Volage-controlled oscillation circuit with an impedance element for carrier-to-noise compensation

In a voltage-controlled oscillation circuit, a parallel resonance circuit which is formed by connecting a coil (an inductive element) and a capacitor (a capacitive element) in parallel with each other is provided between a control terminal and a resonance circuit. A control voltage is inputted at the control terminal and is supplied to the resonance circuit through the parallel resonance circuit, so that the resonance frequency of the resonance circuit is responsive to the control voltage; an oscillation stage of the voltage-controlled oscillation circuit oscillates at that resonance frequency. The oscillation output of the oscillation stage is outputted at an output terminal from a buffer stage through an output matching stage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a voltage-controlled oscillation circuit 
(hereinafter referred to as VCO). More specifically, the present invention 
relates to a VCO, which is supplied with a control voltage for controlling 
its oscillation frequency through an impedance element for C-N 
characteristic (carrier-to-noise ratio) compensation. 
2. Description of the Background Art 
FIG. 1 is a circuit diagram showing an example of a conventional VCO 1. 
Referring to FIG. 1, the VCO 1 includes an oscillation stage 4, a buffer 
stage 5 and an output matching stage 6. The oscillation frequency of the 
oscillation stage 4 is changed in response to a control voltage Vc which 
is applied to a control terminal C. The buffer stage 5 prevents the 
oscillation frequency of the oscillation stage 4 from varying with load 
fluctuation. The output matching stage 6 attains matching with a 
next-stage circuit which is connected with an output terminal P, and 
suppresses higher harmonics. 
The oscillation stage 4 includes a resonance circuit 7. This resonance 
circuit 7 includes a frequency variable varactor diode VD, a coupling 
capacitor C11 and a resonance inductor L2, and the control terminal C 
supplies the control voltage Vc to a cathode of the frequency variable 
varactor diode VD and an end of the coupling capacitor C11 through an 
impedance element Zvc for C-N characteristic compensation having a line 
impedance. An anode of the frequency variable varactor diode VD is 
grounded, while another end of the coupling capacitor C11 is connected to 
an end of the resonance inductor L2 and an end of the coupling capacitor 
C10. Another end of the resonance inductor L2 is grounded. 
The oscillation stage 4 includes an oscillation transistor Q2, and another 
end of the coupling capacitor C10 is connected to its base. Further, the 
base of the oscillation transistor Q2 is supplied with a voltage, which is 
obtained by dividing a power supply voltage V.sub.B by bias resistors R4 
and R5 serially connected between a power supply terminal B and the 
ground, as a bias voltage. A capacitor C9 is connected between a base and 
an emitter of the oscillation transistor Q2, while a resistor R6 and a 
capacitor C8 are connected in parallel between the emitter of the 
oscillation transistor Q2 and the ground. 
The capacitors C9 and C8 form a Colpitts capacitance, and the oscillation 
transistor Q2 forms a Colpitts oscillator with the capacitors C9 and C8 
and the resonance inductor L2, to oscillate at the resonance frequency of 
the resonance circuit 7. 
An oscillation output of the oscillation stage 4 is supplied to the buffer 
stage 5 through the coupling capacitor C7. The buffer stage 5 includes a 
buffer transistor Q1, which is supplied with the oscillation output of the 
oscillation stage 4 in its base. The base of the buffer transistor Q1 is 
also supplied with a voltage, which is obtained by dividing the power 
supply voltage V.sub.B by bias resistors R1 and R2 connected in series 
between the power supply terminal B and the ground, as a bias voltage. A 
collector of the buffer transistor Q1 is connected to the power supply 
terminal B through a choke coil L1 which is included in the output 
matching stage 6. 
The output matching stage 6 includes the choke coil L1, a coupling 
capacitor C1 and an output matching capacitor C2. An end of the coupling 
capacitor C1 is connected to the collector of the buffer transistor Q1, 
while another end thereof is connected to the output terminal. On the 
other hand, an end of the output matching capacitor C2 is connected to the 
output terminal P, while another end thereof is grounded. A high frequency 
bypass capacitor C5 is connected between the power supply terminal B and 
the ground, while another high frequency bypass capacitor C3 is connected 
between the control terminal C and the ground. 
In the VCO 1 having the structure shown in FIG. 1, the capacitance of the 
frequency variable varactor diode VD included in the resonance circuit 7 
is changed in response to the control voltage Vc which is inputted through 
the impedance element Zvc for C-N characteristic compensation. The 
resonance circuit 7 resonates on the basis of the coupling capacitor C11, 
the capacitance of the frequency variable varactor diode VD, and the 
resonance inductor L2, while the oscillation transistor Q2 oscillates at 
the resonance frequency thereof. The oscillation output is supplied to the 
base of the buffer transistor Q1 through the coupling capacitor C7, so 
that the collector of the buffer transistor Q1 outputs the oscillation 
output, which in turn is outputted from the output terminal P through the 
coupling capacitor C1. 
In such a VCO, the impedance element Zvc for C-N characteristic 
compensation is generally connected between the control terminal C and the 
resonance circuit 7. This impedance element Zvc is formed by an inductive 
or resistive element to protect the quality factor of the resonance 
circuit 7 from damping caused by the control voltage Vc, thereby 
maintaining excellent C-N characteristics. It is possible to attain 
excellent C-N characteristics when this impedance element Zvc has a high 
line impedance. 
In a miniature VCO which is employed in a cordless telephone, a portable 
telephone or a pager, the aforementioned impedance element Zvc is mainly 
formed by a coil, a transmission line such as a stripline, or a resistive 
element. When a resistive element is employed as the impedance element 
Zvc, however, the C-N characteristics of the VCO are deteriorated due to 
the influence of thermal noise. Therefore, the impedance element Zvc is 
preferably formed by an inductive element. 
In order to attain a high impedance when the impedance element Zvc is 
formed by an inductance element, however, a coil having a large feature 
size or a stripline which is formed in a wide area is required and hence 
the VCO cannot be miniaturized. 
SUMMARY OF THE INVENTION 
Accordingly, a principal object of the present invention is to provide a 
VCO whose C-N characteristics can be improved with no increase in size, 
while employing an inductive element as an impedance element. 
Briefly stated, the present invention is directed to a voltage-controlled 
oscillation circuit which is supplied with a control voltage for 
controlling its oscillation frequency through an impedance element for C-N 
characteristic compensation. The impedance element includes a parallel 
resonance circuit which is formed by connecting an inductive element and a 
capacitive element in parallel with each other. 
According to the present invention, therefore, it is possible to employ the 
maximum impedance in the parallel resonance circuit as a line impedance. 
Thus, an inductive element having a small impedance such as a coil having 
a small feature size or a stripline having a narrow area can be used to 
provide the line impedance, whereby the C-N characteristics can be 
improved without increasing the VCO in size. 
In a further preferable embodiment of the present invention, a resistive 
element is connected in parallel with the inductive element and the 
capacitive element forming the parallel resonance circuit, to adjust the 
resonance frequency. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 is an electric circuit diagram showing a VCO 10 according to an 
embodiment of the present invention. In the embodiment shown in FIG. 2, a 
parallel circuit 11 which is formed by a coil L3 of an inductive element 
and a capacitor C12 of a capacitive element is connected between a control 
terminal C and a resonance circuit 7 for serving as an impedance element 
for C-N characteristic compensation. The remaining structure of this 
embodiment is identical to that shown in FIG. 1. The resonance frequency 
of the parallel resonance circuit 11 is selected to be substantially equal 
to that of the resonance circuit 7 provided in an oscillation stage 4. 
FIG. 3 illustrates resonance characteristics of the VCO 10 shown in FIG. 2. 
The resonance frequency of the resonance circuit 7 is changed in response 
to a control voltage as clearly understood from the resonance 
characteristics of the resonance circuit 7 shown by dotted lines in FIG. 
3. In order to cope with this change, the quality factor of the parallel 
resonance circuit 11 is so reduced that the same has a flat resonance 
characteristic as shown by a solid line in FIG. 3. Thus, it is possible to 
employ an inductive element having a small impedance such as a coil having 
a small feature size or a stripline having a narrow area by employing an 
impedance at the resonance frequency of the parallel resonance circuit 11 
as the line impedance, thereby improving the C-N characteristics without 
increasing the VCO 10 in size. 
According to an experiment made by the inventors, it was possible to 
confirm improvement of C-N characteristics by 4.4 dB in a sample of the 
embodiment shown in FIG. 2 employing a parallel resonance circuit 11 which 
was formed by a coil L3 of 0.33 .mu.H and a capacitor C12 of 22 pF for 
serving as a line impedance Zvc for C-N characteristic compensation, as 
compared with a conventional sample employing only a coil L of 0.33 .mu.H. 
FIG. 4 is an electric circuit diagram showing a VCO 14 according to a 
further embodiment of the present invention. In the embodiment shown in 
FIG. 4, a resistor R3 is connected in parallel with a circuit 
corresponding to the parallel resonance circuit 11 shown in FIG. 2 for 
further flattening its resonance characteristics. The remaining structure 
of this embodiment is similar to that shown in FIG. 2. Thus, the resistor 
R3 is connected in parallel with a coil L3 and a capacitor C12 to form a 
parallel resonance circuit 113, whereby it is possible to reduce the 
quality factor of the parallel resonance circuit 113 and flatten its 
resonance characteristics, thereby attaining a high impedance in a wide 
frequency band. 
In the aforementioned embodiments, circuit parts other than the parallel 
resonance circuits 11, 111, 112 and 113 are arbitrarily changeable. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.