Oscillators – Plural oscillators
Reexamination Certificate
1999-08-25
2001-06-19
Mis, David (Department: 2817)
Oscillators
Plural oscillators
C331S03600C, C331S1170FE, C331S172000, C331S17700V, C331S17700V, C455S262000, C455S318000
Reexamination Certificate
active
06249190
ABSTRACT:
FIELD OF THE INVENTION
The field of the present invention relates generally to differential oscillators and more specifically, to differential dual Colpitts oscillators.
BACKGROUND OF THE INVENTION
An oscillator generates a periodic signal. Accordingly, an oscillator must have a self-sustaining mechanism that allows its own noise to grow and eventually become a periodic signal. Oscillators having a periodic signal whose frequency falls in the radio frequency range (RF) are often referred to as RF oscillators.
Many RF oscillators use feedback circuits to generate the periodic signal. In these RF oscillators, a frequency-selective network such as an inductor-capacitor (LC) tank is included in the feedback loop in order to stabilize the frequency. The frequency-selective network is also called a “resonator.” The nominal frequency of oscillation is often determined by the characteristics of the circuit including, for example, the resonance frequency of the LC tank.
Most discrete RF oscillators incorporate only one active device (e.g., a transistor). There are two reasons for using a one-transistor topology: noise is minimized and costs are reduced.
FIG. 1
illustrates common collector configuration of a traditional Colpitts oscillator that is well known in the art. This Colpitts oscillator has only one transistor, a bipolar junction transistor
12
. The transistor
12
has its collector connected to a voltage source V
cc
. The base of the transistor
12
is connected to an inductor
14
via node
16
. The inductor
14
has an internal resistance, shown representatively by resistor
18
. Resistor
18
is not a resistance separate from the internal resistance of the inductor
14
. A resistor
20
is connected between the voltage source V
cc
and the node
16
. Node
16
is also connected to one side of a capacitor
22
and the other side of the capacitor is connected to the emitter of the transistor
12
through nodes
24
and
26
. A capacitor
28
is connected between node
24
and ground. A resistor
30
is connected between node
26
and ground.
Such standard Colpitts oscillators are well known and their characteristics have been well studied. Colpitts oscillators behave in a predictable fashion and are easy to implement. Nodes
24
and
26
are physically the same node and carry the output voltage.
One advantage of this Colpitts oscillator is that it has a low output impedance and therefore is less influenced by the circuits which follow it. However, the output signal on node
26
is a single signal. Hence, if the circuit designer requires differential signals, this Colpitts oscillator cannot output such signals. Therefore, there is a need to have an oscillator circuit that can output differential voltage signals which are accurate and have good harmonic content. A signal having good harmonic content is one that has a primary resonant frequency and whose higher order harmonic frequencies are suppressed. It is desirable to have a “balanced” signal, that is, one whose two components are 180 degrees out of phase. Differential voltage signals that are not precisely out of phase result in reduced signal amplitude or phase errors which may degrade the quality of systems that use oscillators. For example, telecommunication and cellular telephone systems that use noisy or inaccurate oscillators may suffer from perceptibly degraded voice qualities.
The phase noise of an oscillator based on a LC tank usually depends on the Q of the tank. The higher the Q of the LC tank, the sharper the resonance and the lower the phase noise skirts. The Q represents how much energy is lost as the energy is transferred from the capacitor to the inductor and vice versa. The Q, phase noise and other attributes of Colpitts oscillators have been well studied and are well known to those of skill in the art.
Oscillators may be used to form other devices including voltage controlled oscillators (VCOs). On a larger scale, oscillators may be used in wireless communication systems such as mobile radio communication systems and cellular telephone systems. Hence, improvements in oscillators lead to improvements in other systems.
SUMMARY OF THE INVENTION
In accordance with the purpose of the invention as broadly described therein, there is provided a differential oscillator.
In particular, a first embodiment of a differential oscillator has a first Colpitts oscillator that is coupled to a mirror image Colpitts oscillator through a coupling network comprising two inductors, a first resistor that is connected between the base of the transistor of the first Colpitts oscillator and the voltage source, and a second resistor that is connected between the base of the transistor of the second Colpitts oscillator and the voltage source.
A second embodiment of the differential oscillator comprises a first Colpitts oscillator that is coupled to a mirror image Colpitts oscillator through a coupling network comprising a cross-coupled transformer having two inductors, two DC blocking capacitors connected to the base of the transistors of the Colpitts oscillators, a first resistor that is connected between the base of the transistor of the first Colpitts oscillator and the voltage source, and a second resistor that is connected between the base of the transistor of the second Colpitts oscillator and the voltage source.
A third embodiment of the differential oscillator comprises a first Colpitts oscillator that is coupled to a mirror image Colpitts oscillator through a coupling network comprising a single inductor and a resistor that is connected between the voltage source and the midpoint of the inductor.
A fourth embodiment of the differential oscillator comprises a first Colpitts oscillator that is coupled to a mirror image Colpitts oscillator through a coupling network comprising two substantially identical inductors and a resistor that is connected between the voltage source and a node between the two inductors.
The differential oscillator may be used to form a VCO. In particular, a first embodiment of such a VCO comprises a first Colpitts oscillator, a mirror image Colpitts oscillator that is coupled to the first Colpitts oscillator through an inductor, and a varactor or variable capacitor coupled across the inductor. A resistor may be connected between the midpoint of the inductor and a voltage source in order to bias the transistors.
A second embodiment of a VCO comprises a first Colpitts oscillator, a mirror image Colpitts oscillator that is coupled to the first Colpitts oscillator through two identical inductors, and two varactors or variable capacitors coupled in series across the inductors. This second embodiment further includes DC blocking capacitors coupled to the varactors and a voltage control signal to control the varactors.
The differential oscillator may be used in any kind of system. For example, it can be used in a multi-band transceiver for transmitting and receiving RF signals in one of a plurality of frequency bands. One embodiment of a transceiver that uses the differential oscillator comprises a transmit circuit which modulates a voice or data signal onto a carrier signal. The carrier signal is a differential very high frequency (VHF) signal outputted from a differential dual Colpitts oscillator, such as the fourth embodiment described above. This modulated carrier signal is filtered and processed and transmitted as a radio frequency signal. The transceiver includes a receiver circuit that receives radio frequency signals and amplifies and filters them. The receiver circuit isolates the voice or data signal from the carrier signal by using filters and mixers. To do so, the receiver circuit uses a differential ultra high frequency (UHF) signal generated by a differential dual Colpitts oscillator, such as the fourth embodiment described above. By using differential voltage signals, the transceiver is more immune to external noise and interference because differential signals tend to cancel the effects of external noise and interference on the transceiver.
A first, separate aspect of the differential oscillato
Domino William J.
Oskowsky Mark
Rozenblit Dmitriy
Conexant Systems Inc.
Lyon & Lyon LLP
Mis David
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