Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-12-04
2002-06-11
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S11300A, C331S109000, C331S17700V, C331S144000
Reexamination Certificate
active
06404296
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electronic oscillators and, in particular, to a source-coupled oscillator capable of operating over an exceptionally wide frequency range with a minimal change in amplitude.
BACKGROUND OF THE INVENTION
FIG. 1
is a circuit diagram of a known source-coupled oscillator
10
, which uses AC source-coupled differential-current switches (MESFETs X
1
, X
2
) to steer current to loads L
1
, L
2
. The voltages at loads L
1
, L
2
are buffered by source-follower MESFETs X
3
, X
4
, level shifted by level shift devices Z
1
, Z
2
, and cross-coupled to the gates of the opposite ones of MESFETs X
1
, X
2
, respectively. Constant current source S
5
provides a current for source-follower X
3
and level shift device Z
1
, and constant current source S
6
provides a current for source-follower X
4
and level shift device Z
2
. Level shift devices Z
1
and Z
2
can be diodes, batteries or any other component capable of inserting a constant voltage drop into the circuit.
The frequency of oscillator
10
is proportional to the size of the currents I
1
, I
2
supplied by current sources S
1
, S
2
, respectively, and inversely proportional to the capacitance of capacitor C
1
. The period of the oscillations (T) can be expressed by the relationship:
T
~(
V/I
)×
C
1
where V is the signal swing ((I
1
+I
2
)×Lx), I is the magnitude of the currents I
1
, I
2
, and C
1
is value of capacitor C
1
. This is a capacitive slew rate relationship regulating the discharge time of capacitor C
1
by the currents I
1
, I
2
. Since, in this example, I
1
is equal to I
2
, the duty cycle of the oscillations is 50%. If I
1
is not equal to I
2
, then the duty cycle of the oscillation will be equal to I
1
/I
2
.
Oscillator
10
is essentially controlled by positive feedback paths which run: (a) from node
1
at the terminal of load L
1
, through source-follower MESFET X
3
and level shift device Z
1
to the gate of MESFET X
2
; and then from node
2
at the terminal of load L
2
, through MESFET X
4
and level shift device Z
2
to the gate of MESFET X
1
; and (b) from node
2
at the terminal of load L
2
, through source-follower MESFET X
4
and level shift device Z
2
to the gate of MESFET X
1
, and then from node
1
through source-follower MESFET X
4
and level shift device Z
1
to the gate of MESFET X
2
. The net result is that an increase in the current through MESFET X
1
tends to turn MESFET X
1
further on, and an increase in the current through MESFET X
2
tends to turn MESFET X
2
further on.
When oscillator
10
is initially turned on, noise fluctuations cause the currents through MESFETs X
1
and X
2
to vary (i.e., because of noise, it is impossible for the currents through MESFETs X
1
and X
2
to remain perfectly constant). Assume that initially the current through MESFET X
1
is increasing. With the current through MESFET X
1
and load L
1
is increasing, the voltage at node
1
falls. This falling voltage is coupled through source-follower MESFET X
3
and through level shift device Z
1
to the gate of MESFET X
2
. Since in this embodiment MESFETs X
1
and X
2
are N-channel devices, the effect of lowering the voltage at the gate of MESFET X
2
is to reduce the source-to-gate voltage V
gs
of MESFET X
2
, thereby reducing the current through MESFET X
2
. As the current through MESFET X
2
becomes smaller, the voltage at node
2
rises. This increasing voltage is coupled through source-follower MESFET X
4
and level shift device Z
2
to the gate of MESFET X
1
. This increases V
gs
in MESFET X
1
and further increases the size of the current through MESFET X
1
.
Capacitor C
1
transmits the voltage at the source of MESFET X
1
to the source of MESFET X
2
, following the normal capacitive lag. As MESFET X
1
turns on, the voltage at its source rises, biasing the left side of capacitor C
1
positively. This rising voltage is transmitted, to the source of MESFET X
2
and reinforces the reduction of V
gs
in MESFET X
2
. The right side of capacitor C
1
is biased negatively.
This process continues until MESFET X
1
is fully turned on and MESFET X
2
is fully turned off. The circuit is now halfway through one cycle of oscillation. With MESFET X
2
off, current source S
2
draws current from the right side of capacitor C
1
. This starts to pull the voltage at the source of MESFET X
2
down, increasing V
gs
in MESFET X
2
and beginning to turn MESFET X
2
on. As the current through MESFET X
2
increases, the voltage at node
2
begins to fall, and this falling voltage is transmitted through source-follower MESFET X
4
and level shift device Z
2
to the gate of MESFET X
1
. This reduces the V
gs
of MESFET X
1
and begins to turn MESFET X
1
off. As the current through MESFET X
1
decreases, the voltage at node
1
rises. This rising voltage is transmitted through source-follower X
3
and level shift device Z
1
to the gate of MESFET X
2
, further increasing the V
gs
of MESFET X
2
. At this point in the cycle, the increasing current through MESFET X
2
begins to charge the right side of capacitor C
1
. This increasing voltage is transmitted to the left side of capacitor C
1
, further reducing the V
gs
of MESFET X
1
. The process continues until MESFET X
1
is fully turned off and MESFET X
2
is fully turned on, completing one full cycle of oscillation. The current I
1
then starts to discharge the left side of capacitor C
1
, and the cycle is repeated.
In this conventional oscillator, the frequency is set by adjusting the magnitude of the currents I
1
and I
2
and/or the size of capacitor C
1
. The frequency increases as I
1
and I
2
increase and decreases as I
1
and I
2
decrease. A problem with this arrangement, however, is that as the size of I
1
and I
2
varies, the amplitude of the output signal also varies. For example, the signal swing at node
1
is directly related to the magnitude of (I
1
+I
2
)×L
1
), and the signal at node
1
is translated through source-follower MESFET X
3
and level shift device Z
1
to become the OUT
1
signal. Likewise, the signal swing at node
2
directly related to the magnitude of (I
2
+I
1
)×L
2
, and the signal at node
2
is translated through source-follower MESFET X
4
and level shift device Z
2
to become the OUT
2
signal. Thus increasing (or decreasing) the frequency of oscillator
10
has the undesirable side effect of also increasing (or decreasing) the amplitude of the output signal. Increasing amplitude means increasing output slew times, and decreasing amplitude means decreasing output slew times. Therefore, as the current is increased to increase the frequency of the oscillator (or decreased to decrease the frequency of the oscillator), the change in the voltage swing that the circuit must slew through is acting in opposition to the desired change in frequency. This limits the oscillator to a much narrower tuning range than would be assumed from the relationship T~(V/I)×C
1
. The gain issue becomes a problem as the current is reduced to a level below that at which the source-coupled amplifier's gain drops below one. The problem of varying amplitude also creates issues with available bias and supply constraints.
There is, accordingly, a real need for a source-coupled oscillator that is able to operate at a substantially fixed amplitude over a wider frequency range than a conventional oscillator of the kind described above.
SUMMARY OF THE INVENTION
In accordance with this invention, a source-coupled oscillator includes a first MESFET connected with a first load in a first current path and a second MESFET connected with a second load in a second current path. The first MESFET is supplied with a constant current (I
1
) and the second MESFET is supplied with a constant current (I
2
). A capacitor is connected between a source of the first MESFET and a source of the second MESFET. A first feedback path is connected between the first load and a gate of the second MESFET; and a second feedback path is connected between the second load and a gate of the first MESF
Bowman Tyler
Davenport William H.
Kinkead Arnold
TriQuint Semiconductor Inc.
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