Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-07-28
2001-10-16
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S1160FE, C331S158000, C331S179000
Reexamination Certificate
active
06304152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital-control oscillator circuit provided with an oscillator element and capacitor element, capable of changing the oscillation frequency by changing the capacitance of the load capacitor through a digital signal.
2. Description of the Related Art
An oscillator circuit employing an oscillator element such as a quartz oscillator or a ceramic oscillator provides an oscillation frequency of a high accuracy and a high stability. The oscillator element needs a load. A capacitive element is commonly employed for the load. A change in a capacitance of the capacitive element (hereinafter, referred to as a load capacitor) causes a change in an oscillation frequency of the oscillator circuit.
Oscillator circuits, even when produced in a settled manufacturing process, sometimes have a fluctuation in the oscillation frequency. When the fluctuation goes beyond a tolerance range, the oscillation frequency is adjusted by varying the capacitance of the load capacitor so as to have the frequency fall within a permissible range.
Japanese Patents Laid-open S57-132406 and H05-218738 (hereinafter referred to as D1 and D2, respectively) describe oscillator circuits capable of adjusting an oscillation frequency by changing the capacity of the load capacitor. D1 describes a technique to change the capacitance of the load capacitor depending on the characteristic of the piezoelectric oscillator, while D2 describes a technique to change the capacitance of the load capacitor depending on the temperature variation.
FIG. 1
shows a traditional digital-controlled oscillator circuit of the Colpitts type. The oscillator circuit has a parallel-resonance tank circuit made up of a crystal oscillator
1
, a first capacitor circuit
2
and a second capacitor circuit
3
connected in parallel.
Crystal (quartz) oscillator
1
is connected between gate line
11
and the ground potential.
The oscillator circuit is further provided with a source-follower circuit
4
made up of an N-MOS transistor
9
with a source thereof connected with an emitter-grounded constant-current source
10
.
First capacitor circuit
2
has two voltage-dividing capacitors
5
and
6
in series with one electrode of capacitor
5
connected to the gate of transistor
9
, their common connection
13
connected to output
14
of source follower
4
and another electrode of capacitor
6
being grounded.
First capacitor circuit
2
acts as a main load capacitor. The capacitors
5
and
6
of first capacitor circuit
2
have capacitances C
1
and C
2
, respectively.
The values of C
1
and C
2
are determined from both the desired oscillation frequency and the predetermined ratio of C
1
to C
2
, wherein the ratio C
1
/C
2
is determined so as to ensure a stable oscillation of the oscillator circuit.
Second capacitor circuit
3
has capacitor
7
of capacitance C
3
and switch
8
for connecting capacitor
7
in parallel to or disconnecting it from the tank circuit.
The tank circuit is connected between gate line
11
and the ground potential.
The bias potential for the gate line
11
is supplied from the junction of two voltage-dividing resistors
15
and
16
connected in series between voltage source VDD and the ground potential.
FIG. 2
illustrates the constitution of the Colpitts oscillator circuit shown in FIG.
1
: (a) the basic circuit; (b) the equivalent circuit.
Since the transistor amplifier is a source-follower, the basic circuit of the Colpitts oscillator circuit is represented in a drain-grounded configuration.
In FIG.
2
(
a
), the reactance of crystal oscillator
1
is represented as L, and the dotted lines represent on/off of switch
8
. In FIG.
2
(
b
), X
2
denotes the equivalent reactance to the parallel reactances of crystal oscillator
1
and capacitor
7
. It follows that X
2
equals jL&ohgr; if switch
8
is off and equals −j[C
3
&ohgr;−(L&ohgr;)
−1
]
−1
if switch
8
is on. In the figure, v
A
and v
B
denote an input voltage (gate-source voltage) and an output voltage (source-ground voltage), respectively. The g
m
and g
d
stand for a transconductance and an internal conductance, respectively, and g
m
v
A
represents a voltage-controlled current source, as usual.
The equivalent circuit gives an admittance y
0
=i
o
/v
B
as viewed from 1 and 1′ as follows:
y
0
=
1
j
X
2
-
j
1
C
1
ω
+
-
j
1
C
1
ω
j
X
2
-
j
1
C
1
ω
g
m
+
g
d
+
j
C
2
ω
=
Real
(
y
0
)
+
j
Imaginary
(
y
0
)
,
(
1
)
where
Real
(
y
0
)
=
-
1
X
2
C
1
ω
-
1
g
m
+
g
d
(
2
)
Imaginary
(
y
0
)
=
-
C
1
ω
X
2
C
1
ω
-
1
+
C
2
ω
,
(
3
)
X
2
=jL&ohgr; when switch
8
is off, and
X
2
=
-
L
C
3
L
ω
=
1
C
3
ω
when switch
8
is on.
Equating Imaginary(y
0
) to 0 gives the frequency condition
X
2
&ohgr;[C
1
C
2
/(
C
1
+C
2
)]=1. (4)
Generation of a stable oscillation requires further condition for sustaining a resonance oscillation without decay. This condition will hereinafter be referred to as an oscillation condition.
The oscillation condition is given by the inequality
Real(
y
0
)<0, (5)
i.e.,
-
1
X
2
C
1
ω
-
1
g
m
+
g
d
<
0
(
6
)
Substitution of equation (4) to equation (6) yields the oscillation condition for a resonance frequency:
C
1
C
2
g
m
g
d
=
C
1
C
2
μ
>
1
(
7
)
It is to be noted that the oscillation condition (5) is independent of X
2
at the resonance frequency as represented in inequality (7), while it relates to X
2
as represented in inequality (6) when the frequency is not at a resonance frequency.
It is an object of the present invention to provide a digital-control Colpitts crystal oscillator circuit of a modified type capable of changing an oscillator frequency sustaining a stable oscillation.
SUMMARY OF THE INVENTION
As is well known, a Colpitts oscillator circuit has: a transistor amplifier; voltage-dividing first capacitor and second capacitor with the first capacitor connected between an output of said transistor amplifier and a control electrode of the transistor amplifier and the second capacitor connected between the output of the transistor amplifier and an AC-ground potential; and an oscillator element connected between the control electrode of the transistor amplifier and the AC-ground potential.
In this circuitry, the first and second capacitors are connected in series through the output of the transistor amplifier, and the series-connected first and second capacitors act as a load capacitor of the parallel resonance tank circuit. Thus, changing the equivalent capacitance of the series capacitors caused the resonance frequency.
However, an arbitrary change of the equivalent capacitance does not always guarantee sustention of a stable oscillation.
The analysis of the Colpitts oscillation circuit shows that the feedback gain of the Colpitts oscillation circuit, namely the ratio of the input voltage to output ratio of the transistor amplifier, is proportional to the ratio of the capacitance of the first capacitor to that of the second capacitor, at a resonance frequency.
Thus, changing the oscillation frequency while sustaining a stable oscillation can be attained by configuring the capacitance of the load capacitor to be variable under the condition that the ratio of the capacitance of a first capacitor to that of a second capacitor is kept unchanged at a prescribed value. The value of the ratio is prescribed so that a stable oscillation is sustained.
The Colpitts oscillator circuit of the present invention has a resonance circuit featured by a load capacitor: the capacitance of the load capacitor is variable under the condit
Nakamura Kouji
Takahashi Yutaka
Foley & Lardner
Mis David
NEC Corporation
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