Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-10-04
2002-09-03
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S17700V, C331S179000
Reexamination Certificate
active
06445257
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to voltage-controlled oscillators. More particularly, the present invention relates to a fuse-trimmed tank circuit for an integrated voltage-controlled oscillator.
BACKGROUND OF THE INVENTION
A voltage-controlled oscillator (VCO) is an electronic device which provides a periodic signal where the frequency of the periodic signal is related to the level of an input voltage control signal supplied to the VCO. There are numerous applications for VCOs, including modulating and demodulating radio frequency communication signals, such as in a radio frequency transceiver. A VCO typically includes a resonant tank circuit which resonates so as to form the periodic signal. A conventional resonant tank circuit used in an integrated VCO includes a parallel combination of an inductor and a capacitor along with a cross-coupled transistor pair.
FIG. 1
illustrates a resonant tank circuit
100
of the prior art. More particularly, the tank circuit
100
includes inductors L
1
and L
2
, each having a first terminal coupled to a supply voltage V
CC
. A second terminal of the inductor L
1
forms a node N
1
which is coupled to a first terminal of a capacitor C
1
, to a collector of a transistor Q
1
and to a base of a transistor Q
2
. A second terminal of the inductor L
2
forms a node N
2
which is coupled to a second terminal of the capacitor C
1
and to a collector of the transistor Q
2
and to a base of the transistor Q
1
. Emitters of the transistors Q
1
and Q
2
are coupled together and to a first terminal of a resistor R
t
. A second terminal of the resistor R
t
is coupled to a ground node. An output from the tank circuit
100
is taken across the capacitor C
1
and supplied to a buffer circuit
102
.
In operation, a voltage signal formed across the capacitor C
1
is generally a sinusoid which oscillates at the resonant frequency of the tank circuit
100
. When the node N
1
is at a higher voltage level than the level of the node N
2
, the transistor Q
2
has a higher bias voltage than the transistor Q
1
. Accordingly, nearly all of the current through the resistor R
t
passes through the right side of the tank circuit
100
(through the inductor L
2
and the transistor Q
2
). This tends to reinforce the voltage at the node N
1
being higher than the voltage at the node N
2
. Accordingly, this results in positive feedback in the tank circuit
100
.
Eventually, however, because there is little or no current passing through the inductor L
1
and the transistor Q
1
, the voltage at the node N
2
tends to rise relative to the level at the node N
1
. In response, the bias on the transistor Q
1
increases while the bias on the transistor Q
2
decreases. This reduces the current in the right side of the tank circuit
100
and increases the current in the left side (through the inductor L
1
and the transistor Q
1
). Eventually, nearly all of the current through the resistor R
t
passes through the left side which reinforces the voltage at the node N
2
being higher than the voltage at the node N
1
, through positive feedback.
Because there is little or no current passing through the right side of the tank circuit
100
, the voltage at the node N
1
tends to rise relative to the level at the node N
2
. In response, the bias on the transistor Q
2
increases while the bias on the transistor Q
1
decreases. Accordingly, the above-described cycle repeats. In this manner, current is alternately steered through the right and left sides of the tank circuit
100
, thereby forming a sinusoidal signal across the capacitor C
1
.
The conventional oscillator circuit illustrated in
FIG. 1
has the drawback in that it has a gain peak which is relatively distant from the zero phase crossing, which can result in frequency drift during settling. The amplifier section of the oscillator formed by Q
1
, Q
2
and R
t
has a relatively high capacitive output susceptance (low output capacitive impedance) which reduces tuning range. In addition, the tank circuit
100
requires the buffer circuit
102
which can increase phase noise by loading the tank circuit
100
.
Therefore, what is needed is an improved oscillator circuit which does not suffer from the drawbacks exhibited by the prior art oscillator circuit of FIG.
1
.
In addition, so that a carrier signal used for transmitting signals in a wireless communication system is appropriately modulated or demodulated, it is important that the nominal center frequency of a VCO used in the system be equivalent to the center frequency of the frequency band covered. The center frequency, however, will generally be affected by process variations which affect the values of the various components of the VCO. Accordingly, the VCO must generally be tunable to compensate for these process variations.
Accordingly, what is needed is an improved technique for tuning a VCO.
An integrated VCO is a VCO which is formed as a portion of an integrated circuit, rather than with discrete components. Because the reactive elements of an integrated VCO are not as accessible as those of a VCO which is formed of discrete components, this creates a difficulty in tuning an integrated VCO.
Therefore, what is further needed is an improved technique for tuning an integrated VCO.
SUMMARY OF THE INVENTION
The invention is a fuse-trimmed tank circuit for an integrated voltage-controlled oscillator (VCO). An improved oscillator circuit in accordance with the present invention includes an L-C portion; a cross-coupled pair of transistors, whose collectors are coupled to the L-C portion; a capacitor coupled across the emitters of the cross-coupled pair of transistors; and a pair of resistors, where each resistor is coupled between a corresponding one of the emitters and a ground node.
The capacitor coupled across the emitters of the cross-coupled pair of transistors counteracts the effects of phase lag caused by the transistors. Accordingly, this oscillator circuit arrangement provides improved performance in that it results in a high small-signal loop gain which provides reliable start-up; has a peak gain which is closer to a zero phase crossing than prior tank circuits, which reduces frequency drift during settling; and has a low capacitive output susceptance. Also, a periodic output signal formed by the VCO can be taken at the emitters of the cross-coupled pair, which avoids having to utilize a buffer which could increase phase noise.
The present invention also provides a technique for trimming the VCO so as to adjust its center frequency. Capacitance for the L-C portion of the tank circuit is provided by one or more varactor diodes. When the VCO is operational, the varactors are under reverse bias conditions. Accordingly, the varactors act as capacitive elements. One or more of the varactors has an associated fuse coupled in series with the corresponding varactor. To trim the center frequency of the VCO, the capacitance of the L-C portion is adjusted by selectively blowing the fuses. This reduces the capacitance of the L-C portion by an amount equivalent to the capacitance of the varactors associated with the blown fuses.
Under normal operating conditions, currents through the fuses are sufficiently low so as to avoid blowing any of the fuses. To blow a fuse, however, a dc voltage is applied to forward bias the corresponding varactor such that the resulting current is sufficiently high to blow the fuse. In the preferred embodiment, the varactors are arranged in pairs, where each varactor of the pair contributes to the capacitance of one or the other sides of the tank circuit for maintaining its symmetrical and differential nature. Preferably, the cathodes of each pair of varactors are coupled together and to a corresponding switch. The dc voltage is selectively applied to the pair of diodes via the corresponding switch. Accordingly, the two fuses associated with each varactor of the pair are blown together.
In the preferred embodiment, the VCO is fully integrated into an integrated circuit. Accordingly, the present invention has the advantage of not r
Cox Robert
Luff Gwilym
Haverstock & Owens LLP
Micro Linear Corporation
Mis David
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