JPH01317004A - Switched capacitor circuit and digital temperature compensated crystal oscillation circuit using said circuit - Google Patents

Abstract

PURPOSE:To devise the circuit so as to change over even a minute capacitance less than a stray capacitance of a switch and a wire by adopting the constitution such that plural series circuits each comprising a capacitor and a switch and at least one of series circuits each comprising a capacitor, a resistor and a switch are connected in parallel. CONSTITUTION:Plural series circuits each comprising a capacitor 125-127 and a switch 115-117 and at least one of series circuits each comprising a capacitor 121-124, a resistor 131-134 and a switch 115-117 are connected in parallel. Since the resistor in the series connection of the capacitor and the resistor limits the charge/discharge speed of the capacitor, the effective capacitance is changed depending on the frequency. The relation between the capacitance of the entire circuit and the frequency depends on the capacitor at a low frequency but the capacitance is reduced effectively through less carrier charged/discharged from/to the capacitor because the succeeding discharge period is transited before the charging of the capacitor is finished at a high frequency. Thus, even a minute capacitance is changed over without the effect of a stray capacitance by selecting a capacitor and a resistor in matching with the operating frequency.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a switched capacitor circuit that selects a plurality of capacitance values by opening and closing a switch, and also relates to a digital temperature compensated crystal oscillator constructed using the switched capacitor circuit. Regarding circuits.

[Background of the invention]

Various types of crystal oscillators are often used as frequency sources in communication equipment. Among these, in the case of mobile communication devices such as automobile radios, a temperature compensated crystal oscillator (hereinafter referred to as TCXO) is used because the operating temperature environment is not constant. Recently, there has been a strong demand for higher accuracy in temperature compensation.
To give an example of the specifications, the accuracy is oscillation frequency 12.8 MHz±2 pIm in the temperature range -20 to +80°G. Since it is industrially extremely difficult to achieve such precision through analog processing, digital TCXOs (hereinafter referred to as DTCXOs) are attracting attention.

[Conventional technology]

DTCXO temperature compensation digitally encodes the temperature,
A temperature compensation value corresponding to this digital code is obtained, the temperature compensation value is then converted into an analog value, and the variable capacitance element is controlled using this analog value (temperature information) to compensate the oscillation frequency.

In recent years, in order to make DTCXOs more integrated into LSIs, smaller in size, and with lower power consumption, switched capacitor circuits have been developed that integrate analogization of temperature compensation values and control of variable capacitance elements among the constituent parts of DTCXOs. - An example is shown in Fig. 4 (Takehiko Uno, Yoshio Shimoda, "Digital CMO8 crystal oscillator circuit using RSW-C array" 1981 Institute of Electronics and Communication Engineers National Conference Lecture Proceedings 649).

In Figure 4, temperature information, which is a digital quantity, is transferred to the inverter 4.
11 to 416, selectively turns on and off switches 421 to 426 corresponding to the input temperature information, and connects a selected capacitor among capacitors 461 to 436 to crystal oscillation circuit 400, thereby changing the oscillation frequency. Compensation is being provided.

In this temperature compensated crystal oscillator circuit, if the capacitance value of capacitor 461 is C, capacitor 462.436.434
．． The capacitance values of 435.466 are 2C, 4C, and 8C, respectively.
, 16C, and 32C, and can perform temperature compensation for a wide range of oscillation frequencies.

[Problem to be solved by the invention]

However, when attempting to perform highly accurate temperature compensation with the crystal oscillator circuit shown in Figure 4, the capacitance of the capacitors (especially capacitors 431, 432, etc. with small capacitance values) is usually 0. .04 pF or more) and the stray capacitance of wiring (in some cases, 0.1 pF or more). For this reason, it has not been possible to perform highly accurate temperature compensation with such a crystal oscillation circuit.

The present invention has been made to solve these problems, and provides a switched capacitor circuit that allows minute capacitance values to be switched, and a highly accurate switched capacitor circuit configured using this switched capacitor circuit over a wide temperature range. The purpose of this invention is to provide a digital temperature compensated crystal oscillator circuit that can compensate the oscillation frequency.

[Means to solve the problem]

The switched capacitor circuit of the present invention is configured by connecting in parallel a plurality of series circuits of a capacitor and a switch, and at least one series circuit of a capacitor, a resistor, and a switch. However, the resistance in this case means a resistance different from the on-resistance of the switch.

Further, the digital temperature compensated crystal oscillation circuit of the present invention includes at least a temperature information creation section that creates digital temperature information, the above-mentioned switched capacitor circuit, and a crystal oscillation circuit. The digital temperature compensated crystal oscillator circuit of the present invention inputs the digital temperature information created by the temperature information creation section to the switched capacitor circuit, and in the switched capacitor circuit connects the capacitor and/or capacitor and resistor corresponding to the input temperature information. The oscillation frequency is temperature-compensated by selectively turning the switch on and off and connecting it to the crystal oscillation circuit.

[Effect]

The principle of the switched capacitor circuit of the present invention will be explained using FIG.

When a capacitor and a resistor are connected in series, the resistor limits the charging and discharging speed of the capacitor, so the effective capacitance value changes depending on the frequency. The situation is shown in Figure 3.

In Figure 3 (al), a capacitor 301 with a capacitance value C1 and a resistor 602 with a resistance value R1 are connected in series.If the capacitance value C1 of the capacitor 301 is made much larger than the stray capacitance of the wiring, the stray capacitance can be eliminated.

The relationship between the capacitance value and frequency of this entire circuit is shown in Figure 3 (b
) As shown in (, 1/C, R), on the lower frequency side by about two orders of magnitude or more, it is equal to the value of C1, but on the high frequency side, the value shifts to the next discharge cycle before the capacitor 301 finishes charging. Therefore, fewer carriers are charged and discharged into the capacitor 301, and the effective capacitance value is reduced.1/C,R.

At frequencies higher than the frequency determined by the capacitor 601
carriers can hardly follow it, and the overall capacitance value approaches 0.

Therefore, by selecting a capacitor and resistor that match the frequency used, even extremely small capacitance values can be achieved without the influence of stray capacitance.

〔Example〕

Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 1 shows a switched capacitor circuit according to the present invention.
FIG. 2 is a circuit diagram showing an example applied to a CXO. In FIG. 1, a feedback resistor 102 and a crystal resonator 103 are connected in parallel between the gate and drain of an oscillation inverter 101.
Capacitors 104 and 105 are connected between the gate and the drain and the electrodes, respectively, and a 7-bit switched capacitor circuit is connected in parallel with the capacitor 105 on the drain side. Each of the switches 111 to 117 is opened and closed by a digital signal containing temperature information created by the temperature information creation section 140.

Crystal resonator 103 is an AT cut crystal resonator (12,8M
Hz), there is usually a change in the oscillation frequency of ±15 C over a temperature range of -20 to +80°C. This 30-
In order to compensate for the change in the oscillation frequency with a 7-pit switched capacitor circuit, since 30, -2 = 0.23, the compensation amount for the smallest bit may be set to about 0.25 pIII.

Therefore, the amount of compensation for each bit is 0 for the 1st bit, 25 p
IIXI, 2nd bit is 0.5-13th bit is 1 helmet,
The 4th bit is 2 ppm, the 5th bit is 41)P, the 6th bit F is 3 ppm, and the 7th bit is 16-.

Capacitors 104.105 on the gate and drain sides
The capacitance value of both is 14pF, and 12.8M Flz
The voltage amplification factor and output impedance of the inverter 101 during stable oscillation are 2.0 and 1.0, respectively. OkΩ
In this case, the capacitance change to achieve the compensation amount for each bit is 0.021)F for the first bit.

2nd bit is 0.049F, 3rd bit is 0.099F
, 4th bit is 0.18pF, 5th bit is 0.369
F, 6th bit is 0.72pF, 7th bit is 1.50
It needs to be pF.

As mentioned above, the stray capacitance of switches and wiring is large,
Only capacitors and switches can realize the value after the 5th bit of 0.36 pF, and the 1st to 4th bits cannot be realized by conventional technology or have a large error and are not practical.

In the present invention, the first bit capacitor 121 and resistor 161
are each 1.4 pF + 40Ω, and the second bit capacitor 122 and resistor 132 are each 1.1 pF + 4
0kfl, 3rd bit capacitor 126 and resistor 1
36 to 1.2 pF each.

30Ω, 4th bit capacitor 124 and resistor 1
By setting 34 to 1.3pFt20 and Ω, the effective values are 0.02pF and 0.04pF.

This achieves 0.09 pF and 0.18 pF%.

In the above embodiment, resistors are used only for the 1st to 4th pits, but it is of course possible to use resistors for all bits. Furthermore, the capacitor, resistor, and switch only need to be connected in series, and the order of connection does not matter. If the structure, frequency, etc. of the oscillation inverter change, the value of the required minute capacitance will also change, but in any case, the required switched capacitor circuit can be realized by the present invention.

Further, the switched capacitor circuit of the present invention may be connected to either the input side or the output side of the crystal oscillation circuit 1000, or both the input side and the output side.

FIG. 2 is a circuit diagram showing an embodiment in which the switched capacitor circuit according to the present invention is applied to a tuning circuit. In FIG. 2, a parallel condensed circuit consisting of a coil 202 and a capacitor 206 is connected between an antenna 201 and the ground, and a 5-pit switched capacitor circuit is connected in parallel with the capacitor 206. Each of the switches 211 to 215 is opened and closed by a digital signal including temperature information, but that part is omitted.

In this example, the 4th and 5th bits to which no resistors are connected are used to roughly select the frequency, and the selectivity is improved by switching the 1st to 3rd bits to which resistors 261 to 236 are connected. This method achieves high precision in the tuning frequency. In this manner, the switched capacitor circuit of the present invention enables higher precision in circuits that require switching of minute capacitances.

〔Effect of the invention〕

As is clear from the above explanation, according to the present invention, it is possible to realize a switched capacitor circuit that can switch even a small capacitance value that is less than the stray capacitance of a switch or wiring, and the effect is very large. big. The effect is particularly great when applied to temperature-compensated crystal pumps that require high precision.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a temperature compensated crystal oscillation circuit using a switched capacitor circuit according to the present invention, and FIG. 2 is a circuit diagram showing an embodiment of a tuning circuit using a switched capacitor circuit according to the present invention. FIG. 4 is a circuit diagram and characteristic diagram for explaining the working principle of the invention, and FIG. 4 is a circuit diagram of a digital temperature-compensated crystal oscillation circuit according to the prior art. 103...Crystal oscillator, 111-117...Switch. 121-127 --- Capacitor, 131-164
...Resistor, 140...Temperature information creation section, 201...Antenna, 211-215...Switch. 221-225...Capacitor, 231-23
3...Resistance. Day 1 '5'' Liquid count

Claims (2)

[Claims] (1) A switched capacitor circuit configured by connecting in parallel a plurality of series circuits of a capacitor and a switch and at least one series circuit of a capacitor, a resistor, and a switch. (2) A digital temperature-compensated crystal oscillation circuit comprising at least a temperature information creation section that creates digital temperature information, the switched capacitor circuit according to claim 1, and a crystal oscillation circuit.