Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-10-30
2004-06-15
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S1160FE, C331S1170FE, C331S167000, C331S10800D, C331S046000, C331S045000, C330S253000
Reexamination Certificate
active
06750727
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to high frequency oscillator circuits. More particularly, this invention relates to metal oxide semiconductors (MOS) oscillators having low phase noise.
2. Description of the Related Art
Inductive/capacitive (LC) oscillators are important elements of any Radio Frequency (RF) communication devices, such as transmitters, where the LC oscillators are used as master oscillators, or as receivers where the LC oscillators are used as local oscillators. An important performance benchmark of and LC oscillator is the phase noise characteristic. An oscillator with a lower phase noise indicates that the oscillator produces lower spurious energy outside the desired fundamental signal tone.
Phase noise is produced as a result of low frequency noise signal found in active elements used in the oscillator. This low frequency signal is modulated (up converted) by the fundamental signal tone, resulting in the spreading of the oscillator frequency energy beyond the intended target frequency. This low frequency noise signal is often referred to as flicker noise (commonly referred to in the literature as 1/f) in bipolar and Metal Oxide Semiconductor (MOS) transistors. The 1/f noise energy in bipolar transistors is known to be significantly less than that of MOS transistors. This is the reason why practically all low phase noise LC oscillators are built using bipolar transistors or even more esoteric transistors such as Galium-Arsenide devices.
Complementary MOS (CMOS) based LC oscillators are now being investigated again for application to systems-on-a-chip (SOC) devices for RF communication applications. LC oscillators of the prior art fall far short of the minimum performance requirements of many of today's wireless communication systems.
A typical example of an LC oscillator in MOS technology is shown in FIG.
2
. It is based on cross-coupled NMOS transistors M
1
and M
2
, a pair of inductors L
1
and L
2
, and capacitor C
1
and C
2
tuning elements. PMOS transistors, which usually have slightly lower 1/f noise characteristics, can be used to replace the NMOS transistors M
1
and M
2
at a slight increase in power dissipation and lower maximum operating frequency.
A review of a general form of the criteria for designing an oscillator circuit of the prior art is shown in FIG.
1
. The necessary components of an oscillator are a frequency dependent gain circuit
100
, a frequency dependent feedback circuit
105
, and a combining block
110
. The output V
0
120
of the gain circuit
100
is the input to the feedback circuit
105
. The input signal V
1
115
is combined in the combining block
110
with the output V
fb
107
of the feedback circuit
105
to form the input
112
of the gain circuit
100
.
The gain of the gain block
100
is designated (G(j&ohgr;) and the gain of the feedback circuit
105
is designated H(j&ohgr;). These gains G(j&ohgr;) and H(j&ohgr;) describe the relationship of their respective output signals V
o
120
and V
fb
107
to their respective input signals
112
and V
o
120
. Therefore, the output signal V
o
120
becomes
V
o
=
V
i
G
(
j
ω
)
1
+
G
(
j
ω
)
H
(
j
ω
)
For an oscillator, the output signal V
o
120
must be nonzero even if the input voltage V
1
115
is zero. For this to be true, then
1+
G
(
j
&ohgr;)
H
(
j
&ohgr;)=0
or
G
(
j
&ohgr;)
H
(
j
&ohgr;)=−1
That is, the magnitude of the open-loop transfer function must be equal to 1 and the phase shift of the gain circuit
100
and the feedback circuit
105
must be 180°.
In
FIG. 2
, the gain circuit of the oscillator is formed by the differentially cross-connected pair of transistors M
1
and M
2
and the constant current source I
1
. The frequency dependent gain determining impedances are formed by the inductors L
1
and L
2
and the capacitors C
1
and C
2
.
The feedback circuit is accomplished by the connecting of the drain of the NMOS transistor M
1
to the gate of the NMOS transistor M
2
and the drain of the NMOS transistor M
2
to the gate of the NMOS transistor M
1
. This forms a cross-coupled differential oscillator.
A CMOS oscillator of the prior art is illustrated in FIG.
3
. In this case, the gain circuit is formed by the differentially connected pair of NMOS transistors M
1
and M
2
, the differentially connected pair of PMOS transistors M
3
and M
4
, and the current sources I
1
and I
2
. As described above, the frequency dependent gain determining impedances are formed by the inductors L
1
and L
2
and capacitors C
1
and C
2
.
The fundamental frequency f
0
of a cross coupled differential oscillator is determined by the formula:
ω
=
1
L
eff
C
eff
such
that
f
o
=
1
2
π
L
eff
C
eff
where:
L
elf
is the value of the effective inductance of the inductors L
1
and L
2
.
C
eff
is the value of the effective capacitance of the capacitors C
1
and C
2
.
For the structure of the design where the inductors are mutually coupled then the effective inductance is:
L
eff
=4
L
1
=4
L
2
The effective capacitance of the capacitors C
1
and C
2
is the parallel combination of the two capacitors C
1
and C
2
and is:
C
eff
=½
C
1
=½
C
2
Combining the above, the frequency of the oscillators of
FIGS. 2 and 3
is:
f
o
=
1
2
π
2
L1C2
.
It should be noted that the capacitances C
1
and C
2
included the parasitic capacitances of the oscillator circuit.
It is well known in the art that phase noise is the result of small perturbations in phase due to small random shifts in oscillator frequency. These shifts are caused by thermal noise, shot noise, and flicker noise (1/f noise). These noises are functions of the device characteristics of the NMOS transistors M
1
and M
2
of
FIGS. 2 and 3
and the PMOS transistors M
3
and M
4
of FIG.
3
. The phase noise is modeled as small voltage sources Vn
1
and Vn
2
at the gates of the NMOS transistors M
1
and M
2
of
FIGS. 2 and 3
and voltage sources Vp
1
and Vp
2
at the gates of the PMOS transistors M
4
and M
4
of FIG.
3
.
The flicker noise (1f
oise) is a function of the active device characteristics of the NMOS transistors M
1
and M
2
of
FIGS. 1 and 2
and PMOS transistors M
3
and M
4
of FIG.
3
.
The advancements in scaling of the device features in semiconductor processing allow multi-gigahertz operating frequencies to be readily achievable. Unfortunately, the same scaling down of MOS transistors have the opposite effect on the 1/f noise characteristics. The smaller device geometries are, the higher the 1/f noise components, leading to higher phase noise on the final oscillator.
“A 1.8 Ghz CMOS Voltage-Controlled Oscillator”,—Razavi, B., Digest of Technical Papers, 43rd ISSCC, 1997, pp. 388-389 and shown in
FIG. 4
describes a structure of having multiple oscillators OSC
1
and OSC
2
coupled together to oscillate in quadrature or 90° out-of-phase. The oscillator OSC
1
and OSC
2
are structured and function as described in FIG.
2
. The differential pair of NMOS transistors M
3
and M
4
and the current source
12
form a first coupling circuit. The first coupling circuit has an in-phase input that is formed by the gate of the NMOS transistors M
3
and a out-of-phase input that is formed by the gate of the NMOS transistors M
4
. The first coupling circuit has a in-phase output that is formed by the drain of the NMOS transistor M
4
and an out-of-phase output that is formed by the drain of the NMOS transistor M
3
. The in-phase input of the first coupling circuit is connected to the drain of the NMOS transistor M
5
and the gate of the NMOS transistor M
6
. The out-of-phase input of the first coupling circuit is connected to the drain of the NMOS transistor M
6
and the gate of the NMOS transistor M
5
. The in-phase output of the first coupling circuit is connected to the drain of the NMOS transistor M
2
and the ga
Kinkead Arnold
Marvell International Ltd.
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