtains an output from the phase-locked voltage-controlled crystal oscillation circuit, wherein the voltage-controlled crystal oscillation circuit has a vibrator excitation level characteristic. A crystal oscillator, characterized by being an oscillation circuit composed of a flat crystal resonator. 2. A crystal oscillation circuit with an oven controlled by a thermostat and a voltage-controlled crystal oscillation circuit whose oscillation frequency is controlled by a control voltage, wherein the output of the voltage-controlled crystal oscillation circuit is a phase-locked loop. A crystal oscillator which is phase-synchronized with the constant-temperature-controlled crystal oscillator circuit output and obtains an output from the phase-locked voltage-controlled crystal oscillator circuit, immediately after power-on, is supplied with a predetermined control voltage stored in advance. From the applied voltage controlled crystal oscillation circuit, and from when the control voltage detected from the phase locked loop falls below a preset threshold, the phase locked loop controlled by the phase locked loop is used. A crystal oscillator characterized by obtaining an output from a voltage-controlled crystal oscillation circuit.