Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-04-06
2002-09-10
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06448689
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a piezoelectric oscillation circuit; in particular, to an inexpensive piezoelectric oscillation circuit that has excellent aging characteristics.
PRIOR ART
Crystal oscillators that have excellent frequency stability (hereafter referred to as “aging characteristics”) despite years of changes generally comprise an AGC circuit such as that shown in
FIG. 4
in order to operate a crystal vibrator at low excitation levels, to restrain the driving stress exerted upon on a crystal vibrator.
FIG. 4
is a circuit diagram of a crystal oscillator comprising a conventional AGC circuit.
In crystal oscillator
100
shown in the figure, output from an oscillation circuit
101
enclosed by a broken line is supplied to a buffer circuit
103
in the next stage via a capacitor
102
; output from buffer circuit
103
is applied to an AGC circuit
104
enclosed by -a broken line; and a signal at an anode of a diode
116
which corresponds to an output from AGC circuit
104
is returned to oscillation circuit
101
.
Oscillation circuit
101
is of a general Colpitts type: a crystal vibrator
106
with one end grounded via a capacitor
105
has the other end connected to a base of a transistor
107
, a series circuit composed of capacitors
108
and
109
as well as a resistor
110
are connected between the base and the ground, and a connection midpoint of the series circuit composed of capacitors
108
and
109
is connected to an emitter of transistor
107
, the emitter being grounded via a resistor
111
.
The base of transistor
107
is connected to the output of AGC circuit
104
via a resistor
112
, and a collector of transistor
107
is connected to a power supply Vcc via a resistor
113
.
The AGC circuit
104
is comprised of resistors
114
,
120
,
121
,
122
, capacitors
115
,
118
,
119
, diodes
116
and
117
. A cathode end of the diode
116
and an anode end of the diode
117
connect to an output end of buffer circuit
103
via a series circuit of resistor
114
and capacitor
115
. The other ends of the diodes are grounded via capacitor
118
and
119
, respectively and are connected together via a resistor
120
. The anode end of diode
116
is connected to the base of transistor
107
via resistor
112
included in oscillation circuit
101
. Additionally, a series circuit composed of resistors
121
and
122
is connected between the power supply Vcc and the ground and has its connection midpoint connected to a cathode of diode
117
.
Buffer circuit
103
is connected to an output of the oscillator via a series circuit composed of a resistor
123
and a capacitor
124
.
The operation of the crystal oscillator will be described below. The oscillation circuit
101
is of the general Colpitts type; since the particulars of this circuit are well known, a description is omitted.
When an excitation signal output from oscillation circuit
101
is applied to AGC circuit
104
via buffer
103
, a plus half cycle of the signal is directed to the ground via diode
117
, and a minus half cycle of the signal is directed to the ground via diode
116
. Accordingly, a current A is generated between terminals of resistor
120
due to the occurrence of such a voltage that provides a low voltage at the terminal connected to diode
116
.
In addition to current A, a current B determined on the basis of a voltage generated at a connection midpoint between resistor
121
and resistor
122
is supplied to resistor
120
. As a result, a total current obtained by subtracting current B from current A is supplied to a base current flowing through transistor
107
, and the base current determined by the total current causes the crystal vibrator
106
to generate an excitation signal of a desired fixed level.
In the above operational state, if the level of the excitation signal from crystal vibrator
106
increases, the level of the excitation signal supplied to AGC circuit
104
increases and the voltage across resistor
120
rises to increase the value of current A, thereby reducing the total current (current B—current A) and the base current of transistor
107
.
The decrease in the base current of transistor
107
reduces the gain of transistor
107
, reducing the output level (the collector output level of transistor
107
) of oscillation circuit
101
and thus reducing the emitter voltage (AC component) level of transistor
107
. Consequently, the level of a signal generated between the terminals of capacitor
109
falls, reducing the level of the excitation signal from crystal vibrator
106
.
If the level of the excitation signal from crystal vibrator
106
increases, then an operation that is the reverse of the above-described operation occurs. A description of this operation is omitted.
The above operation is repeated to have the level of the excitation signal from the crystal vibrator
106
being stable to the desired value.
However, since the electronic parts (including the crystal vibrator) constituting the oscillation circuit and AGC circuit have varying characteristics, the circuit must be configured to obtain a low and desired excitation level—that is, an optimal excitation level—taking into account the characteristics of the individual parts. This prevents cost-efficient production of the crystal oscillator.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above problems to provide an inexpensive crystal oscillator having excellent aging characteristics.
To resolve this problem, the present invention provides a piezoelectric oscillator comprising a Colpitts circuit including a piezoelectric vibrator and a load capacitance comprising at least a capacitor in the oscillating loop, and a limiter circuit, wherein an excitation signal from the piezoelectric vibrator is supplied to the limiter circuit. Based on variations in excitation signal, the limiter At controls an inter-terminal impedance of said capacitor which is connected to said limiter and thus clips (the sill level outside the range of a fixed is eliminated) the level of the excitation signal.
In this configuration, the limiter circuit comprises a series circuit comprising two diodes connected in series so that a forward current of the diodes flows from a power supply toward a ground, and a capacitor connected in parallel win the series circuit, with a connection midpoint between the two diodes connected to one end of the capacitor.
Moreover, the capacitor is inserted and connected between the piezoelectric vibrator and the ground; the capacitor includes two divided capacitors connected in series; the limiter circuit comprises a series circuit comprising two diodes connected in series so that a forward current of the diodes flows from a power supply toward a ground; and a capacitor connected in parallel with the series circuit, and a connection midpoint between the two divided capacitors and a connection midpoint between the two diodes are connected together via a capacitor.
Moreover, the present invention provides a Colpitts circuit comprising a transistor circuit having a resistance element connected to an appropriately biased emitter, a piezoelectric vibrator inserted between a base and a ground of he transistor circuit, and two divided capacitors, one of which is connected between the base and emitter of the transistor, with the other connected in parallel with the emitter resistor, wherein a resistor of a desired value and two diodes are inserted in series between a power supply and the ground, the Colpitts circuit includes a capacitor for earthing the diode series circuit toward a high frequency, and the two diode connection points and the emitter of the transistor are connected to obtain an alternate current.
REFERENCES:
patent: 3629726 (1971-12-01), Popescu
patent: 4277710 (1981-07-01), Harwood et al.
patent: 4578650 (1986-03-01), Watson
patent: 5113116 (1992-05-01), Wilson
patent: 5216338 (1993-06-01), Wilson
Budd Mark O.
Koda & Androlia
Toyo Communication Equipment Co., Ltd.
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