Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-10-12
2003-04-08
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S03600C, C331S11300A, C331S17700V, C331S175000
Reexamination Certificate
active
06545555
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electronic oscillator circuits, and more particularly, to differential voltage controlled oscillator circuits.
In a tuned oscillator, the frequency of oscillation may be varied by means of a control signal. The term voltage controlled oscillator (hereinafter referred to as VCO) generally refers to a class of oscillators whose frequency is a function of the voltage of the input signal. Thus, conventional VCOs are constructed to generate an output signal having a frequency that is a function of the voltage of the input signal. VCOs are used in a wide variety of applications, and are particularly useful in transmitters and receivers to generate an oscillation at a specific frequency.
A conventional differential controlled oscillator
1
topology is shown in FIG.
1
. This conventional differential VCO
1
topology includes biasing circuitry
2
, an oscillator core
4
, a tuning circuit
6
, inductors
8
a
,
8
b
and output capacitors
10
a
,
10
b.
The biasing circuitry generally provides the voltage drop that serves as the bias for core transistors
18
,
20
. The biasing circuitry comprises a bias resistor
12
or optionally a current source that is connected between ground
30
and the emitters of the core transistors
18
,
20
, and two bias resistors
14
,
16
that are connected between the bases of the core transistors
18
,
20
. The biasing resistor
12
also provides some common mode rejection to the differential pair of the core transistors
18
,
20
. The two bias resistors
14
,
16
provide a voltage input at the bases
18
b
,
20
b
of the core transistors
18
,
20
, respectively.
The oscillator core
4
may comprise, for example, at least one pair of bipolar junction transistors
18
,
20
(referred to above as core transistors), a first pair of capacitors
22
,
24
, a capacitor
26
cross-coupled between the base
18
b
of core transistor
18
and the collector
20
c
of core transistor
20
, and a second capacitor
28
cross-coupled between the base
20
b
of core transistor
20
and the collector
18
c
of core transistor
18
. The two capacitors
22
,
24
are connected between the bases of bipolar junction transistors
18
,
20
and the AC ground node
30
, respectively. Two cross-coupled linear capacitors
26
,
28
provide a positive feedback loop to produce oscillations. The ratio between the two linear capacitors
26
,
28
to the capacitors
22
,
24
, respectively sets the amount of positive feedback in the oscillator. The core transistors
18
,
20
provide gain in the VCO
1
, and emitters
18
e
and
20
e
of each core transistor
18
,
20
are coupled to an AC ground node
30
by bias resistor
12
.
The VCO
1
essentially functions as a signal generator that generates oscillations at a specific oscillation frequency (w). A tuning circuit
6
may comprise, for example, a pair of varactors
34
a
,
34
b
and a pair of bias resistors
36
a
,
36
b
connected in parallel across the varactors
34
a
,
34
b
. A capacitor
38
a
is coupled to the varactor
34
a
, and is connected between the collector
18
c
and the bias resistor
36
a
. Another capacitor
38
b
is coupled to the varactor
34
b
, and is connected between the collector
20
c
and the bias resistor
36
b
. The inductors
8
a
,
8
b
bias the collectors
18
c
,
20
c
of the core transistors
18
,
20
, and have some influence over the loop gain. Inductors
8
a
,
8
b
combine with the tuning circuit
6
create a variable tank circuit in the VCO
1
. In other words, the inductors
8
a
,
8
b
, along with the total capacitance of the oscillator core
4
, may be used to set the oscillation frequency of the VCO
1
which is
w
=
1
LC
.
The inductance (L) of the VCO is determined primarily by the values of the inductors
8
a
,
8
b
, while the capacitance (C) of the VCO is all of the capacitance in the VCO
1
. For example, the values chosen for the linear capacitors
26
,
28
influence the oscillation frequency (w) of the VCO
1
since those linear capacitors are placed across the nodes of the tank circuit. Importantly, the total capacitance (C) of the VCO
1
can be varied via the tuning circuit
6
since each varactor
34
a
,
34
b
in the tuning circuit
6
can be characterized by a voltage-sensitive capacitance. The control voltage V
tune
32
may be used to change the voltage across these varactors
34
a
,
34
b
, which in turn changes the capacitance of the varactors
34
a
,
34
b
to thereby change the total capacitance of the VCO
1
. Thus, by changing the control voltage V
tune
32
, the oscillation frequency (w) of the VCO
1
is varied.
The output coupling capacitors
10
a
,
10
b
are connected between the output of the VCO
1
and the output terminals
40
a
,
40
b
of the VCO
1
. The capacitors
10
a
,
10
b
couple the output signal to the next circuit, and determine how much signal will be coupled to the output. The total output signal is the difference between output A
40
a
and output B
40
b
or vice versa.
The performance of an oscillator may be characterized by a number of attributes, including spectral purity, frequency stability, and signal-to-noise ratio. To achieve frequency stability it is desirable to reduce the amount of phase noise. One problem that typically occurs in the VCO shown in
FIG. 1
is that the output may exhibit an unacceptable level of phase noise which decreases the signal-to-noise ratio of the VCO.
The signal-to-noise ratio may be increased by increasing current in the VCO to increase the amount of signal with respect to phase noise. However, a 6 dB improvement in phase noise typically requires doubling of current in the VCO. Such an increase in current consumption is not an option in many applications. Accordingly, there is a need in the art for an improved VCO with reduced phase noise that may be implemented in a simple, inexpensive circuit. It would be highly desirable to reduce phase noise without an appreciable increase in current consumption. It is also highly desirable to accomplish both of these objectives without substantially increasing the cost or size of the VCO.
SUMMARY OF THE INVENTION
Differential voltage controlled oscillators are provided including a differential oscillator core, a first linear capacitor, a second linear capacitor, a first phase noise reduction device, and a second phase noise reduction device. The differential oscillator core has a first output that is capacitively coupled to a first differential input thereof by a first semiconductor junction having a first non-linear capacitance, and a second output that is capacitively coupled to a second differential input thereof by a second semiconductor junction having a second non-linear capacitance. The first linear capacitor is electrically cross-coupled from the first differential input to the second output, while the second linear capacitor electrically cross-coupled from the second differential input to the first output. The cross-coupled first and second linear capacitors provide a positive feedback loop to generate oscillations. The ratio between the first and second linear capacitors sets the amount of positive feedback in the oscillator. The first phase noise reduction device has a first non-linear capacitance characteristic, and is electrically cross-coupled from the first differential input to the second output. The second phase noise reduction device has a second non-linear capacitance characteristic, and is electrically cross-coupled from the second differential input to the first output. By using the first and second phase noise reduction devices to add non-linear junction capacitance to each side of the differential structure, the non-linear junction capacitances of the first and second semiconductor junctions are effectively canceled out to reduce the phase noise of the oscillator.
The first and second phase noise reduction devices are preferably p-n junction devices having non-linear capacitance characteristics that are preferably matched to the first and second non-linear capacitances over the ent
Bengtsson Erik
Justice Scott
Kinkead Arnold
Myers Bigel & Sibley & Sajovec
Telefonaktiebolaget LM Ericsson
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