Oscillators – With device responsive to external physical condition – Temperature or light responsive
Reexamination Certificate
2003-02-19
2004-08-03
Mis, David (Department: 2817)
Oscillators
With device responsive to external physical condition
Temperature or light responsive
C331S068000, C331S158000, C331S176000
Reexamination Certificate
active
06771135
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator (TCXO), and more particularly, to a temperature-compensated crystal oscillator which maintains a satisfactory phase noise characteristic and facilitates adjustments to its oscillation frequency.
2. Description of the Related Arts
A temperature-compensated crystal oscillator compensates variations in frequency due to a frequency-temperature characteristic of a quartz crystal unit to stably maintain the oscillation frequency notwithstanding a change in ambient temperature. Such a temperature-compensated crystal oscillator is widely used as a frequency source particularly in a portable telephone and the like which is used in a mobile environment. In recent years, the temperature-compensated crystal oscillator is required to excel in the phase noise characteristic, needless to say that it must be small in size and low in cost.
FIG. 1
illustrates an exemplary configuration of a conventional temperature-compensated crystal oscillator of a voltage-controlled type. The illustrated temperature-compensated crystal oscillator comprises voltage-controlled crystal oscillator (VCXO)
1
; compensating voltage generator (COMP)
2
for generating a compensating voltage which is applied to the voltage-controlled crystal oscillator
1
as a control voltage; low pass filter (LPF)
3
; and switching element
4
. Voltage-controlled crystal oscillator
1
generally comprises crystal unit
5
; two voltage variable capacitance elements connected to both ends of crystal unit
5
, respectively, to form a resonator circuit together with crystal unit
5
; and inverter
7
for oscillation which amplifies a current of the resonator circuit and feeds back the amplified current. Variable capacitance diodes
6
are used herein for the voltage variable capacitance elements, and each has a grounded anode and a cathode connected to crystal unit
5
. Feedback resistor
8
is inserted to connect an input terminal to an output terminal of inverter
7
. The input terminal and output terminal of inverter
7
are connected to the resonator circuit, respectively, through DC blocking capacitors
9
. Oscillation output Vout is generated from the output terminal of inverter
7
.
Crystal unit
5
is made, for example, of an AT-cut quartz crystal blank, and its frequency-temperature characteristic is represented by a cubic function curve as indicated by curve A in FIG.
2
. In
FIG. 2
, the vertical axis represents a frequency deviation &Dgr;f/f, where f is the frequency at 25° C. Due to the use of such crystal unit
5
having the frequency-temperature characteristic as illustrated, the crystal oscillator also exhibits a similar frequency-temperature characteristic in its oscillation frequency.
Compensating voltage generator
2
comprises a temperature sensor connected to power supply Vcc for detecting, for example, an ambient temperature; and a cubic function generator for generating a voltage which changes in accordance with a cubic function in response to a detected temperature. Compensating voltage generator
2
generates compensating voltage Vc in response to the ambient temperature. Compensating voltage Vc is applied to the cathode of each variable capacitance diode
6
through high frequency blocking resistor
10
. Each of variable capacitance diode
6
changes the capacitance between the anode and cathode terminals in response to compensating voltage Vc applied thereto, resulting in a change in an equivalent series capacitance (load capacitance) viewed from crystal unit
5
to cause a change in the oscillation frequency. Compensating voltage Vc is set herein to change in response to the temperature in accordance with a cubic function curve, as indicated by curve B in
FIG. 2
, to compensate the crystal oscillator for the frequency-temperature characteristic so that the characteristic curve becomes flat.
Low pass filter
3
, which comprises a CR time constant circuit composed of capacitor (C)
11
and resistor (R)
12
, is inserted between voltage-controlled crystal oscillator
1
and compensating voltage generator
2
. Compensating voltage Vc from compensating voltage generator
2
is applied to variable capacitance diodes
6
through low pass filter
3
, thereby removing low frequency noise components possibly included in compensating voltage Vc.
Low pass filter
3
comprising a CR time constant circuit, provided as mentioned above, causes a delay in applying compensating voltage Vc to variable capacitance diodes
6
upon start of the temperature-compensated crystal oscillator, thereby exacerbating the starting characteristic of the oscillator. To solve this problem, the temperature-compensated crystal oscillator has switching element
4
connected in parallel with low pass filter
3
such that low pass filter
3
is short-circuited upon starting. Specifically, switching element
4
is connected in parallel with resistor
12
of the time constant circuit, and short-circuits resistor
12
only in the event of starting the temperature-compensated crystal oscillator, and is turned off after the starting. This avoids the delay in the operation upon starting due to the CR time constant circuit, resulting in a satisfactory starting characteristic. Switching element
4
is controlled by starting control circuit (START CNTL)
13
connected to power supply Vcc.
Starting control circuit
13
comprises PNP transistor
18
connected between power supply Vcc and switching element
4
; resistor
19
connected between the base of PNP transistor
18
and a ground point; and capacitor
20
connected between the base and power supply Vcc, for example, as illustrated in FIG.
3
. With this circuit configuration, PNP transistor
18
conducts to turn on switching element
4
when the crystal oscillator is powered on. Subsequently, the base voltage increases to the same potential as power supply Vcc by a time constant determined by capacitor
20
and resistor
19
to turn off PNP transistor
18
, thereby turning off switching element
4
.
The respective circuits, which form the temperature-compensated crystal oscillator, are generally integrated in a single IC (integrated circuit) chip except for crystal unit
5
.
Then, the temperature-compensated crystal oscillator is assembled, for example, as illustrated in
FIG. 4
, by securing IC chip
15
on the bottom of a recess formed in container body
14
made of laminated ceramic, for example, by face down bonding, and securing one end of quartz crystal blank
5
A, which constitutes crystal unit
5
, on a step formed on the recess with a conductive adhesive. Excitation electrodes
17
are formed on both main surfaces of crystal piece
5
A. Then, one of excitation electrodes
17
on crystal blank
5
A is irradiated with an ion beam, as indicated by arrow P in
FIG. 4
, to reduce the thickness of excitation electrode
17
in order to adjust the oscillation frequency of the oscillator, such that the oscillation frequency matches a reference frequency. Stated another way, the oscillation frequency is adjusted by decreasing an additional mass to crystal unit
5
. The reference frequency used herein refers to a so-called nominal frequency at which oscillation should be carried out, for example, at a room temperature. After the adjustment is completed for the oscillation frequency, a lid member is placed to cover the recess of container body
14
to encapsulate crystal blank
5
A and IC chip
15
within the recess, thereby completing the temperature-compensated crystal oscillator. A circuit pattern is formed on the surface of the recess of container body
14
for electrically connecting IC chip
15
to crystal blank
5
A, and electrode portions are formed on the outer surface of container body
14
for connecting the temperature-compensated crystal oscillator to an external circuit.
However, the temperature-compensated crystal oscillator in the foregoing configuration suffers from the inability to accurately match the oscillation frequency with the reference frequency
Asamura Fumio
Kubo Kuichi
Knobbe Martens Olson & Bear LLP
Mis David
Nihon Dempa Kogyo Co. Ltd.
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