Voltage-controlled oscillator and method for adjusting frequency shift amount thereof

Abstract: A voltage-controlled oscillator and a method capable of precisely adjusting a frequency shift amount are provided irrespective of a fluctuation in characteristics of elements such as a strip-line. A center tap 19 is formed in a strip-line 16 of a resonator. A switching element 20 is connected to the center tap 19, and this switching element 20 is turned ON, so that the center tap is shortcircuited to the ground so as to vary the oscillating frequency. A slit A1 is conducted in the vicinity of the center tap 19 along the strip-line 16 and directed toward a shortcircuited end 17 of the strip-line, so that a frequency shift amount is adjusted. Also, a slit B1 is conducted in the vicinity of the shortcircuited end 17 along the strip-line 16 and directed toward the center tap 19, so that a frequency shift amount is adjusted. Also, trimming points are provided on the side of a hot terminal 18 from the center tap 19, and also provided on the side of the shortcircuited end 17 from the center tap 19, and further a slit is conducted along an intersecting direction of the strip-line 16, so that a shift amount is adjusted.

Description: BACKGROUND OF THE INVENTION The present invention is related to a voltage-controlled oscillator and a method for adjusting a shift amount of a frequency in a high frequency voltage-controlled oscillator employed in a mobile communication such as a portable telephone. A voltage-controlled oscillator is capable of varying an oscillating frequency in response to a change in control voltages. In such a case that two greatly different frequency bands of oscillating frequencies are used in a voltage-controlled oscillator used in a dual type radio communication apparatus, for instance, an oscillating frequency band existed in the vicinity of 800 MHz and another oscillating frequency band existed in the vicinity of 1.5 GHz when such a wide oscillating frequency is covered by a single voltage-controlled oscillator, a changing ratio (df/dVt) of an oscillating frequency "f" to a change in a control voltage Vt must be increased. That is, a so-called "control voltage sensitivity" should be made high. However, when this control voltage sensitivity is increased, such characteristics as a spurrous response, a lock-up time (initiating time), and C/N would be deteriorated, which are required as PLL characteristics. To solve these problems, Unexamined Japanese Patent Publication 8-316731 has proposed the following method. That is, a strip-line is employed in a resonator, a center tap is formed in this strip-line, and such a control is made as to whether or not the center tap is shortcircuited to the ground in response to ON/OFF states of a shift signal. As a result, the oscillator can be operated at the two greatly separated frequency bands. Also, Unexamined Japanese Patent Publication 7-202638 has proposed such a system that two sets of resonators (inductors) are provided, and these resonators are switched by using a switch so as to shift the frequency. However, as in the conventional oscillating method described in Unexamined Japanese Patent Publication 8-316731, in which the frequency band is greatly varied in response to whether or not the center tap of the strip-line is shortcircuited, there are the following problems. That is, since the fluctuations are present in the strip-line, the shift element, and the capacitors, corresponding to the structural elements of the voltage-controlled oscillator, it is practically difficult to vary the frequency shift amount under stable condition. In other words, even when the adjusting trimming electrode is additionally provided with the resonator made of the strip-line and then this trimming electrode is trimmed so as to adjust the oscillating frequency, it is practically difficult to carry out the adjustment of the shift amount at the same time. Also, as explained in Unexamined Japanese Patent Publication 7-202638, the two resonators are used to be switched by operating the switch, so that two sets of operating bands can be obtained. In this conventional system, since two sets of these resonators are required, there is another problem that the shapes cannot be made compact. US 4,749,963 discloses a strip line loop oscillator including a ring type inductive resonator. Voltage controlled oscillators are mentioned. The ring oscillator comprises slits which are formed along the strip line or in a transverse direction thereof. The slits are for adjusting the resonance frequency by varying the widths or lengths of the line. SUMMARY OF THE INVENTION It is the object of the invention to provide a voltage controlled oscillator and a method capable of precisely adjusting an amount of a frequency shift in a voltage controlled oscillator irrespective of a fluctuation in characteristics of elements such as a strip line. This object is solved by the invention as claimed in the independent claims. Preferred embodiments of the invention are defined in the dependent claims. The shift amount can be decreased in accordance with the length of the slit formed in the strip-line and directed toward the shortcircuited end side thereof in the vicinity of the center tap. The shift amount can be increased in accordance with the length of the slit formed along the strip-line located in the vicinity of the shortcircuited end side of the strip-line and directed toward the center tap. The shift amount can be decreased in accordance with the length of the slit formed in the direction along which the slit intersects the strip-line at such a trimming point formed on the side of the hot terminal of the strip-line from the center tap. Also, the shift amount can be increased in accordance with the length of the slit formed in the direction along which the slit intersects the strip-line at such a trimming point formed on the side of the shortcircuited end of this strip-line from the center tap. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a circuit diagram of a voltage-controlled oscillator for showing a frequency shift amount adjusting method of the voltage-controlled oscillator according to an embodiment mode of the present invention; Figs. 1B to 1D are diagrams for representing a relationship between trimmings, oscillating frequencies, and shift amounts of this voltage-controlled oscillator; Fig. 2A is a schematic outer view for indicating the voltage-controlled oscillator used to perform the method of Fig. 1A; Fig. 2B is a plan view for schematically indicating this voltage-controlled oscillator; Fig. 3A is a circuit diagram of a voltage-controlled oscillator for showing a frequency shift amount adjusting method of the voltage-controlled oscillator according to another embodiment mode of the present invention; and Figs. 3B and 3C are diagrams for representing a relationship between trimmings, oscillating frequencies, and shift amounts of this voltage-controlled oscillator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Fig. 1A is a circuit diagram for representing a voltage-controlled oscillator according to an embodiment of the present invention in combination with a trimming method. Figs. 1B to Figs. 1D indicate relationship diagrams among each of trimming amounts and amounts of frequency shifts. Fig. 2A is a perspective view for showing an outer view of the voltage-controlled oscillator according to this embodiment, in which a strip-line is drawn at a center thereof and from which mounted components are omitted. Fig. 2B is a plan view of this voltage-controlled oscillator. In Fig. 1A, reference numeral 1 shows a control voltage terminal, and reference numeral 2 indicates an oscillating transistor. A high frequency output is derived from an emitter 3 of this oscillating transistor 2. With respect to the transistor 2, there are provided capacitors 4 to 10, an inductor 11, resistors 12 to 14, a varicap diode 15, and a strip-line 16 functioning as a resonator. Both the inductor 11 and the capacitor are used to eliminate a high frequency component from a DC voltage applied to the control voltage terminal 1, and then to apply the eliminated DC voltage to the varicap diode 15. The capacitor 5 is employed so as to cut the DC voltage, and the capacitor 6 is employed so as to adjust a frequency. Also, the capacitor 7 is employed in order that a resonant circuit constructed of the varicap diode 15, the capacitors 5, 6 and the strip-line 16 is coupled to the above-described transistor 2 and a peripheral circuit thereof. In the strip-line 16, reference numeral 17 indicates a shortcircuited end connected to the ground; reference numeral 18 shows a hot terminal connected between the capacitor 5 and the capacitor 6, and reference numeral 19 indicates a center tap formed between the hot terminal 18 and the shortcircuited end 17. A diode 20 functioning as a switching element is connected in series to a capacitor 21. This series circuit is interposed between the center tap 19 and the ground. A shift signal input terminal 22 is connected via a resistor 23 to these connection points. A resistor 24 is such a resistor capable of discharging the capacitor 21. As indicated in Figs. 2A and 2B, a voltage-controlled oscillator 26 is made of a glass epoxy board, or another board. This board is manufactured by that ceramics having a low dielectric constant, which is stacked by either the screen printing method or the sheet stacking layer method, and other conductors such as copper, nickel, and silver are stacked and sintered. The above-explained control voltage terminal 1, a shift signal input terminal 22, an output terminal 27, a ground terminal 28, and a power supply terminal 29 are provided around this board. In such a case that a board is made of a stacked board, the circuit elements other than those formed in this stacked board are mounted on a portion surrounded by a two-dot/dash line 30, and are mutually connected to each other by a conductor pattern. These circuit elements are the above-described capacitors 4 to 10, and 21, inductor 11, and also resistors 11 to 14, 23, 24 in addition to the varicap diode 15 and the diode 20, which are mounted on the circuit of the voltage-controlled oscillator. Also, the strip-line 16 for constituting a resonator is formed on this surrounded portion. A ground conductor located opposite to this strip-line is formed within the stacked board, or on a rear surface of this surrounded portion. A trimming point 32 is formed between a center tap 19 of the strip-line 16 and a hot terminal 18. A portion of this trimming point 32 is projected from the strip-line to a side portion. Another trimming point 33 is formed over a substantially entire length of the strip-line between the center tap 19 and the shortcircuited end 17. This trimming point 33 is projected to the side of the center tap 19. In Fig. 1A, when a shift signal having a value of, for example, approximately 3V is applied from the shift signal input terminal 22, the center tap 19 is electrically shortcircuited to the ground in the AC manner, so that the inductance value of the strip-line 16 is decreased and therefore the resonant frequency is increased. The resonant frequency can be greatly changed by turning ON/OFF this shift signal. In Fig. 1A and Fig. 2B, symbols "A1", "B1", and "C1" indicate slits as trimming traces used to adjust a shift amount and a frequency. The slit "A1" is formed in the vicinity of the center tap 19 along the strip-line 16, and directed toward the side of the shortcircuited end 17 of this strip-line 16. In the case of this slit A1, when a length of a notch of this slit directed from the side of the center tap 19 to the shortcircuited end 17 is increased, a current path defined from the hot terminal 18 to the center tap 19 is increased under such a condition that the center tap 19 is shortcircuited to the ground. As a result, the inductance value of the strip-line is increased in accordance with the length of the slit A1. As a consequence, when the shift signal is set to ON, a change in the oscillating frequencies with respect to the notch length of the slit A1 is large. On the other hand, when the shift signal is set to OFF, as shown in the drawings, since the slit A1 is formed in the vicinity of the center tap 19, a changing degree of widths of the strip-line 16 in another current path defined from the side of the center tap 19 to the shortcircuited end 17 is decreased. A change in the oscillating frequencies is decreased with respect to a change in the notch lengths of the slit A1. As a result, a shift amount of the oscillating frequencies can be adjusted by adjusting the length of the notched slit A1. The shift amount of the oscillating frequencies is equal to a difference between the oscillating frequencies when the shift signal is set to ON/OFF states. The slit C1 by the trimming is formed at the trimming point 32 between the center tap 19 and the hot terminal 18 in a direction along which the strip-line is intersected by this slit from an edge located opposite to a projection portion of this strip-line toward the side direction. As indicated in Fig. 1D, the larger the length of the slit C1 by the trimming is extended, the larger the inductance value is increased. As a result, the oscillating frequency is lowered. It should be noted that the longer the length of the slit C1 is extended, the slightly smaller the shift amount becomes. In the case that the oscillating frequency is adjusted in this voltage-controlled oscillator, the strip-line 16 is set in such a manner that the oscillating frequency oscillated when the shift signal is set to OFF becomes higher than a target value, and also the shift amount when the shift signal is set to OFF becomes higher than a target value. Then, first of all, while the oscillating frequency is monitored under such a condition chat the shift signal is set to the OFF state, the trimming of the slit C1 is carried out until the oscillating frequency reaches the target value. Subsequently, while monitoring a difference between the oscillating frequency produced when the shift signal is set to the OFF state, and the oscillating frequency produced when the shift signal, is set to the ON state, namely monitoring a shift amount, the slit A1 is conducted. When the shift amount reaches the target value, the trimming of the slit A1 is stopped. It should be noted that the adjustment or this oscillating frequency, namely the adjustment of the shift amount may be alternatively achieved. Contrary to the above-described manner, the shift amount may be first adjusted by the slit A1, and thereafter the slit C1 may be conducted. Furthermore, in order to adjust a shift in the oscillating frequency caused by forming the slit C1, after the slit C1 has been conducted, the slit A1 may be furthermore conducted so as to perform a fine adjustment. Alternatively, the adjustment may be carried out in this order of the slit C1 trimming, the slit A1 trimming and the slit C1 trimming. On the other hand, the above-described slit B1 is formed in the vicinity of the shortcircuited end 17 of the strip-line 16 along this strip-line 16 in the direction of the center tap 19. Since the width of the strip-line 16 becomes narrow, an inductance value is increased in accordance with the length of the slit B1. As a consequence, as indicated in Fig. 1C, the oscillating frequency produced when the shift signal is set to the OFF state is lowered. Also, as shown in Fig. 1C, when the shift signal is set to the ON state, the center tap 19 is electrically shortcircuited in the AC manner. Thus, there is a small influence in the oscillating frequency, cased by the slit B1. As a consequence, the shift amount can be increased in accordance with the length of the slit B1. In the case that shift amount is adjusted by employing this slit B1, while the shift amount is set to be smaller than a target value, the length of the slit B1 is gradually extended until the shift amount reaches this target value by monitoring the shift amount. In this case, there is a small change in the oscillating frequencies by the trimming of the slit 31 when the shift signal is set to the ON state. As a consequence, the adjustment of the oscillating frequency by the trimming of the slit C1 is carried out under such a condition that the shift signal is set to the ON state. In other words, the adjustment of the slit oscillating frequency, namely the adjustment of the shift amount is preferably carried out by any one of the below-mentioned fine adjustments, i.e., the slit C1 trimming to the slit B1 trimming under ON state of the shift signal, and the shift amount adjustment achieved by the slit C1 trimming to the slit B1 trimming, and thereafter the fine adjustment by the slit C1 trimming. With employment such an adjustment, both the shift amount and the oscillating frequency can be precisely adjusted. Also, since only one resonator made of the stripline is employed, it is possible to avoid a large-scaled voltage-controlled oscillator. Second Embodiment Fig. 3A is a circuit diagram of a voltage-controlled oscillator according to another embodiment of the present invention, and a trimming method thereof. Fig. 3B and Fig. 3C represent trimming amounts and adjustment amounts of shift amounts. In the embodiment of Fig. 3A, a trimming point 34 is formed on the side of the hot terminal 18 of the strip-line 16 from the center tap 19. Another timing point 35 is formed on the side of the shortcircuited end 17 from the center tap 19. These trimming points 34 and 35 are made by projecting portions of the strip-line 16 toward the side direction. A slit A2 and another slit B2 are formed in at least one of these trimming points 34 and 35 in a direction along which these slits A2 and 32 intersect the stripline 16. As indicated in Fig. 33, a relationship between the slit A2 and the shift amount at the trimming point 34 formed on the side of the hot terminal 18 is given as follows. The longer the length of the slit A2 is extended, the slightly smaller the shift amount becomes. However, a change amount is small. On the other hand, as indicated in Fig. 3C, an oscillation frequency is lowered when the shift signal is set to the OFF state in accordance with the length of the slit B2 of the trimming point 35 on the side of the shortcircuited end 17. When the shift signal is set to the ON state, the center cap 19 is shortcircuited to the ground. As a result, the length of the trimming gives no influence to the oscillating frequency. As a consequence, the slit A2 is trimmed while the shift signal is set to the ON state. Thereafter, the shift amount is preferably adjusted by trimming the slit B2. Alternatively, when the present invention is executed, a transistor may be employed to shortcircuit the center tap 18 instead of the above-described diode 20 and capacitor 21. In accordance with the present invention, the center tap is formed on the strip-line. The inductance value of this strip-line may be greatly made different from each other by shortcircuiting this center tap to the ground, or not by shortcircuiting the center tap to the ground. The voltage-controlled oscillator can be used in the two largely changed frequency bands. Furthermore, the shift amount of the oscillating frequency can be adjusted by trimming the slit from the center tap of the strip-line to the shortcircuited end by trimming the slit from the shortcircuited end to the center tap, otherwise by trimming the slit in the direction along which the stripline is intersected by the slit from the center tap to the hot terminal side, or from the center tap to the shortcircuited end. The shift amount can be precisely adjusted. Also, since only one resonator is employed, it is possible to avoid the large-scaled voltage-controlled oscillator.

Claims: A voltage-controlled oscillator comprising: a strip-line (16) used as a resonator; a center tap (19) provided in the strip-line of said resonator; a switching element (20) connected to said center tap; and at least one slit (A1, B1, C1, A2, B2) formed in said strip-line to change an inductance thereof; wherein said center tap is electrically shortcircuited to the ground by turning on said switching element so as to vary an oscillating frequency between a first frequency when said switching element is turned off and a second frequency when said switching element is turned on; wherein said at least one slit includes a slit (A1) formed along the strip-line located in vicinity of said center tap (19) and directed to a shortcircuited end side (17) of the strip-line to thereby adjust an amount of a frequency shift which is equal to the difference between the first frequency and the second frequency. The voltage-controlled oscillator as claimed in claim 1, wherein said at least one slit includes another slit (C1) formed in a transverse direction of said strip-line. The voltage-controlled oscillator as claimed in claim 1 or 2, wherein said at least one slit includes another slit (B1) formed along the strip-line located in the vicinity of said shortcircuited end side (17) of the strip-line and directed to said center tap (19) to thereby adjust an amount of a frequency shift. A voltage-controlled oscillator comprising: a strip-line (16) used as a resonator; a center tap (19) provided in the strip-line of said resonator; a switching element (20) connected to said center tap; and at least one slit (A1, B1, C1, A2, B2) formed in said strip-line to change an inductance thereof; wherein said center tap is electrically shortcircuited to the ground by turning on said switching element so as to vary an oscillating frequency between a first frequency when said switching element is turned off and a second frequency when said switching element is turned on, wherein a trimming point (32, 34) is formed on a side of a hot terminal (18) of said strip-line from said center tap (19) and another trimming point (35) is formed on a side of a shortcircuited end (17) of said strip-line from said center tap (19); and said at least one slit includes a slit (C1, A2, B2) formed in at least one of said trimming points in a transverse direction of said strip-line to thereby adjust an amount of the frequency shift between sind first frequency and said second frequency. The voltage-controlled oscillator as claimed in claim 4, wherein said at least one slit includes another slit (A2, B2) formed in the other one of said trimming points in a transverse direction of the strip-line. A method for adjusting a frequency shift amount of a voltage-controlled oscillator having a structure that a strip-line (16) is employed as a resonator; a center tap (19) is provided in the strip-line of said resonator; a switching element (20) is connected to said center tap; and said center tap is electrically shortcircuited to the ground by turning on said switching element so as to vary an oscillating frequency between a first frequency when said switching element is turned off and a second frequency when said switching element is turned on; the method comprising the step of: forming at least one slit (A1, B1, C1, A2, B2) in the strip-line to change an inductance thereof to thereby adjust the frequency shill amount; wherein said at least one slit (A1) is formed along the strip-line located in vicinity of said center tap and directed to a shortcircuited end side (17) of the strip-line to thereby adjust a frequency shift amount which is equal to the difference between the first frequency and the second frequency. The method as claimed in claim 6, wherein another slit (C1) is formed in a transverse direction of said strip-line. The method as claimed in claim 6 or 7, wherein another slit (B1) is formed along the strip-line located in the vicinity of a shortcircuited end side (17) of the strip-line and directed to said center tap (19) to thereby adjust the frequency shift amount. A method for adjusting a frequency shift amount of a voltage-controlled oscillator having a structure that a strip-line (16) is employed as a resonator; a center tap (19) is provided in the strip-line of said resonator; a switching element (20) is connected to said center tap; and said center tap is electrically shortcircuited to the ground by turning on said switching element so as to vary an oscillating frequency between a first frequency when said switching element is turned off and a second frequency when said switching element is turned on the method comprising the step of: forming at least the slit (A1, B1, C1, A2, B2) in the strip-line to change an inductance thereof to thereby adjust the frequency shift amount; wherein a trimming point (32, 34) is formed on a side of a hot terminal (18) of said strip-line from said center tap (19) and another trimming point (35) is formed on a side of a shortcircuited end (17) of said strip-line from said center tap (19); and said at least one slit (C1, A2, B2) is formed in at least one of said trimming points in a transverse direction of said strip-line, to thereby adjust an amount of the frequency shift between said first frequency and said second frequency. The method as claimed in claim 9, wherein another slit (A2, B2) is formed in the other one of said trimming points in a transverse direction of the strip-line.

Inventors: ENDO TAKESHI, KATSUMATA MASASHI, ENDO, TAKESHI, KATSUMATA, MASASHI
Applicants: TDK CORP, TDK CORPORATION
Application year: 1999
Main classification: H03B5
IPC codes: H03B5, H03B1, H01P7