Oscillators – With synchronizing – triggering or pulsing circuits – Triggering or pulsing
Reexamination Certificate
2002-09-05
2003-12-23
Mis, David C. (Department: 2817)
Oscillators
With synchronizing, triggering or pulsing circuits
Triggering or pulsing
C331S1170FE, C331S175000
Reexamination Certificate
active
06667666
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of synchronous oscillators. Such oscillators are used as building blocks in a wide variety of circuit applications, including (but not limited to) clock recovery, modulation, encoding, decoding, signal detectors and discriminators, and direct sequence, spread spectrum (DSSS) systems. It is also ideal for broadband communications, in AM to FM conversion, FM to digital conversion, etc.
BACKGROUND TO THE INVENTION
The synchronous oscillator is well known, and is described, for example, in U.S. Pat. Nos. 4,274,067, 4,335,404 and 4,356,456 as well as in numerous papers published by the present inventor.
The basic building blocks for a synchronous oscillator are shown, for example, in
FIG. 1
, and consist of an amplifier
10
, a high-Q tank circuit
12
, and a positive feedback path
14
through a feedback capacitor
16
, and a second feedback path
18
.
The circuit uses regeneration to provide an output which is regeneratively linked to the input signal. By a combination of positive and negative feedback, the input signal sensitivity and the regeneration gain are inversely proportional to the input signal.
The synchronous oscillator has a natural resonant frequency at which oscillation occurs in the absence of an input signal. However, when an input signal is applied, the oscillator very quickly “tunes” to match the input signal. This occurs by regenerative gain of the input signal; when the amplitude of the regenerated signal exceeds the amplitude of the natural oscillations, then the natural oscillations are replaced by the regenerating signal, which is the input signal multiplied by the regeneration gain.
An example of a conventional synchronous oscillator circuit is illustrated at
20
in FIG.
2
. An input transistor receives an input signal (Vi) at its base. The collector of the input transistor T
1
is coupled to the emitter of an oscillator transistor T
2
. The input transistor T
1
acts as an input buffer amplifier for the input signal, and injects the input signal into the oscillator transistor T
2
.
The collector of the oscillator transistor T
2
is coupled to one end of an inductor L of a tank circuit
22
. The tank circuit
22
comprises the inductor L coupled in parallel with series connected capacitors C
2
and C
3
. The other end of the inductor L is coupled through a second inductor Lc to the positive supply rail Ve.
The synchronous oscillator also includes first and second feedback paths
24
and
26
. The first feedback path
24
is provided by a feedback capacitor C
1
coupled between the tank circuit
22
and the base of the oscillator transistor T
2
. This provides positive feedback from the tank circuit
22
to the input of the oscillator transistor T
2
, to provide regenerative feedback in the circuit. The second inductor Lc acts as a choke to provide a buffer for the power supply, and also forces the feedback though the feedback capacitor C
1
to the base of the oscillator transistor T
2
. Feedback to the transistor is positive to enhance oscillations.
The second feedback path
26
is provided by a connection from the emitter of the oscillator transistor T
2
to a node
28
between the series connected capacitors C
2
and C
3
of the tank circuit
22
.
The transistors T
1
and T
2
are biased by respective resistors R
1
and R
2
coupled to the positive supply rail Ve.
The output from the circuit may be taken at any suitable point driven by the oscillator transistor T
2
. In the illustrated circuit the output is taken from the second feedback path
26
, coupled by a D.C. blocking output capacitor Co.
The main regenerative feedback in the circuit is that provided by the first feedback path
24
through the feedback capacitor C
1
. However, this is believed not be continuous positive feedback, but instead occurs in samples or bursts (depicted schematically at
30
), due to class “C” operation of the oscillator transistor T
2
.
Certain selected characteristics of the synchronous oscillator
20
are illustrated in
FIGS. 3 and 4
.
FIG. 3
shows the relation between the regenerative gain g
m
of the oscillator, and the Q of the tank circuit, with varying frequency. The maximum Q occurs at the resonant frequency &ohgr;
0
. However this also corresponds to the minimum amount of regenerative gain. As the Q decreases, the regenerative gain increases. The product of the two represents the overall gain of the circuit. Therefore, it can be seen that the gain has an extremely wide, and flat, characteristic, representing a linear response over a very broad bandwidth. Such a characteristic is highly advantageous in broadband communications.
In
FIG. 3
, the frequency response drops sharply at the edges of the bandwidth. This is a result of the signal becoming so small that it cannot compensate for losses in the tank circuit.
The output of the standard synchronous oscillator varies in phase from +90° to −90° with respect to the input, across the bandwidth of the oscillator. The zero phase difference occurs when the input signal corresponds to the resonant frequency &ohgr;
0
(see FIG.
3
).
To illustrate the high sensitivity and high noise rejection of the circuit,
FIG. 4
a
depicts an input signal which is affected by noise, for discrimination. The input signal is represented by low amplitude input signal
32
which is barely distinguishable from surrounding noise
34
. When fed as an input to the synchronous oscillator
20
of
FIG. 2
, the oscillations rapidly “tune” to the frequency of the input signal
32
, to provide a strongly discriminated output
36
, as shown in
FIG. 4
b.
FIG. 5
illustrates a second example of a conventional circuit, in the form of a coherent phase synchronous oscillator circuit. This is similar to the circuit of
FIG. 3
, but a phase detector
38
is included to detect the phase difference between the input signal and the output signal. The output from the phase detector
38
is fed through an integrator
39
to provide a phase control input to the oscillator transistor T
2
of the synchronous oscillator. In this way, the phase of the output signal can be made identical to that of the input signal to provide a coherent phase output (in contrast to the variable phase characteristic of the circuit shown in FIG.
3
).
However, compared to other circuits, a synchronous oscillator can provide a wide bandwidth, a high input sensitivity and a high noise rejection, which seems not to be matched by other circuits. A good PLL may, for example, have a signal sensitivity of about −25 dBm and a signal to noise ratio sensitivity of +3 dBm. In contrast, a synchronous oscillator may provide a signal sensitivity of −100 dBm and an input signal to noise ratio sensitivity as low as −38 dB.
A further important characteristic of the synchronous oscillator is its energy efficiency. The regenerative feedback results in very little power dissipation, enabling the circuit to operate highly effectively with very low power supply requirements, for example, approximately 2-3 volts.
As explained above, in the conventional synchronous oscillator, the output “tunes” to the same frequency as the input signal.
SUMMARY OF THE INVENTION
It would be desirable to yet further improve the characteristics of a synchronous oscillator.
Broadly speaking, a first aspect of the present invention provides a synchronous oscillator comprising a feedback path which provides negative impedance conversion (NIC).
A second broad aspect of the invention relates to a synchronous oscillator with an additional feedback path from an output node of an amplifier to an input node. The additional feedback path may include or have one or more of the following:
(a) a resonant frequency at least ten times lower than that of a tank circuit of the oscillator;
(b) a resonant frequency at least ten times lower than a fundamental frequency of the oscillator;
(c) a capacitance (or a capacitance component) having a value at least ten times larger than a characteristic capacitance of the tank circuit;
(d) an inductance
Aeschlimann Anthony
Maiorana P.C. Christopher P.
Mis David C.
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