Oscillators – With frequency adjusting means – With voltage sensitive capacitor
Reexamination Certificate
2002-04-29
2004-03-23
Choe, Henry (Department: 2817)
Oscillators
With frequency adjusting means
With voltage sensitive capacitor
C331S03600C
Reexamination Certificate
active
06710669
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage control type oscillating circuit.
2. Description of the Related Art
At present, as a voltage controlled oscillator, there is a constitution used by externally attaching a piezoelectric resonator such as a crystal resonator and a variable capacitance diode or so-to-speak varicap diode to an oscillation amplifying portion integrated into a single chip. As such a voltage controlled oscillator, there are constitutions shown by, for example, FIG.
8
and FIG.
9
. According to a constitution shown in
FIG. 8
, a feedback resistor rf is connected between an input terminal in and an output terminal out of a CMOS inverter iv
1
and a capacitor cg constituting a load capacitance is connected between the input terminal in and a power source terminal VDD (for example, 3V). A connection point of the input terminal in and the capacitor cg is connected with a terminal xin for externally attaching one terminal of a crystal resonator x
1
constituting a piezoelectric resonator. A connection point of the output terminal out and the resistor rf is connected with one terminal of a resistor rd for restricting current flowing in the crystal resonator x
1
, other terminal of the resistor rd is connected with one terminal of a capacitor cc for cutting the direct current components and a capacitor cd constituting a load capacitance on a side of the output terminal out is connected between the one terminal of the capacitor cc and a power source terminal VDD. Further, other terminal of the capacitor cc is connected with a terminal vout for externally attaching an anode of a varicap diode cv. Further, there is provided a terminal xout for externally connecting a cathode of the varicap diode cv and other terminal of the crystal resonator x
1
. The terminal xout is connected with a control terminal ct
1
for applying control voltage to the varicap diode cv via a resistor rb
1
and the terminal vout is connected with the anode of the varicap diode cv and a power source terminal VSS (0 v) via a resistor rb
2
. By externally connecting the crystal resonator x
1
between the terminal xin and the terminal xout and externally connecting the varicap diode cv between the terminal vout and the terminal xout, a voltage controlled oscillator is constituted by connecting a series circuit of the crystal resonator x
1
and the varicap diode cv between the input terminal in and the output terminal out of the CMOS inverter iv
1
functioning as an oscillation amplifying portion, in other words, to the power source terminals VDD via the load capacitances. A capacitance value of the varicap diode cv is changed by the control voltage applied from a control circuit, not illustrated, to the terminal ct
1
to thereby control oscillation frequency.
An explanation will be given of a constitution shown in FIG.
9
. In
FIG. 9
, respective constituent elements in correspondence with respective constituent elements of
FIG. 8
are designated by the same notations. The respective constituent elements other than the feedback resistor rf and the CMOS inverter iv
1
are externally attached. The input terminal in and the output terminal out of the CMOS inverter iv
1
are respectively connected with the terminals xin and xout for external attachment, the terminal xin and the terminal xout are externally attached respectively with the capacitors cg and cd and the crystal resonator x
1
is externally attached between the terminal xin and the terminal xout. Further, the capacitor cc for cutting direct current components and the varicap diode cv are connected in series between the terminal xin and the power source terminals VSS (in
FIG. 9
, constituting external power source terminals set to, for example, 0V) and a connection point of the capacitor cc and the varicap diode cv is connected with the control terminal ct
1
via the resistor rb
1
. Also according to the constitution of
FIG. 9
, the capacitance value of the varicap diode cv is changed by control voltage applied from a control circuit, not illustrated, to the terminal ct
1
to thereby control the oscillation frequency.
According to the voltage controlled oscillators, described above, it is generally requested to operate the crystal resonator x
1
by a low amplitude in order to enlarge a variable width of the oscillation frequency. Therefore, for example, there is a constitution of restricting supply current to the oscillation amplifying portion, that is, the CMOS inverter iv
1
and according to such a constitution, conductance gm of the oscillation amplifying portion is lowered. Further, in order to maintain oscillation starting performance, it is necessary to maintain a negative resistance −RL at a large value to some degree. When the oscillation frequency is designated by notation &ohgr; and the load capacitances are designated by notations Cg and Cd, the negative resistance −RL is represented by −RL≈−gm/(&ohgr;
2
Cd·Cg) and in order to maintain the negative resistance while reducing the conductance gm, it is necessary to reduce the load capacitances Cg and Cd. However, reduction in the load capacitances Cg and Cd amounts to reduction in the variable width of the oscillation frequency. Therefore, ensuring the oscillation starting performance and enlarging the variable width of the oscillation frequency are not compatible with each other.
SUMMARY OF THE INVENTION
Hence, it is an object of the invention to provide a voltage controlled oscillator having a large variable width of an oscillation frequency while ensuring oscillation starting performance.
According to the invention, there is provided a voltage controlled oscillator comprising an inverter connected with a piezoelectric resonator between an input terminal and an output terminal thereof, a varicap diode in which a cathode thereof is connected with a control terminal and an anode thereof or the cathode is connected to the output terminal or the input terminal via a cutting capacitance element for cutting a direct current component, and a switching element for selectively connecting a capacitance element other than the cutting capacitance element in series with a series circuit constituted by the piezoelectric resonator and the varicap diode, wherein the switching element is made OFF when oscillating operation is brought into an initial state.
Further, according to the voltage controlled oscillator of the invention, it is preferable that pluralities of the capacitance elements and the switching elements are provided and the plurality of the capacitance elements are selectively connected by the plurality of the switching elements.
Further, according to the voltage controlled oscillator of the invention, it is preferable that the switching element is an MOS transistor and an on resistance of the MOS transistor is controlled by a control voltage applied to a gate thereof.
Further, it is preferable that the voltage controlled oscillator of the invention is provided with a detecting circuit for generating a detection output by detecting that an oscillation signal of the inverter is provided with a predetermined amplitude value, wherein the switching element is made on in response to generating the detection output.
REFERENCES:
patent: 5030926 (1991-07-01), Walden
patent: 6040744 (2000-03-01), Sakurai et al.
patent: 2002/0024397 (2002-02-01), Fujii et al.
Hasegawa Eiichi
Oyama Kazuhisa
Choe Henry
Jordan and Hamburg LLP
Nippon Precision Circuits Inc.
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