A voltage-controlled oscillator

Abstract: A voltage-controlled oscillator according to the present invention comprises: a transistor (5), where the drain electrode (D) is grounded, the gate electrode (G) is connected to a resonator (3) whose resonant frequency is adjustable according to a voltage applied to a voltage-dependent capacitance diode (D₁) electromagnetically coupled with the resonator (3), an output signal is output from the source electrode (S); a resistor connected (R₂) in gate bias circuit or source voltage supply circuit for detecting a low-frequency noise component generated in the transistor (5), where the detected low-frequency noise component is amplified and fed back to the voltage-dependent capacitance (D₁) so as to cancel a phase-noise component generated in the voltage-controlled oscillator. The above-descried voltage-controlled oscillator may be further provided with a reference crystal oscillator to which the voltage-controlled oscillator is phase-locked. Thus, a microwave voltage-controlled oscillator excellent in phase-noise characteristics of the oscillated frequency is accomplished though there is employed a GaAs FET which is not good in a low-frequency noise characteristic.

Description: This invention relates to a voltage-controlled oscillator employed, for example, in a multichannel microwave transmission apparatus. Description of the Related Arts As a voltage-controlled oscillator, a silicon bipolar transistor generating less low-frequency-noise has been employed in a lower-frequency band, for example, below 12 GHz, and a gallium-arsenide field effect transistor (referred to hereinafter as a GaAs FET) has been preferably employed in a higher-frequency band due to its high maximum frequency of oscillation though its low-frequency noise characteristics is not preferable. The low-frequency noise of the transistor vitally affects a phase-noise characteristics of the oscillated frequency. Accordingly, the phase-noise characteristics of the microwave oscillator employing a GaAs FET is generally worse than that of silicon transistor. A typical circuit diagram of a prior art microwave voltage-controlled oscillator employing a GaAs FET and its noise characteristics are shown in FIG. 1 and FIG. 2, respectively and shown in the European Microwave Digest 1985 p407-412 "Wideband Tunable DR VCO" by Kenzo Wada, et al. Drain electrode D of a GaAs FET 14 is grounded, its gate electrode G is connected to a transmission line 13 whose opposite end is grounded via a DC (direct current) stopping capacitor C₁ and a terminating resistor R₁. An oscillation frequency signal f out is output from its source electrode S via a DC stopping capacitor C₂. Transmission line 13 is electromagnetically coupled with a dielectric resonator 12 having a resonant frequency f o . Dielectric resonator 12 is further coupled with an open stub 11 whose one end is open and another end is connected to a variable-capacitance diode D₁ to which a reverse bias is applied via a choke coil Ch₁. Choke coils Ch₂ and Ch₃ supply a bias voltage to gate electrode G, and a source voltage to source electrode S, respectively. Open stub 11 and variable-capacitance diode D₁ form a sub-resonator circuit and modulates, i.e. modifies, the resonant frequency f o of dielectric resonator 12 by the capacitance change according to the reverse voltage applied to the diode D₁. Thus, the oscillator operates as a voltage-controlled oscillator providing a frequency change of, for example, ±30 MHz for 12 GHz centre frequency. GaAs FET 14 having worse low-frequency noise characteristics compared with that of a silicon transistor generates phase noise components denoted with A on the sidebands, namely ± 1 MHZ, of the output frequency, 12 GHz, as shown as frequency spectrum in Fig. 2. Therefore, it has been seriously requested to provide a microwave voltage-controlled oscillator excellent in its phase-noise characteristics employing a transistor which is not good in a low-frequency noise characteristic. A further known device is shown in Proceedings of the 40th Annual Frequency Symposium, 1986, pages 385-391, "The GaAs FET Oscillator - Its Signal and Noise Performance" by R.A. PUCEL. This shows a circuit for noise compensation in FET oscillators using a dielectric resonator. The F.M. noise of the oscillator is detected and sent back as a negative feedback signal to cancel the noise. SUMMARY OF THE INVENTION It is a general object of the invention to provide a microwave voltage-controlled oscillator excellent in phase-noise characteristics of the oscillated frequency though employing a transistor not good in a low-frequency noise characteristic. According to the present invention, there is provided a voltage-controlled oscillator comprising: a transistor, a first terminal of said transistor being grounded, a second terminal of said transistor being connected to a resonator and an output signal being output from a third terminal of said transistor; and compensating means for detecting a low-frequency noise component generated in said transistor, characterized in that said detected low-frequency noise component is fed to a voltage-dependent reactance so as to cancel a phase-noise component included in said output signal of the voltage-controlled oscillator, and that a resonant frequency of said resonator is adjustable according to a voltage applied to said voltage-dependent reactance of said resonator. The above-described voltage-controlled oscillator may, in a preferred embodiment, be further provided with a reference crystal oscillator to which the voltage-controlled oscillator is phase-locked. The above-mentioned features and advantages of the present invention, together with other objects and advantages, which will become apparent, will be more fully described hereinafter, with references being made to the accompanying drawings which form a part thereof, wherein like numerals refer to like parts throughout. A BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a typical circuit configuration of a prior art microwave voltage-controlled oscillator; Fig. 2 shows phase-noise spectrum in an output of the Fig. 1 prior art oscillator; Fig. 3 shows a circuit configuration of a first preferred embodiment of the present invention; Fig. 4 shows a circuit configuration of a second preferred embodiment of the present invention; and Fig. 5 shows a circuit configuration of third preferred embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to circuit diagrams of preferred embodiments, the present invention is hereinafter described in detail. In a first preferred embodiment of the present invention shown in Fig. 3, a GaAs FET 5 is provided such that: drain electrode D is grounded, gate electrode G is connected to a transmission line 32 whose opposite end is grounded via a DC (direct current) stopping capacitor C₁ and a terminating resistor R₁ typically equivalent to the characteristic impedance of transmission line 32. An oscillation frequency signal frequency f out is output from its source electrode S via a DC stopping capacitor C₂. Transmission line 32 typically formed of a strip line is electromagnetically coupled with a dielectric resonator 3 having a resonant frequency f o . Dielectric resonator 3 is further coupled with an open stub 21 whose one end is open and another end is connected to a variable-capacitance diode D₁ to which a reverse bias +V V is applied via a resistor R₁₀ and choke coil Ch₁. Choke coil Ch₂ supplies a bias voltage to gate electrode G, and choke coil Ch₃ supplies source voltage to source electrode S, while the coke coils prevent the microwave signal. Open stub 21 and variable-capacitance diode D₁ form a sub-resonator circuit and gives an effect on the resonant frequency f o of dielectric resonator 12 by the capacitance change according to the reverse voltage applied to the diode D₁. Thus, the oscillation frequency is slightly deviated from the resonance frequency f o of dielectric resonator 3 by the diode voltage +V V . Electromagnetic couplings between transmission line 32 and dielectric resonator 3 and between the dielectric resonator 3 and open stub 21 are chosen so that the Q value of the oscillation circuit is kept high enough to stably maintain the oscillation frequency and at the same time the variable capacitance of diode D₁ can adequately change the oscillation frequency. A low-frequency noise current flowing into gate electrode G generates a low-frequency noise voltage v nG , for example, 1 µV across a resistor R ₂ typically of 1 kΩ connected between choke coil Ch ₂ and gate bias voltage source -V G . Low-frequency noise voltage v nG is input via capacitor C₃ and resistor R₃ to amplifier 41 having capacitor C₄ and resistor R₄ both in parallel to amplifier 41, where the gain can be adjusted by the value of resistor R₄ and capacitor C₄. The output of amplifier 41 is negatively fed back via capacitor C₅ and choke Ch₁ to diode D1. Accordingly, the oscilation circuit, capacitor C₃, resistor R₃, amplifier 41, capacitor C₅ and variable capacitance diode D₁ constitute a negative feedback loop. Change in the oscillating frequency by the variable-capacitance diode is essentially equivalent to the change in the phase of the oscillating frequency. The adjustments of these feedback circuit elements are carried out so that the phase-noise component in the oscillated microwave signal becomes minimum. Power supply and ground circuit for amplifier 41 are omitted in the figures for simplification. A second preferred embodiment of the present invention is shown in Fig. 4, where resistor R₂ of Fig. 3 is replaced with resistor R₇ typically of 10 Ω connected between source voltage source -V₃ and choke coil Ch₃. A voltage generated across resistor R₇ is proportional to the drain current including the low-frequency current fluctuation. Thus detected low-frequency noise voltage v nS is fed back via capacitor C₃ and resistor R₃ to amplifier 42 having capacitor C₆ and resistor R₆ both in parallel to amplifier 42, where phase of the output signal is inverted, capacitor C₅ and choke Ch₁ to diode D₁. Function of the negative feedback loop is the same as that of Fig. 3 first preferred embodiment. A third preferred embodiment of the present invention is shown in Fig. 5, where the voltage-controlled oscillator 6 according to the first or second preferred embodiment is further phase-locked. Oscillator output f out of 6 GHz is divided by typically sixty thousands (i.e. N₁ = 60,000) by a frequency divider 73 to output 100 kHz. On the other hand, a crystal oscillator 8 generates a reference frequency typically of 10 MHz, which is then divided by 100 (= N₂) by a second frequency divider 71 so as to output a 100 kHz signal. Phases of these two 100 kHz signals from frequency-dividers 73 and 71 are compared in a phase comparator 72, output of which corresponds to the phase difference of the two 100 kHz signals input thereto. Output of phase comparator 72 is input to a low-pass filter 74 having a cut-off frequency typically of 1 kHz which is the loop band width of the phase-locked loop. Output of low-pass filter 74 is added via a resistor R₉ with the feedback signal supplied via resistor R₈ from capacitor C₅, so that the detected phase difference is negatively fed back to the diode D₁, accordingly, the phase difference is kept zero. Thus, the output frequency is phase-locked to the reference frequency of crystal oscillator 8, as well as noise-compensated for the low-frequency noise. The output frequency can also be changed by modifying the number of frequency division N₁ of the frequency divider 73. In the above-described preferred embodiments the output frequency can be frequency-modulated, namely phase-modulated, by a base band signal superposed onto the voltage applied to diode D₁. In the above-described preferred embodiments the low-frequency noise component existing within ± 1 MHz of the oscillation frequency is reduced by 10 db to 20 db. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A voltage-controlled oscillator comprising: a transistor (5), a first terminal of said transistor being grounded, a second terminal of said transistor being connected to a resonator (3) and an output signal being output from a third terminal of said transistor; and compensating means for detecting a low-frequency noise component generated in said transistor, characterized in that said detected low-frequency noise component is fed to a voltage-dependent reactance so as to cancel a phase-noise component included in said output signal of the voltage-controlled oscillator, and that a resonant frequency of said resonator is adjustable according to a voltage applied to said voltage-dependent reactance (D₁) of said resonator. A voltage-controlled oscillator as recited in claim 1, wherein said transistor is a gallium-arsenic field effect transistor. A voltage-controlled oscillator as recited in claim 1, wherein said first terminal is a drain electrode, said second terminal is a gate electrode and said third terminal is a source electrode. A voltage-controlled oscillator as recited in claim 1, wherein said voltage-dependent reactance is a variable-capacitance diode to which a reverse bias is applied, said variable-capacitance diode being electromagnetically coupled with said resonator. A voltage-controlled oscillator as recited in claim 1, wherein said compensating means further comprises phase detecting means (7) for detecting a phase difference between the phase of said output of said voltage-controlled oscillator and the phase of an output of a reference oscillator (8), said voltage fed back to said voltage-dependent reactance of said resonator being controlled so as to keep said detected phase difference essentially zero.

Inventors: SUGAWARA HIDEO, SUGAWARA, HIDEO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H03B5
IPC codes: H03L7, H03B1, H03B5