Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-02-02
2001-09-25
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S1170FE, C331S158000, C331S167000
Reexamination Certificate
active
06294964
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-stability piezoelectric oscillator, and more particularly relates to a Colpitts oscillator having high-stability oscillation characteristics in which a piezoelectric resonator, for example, an SC-cut crystal unit is used, and which can suppress spurious.
2. Description of the Related Art
Piezoelectric oscillators have been used in many fields including a base station for mobile communications, a measuring apparatus, etc. Because the piezoelectric oscillator has an excellent oscillation precision, excellent frequency-temperature characteristics, excellent aging characteristics, etc.
When a doubly rotated crystal resonator, for example, an SC-cut crystal resonator or an IT-cut crystal resonator is used for a piezoelectric unit for use in the piezoelectric oscillator, it is possible to obtain a high-stability piezoelectric oscillator superior in stress sensitivity and thermal shock characteristics and the like, as compared with the case where an AT-cut crystal resonator is used for the piezoelectric unit. Because of such excellent characteristics, SC-cut crystal units or IT-cut crystal units have been studied and put into practical use since twenty-odd years ago.
FIG. 5
is a diagram for showing resonance characteristics of an SC-cut crystal unit. The horizontal axis represents the frequency and the vertical axis represents the reactance.
As is clear from this diagram, the main oscillation is a thickness-shear mode (C mode), and there exist a thickness-twist mode (B mode) and a thickness-longitudinal mode (A mode) and the like as oscillation modes with a higher frequency.
Accordingly, the oscillations of the B mode, the A mode and the like other than the C mode have been a cause of generating various inconveniences as spurious (unwanted response) at the time of constructing an oscillator.
Particularly, as frequency f2 of the B mode adjacent to the C mode as the main oscillation is at a distance of only about 9 to 10% from frequency f1 of the C mode, this has been a cause of generating a frequency jump that the oscillation frequency f1 changes to the frequency f2.
FIG. 6
shows a prior art disclosed in Japanese Patent Application Laid-open No. 9-15740 for solving the above problems. According to this patent application, a collector of a transistor Tr is connected to a power source Vc, and further, one end of a crystal unit Y
1
is connected to the base of the transistor Tr and the other end of the crystal unit Y
1
is connected to the ground through a variable capacitance Cv.
Then, a series circuit of capacitances C
1
and C
2
are connected between the base of the transistor Tr and the ground, and a crystal unit Y
2
is provided between an emitter of the transistor Tr and an intermediate point (division point) of the series circuit of C
1
and C
2
.
In
FIG. 6
, reference symbols R
2
and R
3
represent bleeder resistors, and R
1
represents a feedback resistor (load resistor). An output V
0
is obtained from both ends of the load resistor R
1
.
The crystal unit Y
2
is set such that the series resonance frequency of the crystal unit Y
2
substantially coincides with the oscillation frequency of the main oscillation (C mode) by using, for example, an AT-cut crystal unit.
FIG. 7
is a graph for showing negative resistance characteristics.
FIG. 7
shows a negative resistance a of the oscillation circuit shown in
FIG. 6 and a
negative resistance b of a general Colpitts oscillator for comparison.
In this case, the negative resistance refers to a real number portion of an impedance presented when, for example, in the oscillation circuit shown in
FIG. 6
after excluding the crystal unit Y
1
and the variable capacitance Cv, the oscillation circuit side is looked at from the terminal connecting the crystal unit, that is a connection point of the bleeder resistors R
2
and R
3
and the earth. As the real number portion of the impedance shows a negative resistance in the vicinity of the oscillation frequency, this is called a negative resistance.
It is known that an oscillation is easily generated when the negative resistance becomes larger. However, it is general that the negative resistance is set at a value of about a few times of an effective resistance of the crystal unit for the reasons of minimizing power consumption and restricting unwanted oscillation.
In this case, the effective resistance of the crystal unit refers to a resistance of a crystal unit itself in the resonance frequency of each mode.
The negative resistance a makes the feedback circuit having a frequency selectivity when the crystal unit Y
2
is inserted between the intermediate point of the series circuit of the capacitances C
1
and C
2
and the emitter of the transistor. As compared with the negative resistance characteristic b of the general Colpitts oscillator, the negative resistance characteristic becomes a narrow band. In the frequency (10.0 MHz) of the C mode of the main oscillation, a sufficiently large negative resistance is shown, but a small negative resistance is shown or a positive resistance is shown in the frequency (10.9 MHz) of the B mode. Thus, it is not possible to oscillate in the frequency of the B mode.
FIG. 8
shows a modified embodiment disclosed in the above-described patent application laid-open. This shows an oscillation circuit of the case where the crystal unit Y
2
in
FIG. 6
has been substituted by a series circuit of an inductance L and a capacitance C
3
. The series resonance frequency of the inductance L and the capacitance C
3
is set to coincide with the frequency of the C mode of the main oscillation.
There is described in the above publication that, as shown by a one-dot chain line c in
FIG. 7
, the negative resistance characteristics of this oscillation circuit has a frequency characteristics of a relatively larger than that of the curve a, and shows a sufficiently large negative resistance in the frequency of the C mode of the main oscillation, but, in the frequency of the B mode, this value becomes smaller, and it is not possible to oscillate in this mode.
When the oscillation circuit shown in
FIG. 6
is used, the negative resistance characteristics becomes narrow band characteristics and there occurs no frequency jump phenomenon due to the B mode. However, there has been a problem that as this oscillation circuit requires an AT-cut crystal unit of high stability, the oscillator becomes expensive.
Further, a high-stability piezoelectric oscillator has been actually manufactured as a trial by using the oscillation circuit shown in
FIG. 8
, and negative resistance of this oscillation circuit has been measured. As a result, a negative resistance characteristic as shown in
FIG. 9
has been obtained.
As shown by an arrow mark a in
FIG. 9
, a sufficient negative resistance (equal to or less than −1 k&OHgr;) was obtained in the main oscillation (C mode). However, a large negative resistance (about −400 &OHgr;) was also obtained even in the frequency of the B mode shown by an arrow mark &bgr;. Particularly, when the SC-cut crystal unit is used, the oscillation level of the B mode becomes larger than other cut angles, and some oscillation levels become equal to those of the C mode. Therefore, there has been a problem that there is a risk of an occurrence of a frequency jump phenomenon to the B mode.
SUMMARY OF THE INVENTION
With a view to solving the above-described problems, it is an object of the present invention to provide preventing means for avoiding a frequency jump phenomenon to the B mode at low cost.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a Colpitts oscillator including a piezoelectric unit and an amplifier circuit, wherein a composite circuit network consisting of an inductance and a capacitance is inserted into and connected with an oscillation loop, a series resonance frequency of the composite circuit network is set in the vicinity of the oscillation frequency of the oscillator, and a
Kinkead Arnold
Koda & Androlia
Toyo Communication Equipment Co., Ltd.
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