JPS58184809A - Frequency controlling method of digital control type temperature compensated crystal oscillator - Google Patents

Abstract

PURPOSE:To control a varactor, by giving a resonance frequency temperature coefficient of a crystal oscillator in a digital code series, providing an operating function in a control system, obtaining a resonance frequency deviation at an arbitrary temperature and processing the deviation. CONSTITUTION:A resonance frequency relative deviation DELTAf/f0 is expressed as a plynomial DELTAf/f0=A1DELTAT+A2(DELTAT)<2>+A3(DELTAT)<3>+A4(DELTAT)<4>+A5(DELTAT )<5>+...(1), where DELTAT is the temperature deviation from a reference temperature and An is the n-order temperature coefficient. The reference frequency temperature coefficient An of a crystal oscillator gives deviation information only to the reference value an, then the An is expressed as An=an+deltan...(2). First order temperature coefficients delta11, delta12,...delta1m1; second order temperature coefficients delta21, delta22...delta2m2;...; n-th order temperature coefficients deltan1,...deltanmn are inputted from a temperature coefficient information input terminal 9 to a temperature coefficient generating circuit 10, and the information corresponding to the temperature coefficients are given to an operation processing circuit 11. The circuit 11 obtains the resonance frequency deviation at an arbitrary temperature from equation (1) and performs signal processing.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency control method for a digitally controlled temperature compensated crystal oscillator that has high frequency accuracy using a small amount of crystal resonator temperature characteristic information.

Digitally controlled temperature compensated crystal oscillator (DTCXO)
As shown in FIG. 1, the temperature sensor 2 detects the ambient temperature of the crystal resonator 1, converts the temperature information into a digital code in the temperature encoding circuit 3, and converts the encoded temperature information into a digital code. The signal is input to the signal processing circuit 4, where signal processing is performed that matches the resonant frequency temperature characteristics of the crystal resonator 1, and based on the processing result, the threading value of the variable capacitance element 6, which is a component of the oscillation circuit 5, is controlled. This is a crystal oscillator that obtains a stable frequency over temperature. Since there are manufacturing deviations in the resonant frequency temperature characteristics of a crystal resonator, in order to obtain a stable oscillation frequency, it is necessary to perform signal processing that corresponds to the characteristics of the resonator used.

Therefore, the conventional method of configuring the signal processing circuit 4 is to provide a read-only memory (ROM) 7 as shown in FIG.
The temperature characteristics of the crystal oscillator are measured in advance, and the resonant frequency deviations with respect to the reference temperature are stored in the ROM 7 at many temperature points.The frequency deviation at the temperature is read out based on the temperature information, and the control signal generation circuit 8 generates the control signal. The method used is to obtain

In the conventional method as described above, a large memory capacity is required for the ROM 7. That is, for example, DTC
If the operating temperature range of the XO is -20° to +70°C and data of resonance frequency deviation is encoded in 8 bits every 0.2°C during that time, a capacity of 8x (90x5+1 >=3608 bits is required. Therefore, inputting resonant frequency deviation data for each temperature requires an enormous amount of effort, and a method such as preparing a mask for each crystal photographer and using a mask ROM is required.

In this case, the crystal oscillator and ROM must be used in pairs, which lacks flexibility as it is only possible to replace the crystal oscillator, and the ROM and other circuits must be combined on a single integrated circuit. Since it is difficult to form and consists of a plurality of semiconductor chips, there are problems in terms of miniaturization, low power consumption, etc. Furthermore, since the operating temperature range of the DTCXO is also restricted by the temperature range of data stored in the ROM, if the temperature range deviates from this temperature range, a saturation phenomenon occurs and the error increases rapidly.

On the other hand, as another configuration method, there is a method in which a microprocessor is used instead of the ROM 7 and control is performed by performing arithmetic processing depending on the resonant frequency temperature coefficient value of the crystal resonator and temperature information, but the temperature coefficient value is converted into a decimal number. Since the temperature coefficient is given to the microprocessor as a numerical value, a large capacity is required to store the temperature coefficient, which has the drawback of increasing circuit scale and power consumption.

In order to eliminate the above-mentioned summer point, the present invention has developed a digital system that can realize highly accurate frequency control by providing only a small amount of information regarding the temperature characteristics of the crystal resonator, and also allows the entire circuit to be mounted on one chip. A frequency control method for a controlled temperature compensated crystal oscillator is provided.

The present invention will be explained in detail below with reference to the drawings.

Figure 3 is a diagram showing an example of the resonant frequency temperature characteristics of an AT-cut crystal resonator, where the horizontal axis represents temperature and the vertical axis represents the relative deviation of the resonant frequency with respect to the reference temperature point To, where the reference temperature is
℃. The resonant frequency relative deviation Δf /fo can generally be expressed by the polynomial formula %() (1). Here, ΔT is the temperature deviation from the reference temperature, and An (n=1, 2.3, etc.) is the n-th temperature coefficient. The resonant frequency temperature characteristic of an AT-cut crystal resonator can be expressed with good precision by the third-order term or lower of ΔT in equation (1), and a more accurate approximation can be obtained by subtracting the fifth-order term. Higher-order terms such as fourth order or higher or sixth order or higher may be ignored. Therefore, instead of memorizing the frequency deviation at each temperature, #'i1 to 5th temperature coefficients A+ + A2, A3+ can be used or when high accuracy is required.
By giving A4 and As and calculating the formula (]), the frequency deviation at any temperature can be obtained.

The present invention provides a resonant frequency temperature coefficient of a crystal resonator in a digital code series, provides an arithmetic function in the control system, and calculates the resonant frequency deviation at an arbitrary temperature using equation (1).
The variable capacitance element is controlled by signal processing this, and in order to enable highly accurate control with a small amount of temperature coefficient information, as described below, the resonant frequency temperature coefficient of the crystal resonator is Only deviation information from the reference value is provided. That is, although there are manufacturing variations in the value of the temperature coefficient An, a reference value an is set for the temperature coefficient of each order, and the difference is calculated by δ. Then, An=an+
δn(2) or the difference δ. By introducing a factor pn representing the deviation instead of , it can be expressed as An-pnxan(3). The reference value an is generated as a digital code series within the control system, and the temperature characteristic information corresponding to each crystal oscillator is generated as a deviation or Pn.
is input as a digital code sequence, and A
Find n. δ. Alternatively, since Pn corresponds to the manufacturing deviation from the reference value, it can be given with a relatively low number of bits, and as will be described later, a sufficiently high precision can be obtained with 5 to 10 bits for each order.

FIG. 4 shows an embodiment of the DTCXO according to the present invention, where l is a crystal oscillator, 2 is a temperature sensor, 3 is a temperature encoding circuit, 5 is an excavation circuit, 6 is a variable capacitance element, and 9 is a crystal oscillator. A terminal for inputting temperature coefficient information in the form of a deviation from a reference value, 10 includes a circuit such as a Renstar that generates a digital code sequence of a reference value, and a digital deviation value code sequence indicating the deviation of the temperature coefficient from the terminal 9. and a reference value digital code sequence to generate information corresponding to the resonant frequency temperature coefficient of the crystal resonator; 11;
is an arithmetic processing circuit. i, temperature coefficient information input terminal 9, as indicated by the terminal names in FIG. ,j
m1 piece, the secondary temperature coefficient is δ21. δ22. ...Song, 6
m2 terminals of 2.1 m, and similarly, mn terminals are provided for the n-th temperature coefficient, and the deviation of the temperature coefficient is δ. is encoded, and each terminal is fixed at a logic level of "O" or "l'" based on the result. As will be explained later, the total number of coefficient information input terminals is calculated as follows:
In practice, it is about 20 lines, and about 40 lines for quintic approximation.
Sufficient accuracy can be obtained.

In the embodiment shown in FIG. 4, 3, 5, 10, and 11 can be mounted on one semiconductor chip, and if these circuits are configured with, for example, CMO8, not only the circuit can be made smaller but also the power consumption can be reduced. Electrification can be achieved. Furthermore, since temperature coefficient information is provided through external terminals, replacement of the crystal resonator can be easily handled by changing the terminal connections. On the other hand, if it is difficult to provide an external terminal for inputting temperature coefficient information, the temperature coefficient deviation δ. This can be solved by providing a circuit that has the function of generating a digital code corresponding to . The specific configuration is a very small capacity ROM 6
Alternatively, a programmable logic play (PLA) can be provided, and the required storage capacity is at most 4 even for quintic approximation.
The focus is about 0, which is much less than conventional methods, so it can be mounted on the same chip as other circuit parts.

Next is the temperature coefficient deviation δ. Explain the number of bits required to humiliate. As a result of measuring the resonance frequency temperature characteristics of a large number of AT-cut crystal vibrations f, the temperature coefficient when approximated by a cubic equation is a linear coefficient A.

For (0~-4)×1O-7/℃, quadratic coefficient A2
For (-6 to -16)XIO-10/ (℃)2
, and the third-order coefficient A3 is (9~n) x to-”
/ (°C)3. Therefore, if each wide temperature coefficient is set separately, δl, δ2, and δ3 will be respectively. When δ1 (i = = 1 + 2.3) is encoded with m1 bits, the wrinkled stenog dδ1 is dδ1 - δ1/2°1
Therefore, if δl is encoded with 8 pits and δ2 and δ3 are encoded with 5 bits each, a total of 18 bits,
daI=0.0156, dδ2=0.313,
dδ3 = 0.0625'.

For example, if we consider a temperature range of 25°C ± 45°C, the maximum error value of the frequency relative deviation due to the above quantization step δ(Δf/fo), , is δ(Δf/fo)I
IIM, = dδ, Xl0−7X45+dδ2×1O−1
0X452+dδ3 X 10-” X 453= 1
．． 9 x 10-' That is, the error is 0.2 ppm or less, which is extremely small.

The above example is a case where the resonant frequency temperature coefficient is expressed according to equation (2), but the same applies when equation (3) is used.

In other words, for example, the quadratic temperature coefficient A2 is expressed as A2 = -pnX 6 X 10'/(℃)2(6)
Then, 1≦pn≦2.667, ,
(7), so if we encode this with 5 bits, we get:

A2 (D quantization step is δA2 = 6 X lo ”
x (2,667-1)/25= 0.313X 1o
-to, which matches the case expressed by equation (4).

Even when the resonant frequency temperature characteristic is approximated by a quintic equation, if the temperature coefficient of each order is similarly encoded using 7 to 10 bits, a total of about 40 bits, the error will be reduced in the temperature range of 25°C ± 45°C. The maximum value is about 0.1 ppm. Further, even if the ambient temperature deviates from the above-mentioned temperature range, the resonant frequency at that temperature is determined by calculation, so that a sudden increase in errors due to insufficient storage capacity does not occur.

As explained above, the control method of the digitally controlled temperature-compensated crystal oscillator according to the present invention makes it possible to provide the resonant frequency temperature characteristics of the crystal resonator with high precision using a small number of bits. Also, since the circuit size is small, all the circuits except the crystal oscillator and temperature sensor can be integrated into one.
It is 1liT- to be installed in the integrated circuit of the chip,
Since it provides a small, low power consumption, and highly accurate temperature compensated crystal oscillator, it has a tremendous effect on improving the performance of various communication devices.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a digitally controlled temperature compensated crystal oscillator, Figure 2 is a diagram showing the conventional configuration method of a signal processing circuit for a digitally controlled temperature compensated crystal oscillator, and Figure 3 is a diagram showing the resonant frequency temperature of the crystal resonator. Characteristic Example: FIG. 4 is a diagram showing an example of the configuration of a temperature compensated crystal oscillator according to the control method of the present invention. DESCRIPTION OF SYMBOLS 1... Crystal resonator, 2... Temperature sensor, 3... Temperature encoding circuit, 4... Signal processing circuit, 5... Oscillation circuit, 6... Variable capacitance element, 7... - Read-only memory, 8 - Control signal generation circuit, 9... Temperature coefficient information input terminal, 10... Temperature coefficient generation circuit, 11... Arithmetic processing circuit. Patent Applicant Nippon Telegraph and Telephone Public Corporation Agent 1 Mizutsune Male'I51 Flash, 73 ≧ KUWEI2[p

Claims (1)

[Claims] The resonant frequency temperature characteristic of the crystal resonator is expanded into a polynomial related to temperature, the resonant frequency temperature coefficient corresponding to each order of the polynomial is separated into a reference value and a deviation value, and the information corresponding to the deviation value is converted into a digital deviation value code series. is input to the control system, a digital reference value code series corresponding to the reference value is generated within the control system, and the digital reference value code series and the digital deviation value code series are subjected to arithmetic processing. Information corresponding to the resonant frequency temperature coefficient of the vibrator, and generating an oscillation frequency control signal based on the information corresponding to the resonant frequency temperature coefficient and separately supplied digitally encoded temperature information. A frequency control method for a digitally controlled temperature compensated crystal oscillator.