JPH09307356A - Crystal oscillator circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set the oscillating frequency to be an object frequency with high precision by applying a voltage to variable capacitor selection terminals so as to make a MOS transistor(TR) conductive/nonconductive thereby allowing a varactor diode to adjust fluctuation in the oscillated frequency. SOLUTION: A voltage is applied to variable capacitor selection terminals S1-Sn to make n-channel MOS TRs M1-Mn conductive/nonconductive. Thus, the oscillated frequency is set to the object frequency with high precision by using a plurality of variable capacitor diodes Cval1-n so as to adjust fluctuation in the oscillated frequency due to dispersion in capacitance C1, C2, Cv. Furthermore, the frequency is varied by varying a variable capacitor control voltage Vcnt. Thus, it is prevented that a capacitance variable range is reduced with varying the control voltage Vcnt, the fluctuation in the frequency variable range due to LSI manufacture tolerance is reduced and the oscillated frequency is set to the object frequency with high precision.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal oscillator circuit of a semiconductor integrated circuit, and more particularly to a crystal oscillator circuit that reduces deterioration of a frequency variable range due to manufacturing variations.

[0002]

2. Description of the Related Art FIG. 4 shows a configuration example of a conventional crystal oscillator circuit of this type. Referring to FIG. 4, the conventional crystal oscillation circuit includes a source follower circuit 5, a capacitor C1 connected between an input and an output of the source follower circuit 5, and an output of the source follower circuit 5 and a ground (GND). The connected capacitance C2, the capacitance Cv connected between the input of the source follower circuit 5 and GND, the resistors 3 and 4 for supplying a bias voltage to the input of the source follower circuit 5, and the voltage source 8
A current source 6 of the source follower circuit 5 and a crystal oscillator 2 having one end connected to the input of the source follower circuit 5,
It is composed of a varicap diode Cval connected between the other end of the crystal unit 2 and GND, and the varicap control voltage input terminal V is connected to the varicap diode Cval.
A control voltage for changing the capacitance is supplied from cont via the resistor 1.

In the conventional crystal oscillation circuit shown in FIG. 4, fluctuations in the oscillation frequency due to variations in the capacitances C1 and C2 and the varicap diode Cval are set to the desired oscillation frequency by the capacitance Cv, and the control voltage Vcont is set to the varicap diode. It is composed of a Colpitts type oscillating circuit for changing the oscillation frequency by changing the capacitance value of the varicap diode by giving it to Cval.

The varicap diode Cval changes the capacitance value when a reverse bias is applied and the value thereof is varied, and serves as a voltage control variable capacitance.

Further, the crystal unit can be represented by a series-parallel circuit of LCRs, and an equivalent circuit thereof is one in which a capacitor C, a coil L and a resistor R are connected in series, and a capacitor Co is connected in parallel. When the load capacitance is CL, the oscillation frequency fo is expressed by the following equation (1), and the oscillation frequency changes depending on the load capacitance CL.

[0006]

[Equation 1]

The load capacitance CL is a series capacitance of C1 and C2 //
The capacitor Cv (//: means parallel connection) and the varicap diode Cval connected in series are represented by the following equation (2).

[0008]

[Equation 2]

Now, assuming that the series capacitance of the capacitors C1 and C2 = 16
If the target frequency is oscillated with pF, varicap diode Cval = 32 pF, and capacitance Cv = 16 pF,
From FIG. 3, the variable capacitance value of the varicap diode when the Vcont control voltage is varied is 26.5 pF, and the frequency can be varied by this capacitance change.

If the capacitance values of C1, C2, and Cval are increased by, for example, 25% due to variations in LSI manufacturing, C1 and C
2 series capacitance = 20 pF, Cval = 40 pF,
The capacitance Cv for adjustment is set to 4 pF to obtain the target oscillation frequency. At this time, the variable capacitance value of the varicap diode when the Vcont control voltage is varied is 33 pF from FIG.
It is.

The capacitance value of C1, C2 and Cval is L
If there is a 25% reduction due to SI manufacturing variations, for example, C
The series capacitance of 1 and C2 = 12 pF and Cval = 24 pF, and the adjustment capacitance Cv is set to 28 pF to obtain the desired oscillation frequency. At this time, the variable capacitance value of the varicap diode when the Vcont control voltage is varied is 20 from FIG.
pF.

As described above, the conventional crystal oscillator circuit shown in FIG. 4 has the LS of the internal capacitors C1, C2, and Cval.
I The manufacturing variation is adjusted by the capacitance Cv inside or outside the LSI to obtain the target oscillation frequency. However, as shown in FIG. 3, the varicap diode has a variable control voltage as the capacitance value increases. It has a characteristic that the variable range of the capacitance at the time of time also widens, and the variable range of the capacitance at the time of changing the control voltage narrows as the capacitance value decreases.

[0013]

Therefore, the capacitance value C
When C1, C2 and Cval decrease by 25%, Cva
Since l = 24 pF, the variable range of the capacitance when the Vcont control voltage is variable is 20 pF, which narrows the variable frequency range.

As described above, the above-described conventional crystal oscillation circuit has a problem that the frequency variable range greatly changes when the capacitance value changes due to manufacturing variations.

Further, since adjustment is performed by the capacitance Cv, C
When v is set to a large value, there is a problem that oscillation is stopped.

Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a crystal oscillation circuit that reduces deterioration of a frequency variable range due to manufacturing variations.

[0017]

To achieve the above object, a crystal oscillator circuit according to the present invention comprises a source follower circuit, a first capacitor connected between an input and an output of the source follower circuit, and the source. A second capacitor connected between the output of the follower circuit and the ground, a third capacitor connected between the input of the source follower circuit and the ground, and means for applying a bias voltage to the input of the source follower circuit; A crystal oscillator whose one end is connected to the input of the source follower circuit and the other end is connected to a control voltage input terminal, and a switch which is turned on / off by a control terminal and a voltage control between the other end of the crystal oscillator and the ground. It is characterized in that a plurality of sets of type variable capacitance elements are connected in series and are connected in parallel.

In the present invention, preferably, a source follower circuit and a first capacitor connected between the input and the output of the source follower circuit and a second capacitor connected between the output of the source follower circuit and the GND. A third capacitor connected between the capacitor and the input of the source follower circuit and GND
And a means for applying a bias voltage to the input of the source follower circuit, and a Colpitts type crystal oscillator having one end connected to the input of the source follower circuit and a voltage control variable capacitor connected between the other end and GND. A plurality of n-channel MOS transistors in which the voltage-controlled variable capacitance is composed of a plurality of diodes whose anodes are connected to GND, each cathode is connected to each source, and each drain is commonly connected; Means for applying a variable bias to the drains of the commonly connected n-channel MOS transistors are provided, and a plurality of bits of control signals applied to the gates of the plurality of n-channel MOS transistors and the commonly connected n-channel MOS transistor. The oscillation frequency depends on the bias voltage applied to the drain of the transistor. And controlling the.

According to the present invention, by preparing a plurality of varicap diodes, it is possible to reduce the manufacturing variation of the varicap diodes and suppress the variation of the variable frequency range. Further, since the varicap diode also serves as the adjusting capacitance Cv, the oscillation stop is less likely to occur.

[0020]

1 is a diagram showing a circuit configuration of an embodiment of a crystal oscillation circuit according to the present invention. Referring to FIG. 1, a crystal oscillation circuit according to an embodiment of the present invention includes a source follower circuit 5, a capacitor C1 connected between an input and an output of the source follower circuit 5, an output of the source follower circuit 5, and a GND. A capacitor C2 connected between the two, a capacitor Cv connected between the input of the source follower circuit 5 and GND, resistors 3 and 4 for supplying a bias voltage to the input of the source follower circuit 5, and a voltage source 8. A current source 6 of the source follower circuit 5 and a plurality of varicap diodes Cval1 to Cva whose anodes are connected to GND.
In and a plurality of varicap diodes Cval1 to Cval1.
A plurality of n-channel MOS transistors M1 to Mn each having a source connected to the cathode of Cvaln and a common drain connected thereto, and a varicap for giving a variable bias to the drains of the plurality of commonly connected n-channel MOS transistors M1 to Mn. Control voltage input terminal Vcont
And the crystal oscillator 2 connected between the input of the source follower circuit 5 and the common connection point of the drains of the plurality of n-channel MOS transistors M1 to Mn, and the plurality of n-channel MOS transistors M1. Control voltages are input from the control signals S1 to Sn of a plurality of bits to the gates of Mn to Mn.

In the embodiment of the present invention, in addition to the conventional crystal oscillator circuit shown in FIG. 4, a varicap diode Cv is used.
ali (i = 1 to n) to which n-channel MOS transistors Mi are connected are arranged in parallel with each other to form n-channel MOS transistors Mi (i = 1 to n).
Gates are input terminals for varicap selection Si
(I = 1 to n).

Next, the operation of the circuit according to the embodiment of the present invention will be described.

Variation of the oscillation frequency due to variations in the capacitances C1, C2 and Cval is taken as an input terminal S1 for varicap selection.
By applying a voltage to -Sn to turn on or off the n-channel MOS transistors M1 to Mn, the number of varicap diodes seen from the crystal resonator 2 is changed to set a desired oscillation frequency.

The frequency is changed by changing the voltage of the varicap control voltage Vcont as in the prior art shown in FIG.

Now, suppose that the serial capacitance of C1 and C2 is 16 p.
Assuming that the target frequency is oscillated with F, the varicap diode Cval = 32 pF, and the external capacitance Cv = 16 pF, the variable capacitance value of the varicap diode when the Vcont control voltage is varied is 26.5 pF from FIG. The frequency can be changed by the amount of change.

The capacitance values of the capacitors C1, C2 and Cval are LS.
I If 25% increase due to manufacturing variations, series capacitance of C1 and C2 = 20 pF, Cval = 40 pF, Cv = 16
It becomes pF, and the adjustment capacitance Cval is set to 28 pF to obtain the target oscillation frequency. At this time, the variable capacitance value of the varicap diode when the Vcont control voltage is changed is
From FIG. 3, it is 23.25 pF.

When the capacitance values of the capacitors C1, C2 and Cval decrease by 25% due to variations in LSI manufacturing, the series capacitance of C1 and C2 = 12 pF, Cval = 24 pF, Cv.
= 16 pF, and the adjustment capacitance Cval is set to 36 pF to obtain the target oscillation frequency. At this time, the variable capacitance value of the varicap diode when the Vcont control voltage is variable is 29.75 pF from FIG.

As described above, a plurality of varicap diodes Cval are used for the change of the capacitance value due to the variation in the LSI manufacturing.
Can be set with high accuracy by adjusting with, and the variation of the capacitance value of the varicap diode can be suppressed as compared with the conventional technique shown in FIG. 4, so that the variable range of the capacitance at the time of varying the Vcont control voltage can be prevented from decreasing. It is possible to reduce fluctuations in the frequency variable range due to manufacturing variations.

FIG. 2 shows a circuit configuration of the second embodiment of the present invention, in which a coil L is inserted between the crystal oscillator 2 and the varicap diode. In the circuit of this embodiment, the coil L is used to further widen the frequency variable range.

[0030]

As described above, according to the crystal oscillating circuit of the present invention, the change of the capacitance value due to the variation in the LSI manufacturing is reduced, the fluctuation of the frequency variable range is reduced, and the target frequency is set with high accuracy. There is an effect that can be done.

[Brief description of drawings]

FIG. 1 is a diagram showing a crystal oscillation circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a crystal oscillator circuit according to a second embodiment of the present invention.

FIG. 3 is a diagram showing characteristics of a varicap diode.

FIG. 4 is a diagram showing a conventional crystal oscillation circuit.

[Explanation of symbols]

1, 3, 4 Resistance 2 Crystal oscillator 5, M1, M2-Mn n-channel MOS transistor Vcont Varicap control voltage input terminal 6 Current source 7 Output terminal 8 Voltage source S1, S2-Sn Varicap selection input terminal Cval1, Cval2-Cvaln Varicap diode C1, C2, Cv Capacitor L coil

Claims (2)

[Claims] 1. A source follower circuit, a first capacitor connected between an input and an output of the source follower circuit, a second capacitor connected between an output of the source follower circuit and a ground, and the source follower. A third capacitor connected between the input of the circuit and ground; a means for applying a bias voltage to the input of the source follower circuit; one end connected to the input of the source follower circuit and the other end connected to the control voltage input terminal A plurality of pairs of the crystal oscillator and a switch in which a switch turned on / off by a control terminal and a voltage-controlled variable capacitance element are connected in series are connected in parallel between the other end of the crystal oscillator and the ground. A crystal oscillator characterized by: 2. A source follower circuit, a first capacitor connected between an input and an output of the source follower circuit, a second capacitor connected between an output of the source follower circuit and a ground, and the source follower. A third capacitor connected between the input of the circuit and the ground; a means for applying a bias voltage to the input of the source follower circuit; one end connected to the input of the source follower circuit; and voltage control between the other end and the ground. In a Colpitts-type crystal oscillator to which a variable capacitor is connected, the voltage-controlled variable capacitor is composed of a plurality of diodes whose anodes are connected to ground, each cathode is connected to each source, and each drain is connected to each cathode. A plurality of commonly connected n-channel MOS transistors, and a plurality of commonly connected n-channel MOS transistors A means for applying a variable bias to the drains of the transistors is provided, and the oscillation frequency is controlled by a plurality of bits of control signals applied to the gates of the n-channel MOS transistors and a bias voltage applied to the drains of the commonly connected n-channel MOS transistors A crystal oscillator characterized by variable control.