Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-10-04
2002-03-05
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S175000, C331S17700V, C332S124000, C332S136000, C332S141000
Reexamination Certificate
active
06353370
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of voltage-controlled oscillators (VCOs). More particularly, the present invention relates to the field of modulation of the operating frequency of a voltage controlled oscillator.
BACKGROUND OF THE INVENTION
A voltage-controlled oscillator (VCO) forms a periodic output signal where a frequency of the periodic output signal is related to the level of an input control voltage. The center frequency of a VCO is defined as the frequency of the periodic output signal formed by the VCO when the input control voltage is set to a nominal level, such as zero volts. The input control voltage is then adjusted up or down to control the frequency of the periodic output signal.
VCOs are used in variety of applications, such as for transmitting information according to frequency modulation techniques. To implement frequency modulation using a VCO, the center frequency is set equal to a carrier frequency to be utilized for transmitting the information. The input control voltage is then adjusted up or down in response to an information-carrying signal, thus, forming a frequency modulated signal. The frequency modulated signal is transmitted via a transmission medium to a receiver tuned to the carrier frequency. The information-carrying signal is then recovered by demodulating the received signal.
FIG. 1
illustrates a schematic diagram of a conventional VCO including a cross-coupled transistor pair. A first terminal of an inductor L
1
and a first terminal of an inductor L
2
are coupled to a supply voltage node V
CC
. A second terminal of the inductor L
1
forms a node N
1
and is coupled to a collector of a transistor Q
1
, to a base of a transistor Q
2
, to a cathode of a varactor diode D
1
, and to a cathode of a varactor diode D
2
. A second terminal of the inductor L
2
forms a node N
2
and is coupled to a collector of the transistor Q
2
, to a base of the transistor Q
1
, to a cathode of a varactor diode D
3
, and to a cathode of a varactor diode D
4
.
An anode of the varactor D
1
and an anode of the varactor D
3
are coupled to a first terminal of a resistor R
1
. A second terminal of the resistor R
1
is coupled to receive a tuning control voltage V
tune1
. An anode of the varactor D
2
and an anode of the varactor D
4
are coupled to a first terminal of a resistor R
2
. A second terminal of the resistor R
2
is coupled to receive a modulation control voltage V
mod1
. An emitter of the transistor Q
1
and an emitter of the transistor Q
2
are coupled to a first terminal of a resistor R
3
. A second terminal of the resistor R
3
is coupled to the ground node.
In operation, an output voltage signal V
out1
formed across the nodes N
1
and N
2
is generally a sinusoid which oscillates at the resonant frequency of the VCO. 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
3
passes through the right side of the VCO (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 VCO.
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 VCO 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
3
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 VCO, 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 VCO, thereby forming a the output sinusoidal signal V
out1
across the nodes N
1
and N
2
.
The tuning control voltage V
tune1
is typically adjusted such that the output periodic signal V
out1
oscillates at the desired center frequency when the modulation control voltage V
mod1
is at a nominal level. The modulation control voltage V
mod1
is then adjusted up or down to control the frequency of the output periodic signal V
out1
. A drawback to the conventional VCO illustrated in
FIG. 1
is that the frequency deviation obtained in the output signal V
out1
depends upon capacitance of each of the varactors D
1
-D
4
and the amplitude of the modulation control voltage V
mod1
. As the capacitance of the varactors D
1
and D
3
increases in response to adjusting the tuning control voltage V
tune1
, the frequency deviation obtained at the output signal V
out1
for a given modulation control voltage V
mod1
level is reduced. Conversely, as the capacitance of the varactors D
1
and D
3
is reduced in response to adjusting the tuning control voltage V
tune1
, the frequency deviation obtained at the output V
out1
for a given modulation control voltage V
mod1
level increases. As a result, the VCO exhibits undesired, non-linear behavior.
FIG. 2
illustrates an exemplary graph of tuning control voltage V
tune1
vs. frequency deviation in the output V
out1
in response to changes in the modulation control voltage V
mod1
for the VCO illustrated in FIG.
1
. Thus, when the tuning control voltage V
tune1
is at a level given on the x-axis, and the frequency deviation which results from an incremental (e.g., one millivolt) change in the modulation control voltage V
mod1
is given on the y-axis. As can be seen from
FIG. 2
, the frequency deviation changes in a non-linear fashion with changes in the tuning control voltage V
tune1
. Difficulties can be encountered when attempting to demodulate a signal which has been modulated by a VCO which exhibits such a non-linear characteristic.
Therefore, what is needed is a technique for obtaining a more linear relationship between a tuning control voltage and frequency deviation in a VCO output signal resulting from changes in a modulation control voltage applied to the VCO.
SUMMARY OF THE INVENTION
The invention is a method and apparatus for modulation of a voltage-controlled oscillator (VCO). The VCO receives a tuning control voltage for adjusting a center frequency of an output periodic signal formed by the VCO. In addition, the VCO receives a modulation control voltage for modulating the output periodic signal by a content-carrying signal according to frequency modulation techniques. A frequency deviation obtained in the output periodic signal in response to changes in the modulation control voltage is linearized by forming the modulation control voltage as the result of a linear correction polynomial. The linear correction polynomial is preferably of the form:
V
mod2
=K
0
+(K
1
)(V
tune2
)+m(V
sig
)
where V
mod2
is the modulation control voltage, V
tune2
is the tuning control voltage, V
sig
is the content carrying signal and K
0
, K
1
and m are constants. Appropriate values for the constants K
0
, and K
1
can be determined by measuring the voltage level for V
mod2
required to obtain a desired frequency deviation at various values of the tuning control voltage V
tune2
(e.g., two endpoints and a center value) and with an assumed value of m(V
sig
). Such measurements produce a numerical function which can be approximated with an interpolating polynomial by selecting the values for K
0
and K
1
. The value of m can be selected to achieve a desired proportionality between frequency deviation and amplitude of the content-carrying signal V
Cox Robert
Luff Gwilyn
Haverstock & Owens LLP
Micro Linear Corporation
Mis David
LandOfFree
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