Oscillators – Automatic frequency stabilization using a phase or frequency... – Plural a.f.s. for a single oscillator
Reexamination Certificate
2001-05-29
2003-05-13
Mis, David C. (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Plural a.f.s. for a single oscillator
C331S014000, C331S017000, C331S025000, C331S03600C, C331S03600C, C331S179000, C327S156000, C327S157000, C375S376000, C455S260000
Reexamination Certificate
active
06563387
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a frequency synthesizer used in a wireless mobile appliance, and more specifically, to a frequency synthesizer capable of manufacturing a VCO in an IC form, resulting in low cost.
2. Description of the Related Art
In wireless mobile appliances such as portable telephones, frequency synthesizers are used to generate arbitrary local oscillation frequencies from a reference signal.
In general, as indicated in
FIG. 5
, a frequency synthesizer used in a wireless mobile appliance such as a portable telephone is equipped with a VCO
1
, a prescaler
2
, a counter
3
, a reference frequency divider
5
, a phase comparator
6
, a charge pump
7
, and a loop filter
8
. Both the prescaler
2
and the counter
3
constitute a pulse-swallow type variable frequency divider. The VCO
1
oscillates a signal having a frequency in response to a voltage which is applied to a frequency control voltage terminal of the frequency synthesizer. The prescaler
2
divides a frequency of an output signal (will be referred to as an “f
VCO
” hereinafter) of the VCO
1
. The counter
3
counts the output signal of the prescaler
2
. The reference frequency divider
5
divides a frequency of an output signal (will be referred to as an “f
OSC
” hereinafter) of a reference signal source
4
. The phase comparator
6
compares a phase of an output signal (will be referred to as an “f
div
” hereinafter) of the counter
3
with a phase of an output signal (will be referred to as an “f
ref
” hereinafter) of the reference frequency divider
5
to thereby output a phase difference. The charge pump
7
converts the output signal of the phase comparator
6
into either a voltage or a current. The loop filter
8
averages the output signal of the charge pump
7
.
FIG. 6
is a circuit diagram for representing a basic operation of the VCO disclosed in Japanese Laid-open Patent Application No. Hei-10-261918. This circuit is provided with the capacitor “CO”, the negative-characteristic resistive portion “−R”, and the inductor “L”, which are connected in parallel to each other. This circuit is further equipped with the capacitor “C
1
” and the variable capacitance diode “Cv”, which are connected in the cascade connection. This cascade connection between the capacitor C
1
and the variable capacitance diode “Cv” is connected in parallel to the capacitor CO.
Next, operations of the circuit shown in
FIG. 6
will now be explained. The parallel connection portion constructed of the negative-characteristic resistive portion “−R”, the capacitor “CO”, and the inductor “L” corresponds to a parallel resonant circuit containing an active element which generates electric power of a transistor and the like, to which the power supply voltage is applied. The negative-characteristic resistive portion “−R” is different from the normal resistor, considering such an implication that electric power is generated.
The oscillation frequency of this VCO is expressed by the following formula [1]:
f
VCO
=½&pgr;
L[C
0
+
C
1
·
Cv
/(
C
1
+
Cv
)] [1]
In the case that this voltage-controlled oscillator is employed in the frequency synthesizer of
FIG. 5
, the control voltage is applied to the variable capacitance diode “Cv”, so that the capacitance value of the variable capacitance diode “Cv” is varied. As a result, the oscillation frequency “f
VCO
” is varied.
In such a frequency synthesizer, when the count value of the counter
3
is changed, the frequency “f
div
” is changed in response to this change in the count values. As a result, the phase comparator
6
outputs the phase error. Based upon this phase error, voltage of the frequency control voltage terminal of the VCO
1
is changed via the charge pump
7
and the loop filter
8
so as to vary the frequency “f
VCO
.” As previously explained, the frequency synthesizer constitutes the negative feedback loop, and finally locks the phase when the phase of the frequency “f
ref
” is made coincident with the phase of the frequency “f
div
”, so that the output frequency of the VCO
1
can be made stable.
Generally speaking, in such a frequency synthesizer, a VCO is constituted by a module component. This module component will constitute one of major factors which may impede a compactness of a portable telephone and the like. Thus, such a VCO is desirably manufactured in the form of an IC component. However, in the case that such a VCO is manufactured in the IC component form, an oscillation frequency of this VCO would be largely varied due to manufacturing fluctuations occurred in electronic components for constituting this VCO. As a result, there is such a problem that the frequency synthesizer could not be phase-locked at the desirable frequency.
To solve this problem, there is one method capable of increasing a control sensitivity (namely, change width of oscillation frequency per 1V: unit being [Hz/V]). However, if the control sensitivity is increased, then there is another problem. That is, when the control sensitivity is increased, the frequency synthesizer may be easily and adversely influenced by outer disturbance noise, so that the CPU characteristic would be deteriorated.
Also, there is a further method for solving the above-explained problem. That is, in this method, a plurality of fixed capacitances are connected in parallel to a parallel resonant circuit of a VCO, and when this VCO is manufactured in the IC component form, these fixed capacitances are trimmed by laser and the like so as to set the oscillation frequency of the VCO. However, since the IC is separately adjusted, the manufacturing cost would be increased, resulting in another problem.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above-described conventional problems, and therefore, has an object to provide a frequency synthesizer having a better C/N characteristic, by which a VCO having a wide output frequency range can be manufactured in an IC form, while realizing low cost.
To achieve the above-explained object, a frequency synthesizer, according to the present invention, is featured, by that in a frequency synthesizer having a switching means for switching a capacitor, or an inductor; and equipped with: a voltage-controlled oscillator (will be referred to as a “VCO” hereinafter) for oscillating a signal having a frequency in response to a voltage applied to a control voltage terminal; a first frequency divider for outputting a signal having a frequency which is obtained by dividing a frequency of an output signal derived from the VCO; a second frequency divider for dividing a frequency of a reference signal; a phase comparator for comparing a phase of an output signal of the first frequency divider with a phase of an output signal of the second frequency divider to output a phase difference thereof; and a charge pump for outputting the output signal of the phase comparator via a loop filter to a control voltage terminal of the VCO,
the frequency synthesizer is comprised of: frequency adjusting means for detecting a frequency error between both the output signal of the first frequency divider and the output signal of the second frequency divider, and for switching the capacitor value of the VCO, or the inductor value thereof in accordance with the detection result of the frequency error; and bias control means for applying an arbitrary voltage to the control voltage terminal of the VCO in order that the output signal of the charge pump is brought into a high impedance state when the frequency adjusting means is operated. With employment of such an arrangement, even when there is a manufacturing fluctuation occurred in the electronic components which constitute the VCO, since the resonant frequency of the parallel resonant circuit is varied in response to the actual oscillation frequency of the VCO, the phase can be locked at a desirable frequency. Furthermore, since the VCO can be manufactured in the IC form,
Adachi Hisashi
Hirano Shunsuke
Hiraoka Yukio
Miyahara Yasunori
Yamada Ryoichi
Matsushita Electric - Industrial Co., Ltd.
Mis David C.
Pearne & Gordon LLP
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