CN107257240A - A kind of digital temperature compensating method of crystal oscillator - Google Patents

Abstract

The invention discloses a digital temperature compensation method of a crystal oscillator, which adopts a closed-loop feedback compensation framework. First, determine the binary code B _0i corresponding to the target frequency f ₀ and store it in the microprocessor; when the temperature changes, the microprocessor will measure the output frequency of the VCXO in real time to generate a binary code B _1i , and perform a binary code corresponding to the target frequency Compare to get the binary code of the required compensation information; finally, convert it into a compensation voltage through a digital-to-analog converter and input it to the voltage control adjustment terminal of the VCXO, so that it can output the target frequency to realize temperature compensation. Compared with the digital temperature compensation method of the existing crystal oscillator, the present invention does not need a temperature sensor, but directly converts the frequency deviation related to the temperature in real time into a binary code with a one-to-one correspondence relationship with it, and converts it into a corresponding The compensation voltage is used for temperature compensation, which overcomes the temperature hysteresis problem caused by the asynchronous temperature change of the temperature sensor and the crystal resonator in the existing temperature compensated crystal oscillator (TCXO).

Description

A Digital Temperature Compensation Method for Crystal Oscillator

technical field

The invention belongs to the technical field of crystal oscillators, and more specifically, relates to a crystal oscillator.

Background technique

Temperature Compensated Crystal Oscillator (TCXO, Temperature Compensate Xtal (crystal) Oscillator) is a kind of crystal oscillator that can work in a wide temperature range and maintain the output frequency of the crystal oscillator within a certain accuracy range (10 ^-6 ~ 10 ^-7 magnitude) crystal oscillator. It has the characteristics of low power, can work immediately after starting up, and has high stability. It is widely used in various communications, navigation, radar, satellite positioning systems, mobile communications, program-controlled telephone exchanges, and various electronic measuring instruments.

The existing temperature-compensated crystal oscillator is essentially a voltage-controlled crystal oscillator (Voltage Controlled Xtal (crystal) Oscillator, VCXO) with a temperature compensation network that generates a temperature-dependent compensation voltage. The key device in the uncompensated voltage-controlled crystal oscillator is an AT-cut quartz crystal, and its temperature characteristic curve is approximately a cubic curve, which can be expressed as:

f(T)＝a ₃ (TT ₀ ) ³ +a ₁ (TT ₀ )+a ₀ (1)

Among them, a ₃ is the cubic coefficient term, a ₁ is the first-order coefficient term, and a ₀ is the oscillation frequency of the temperature at the reference temperature T ₀ .

For the frequency linear gain characteristic of the existing voltage-controlled crystal oscillator, it can be approximately expressed as follows:

f(VC)＝-G(VC-VC ₀ )+f ₀ (2)

Among them, G is the gain of the voltage-controlled crystal oscillator, VC is the control voltage of the voltage-controlled crystal oscillator, VC ₀ is the input voltage of the voltage-controlled terminal of the voltage-controlled crystal oscillator, and f ₀ is the oscillation frequency when the input is VC ₀ .

Then, the equation of the compensation voltage VC(T) as a compensation crystal oscillator temperature characteristic can be expressed as:

VC(T)＝A ₃ (TT ₀ ) ³ +A ₁ (TT ₀ )+A ₀ (3)

At this time, A ₃ =a ₃ /G, A ₁ =a ₁ /G, A ₀ is the compensation voltage when the temperature is T ₀ .

In order to realize equation (3), it is necessary to generate a temperature compensation voltage and add it to the voltage-controlled crystal oscillator for temperature compensation to offset the frequency temperature characteristics, so as to obtain a stable frequency output in a wide temperature range and achieve the purpose of temperature compensation .

At present, the digital temperature compensation of the temperature compensated crystal oscillator (TCXO) is mainly performed by the microcontroller to collect data from the temperature sensor and output the compensation voltage. At present, there are mainly two methods:

The first is microprocessor-based temperature compensation. Figure 1 is a structure diagram of a temperature-compensated crystal oscillator based on a microprocessor-based temperature compensation method, which is an open-loop temperature compensation framework. As shown in FIG. 1 , it includes a temperature sensor and conditioning circuit 101 , a microprocessor 102 , a compensation network 103 and a voltage-controlled crystal oscillator 104 . The temperature T is collected and conditioned by the temperature sensor and the conditioning circuit 101, and then sent to the microprocessor 102 to look up the temperature-compensation voltage table according to the temperature to obtain the compensation voltage value, and then the compensation network 103 converts the compensation voltage value into compensation The voltage is input to the voltage-controlled crystal oscillator 104 voltage-controlled voltage control terminal, that is, the varactor device. When the compensation voltage changes, the capacitance value of the varactor device changes accordingly, thereby changing the output frequency of the voltage-controlled crystal oscillator to reach the control frequency. Purpose. It can be seen that it directly inputs a temperature-related compensation voltage to the voltage-controlled voltage control terminal of the voltage-controlled crystal oscillator 104 (to be compensated) to achieve the purpose of temperature compensation. Wherein, the temperature-compensated voltmeter is constructed by collecting in advance the voltage that needs to be compensated to maintain the frequency stability of the voltage-controlled crystal oscillator 104 at different temperatures. For a detailed description, please refer to "Liu Haixia, Yang Yu, Zhou Wei. A New Microcomputer Compensated Crystal Oscillator. Journal of Instrumentation. 2002(S3): 135-136."

The second is temperature compensation based on frequency mixing. Figure 2 is a structural diagram of a temperature-compensated crystal oscillator based on frequency-mixing temperature compensation, which is also an open-loop temperature compensation framework. As shown in FIG. 2 , the temperature compensated crystal oscillator generates a compensation frequency signal whose absolute value is equal to the offset frequency generated by the quartz crystal oscillator 204 that needs to be compensated, and whose sign is opposite to that of the offset frequency generated by the temperature sensor 201 and the compensation frequency generating circuit 202 . The signal after the frequency signal has been rectified by the rectifying circuit 203 and the uncompensated frequency signal output by the quartz crystal oscillator 204 are mixed in the mixer 205 and output, and then passed through another filter 205 to obtain the desired compensated frequency signal. After the frequency signal, so as to achieve the purpose of temperature compensation. The compensation frequency signal generation circuit is mainly composed of temperature sensor, ADC, single chip microcomputer and DAC. For a detailed description, please refer to the Chinese invention patent with the announcement number CN 100471035B issued on March 18, 2009: a temperature compensation method for a quartz crystal oscillator. The inventors are Huang Xianhe, Li Minqiang, Fu Wei, and Tan Feng. No./authorization number: CN200410022680.3". This method has advantages in phase noise characteristics when realizing high-frequency temperature-compensated crystal oscillators (TCXOs), but the composition is relatively complicated and has not been widely used yet.

In summary, the existing temperature compensation methods for crystal oscillators all adopt an open-loop compensation framework, and a temperature sensor is used. The temperature sensor is as close as possible to the crystal resonator on the circuit, and the resonator chip of the crystal resonator It is individually packaged in a closed space, which inevitably produces a temperature hysteresis between the temperature sensor and the resonant chip, resulting in a failure to achieve a breakthrough in the frequency-temperature characteristics of the temperature-compensated crystal oscillator, that is, the TCXO. Especially for crystal oscillators whose output signals are high frequency, this temperature hysteresis problem is more serious, and the compensation accuracy is limited.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies in the prior art and provide a digital temperature compensation method for crystal oscillators to avoid the temperature hysteresis effect caused by the temperature sensor, that is, the output caused by the inconsistency between the temperature collected by the sensor and the real-time temperature of the resonant chip. Signal spectrum error problem.

In order to achieve the above-mentioned purpose of the invention, the digital temperature compensation method of the crystal oscillator of the present invention is characterized in that it comprises the following steps:

(1), determine the binary code B _0i corresponding to the target frequency f ₀

At normal temperature T ₀ , such as 25°C, adjust the control voltage of the voltage-controlled crystal oscillator, that is, the voltage control terminal of the VCXO Make it output the target frequency f ₀ , then measure the frequency through the microprocessor, and save the binary code B _0i of the target frequency f ₀ for comparison and operation;

(2) Determine the binary code corresponding to the frequency offset Δf(T) at the current moment

Due to temperature changes, the output frequency of the voltage-controlled crystal oscillator, namely VCXO, is f(T)=f ₀ ±Δf(T), where the frequency f(T) is the uncompensated real-time output frequency that needs to be compensated, and f ₀ is It is expected that the target frequency output by the voltage-controlled crystal oscillator, Δf(T) is the frequency offset caused by temperature change, it is a function that changes with the change of temperature, if the output frequency increases, then f(T)=f ₀ +Δf(T), if the output frequency decreases, then f(T)=f ₀ -Δf(T), the voltage-controlled crystal oscillator, that is, the VCXO real-time output frequency f(T) signal is sent to the microprocessor Perform frequency measurement and convert to binary code B _1i , then compare binary code B _1i with binary code B _0i corresponding to target frequency f ₀ to obtain binary code ΔB _1i corresponding to frequency offset Δf(T);

(3), determine the compensation voltage V _C

On the basis of step (2), first determine the binary code corresponding to the compensation voltage: if the real-time output frequency f(T)>f ₀ , then the microprocessor outputs B _out = B _0i -ΔB _1i ; if the real-time output frequency f (T)<f ₀ , then the microprocessor outputs B _out =B _0i +ΔB _1i , and then converts the microprocessor output B _out into the corresponding compensation voltage through a digital-to-analog converter

(4), conditioning compensation

The compensation voltage generated by step (3) After passing through the signal conditioning circuit, it is sent to the voltage-controlled voltage input terminal of the voltage-controlled crystal oscillator, that is, VCXO, and finally obtains the expected compensated frequency f'(T), which is the target frequency f ₀ ;

(5), repeating steps (2)-(4) just can obtain the frequency f'(T) signal that obtains compensation under another temperature, and so on, just can realize the real-time temperature compensation to the high-frequency voltage-controlled crystal oscillator.

The purpose of the present invention is achieved like this.

The digital temperature compensation method of the crystal oscillator of the present invention adopts a closed-loop feedback compensation framework. First, determine the binary code B _0i corresponding to the target frequency f ₀ and store it in the microprocessor; when the temperature changes, the microprocessor will measure the output frequency of the VCXO in real time to generate a binary code B _1i , and perform a binary code corresponding to the target frequency Compare to get the binary code of the required compensation information; finally, convert it into a compensation voltage through a digital-to-analog converter and input it to the voltage control adjustment terminal of the VCXO, so that it can output the target frequency to realize temperature compensation.

Compared with the digital temperature compensation method of the existing crystal oscillator, the present invention has the following technical advantages:

1) No temperature sensor is needed, but the frequency deviation related to the real-time temperature is directly converted into a one-to-one correspondence binary code, and converted into the corresponding compensation voltage for temperature compensation. This method can overcome the existing The temperature hysteresis problem caused by the asynchronous temperature change of the temperature sensor and the crystal resonator in the temperature compensated crystal oscillator, that is, TCXO;

2), the present invention adopts a closed-loop compensation framework, which makes it easier to realize real-time high-precision compensation;

3), the compensation process of the present invention is simple, does not need to first collect the data of frequency temperature and compensation voltage like the temperature compensation crystal oscillator of the method, but directly converts the required compensation information into compensation voltage, and the structure of the present invention is relatively simple, Easy integration and mass production;

4) The present invention is well applicable to crystal oscillators of various frequencies, especially for high-frequency crystal oscillators with poor compensation effects in the prior art, the compensation effect is also good.

Description of drawings

Fig. 1 is the structural diagram of the temperature compensated crystal oscillator under the existing microprocessor-based temperature compensation mode;

Fig. 2 is the structural diagram of the temperature compensated crystal oscillator under the existing temperature compensation mode based on frequency mixing;

Fig. 3 is a kind of concrete implementation flowchart of the digital temperature compensation method of crystal oscillator of the present invention;

Fig. 4 is a functional block diagram of hardware constructed according to the method of the present invention.

detailed description

Specific embodiments of the present invention will be described below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

Fig. 3 is a specific implementation flow chart of the digital temperature compensation method of the crystal oscillator of the present invention.

In this embodiment, as shown in FIG. 3, the digital temperature compensation method of the crystal oscillator of the present invention includes the following steps:

Step S1: Determine the binary code B _0i corresponding to the target frequency f ₀

At normal temperature T ₀ , such as 25°C, adjust the control voltage of the voltage-controlled crystal oscillator, that is, the voltage control terminal of the VCXO Make it output the target frequency f ₀ , then measure the frequency through the microprocessor, and save the binary code B _0i of the target frequency f ₀ for comparison and operation;

Step S2: Determine the binary code corresponding to the frequency offset Δf(T) at the current moment

Due to the temperature change, the output frequency of the voltage-controlled crystal oscillator (VCXO) at the current moment is f(T)=f ₀ ±Δf(T), where the frequency f(T) is the uncompensated real-time output frequency that needs to be compensated, f ₀ is the target frequency expected to be output by the voltage-controlled crystal oscillator, and Δf(T) is the frequency offset caused by temperature changes. It is a function that changes with temperature changes; The signal with frequency f(T) is sent to the microprocessor for frequency measurement and converted into binary code B _1i , and then the binary code B _1i is compared with the binary code B _0i corresponding to the target frequency f ₀ to obtain the frequency offset Δf (T) corresponding binary code ΔB _1i ;

Step S3: Determine the compensation voltage V _C

On the basis of step S2, first determine the binary code corresponding to the compensation voltage: if the real-time output frequency f(T)>f ₀ , then the microprocessor outputs B _out =B _0i -ΔB _1i ; if the real-time output frequency f(T )<f ₀ , then the microprocessor outputs B _out =B _0i +ΔB _1i , and then converts B _out into the corresponding compensation voltage through a digital-to-analog converter Among them, if the real-time output frequency f(T)<f ₀ , the compensation voltage If the real-time output frequency f(T)>f ₀ , the compensation voltage ΔV(T) is the voltage corresponding to the frequency offset Δf(T);

Step S4: Conditioning Compensation

The compensation voltage generated in step S3 will be After passing through the signal conditioning circuit, it is sent to the voltage-controlled voltage input terminal of the voltage-controlled crystal oscillator, that is, VCXO, and finally obtains the expected compensated frequency f'(T), which is the target frequency f ₀ ;

Step S5: Repeat steps S2-S4 to obtain a compensated frequency f'(T) signal at another temperature, and so on, to realize real-time temperature compensation of the high-frequency voltage-controlled crystal oscillator.

In this embodiment, according to the block diagram of the hardware set up according to the method of the present invention as shown in Figure 4, it includes: a voltage-controlled crystal oscillator, namely VCXO 301, a power divider 302, a microprocessor 303, a digital-to-analog converter 304, Signal conditioning circuit 305 . The voltage-controlled crystal oscillator, namely VCXO 301, is mainly composed of a quartz resonator, a varactor diode and an oscillation circuit. Its working principle is to change the capacitance of the varactor diode by controlling the voltage, thereby "pull" the frequency of the quartz resonator to achieve the frequency The purpose of modulation; the power divider 302 is to divide the output signal of the voltage-controlled crystal oscillator, that is, the VCXO 301, into two paths, wherein one path is normally output, and the other path is input to the microprocessor 303; the microprocessor 303 performs signal processing on the input signal Measure the frequency and compare and calculate the binary code corresponding to the frequency to obtain the output binary code B _out ; the digital-to-analog converter 304 converts the binary B _out into a compensation voltage V _C , adjusts the output and inputs it to the voltage-controlled crystal through the signal conditioning circuit 305 The voltage-controlled voltage control terminal of the oscillator 301 performs compensation.

Although the illustrative specific embodiments of the present invention have been described above, so that those skilled in the art can understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, As long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are included in the protection list.

Claims (1)

1. a kind of digital temperature compensating method of crystal oscillator, it is characterised in that comprise the following steps：

(1) target frequency f, is determined₀Corresponding binary coding B_0i

In normal temperature T₀, at 25 DEG C, adjustment VCXO is the control voltage at the voltage-controlled ends of VCXOIt is set to export target Frequency f₀, frequency measurement is then carried out by microprocessor, and by target frequency f₀Binary coding B_0iPreserve to carry out Compare and computing；

(2) the corresponding binary codings of current time frequency shift (FS) Δ f (T), are determined

Due to the change of temperature, VCXO is that VCXO output frequency is f (T)=f₀± Δ f (T), wherein, frequency f (T) it is not compensate and need the real-time output frequency of compensation, f₀It is the target frequency for expecting VCXO output, Δ f (T) it is the frequency shift (FS) as caused by temperature change, it is a function, variation with temperature and changed, if output frequency liter Height, then f (T)=f₀+ Δ f (T), if output frequency is reduced, f (T)=f₀- Δ f (T), is VCXO by VCXO The frequency exported in real time is progress frequency measurement in f (T) signal feeding microprocessor and is converted to binary coding B_1i, then By binary coding B_1iWith target frequency f₀Corresponding binary coding B_0iIt is compared, obtains frequency shift (FS) Δ f (T) corresponding Binary coding Δ B_1i；

(3) offset voltage V, is determined_C

On the basis of step (2), it is first determined the binary coding corresponding to offset voltage：If real-time output frequency f (T) ＞ f₀, then microprocessor output B_out=B_0i-ΔB_1i；If real-time output frequency f (T) ＜ f₀, then microprocessor output B_out=B_0i+Δ B_1i, microprocessor is then exported by B by digital analog converter_outBe converted to corresponding offset voltage

(4), conditioning compensation

The offset voltage that step (3) is producedAfter signal conditioning circuit, voltage-controlled crystal (oscillator) vibration is sent to Device is VCXO voltage-controlled voltage input end, finally obtains and it is expected that what is obtained has been compensated for frequency f ' (T) i.e. target frequency f₀；

(5), repeat step (2)-(4) can be obtained by frequency f ' (T) signal that compensation is obtained at another temperature, the like, The real-time temperature compensation to high frequency VCXO can just be realized.