Oscillators – With frequency adjusting means – With voltage sensitive capacitor
Reexamination Certificate
2002-08-30
2004-05-04
Wells, Kenneth B. (Department: 2817)
Oscillators
With frequency adjusting means
With voltage sensitive capacitor
C331S03600C, C331S1170FE, C331S1170FE
Reexamination Certificate
active
06731182
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage-controlled oscillator of which the oscillation frequency is varied according to a tuning voltage, and in particular to a voltage-controlled oscillator for use in a high-frequency communication apparatus or the like.
2. Description of the Prior Art
Voltage-controlled oscillators are generally used in high-frequency communication apparatus and the like, particularly as oscillators for frequency synthesizers serving as local oscillators in wireless communication apparatus. Now, a conventional voltage-controlled oscillator will be described.
FIG. 4
shows a typical configuration of a conventional voltage-controlled oscillator. The voltage-controlled oscillator includes a resonant portion
1
and an active portion
2
.
The resonant portion
1
is a circuit having an inductor L
1
and a variable-capacitance device C
1
, such as a variable-capacitance diode, connected in parallel. The symbol RT represents the sum of the parasitic resistance of the inductor L
1
and the parasitic resistance of the variable-capacitance device C
1
. One of the nodes at which the inductor L
1
and the variable-capacitance device C
1
are connected together is connected to an output terminal S
1
, and the other is connected to an output terminal S
2
.
A reference voltage supply terminal
4
is connected to the half point of the inductor L
1
. A reference voltage V
CC
is supplied to the reference voltage supply terminal
4
. The capacitance of the variable-capacitance device C
1
is controlled by a tuning voltage V
tun
that is applied to a control terminal
5
.
The active portion
2
includes NPN-type transistors Q
1
and Q
2
, bias resistors R
1
and R
2
, signal level adjustment capacitors C
21
, C
22
, C
31
, and C
32
, and a DC (direct-current) current source
3
.
The collector of the transistor Q
1
is connected to the output terminal S
1
, and the collector of the transistor Q
2
is connected to the output terminal S
2
.
The base of the transistor Q
1
and the collector of the transistor Q
2
are connected together through the capacitor C
21
, and the base of the transistor Q
2
and the collector of the transistor Q
1
are connected together through the capacitor C
22
.
The base of the transistor Q
1
is connected through the bias resistor R
1
to a bias voltage supply terminal
6
, and is also grounded through the capacitor C
31
. The base of the transistor Q
2
is connected through the bias resistor R
2
to the bias voltage supply terminal
6
, and is also grounded through the capacitor C
32
.
The emitters of the transistors Q
1
and Q
2
are connected together, and are grounded through the DC current source
3
.
The voltage-controlled oscillator configured as described above operates as follows. The resonant portion
1
outputs a signal having a given resonant frequency to the output terminals. The negative resistance of the active portion
2
is canceled by the resistance RT and the load resistance connected between the output terminals S
1
and S
2
, and thus the output voltage V
o
appearing between the output terminals S
1
and S
2
is a standing wave. In this way, it is possible to obtain an output voltage V
o
having the desired frequency according to the V
tun
.
One of the parameters that describe the performance of the resonant portion
1
is the Q factor. If the impedance of the resonant portion
1
is assumed to be Z, then the Q factor is given by formula (1) below.
Q=Image (Z)/Real (Z) (1)
On the other hand, if the offset frequency from the oscillation frequency is assumed to be &Dgr;f, then the phase noise n(&Dgr;f) of the voltage-controlled oscillator is approximated by formula (2) below.
n
(&Dgr;
f
)∝1/[
Q
2
×(&Dgr;
f
)
2
] (2)
Hence, the higher the Q factor of the resonant portion
1
, the more the phase noise can be reduced. For this reason, it is generally believed that portable communication apparatus require the use of voltage-controlled oscillators of which the resonant portion has a high Q factor.
On the other hand, in portable communication apparatus, to make them compact and inexpensive, voltage-controlled oscillators need to be formed as integrated circuits. However, monolithic passive devices have low Q factors, and therefore forming the resonant portion of a voltage-controlled oscillator as an integrated circuit lowers the Q factor of the resonant portion, degrading the phase noise characteristics of the voltage-controlled oscillator.
Japanese Patent Application Laid-Open No. H10-145190 proposes to solve this problem by adopting a circuit configuration that reduces phase noise, and by forming an inductor out of bonding wire.
FIG. 5
shows a voltage-controlled oscillator disclosed in Japanese Patent Application Laid-Open No. H10-145190 mentioned above. Here, such circuit elements and signals as are found also in
FIG. 4
are identified with the same reference numerals and symbols, and overlapping explanations will not be repeated.
The voltage-controlled oscillator of
FIG. 5
differs from the voltage-controlled oscillator of
FIG. 4
in the following respects. The inductor L
1
is replaced with inductors L
11
and L
12
. The variable-capacitance device C
1
is replaced with variable-capacitance diodes C
11
and C
12
. The DC current source
3
is replaced with a resistor R
3
. The capacitors C
31
and C
32
are abolished. Between the inductor L
11
and the variable-capacitance diode C
11
, a high-pass filter
7
composed of a capacitor and a resistor is inserted. Between the inductor L
12
and the variable-capacitance diode C
12
, a high-pass filter
8
composed of a capacitor and a resistor is inserted. The cathodes of the variable-capacitance diodes C
11
and C
12
are connected together, and are then connected through a resistor R
4
to the tuning voltage supply terminal
5
.
In the voltage-controlled oscillator of
FIG. 5
, the high-pass filters
7
and
8
provided in the resonant portion serve to separate the resonant portion from low-frequency fluctuation of the reference voltage V
CC
, thereby reducing phase noise. However, the provision of the high-pass filters
7
and
8
in the resonant portion tends to lower the Q factor of the resonant portion as a whole and thus eventually increase phase noise n(&Dgr;f).
Japanese Patent Application Laid-Open No. H10-145190 mentioned above also discloses a configuration in which, for the sake of integration, the inductors L
11
and L
12
are replaced with bonding wires. This configuration, however, suffers from lower reliability of the oscillator, when mounted, and increased variations in the inductances of the inductors.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a voltage-controlled oscillator that offers satisfactory phase noise characteristics even when formed as an integrated circuit.
To achieve the above object, according to the present invention, a voltage-controlled oscillator is provided with a resonant portion including at least one variable-capacitance device and at least one inductor, an active portion including differential pair transistors and supplying electric power to the resonant portion in such a way as not to attenuate the oscillation of the resonant portion, and a current source connected to the emitters or sources of both of the differential pair transistors. Here, the current source is a circuit having a resistor and a capacitive device connected in parallel.
REFERENCES:
patent: 6046647 (2000-04-01), Nelson
patent: 6542043 (2003-04-01), Cao
Birch & Stewart Kolasch & Birch, LLP
Sharp Kabushiki Kaisha
Wells Kenneth B.
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