Digital compensation crystal oscillator

Abstract

The invention discloses a digital compensation crystal oscillator, and relates to the technical field of crystal oscillators. The bimorph oscillator comprises a bimorph resonator, an oscillation circuit, a frequency offset compensation circuit and a frequency output module. The bimorph resonator comprises an ACcut crystal resonator and an ATcut crystal resonator; the oscillation circuit comprises an AC oscillation circuit, an AT oscillation circuit and a frequency tuning circuit; wherein the AC oscillation circuit is connected with the ACcut crystal resonator and outputs frequency to the frequency offset compensation circuit; the ATcut crystal resonator, the frequency tuning circuit and the AT oscillating circuit are connected in sequence and output frequency to the frequency output module; the frequency offset compensation circuit receives a frequency value transmitted by the AC oscillation circuit, converts the frequency value into a frequency offset compensation value of the AT oscillation circuit, and outputs the frequency offset compensation value to the control end of the frequency tuning circuit. According to the invention, all-digital high-precision temperature measurement is realized, and the crystal oscillator is used for realizing high-stability crystal oscillator products.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of crystal oscillator, in particular to a digital compensation crystal oscillator. BACKGROUND

[0002] As a cost-effective frequency source, crystal oscillator (hereinafter referred to as crystal) is widely used in various communication equipment, instruments and meters, navigation positioning equipment, etc. The temperature compensated crystal oscillator is a high performance series of crystal oscillator, which uses a built-in temperature sensor to measure the change of ambient temperature in real time, and then adjusts the frequency temperature characteristic of the AT_cut crystal resonator to within ±500~1000ppb through a compensation circuit, so as to ensure a high frequency stability in the full temperature range.

[0003] The temperature data of the traditional temperature compensated crystal oscillator is generally measured by a temperature sensor on a CMOS integrated circuit or a thermistor. Due to the thermal hysteresis of the temperature sensing element and the AT_cut wafer to be measured, and the analog signal noise, the accuracy is inevitably reduced. It is generally believed that the accuracy of the CMOS integrated circuit temperature sensor can only reach 1E-2K, so the typical value of the temperature stability of the traditional temperature compensated crystal oscillator after compensation is only about ±250ppb.

[0004] The patent with application number 202510192263.5 discloses a high-temperature-resistant thermistor crystal resonator without sealing, and the thermistor is placed in the crystal resonator cavity to provide an accurate real-time temperature signal for stable operation in a high-temperature environment. Although the temperature sensing element of the product is as close as possible to the AT_cut wafer to be measured, the thermistor still uses an analog temperature sensing method, which cannot avoid the temperature measurement accuracy problem caused by electrical noise. SUMMARY

[0005] Therefore, the present application provides a digital compensation crystal oscillator. The present application realizes full-digital high-precision temperature measurement, and is used for realizing a high-stability crystal oscillator product.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0007] A digital compensation crystal oscillator, comprising a double wafer resonator, an oscillation circuit, a frequency offset compensation circuit and a frequency output module;

[0008] The double wafer resonator comprises an AC_cut crystal resonator and an AT_cut crystal resonator;

[0009] The oscillation circuit comprises an AC oscillation circuit, an AT oscillation circuit and a frequency tuning circuit;

[0010] The AC oscillation circuit is connected with the AC_cut crystal resonator, and outputs a frequency to the frequency offset compensation circuit; the AT_cut crystal resonator, the frequency tuning circuit and the AT oscillation circuit are sequentially connected, and output a frequency to the frequency output module;

[0011] The frequency offset compensation circuit receives the frequency value transmitted by the AC oscillation circuit, converts it into a frequency offset compensation value of the AT oscillation circuit, and outputs it to the control end of the frequency tuning circuit.

[0012] Further, the double wafer resonator comprises a ceramic cavity 4 and a metal cover plate 1 above the ceramic cavity 4, and the metal cover plate 1 is welded to the ceramic cavity 4 by resistance welding;

[0013] The ceramic cavity 4 is provided with two layers of stepped ceramic structures 7, an AC wafer 2 and an AT wafer 3; wherein, the two layers of stepped ceramic structures 7 are respectively provided with an AC dispensing point 5 and an AT dispensing point 6; the AC wafer 2 and the AT wafer 3 are fixed on the AC dispensing point 5 and the AT dispensing point 6 by conductive glue respectively; in addition, the ceramic cavity 4 and the two layers of stepped ceramic structures 7 are provided with a pre-set circuit, and the AC wafer 2 and the AT wafer 3 form an AC_cut crystal resonator and an AT_cut crystal resonator respectively, and are connected with the outside.

[0014] Further, the frequency offset compensation circuit comprises a frequency measurement unit, a digital filter unit and a DAC conversion unit;

[0015] The digitally compensated crystal oscillator passes through a pre-adjustment stage:

[0016] The digitally compensated crystal oscillator is placed in a temperature chamber, a plurality of groups of temperature data are set for testing, for each group of temperature data , the frequency value output by the AC oscillation circuit at this time is measured , and finally the linear relationship between the frequency of the AC_cut crystal and the temperature is fitted: , is the corresponding linear coefficient;

[0017] For each group of temperature data , the frequency value preset by the AT oscillation circuit at this time is measured , and the actual output frequency value is measured, and the frequency offset value is further calculated; finally, the Nth function relationship between the frequency offset value of the AT_cut crystal and the temperature is fitted:

[0018]

[0019] wherein, is the coefficient corresponding to the Nth power term; ​

[0020] After the debugging stage, the frequency and temperature linear relationship of the AC_cut crystal and the Nth function relationship between the frequency deviation value of the AT_cut crystal and the temperature are saved to the frequency measurement unit. In actual work, the frequency measurement unit receives the frequency value transmitted by the AC oscillation circuit , calculates the corresponding frequency deviation value of the AT_cut crystal , and inputs the frequency deviation value to the digital filter unit. After digital filtering by the digital filter unit, the digital-to-analog conversion is performed by the DAC conversion unit and output to the control end of the frequency tuning circuit.

[0021] Compared with the prior art, the beneficial effects of the present application are as follows:

[0022] 1. The present application encapsulates the temperature sensing core AC_cut wafer and the main vibration core AT_cut wafer into a double-wafer resonator in a cavity. The temperature sensing device AC_cut wafer and the At_cut wafer as the main vibration unit are almost at the same point in the thermal structure, which greatly reduces the temperature measurement error introduced by the hysteresis problem.

[0023] 2. The present application realizes full-digital temperature measurement technology by measuring the frequency of the AC_cut wafer. Compared with the traditional digital-analog hybrid temperature measurement technology, the precision is improved from 1E-2K to 5E-4K.

[0024] 3. On the basis of accurate temperature sensing and accurate temperature measurement, the present application uses digital frequency modulation to realize the temperature characteristic compensation of the AT_cut crystal oscillator, thereby realizing a temperature compensation crystal oscillator product with ±50ppb. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a schematic diagram of the overall structure of a digital compensation crystal oscillator in an embodiment of the present application.

[0026] Figure 2 It is a schematic diagram of the overall structure of a digital compensation crystal oscillator in an embodiment of the present application. Figure 1

[0027] It is a schematic diagram of the overall structure of a digital compensation crystal oscillator in an embodiment of the present application. Figure 3

[0028] It is a schematic diagram of the overall structure of a digital compensation crystal oscillator in an embodiment of the present application. Figure 4 DETAILED DESCRIPTION

[0029] The content of the present application will be further described below in conjunction with the drawings and specific embodiments.

[0030] A digital compensation crystal oscillator, such as Figure 1 ​As shown, it comprises a double wafer resonator, an oscillation circuit, a frequency offset compensation circuit and a frequency output module.

[0031] (1) A double wafer resonator is used as a main vibration element, and an AT_cut wafer and an AC_cut wafer are assembled in a resonant cavity. The conductive glue and the wafer body for fixing the two wafers need to be closely close to reduce thermal hysteresis and temperature error. As shown in Figure 2 The AC&AT double wafer resonator comprises a ceramic base, a metal cover plate, an AC wafer, an AT wafer, a step, an AC wafer dispensing point and an AT wafer dispensing point. The ceramic base 4 has a step 7; the step 7 has an AC dispensing point 5 and an AT dispensing point 6; the AT wafer 3 is fixed on the AT wafer dispensing point 6 by conductive glue; the AC wafer 2 is fixed on the AC wafer dispensing point 5 by conductive glue; and the metal cover plate 1 is resistance-welded to the ceramic base 4.

[0032] (2) The resonator is connected to the respective oscillation circuit, and is debugged as follows: the AC oscillation circuit and the AC_cut crystal resonator can output a predetermined frequency signal; the AT oscillation circuit and the AT_cut crystal resonator can output a predetermined frequency signal; the DAC output of the MCU is pulled to 0~Vcc voltage, and the frequency tuning range of the AT oscillation circuit is about 50ppm. As shown in Figure 1 The oscillation circuit comprises an AC oscillation circuit, an AT oscillation circuit and a frequency tuning circuit. One end of the AC oscillation circuit is connected to the AC_cut crystal resonator, and the other end is connected to the frequency test part of the MCU circuit; one end of the AT oscillation circuit is connected to the frequency tuning circuit, and the other end is connected to the frequency test part and the frequency output module of the MCU circuit; the frequency tuning circuit is connected to the AT_cut crystal resonator, and the control end is connected to the DAC part of the MCU circuit.

[0033] In this embodiment, the frequency output module is a single-channel inverter, which plays a role of isolation between the oscillation circuit and the subsequent load;

[0034] (3) The frequency of the AC oscillation circuit is measured by using a digital frequency measurement technology, and the temperature data is calculated according to the frequency-temperature model of the AC wafer As shown in Figure 1 The frequency measurement circuit can use but is not limited to the MCU circuit. In this embodiment, the frequency of the AC_cut crystal changes linearly with temperature, and the slope is +22ppm / ℃, which can be used to represent the change of temperature. Through the frequency test module in the MCU circuit, the frequency measurement accuracy can reach about 0.01ppm after digital filtering, so that high-precision temperature measurement of about 5E-4K can be realized.

[0035] As shown in Figure 4 The frequency offset value of the AT_cut crystal has a multiple function relationship with temperature:

[0036] ,

[0037] Wherein, in this embodiment, N is 7, the variation P-P is about 40ppm in the range of -40~85℃, and the tuning ability of 50ppm for AT oscillator is already possessed by the operation circuit in 2). The AT oscillator is taken as input, and the frequency-temperature curve of AT oscillator is pre-stored in the memory of the operation circuit. The operation circuit is taken as input, and the frequency-temperature curve of AT oscillator is pre-stored in the memory of the operation circuit. The output frequency of the temperature-compensated crystal oscillator is tuned and compensated, and high-stable frequency output of <±50ppb in the range of -40~85℃ is realized. As shown in the figure, Figure 3 The temperature characteristic of the temperature-compensated crystal oscillator after compensation can be improved to within ±50ppb (-40~85℃).

[0038] Those skilled in the art will appreciate that the embodiments described are for the purpose of helping the reader understand the principles of the present application, and should be understood as not limiting the scope of protection of the present application. The present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application should be included in the scope of protection of the claims of the present application.

Claims

1. A digitally compensated crystal oscillator, characterized by, The double wafer resonator, the oscillation circuit, the frequency offset compensation circuit and the frequency output module are included. The double wafer resonator includes an AC_cut crystal resonator and an AT_cut crystal resonator. The oscillation circuit includes an AC oscillation circuit, an AT oscillation circuit and a frequency tuning circuit. The AC oscillation circuit is connected with the AC_cut crystal resonator and outputs a frequency to the frequency offset compensation circuit; the AT_cut crystal resonator, the frequency tuning circuit and the AT oscillation circuit are sequentially connected and output a frequency to the frequency output module. The frequency offset compensation circuit receives the frequency value transmitted by the AC oscillation circuit, converts it into the frequency offset compensation value of the AT oscillation circuit and outputs it to the control end of the frequency tuning circuit.

2. A digitally compensated crystal oscillator according to claim 1, wherein, The double wafer resonator includes a ceramic cavity (4) and a metal cover plate (1) above the ceramic cavity (4), and the metal cover plate (1) is welded to the ceramic cavity (4) by resistance welding. The ceramic cavity (4) is provided with two layers of stepped ceramic structures (7), an AC wafer (2) and an AT wafer (3); wherein the two layers of stepped ceramic structures (7) are respectively provided with an AC dispensing point (5) and an AT dispensing point (6); the AC wafer (2) and the AT wafer (3) are fixed on the AC dispensing point (5) and the AT dispensing point by conductive glue respectively; in addition, the ceramic cavity (4) and the two layers of stepped ceramic structures (7) are provided with preset circuits, which form the AC_cut crystal resonator and the AT_cut crystal resonator with the AC wafer (2) and the AT wafer (3) respectively, and are connected with the outside.

3. A digitally compensated crystal oscillator according to claim 2, wherein, The frequency offset compensation circuit includes a frequency measurement unit, a digital filter unit and a DAC conversion unit. The digitally compensated crystal oscillator passes through a pre-adjustment stage: The digital compensated crystal oscillator is placed in a temperature chamber, multiple sets of temperature data are set for testing, for each set of temperature data , the frequency value of the AC oscillation circuit output at this time is measured , and finally the linear relationship between the frequency and the temperature of the AC_cut crystal is fitted: , is the corresponding linear coefficient; For each set of temperature data , the frequency value of the preset output of the AT oscillation circuit at this time is measured , and the frequency value of the actual output is measured , and the frequency deviation value is further calculated ; finally, the Nth function relationship between the frequency deviation value of the AT_cut crystal and the temperature is fitted. ； wherein is the coefficient corresponding to the power term After the debugging stage, the frequency and temperature linear relationship of the AC_cut crystal and the Nth function relationship between the frequency deviation value of the AT_cut crystal and the temperature are saved to the frequency measurement unit In actual work, the frequency measurement unit receives the frequency value transmitted by the AC oscillation circuit , calculates the corresponding frequency deviation value of the AT_cut crystal, and inputs the frequency deviation value to the digital filter unit; after digital filtering by the digital filter unit, the digital-to-analog conversion is performed by the DAC conversion unit and output to the control end of the frequency tuning circuit.

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