Oscillators – With distributed parameter resonator
Reexamination Certificate
2000-11-14
2002-04-30
Kinkead, Arnold (Department: 2817)
Oscillators
With distributed parameter resonator
C331S17700V, C331S1070SL, C331S1070DP, C331S143000, C331S143000, C331S143000
Reexamination Certificate
active
06380814
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage-controlled oscillator capable of varying its oscillation frequency by voltage control and a communication device employing the same.
2. Description of the Prior Art
In general, a voltage-controlled oscillator employing a variable capacitive element such as a varactor diode is proposed.
FIG. 14
is a plan view of a conventional voltage-controlled oscillator disclosed in Japanese Patent Laying-Open No. 57-87209 (1982).
Referring to
FIG. 14
, a metal-semiconductor field-effect transistor (MESFET; hereinafter referred to as a transistor)
55
is formed on a dielectric substrate
51
. Microstrip lines
52
,
53
and
54
connected with a source electrode S, a gate electrode G and a drain electrode D of the transistor
55
respectively are also formed on the dielectric substrate
51
. A gate-side stabilizing resistor
57
is connected to an end of the microstrip line
53
.
A dielectric resonator
56
is arranged on the dielectric substrate
51
to be electromagnetically coupled with the microstrip lines
53
and
54
. A further microstrip line
59
is formed on the dielectric substrate
51
to be electromagnetically coupled with the dielectric resonator
56
. This micro strip line
59
has a length half the wavelength corresponding to a prescribed frequency (this wavelength is hereinafter referred to as a half wavelength), and approximates to the dielectric resonator
56
on its middle point. Thus, the middle point of the microstrip line
59
defines a node
70
with the dielectric resonator
56
.
A first end
59
a
of the microstrip line
59
is open. A further microstrip line
60
is arranged on a second end
59
b
of the microstrip line
59
through a varactor diode
58
. The microstrip line
60
has a length quarter the wavelength corresponding to the prescribed frequency (this length is hereinafter referred to as a quarter wavelength).
A cathode C of the varactor diode
58
is connected to the second end
59
b
of the microstrip line
59
, and an anode A is connected to a first end
60
a
of the microstrip line
60
. A second end
60
b
of the microstrip line
60
is open.
In the voltage-controlled oscillator shown in
FIG. 14
, the transistor
55
amplifies a small microwave signal generated on the gate electrode G and outputs the amplified microwave signal to the drain electrode D. The microstrip lines
54
and
53
and the dielectric resonator
56
form a band-pass filter. The microwave signal output to the drain electrode D is positively fed back to the gate electrode G through this band-pass filter. Thus, microwave power oscillating at a constant oscillation frequency is obtained. This oscillation frequency depends on the resonance frequency of the dielectric resonator
56
.
A control voltage is applied across the cathode C and the anode A of the varactor diode
58
. The capacitance value of the varactor diode
58
varies with the control voltage applied across the cathode C and the anode A.
The dielectric resonator
56
and the microstrip line
59
are electromagnetically coupled with each other, and the resonance frequency of the dielectric resonator
56
varies with the capacitance value of the varactor diode
58
. Therefore, this voltage-controlled oscillator can vary the oscillation frequency by changing the control voltage applied across the cathode C and the anode A.
In the aforementioned conventional voltage-controlled oscillator, the microstrip line
59
has the length corresponding to the half wavelength and the open first end
59
a
, and hence the central node
70
is shorted (in a shorted state) in a high-frequency manner while the second end
59
b
is open (in an open state) in a high-frequency manner. The microstrip line
60
has the quarter wavelength and the open second end
60
b
, and hence the first end
60
a
is shorted (in a shorted state) in a high-frequency manner.
In such a structure of the voltage-controlled oscillator, the dielectric resonator
56
is electromagnetically coupled with the varactor diode
58
through the microstrip line
59
and hence the microwave power oscillating at the constant oscillation frequency partially reaches the varactor diode
58
. The anode A of the varactor diode
58
is grounded in a high-frequency manner so that the potential thereof is regularly kept at zero. On the other hand, the cathode C of the varactor diode
58
is open in a high-frequency manner, and hence a voltage resulting from the microwave power is superposed on the control voltage. Thus, the following potential difference V
va
is caused between the cathode C and the anode A of the varactor diode
58
:
V
va
=V
c
+V
po
·sin(2&pgr;ft)
where V
c
represents the control voltage applied across the cathode C and the anode A of the varactor diode
58
, f represents the oscillation frequency, V
po
represents the amplitude of the voltage resulting from the microwave power oscillating at the oscillation frequency f and t represents the time.
As understood from the above equation, the potential difference V
va
between the cathode C and the anode A of the varactor diode
58
fluctuates, followed by fluctuation of the capacitance value of the varactor diode
58
. Consequently, the oscillation frequency f also fluctuates to deteriorate phase noise characteristics of oscillating waves as a result.
The capacitance value of the varactor diode
58
is nonlinear with respect to the voltage. When the potential difference between the cathode C and the anode A of the varactor diode
58
having such nonlinearity fluctuates, baseband noise of the transistor
55
and the varactor diode
58
is converted to a frequency close to the oscillation frequency f, to deteriorate the phase noise characteristics of the oscillating waves as a result. As the dielectric resonator
56
and the microstrip line
59
are strongly coupled with each other, the part of the microwave power reaching the varactor diode
58
increases to more remarkably deteriorate the phase noise characteristics.
In order to reduce such deterioration of the phase noise characteristics caused by the voltage superposed on the controlled voltage for the varactor diode, a countermeasure of connecting two varactor diodes in parallel with each other in opposite polarity is proposed as disclosed in Japanese Patent Laying-Open No. 4-223601 (1192), for example.
However, this structure requires two varactor diodes having completely identical voltage dependency of capacitance values. If the capacitance values of the varactor diodes are asymmetrical with respect to a control voltage, the composite capacitance value of the two varactor diodes fluctuates due to fluctuation of potential differences between cathodes and anodes. Thus, this means cannot solve the problem of fluctuation of the oscillation frequency.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a voltage-controlled oscillator reduced in deterioration of phase noise characteristics.
Another object of the present invention is to provide a communication device having a high communication quality resulting from a reduction in deterioration of phase noise characteristics.
A voltage-controlled oscillator according to an aspect of the present invention comprises an oscillation part performing oscillation, a resonance circuit resonating with the oscillation frequency of the oscillation part and a modulation circuit for modulating the oscillation frequency of the oscillation part within an oscillation band by changing the resonance frequency of the resonance circuit, while the modulation circuit includes a coupling part coupled with the resonance circuit in a high-frequency manner and a variable capacitive element having a pair of electrodes subjected to application of a control voltage, and the input impedance of the coupling part as viewed from the side of the resonance circuit at a frequency within the oscillation band is substantially set in a shorted state while one of the pair of electrodes of the variable capacitive element is conn
Armstrong Westerman & Hattori, LLP
Kinkead Arnold
Sanyo Electric Co,. Ltd.
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