Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2001-07-31
2003-08-12
Pascal, Robert (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S132000, C331S158000, C331S160000, C331S167000
Reexamination Certificate
active
06606007
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic oscillators, and more particularly to a circuit and method for a quartz crystal oscillator. Described herein are means for limiting loading effects in a crystal oscillator, thereby improving the crystal resonator quality factor (or Q). This is advantageous, since a higher Q is generally associated with improved frequency stability and lower phase jitter. Such precision oscillators are widely used as timing sources in various digital integrated circuits.
2. Description of the Related Art
Modern high-speed digital systems typically employ clock-based timing circuitry. A clock is commonly used to supply a timing reference to synchronize the state changes in logic devices, such as counters, flip-flops, etc. More complex integrated circuits (ICs), such as microprocessors, typically include clock circuitry internal to the IC for this purpose. A clock circuit is a form of electronic oscillator—i.e., a circuit that generates a periodic time-varying output. The most important characteristic for an oscillator used in a clock circuit is that the frequency of oscillation be very consistent, both on a long term and a short term basis. If the frequency of an oscillator changes slowly over time, as a result of temperature, aging effects, etc, the oscillator is said to “drift.” In many applications, especially those in which the oscillator is being used for timing purposes, drift is extremely undesirable. For this reason, oscillators are often made to operate within an oven, so that variations in the ambient temperature can be avoided. Short term fluctuations in the operating frequency are usually characterized as “phase jitter,” since they may occur within a span of a few cycles and may be modeled as a noise source interfering with an ideal oscillator.
Oscillators depend on the use of regenerative feedback from the output of the oscillator back to its input. In this context, the term “regenerative” refers to the fact that the magnitude and phase of the feedback signal are such that it reinforces the input signal, thereby sustaining oscillation. This principle is illustrated in FIG.
1
. In the elementary oscillator represented in
FIG. 1
, an amplifier
50
increases the magnitude of any signal present at its input by a gain factor A. A phase shift network
52
phase-shifts the signal at the output of the amplifier by an angle dependent on the frequency &phgr;(&ohgr;) of the signal. A summing junction
54
combines the input signal with the phase-shifted, amplified version of the input signal (i.e., the feedback signal), and couples the composite signal into the amplifier
50
.
The phase shift network
52
in any oscillator contains reactive components (i.e., capacitors and/or inductors), to achieve a frequency dependent phase shift. Reactive components are fundamentally different from resistive components, since they can store (but not dissipate) energy. Resistive components, on the other hand, cannot store energy and dissipate electrical energy in the form of heat. Furthermore, the voltage across an inductor or capacitor leads or lags the current through the inductor or capacitor by &pgr;/2 radians (or, equivalently, 90°), respectively. In contrast, the voltage across a resistor is always in phase with the current. The ratio of voltage across an inductor or capacitor to the current through the inductor or capacitor is known as the inductive or capacitive reactance, respectively. By the same token, the ratio of the voltage to the current in a resistor defines its resistance. In general, for a network containing a combination of resistance and capacitive or inductive reactance, the ratio of voltage to current is known as the impedance.
In a system such as that represented in
FIG. 1
, oscillation may be induced by an externally applied signal, or by noise, which is always present in an electronic circuit. Noise arising from random physical processes (e.g., thermally-induced molecular motion) is comprised of an entire band of frequencies. A small signal
56
represents one of the frequency components associated with random noise at the input of the summing junction
54
. The random signal
56
passes through amplifier
50
, emerging at the output
58
with increased amplitude and a different phase angle. Note that the phase relationship between the input signal
56
and the amplified signal
58
is not conducive to sustained oscillation. Whereas the input signal
56
is initially increasing, the amplified version
58
is decreasing. Consequently, if these signals were combined in the summing junction
54
, the noise signal
56
would not be reinforced by the feedback signal
58
. However, the effect of the phase shift network
52
is to further alter the phase angle of the amplifier output, producing a signal
60
that has the same phase angle as the initial random signal
56
. When the in-phase, amplified signal
60
is combined with the input signal
56
in the summing junction
54
, it reinforces the input signal. Under these circumstances, the oscillator will generate the signal continuously.
For continuous oscillation to occur, an oscillator must provide a phase shift of 2&pgr; radians (or, equivalently, 360°) at the frequency of oscillation. The phase shift is necessary to reinforce the input signal, as described above. As mentioned earlier, the phase angle associated with phase shift network
52
is frequency dependent. Thus, there is just one (fundamental) frequency at which the phase angle of the feedback signal will be 2&pgr; radians, and the oscillator operates at only this frequency. In practice, the frequency of an oscillator can be made adjustable, by incorporating variable reactive components in the phase shift network. In addition to the necessary phase shift, the oscillator must also have sufficient gain to overcome losses in the resistive components of the oscillator. Without the gain provided by the amplifier, these losses would eventually attenuate the oscillatory signal.
There are a variety of ways to create a oscillator. A classic approach, known as an RC ring oscillator, consists of series-connected phase shift stages, in which the combined phase shift is sufficient to achieve oscillation at the desired operating frequency. For example, an RC ring oscillator can be formed by connecting four stages in series, each stage having a phase shift of &pgr;/2 radians at the desired frequency. By connecting the output of the fourth stage to the input of the first, an overall phase shift of 2&pgr; radians results. If there is sufficient gain, the RC ring oscillator will sustain oscillation. Although this technique is straightforward, it tends to be noisy and lacks sufficient frequency stability for many applications.
A better approach, the LC oscillator, uses both inductors and capacitors in the phase shift network to obtain the necessary 2&pgr; radians of phase shift. (“LC” oscillators are so named because the traditional symbols for inductance and capacitance are L and C, respectively).
Inductors and capacitors are complementary. Inductive reactance is positive, while capacitive reactance is negative. Moreover, inductive reactance increases in magnitude with frequency, while capacitive reactance decreases. The impedance of a circuit comprising a series combination of an inductor and capacitor is given by:
Z
(
ω
)
=
j
ω
·
L
-
j
ω
·
C
where Z is impedance (in ohms), L is inductance (in Henries), C is capacitance (in Farads), and &ohgr; is frequency (in radians per second). Note that Z(&ohgr;) becomes zero when &ohgr;
2
LC=1.
ω
0
=
1
L
·
C
The frequency &ohgr;
0
at which this occurs is known as the resonant frequency for the given LC pair, and the resonant LC network is referred to as a “tank” circuit. Equivalently, the resonant frequency is defined as the frequency at which the inductive and capacitive reactances cancel. A tank circuit operating at its resonant frequency is said to be “resonating” or “at r
Conley Rose & Tayon
Glenn Kimberly E
LSI Logic Corporation
Pascal Robert
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