A voltage controlled oscillator

Abstract: A voltage controlled oscillator is provided comprising resonant circuit means (301) and an active circuit means (303) having an active element T1, the input of the active circuit means being connected to the resonant circuit means (301). Variable capacitance ratio means (304) are connected to the active circuit means (303) in order to oscillate the active element T1 at a resonance frequency of the resonance circuit means (301). Variable control means are provided for changing the resonance frequency, so that the oscillator frequency of the active element T1 can be changed substantially linearly over a wide bandwidth, to thereby retain the amount of positive feed back at a predetermined level against the oscillator frequency of the active circuit means (303).

Description: The present invention relates to a voltage controlled oscillator (VCO) for use in a generator of highly pure signals. Recent synthesized signal generators for a quasi-microwave region of 10 KHz to 2,700 MHz are required not only to be multifunctional but also to suppress SSB phase noise (C/N) as low as possible in order to provide a generator of highly pure signals. To this end, a set of circuit devices, such as a frequency synthesizer, a VCO, a PLL, (a SG,) a mixer and a frequency and phase detector, which are used in a synthesized signal generator must be configured to be effective in suppressing noise individually or in combination. Moreover, the set of circuit devices must be configured to meet requirements for low noise, miniaturization and simplification. However, conventional frequency synthesizers using a PLL and other circuit devices have not been configured yet to meet such requirements. The problems associated with a conventional voltage controlled oscillator will be described. The European Patent Application EP-A-0 125 586 discloses an oscillator circuit for television reception. Resonant circuit means are connected to the base of a transistor via a first variable capacitance diode. Variable capacitance means are also connected in positive feedback manner between the input and the output of the transistor. In addition, a third variable capacitance diode is provided between the resonant circuit and the transistor for controlling capacitive coupling therebetween. Control means are also provided for adjusting the resonance frequency of the resonance circuit. Another VCO used in a phase locked loop (PLL) circuit will now be described. A oscillator the output frequency of which is variably controlled by an external voltage may be arranged as shown in Fig. 1. The VCO of Fig. 1 is a Colpitts oscillator and comprised of a resonant circuit 51 connected to an input terminal via a choke coil L11 and having a coil L12 and variable capacitance diodes D11 and D12 connected between the respective terminals of the coil L12 and ground, an active circuit 52 connected to the resonant circuit 51 via a coupling capacitor C11 and oscillating mainly at a frequency corresponding to the resonant frequency of the resonant circuit 51, a buffer circuit 53 for taking out an oscillating output of the active circuit 52, and a feedback circuit 54 connected between the input and output of the active circuit 52 and having capacitors C12 and C13 for determining a quantity of feedback on the basis of their capacitance ratio. The application of a control voltage of the variable capacitance diodes D11 and D12 from the input terminal 50 controls the frequency at which the active circuit 52 oscillates. An output signal of the active circuit 52 is taken out via the buffer circuit 53. With the oscillator described above, the capacitance ratio of the capacitors C13 and C12 of the feedback circuit 54 is fixed and the quantity of feedback is also fixed independently of frequency. With such a prior art as described above, since the quantity of feedback between the input and output of the active circuit 52 is fixed, the frequency range of oscillation is determined only by the quantity of feedback. Following experiments were therefore performed: the capacitance of the capacitor C13 was set larger than usual for low frequency oscillation; and the capacitance of the capacitor C13 was set smaller than usual for high-frequency oscillation. The experimental results are illustrated in Figs. 2 through 5. In the case of the low frequency oscillation, the linear frequency versus control voltage characteristic could be obtained up to about 1170 MHz when the control voltage was increased gradually as shown in Fig. 2. Thereafter the oscillation frequency saturated so that high frequency oscillation could not be achieved. Also, as shown in Fig. 3, the output level decreased as the oscillation frequency increased. In the case of the high frequency oscillation, abnormal oscillation occurred when the magnitude of the control voltage was small and thus the oscillation frequency was low as shown in Fig. 4. This resulted in great level variations in a low frequency range from 600 to 700 MHz as shown in Fig. 5. There is no problem in particular when the VCO is restrictively used at either a high frequency or a low frequency. In communication equipment needing frequencies the band of which is an octave or more wide with a high speed processing and a compact size, broadband oscillation ranging from a low frequency up to a high frequency needs be performed by a single VCO. To this end, a plurality of oscillators with fixed capacitors adapted for different frequencies and a selector connected to the oscillators may be provided. In this case, the selector can select one of the oscillators to provide a desired frequency. With such an arrangement, however, the provision of the plural oscillators and the selector leads to circuit complexity and an increase in cost. An object of the present invention to provide a voltage controlled oscillator which permits its control voltage versus oscillation frequency characteristic to be linear over a broadband of frequencies including a low frequency and a high frequency and amplitude variations in its output to be reduced without making its circuit arrangement complex for its adaptation to the frequency synthesizer. According to the present invention there is provided a voltage controlled oscillator as defined in claim 1. The present invention will be more fully understood from the following detailed description of embodiments in conjunction with the accompanying drawings, in which: Fig. 1 illustrates the circuit arrangement of a conventional voltage controlled oscillator; Fig. 2 illustrates the voltage versus frequency characteristic of an oscillator used in a low-frequency band; Fig. 3 illustrates the frequency versus output amplitude characteristic of an oscillator in the low frequency band; Fig. 4 illustrates the voltage versus frequency characteristic of an oscillator in the high frequency band; Fig. 5 illustrates the frequency versus output amplitude characteristic of an oscillator in the high frequency band; Fig. 6 is a schematic diagram of a signal generator of the type which would make use of the VCO of the present invention; Fig. 7 illustrates a voltage controlled oscillator according to an embodiment of the present invention; Fig. 8 illustrates the voltage versus frequency characteristic of the VCO of Fig. 7; Fig. 9 illustrates the frequency versus output amplitude characteristic of the VCO of Fig. 7. Before discussing specific embodiments, an illustration of a signal generator of the type which may use a VCO of the present invention will be given with reference to Fig. 1. Fig. 1 is a block diagram of a synthesized signal generator for use in a quasi-microwave frequency band from 10 KHz to 2700 MHz which can achieve the SSB phase noise characteristic of -140 dBc/Hz at 20 KHz detuning point at 1 GHz by means of multiple loops. This SSG is basically divided into two major sections, a synthesizer section and an output section. The synthesizer section is responsive to a 10 MHz reference signal to output an octave signal of 640 - 1350 MHz/5mHz and a 200 MHz local signal for a beat band. In the output section receiving the two output signals of the synthesizer section, a frequency doubler 101, a frequency divider 102 and a mixer 103 produce a broadband signal of 10 KHz - 2.7 GHz/10mHz which is, in turn, applied to an automatic level control (ALC) circuit where it is amplitude modulated and its output level is controlled between -143 dBm and +17 dBm. The multiple loops that the SSG uses is inherently complex but can be represented simply in functional blocks like the synthesizer section of Fig. 1. Actually the multiple loops has one loop for a reference signal generator 105, two loops for a high frequency synthesizer 106, seven loops for a low frequency synthesizer 107 and one loop for a sum loop (PLL) 108, that is, a total of eleven loops. In order to incorporate functions which are to form standards of future signal generators, such as low noise, broadband, various modulating functions, rich sweep functions, fast frequency switching and low leakage, into an ordinary size (177H x 426W x 451Dmm, 32 kg or less) with such a circuit scale, the SSG simultaneously pursues two themes: noise reduction; and high-speed operation, miniaturization and simplification. Techniques for realizing those themes are summarized as follows. For noise reduction, (1) A reference signal has its noise reduced at one point and is multiplied simply. (2) A PLL has its frequency band widened and its noise reduced by means of an AC loop. (3) An interpolation signal is synthesized at a high frequency and has its frequency divided to reduce noise. (4) An FM VCO has its maximum deviation made large and has its frequency divided after addition. (5) A block diagram is modified according to FM deviation. For high speed operation, miniaturization and simplification of circuit, (1) The use of an octave VCO. (2) Non-adjustment of LOCk by means of a frequency detector. (3) The simplification of a control line by means of a serial data transmission line. (4) The high speed data transfer by means of an optical link. (5) The high speed setting by means of an LSI dedicated to frequency data calculation. (6) The full use of surface mount components. (7) The patterning of connections and the like. Fig. 7 is a circuit diagram of a VCO according to an embodiment of the present invention. The VCO (voltage controlled oscillator) of the embodiment forms a Colpitts oscillator and is used in communication equipment requiring the band of frequencies an octave or more wide. The VCO has its output frequency controlled by an externally applied control voltage and comprises an oscillator circuit 305, which is comprised of a resonant circuit 301, a coupling control circuit 302, an active circuit 303 and a variable capacitance ratio circuit 304, and a buffer circuit 306 for taking out the output of the oscillator circuit 305. The resonant circuit 301 comprises a coil L and variable capacitance diodes D1 and D2 for connecting the respective ends of the coil L to ground and has its input connected to an input terminal 307 via a choke coil L2 and its output connected to the coupling control circuit 302, the active circuit 303 and the variable capacitance ratio circuit 304. The variable capacitance diodes D1 and D2 have their barrier capacitances controlled by a control voltage supplied from the input terminal 307 through the choke coil L2, thereby controlling the oscillation frequency. The coupling control circuit 302 is connected between the resonant circuit 301 and the active circuit 303 and comprises a coupling capacitor C2 and a series combination of a variable capacitance diode D3 and a capacitor C1 and a choke coil L3 connected between the junction of D3 and C1 and ground. The coupling control circuit 302 controls the barrier capacitance of the variable capacitance diode D3 through the choke coil L3 so that the resonant circuit 301 is caused a resonance with a higher frequency. That is, capacitance in the variable capacitance ratio circuit 304 and capacitance in the active circuit 303 connect to capacitance in the coupling control circuit 302 in a serial manner. It is understood that this total capacitance becomes an additional capacitance to the resonant circuit 301. Therefore, the additional capacitance is reduced since the capacitance of the coupling control circuit 302 is reduced corresponding to the higher control voltage, then the resonant circuit 301 is caused the resonance with the higher frequency. In relation to opposite, the resonant circuit is caused a resonance with a lower frequency according to the lower control voltage. Furthermore, in the case of capacitive coupling, the coupling strength varies with frequency because capacitor impedance varies with frequency. For this reason, the capacitance is made small when the oscillation frequency is high and vice versa so as to make the coupling strength constant. The active circuit 303 is coupled to the resonant circuit 301 via a coupling control circuit 302 so that it oscillates at the resonant frequency of the resonant circuit. In this example, the active circuit includes an npn transistor T1 having its base connected to ground via a series combination of a resistor R1 and a capacitor C3. The transistor T1 has its collector connected to ground and its emitter connected to a power supply via a resistor R2. Between the resistor R2 and ground is connected a capacitor C4. The emitter of the transistor T1 is connected to the buffer circuit 6 via a coupling capacitor C5. The coupling capacitor C5 has its input end connected to ground via a capacitor C6 and its output end connected to ground via a resistor R3. Between the junction point P2 between the capacitor C5 and the buffer circuit 306 and the power supply is connected a resistor R4. Between the resistor R4 and the resistor R1 connected to the base of the transistor T1 are connected resistors R5 and R6 in the form of L. The resistors R4 and R5 forms a voltage divider. In this active circuit 3, the transistor T1 oscillates at a frequency which varies with the magnitude of the control voltage applied and its output is send to the buffer circuit 306. The variable capacitance ratio circuit 305 has a choke coil L4 having its end connected to the input end of the coupling capacitor C2 and a capacitor C7 having its end connected to the output end of the coupling capacitor C2. A capacitor C8 is connected between the choke coil L4 and the capacitor C7. The junction between the capacitors C7 and C8 is connected to the emitter of the transistor T1. Between the choke coil L4 and ground is connected a variable capacitance diode D4 the barrier capacitance of which is controlled by the control voltage supplied from the input terminal 307. By varying the capacitance ratio of the variable capacitance diode D4 to the capacitors C7, C8, the quantity of feedback from the emitter to the base of the transistor T1, which depends on their capacitive impedances, is controlled to become constant. That is, when the capacitance of the variable capacitance diode D4 is made small when the oscillation frequency is high and vice versa so as to control the capacitance ratio of capacitor C7 to capacitor C8 and variable capacitance diode D4, whereby the quantity of feedback which depends on their capacitive impedances is made constant independently of the oscillation frequency. The buffer circuit 306 includes an npn transistor T2 for amplifying an output signal of the active circuit T1. A capacitor C9 is connected to the collector of the transistor T2 and a resistor R7 is connected between the collector of T2 and ground. In addition, a capacitor C10 is connected between the emitter of T2 and ground, and a resistor R8 is connected between the emitter of T2 and the power supply. With the voltage controlled oscillator constructed as described above, when a control voltage is applied to the choke coil L2, the barrier capacitances of the variable capacitance diodes D1 and D2 of the resonant circuit 301 vary so that the oscillation frequency varies. The oscillator output is taken from the active circuit 303 via the buffer circuit 306. The quantity of feedback associated with the transistor T1 in the active circuit 303 is kept substantially constant by virtue of the variable capacitance-ratio circuit 304. That is, a control voltage raises the oscillation frequency, the capacitance of the variable capacitance diode D4 is made small, thereby making the quantity of feedback of the transistor T constant and causing the oscillation frequency to vary linearly. When the control voltage is small in magnitude and hence the oscillation frequency is low, on the other hand, the capacitance of the variable capacitance diode D4 is controlled to a large extent so as to keep the quantity of feedback of transistor T constant and stable the oscillation in case of higher and lower oscillation frequencies. Resultant, the oscillation frequency is allowed to vary linearly. Fig. 8 illustrates the control voltage versus frequency characteristic and Fig. 9 illustrates the frequency versus output level characteristic. As can be seen from those characteristics, according to the oscillator described above, linear characteristics is obtained over a wide frequency range and the total Q is kept constant. This will also allow the C/N to be kept constant and high level oscillator outputs to be obtained. Using a variable capacitance diode in the variable capacitance ratio circuit 304 the oscillator described above is simple in construction and inexpensive as compared with another arrangement in which a plurality of oscillators are switched by means of a switch circuit. Though, in the above embodiment, npn transistors T1 and T2 are used in the active circuit 3 and buffer circuit 306, field effect transistors may be used instead.

Claims: A voltage controlled oscillator comprising: a resonant circuit (301) having a coil (L1) and at least one first variable capacitance diode (D1), for resonating the circuit within a predetermined variable frequency range; an active circuit (303) having an input, an output, and an active element (T1) connected therebetween, said input being connected to said resonant circuit (301) to receive a resonant output therefrom; a variable capacitance ratio circuit (304) having a second variable capacitance diode (D4) and at least one capacitor (C7, C8) connected in a positive feedback manner between said input and said output of said active circuit (303) to oscillate said active element (T1) at the resonant frequency of said resonant circuit (301); coupling control circuit (302) including a coupling capacitor (C2) and a third variable capacitance diode (D3) for providing capacitance coupling between the resonant circuit (301) and the active circuit (303), and variable control means adapted to change the resonant frequency of said resonant circuit means (301) by applying control voltages to said first, second and third variable capacitance diodes (D1, D4, D3), characterized in that said second variable capacitance diode (D4) is connected in reverse bias manner between an end of said coil (L1) and ground by means of a first choke coil (L4) ; wherein said variable control means are adapted to control the ratio between the capacitance of said at least one capacitor (C7, C8) and that of the second variable capacitance diode (D4), such that the amount of said positive feedback is maintained substantially constant independent of the oscillation frequency; said third variable capacitance diode (D3) is connected in reverse bias manner between said end of said coil (L1) and ground by means of a second choke coil (L3) and a capacitor (C1) is connected between the active circuit (303) and the connecting point of said third variable capacitance diode (D3) and said second choke coil (L3), wherein said variable control means are adapted to control the capacitance ratio between said coupling capacitor (C2) and said third variable capacitance diode (D3) such that the strength of the capacitance coupling between the resonant circuit (301) and the active circuit (303) remains substantially constant independent of the oscillation frequency, wherein the oscillation frequency of said active element (T1) is controlled to be substantially linear with respect to control voltage over a wide bandwidth of no less than one octachord. A voltage controlled oscillator according to Claim 1, wherein said coil (L1) has two ends, and said resonant circuit means (301) includes two first variable capacitance diodes (D1, D2) respectively connected in a reverse bias manner to said ends of said coil (L1) and ground; said control voltage being supplied from variable control means (307) at the input end of said coil (L1); and said resonant output and said control voltage being extracted from the other end of said coil (Ll). A voltage controlled oscillator according to Claim 1 or 2, wherein said at least one capacitor (C7, C8) of said variable capacitance ratio circuit (304) includes a first capacitor (C7) connected between said input and said output of said active circuit means (303); a second capacitor (C8) connected between the connecting point of said first choke coil (L4) and said second variable capacitance diode (D4), and said active circuit means (303) ; and control means adapted to control the capacitance of ratio between said first capacitor (C7) and that of the second capacitor (C8) and the second variable capacitance diode (D4). A voltage controlled oscillator according to one of Claims 1 to 3, further comprising: a buffer circuit (306) for receiving an output from said active circuit (303) and for outputting said output at almost a linear level over the whole bandwidth of the oscillatory frequency of said active element (T1).

Inventors: MOTOYAMA HATSUO, SAEKI HIROSHI, MOTOYAMA, HATSUO, SAEKI, HIROSHI, ANRITSU OKIHARA-RYOU
Applicants: ANRITSU CORP, ANRITSU CORPORATION
Application year: 1990
Main classification: H03L7
IPC codes: H03B19, H03B1, H03B5, H03B21, H03L7