Abstract:
A quadrature oscillator for generating quadrature and differential double-frequency signals is disclosed. This oscillator is formed by PMOS and NMOS transistors, complementary devices, etc. The circuit is designed by using the differential circuit structures, two LC tanks and the technology of current reuse. As the current pass through most devices and selectively coupling the specific terminals, therefore, it has advantages of less area requirement in circuit design, low power dissipation and low phase noise of output.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a quadrature oscillator, more particularly, to an an oscillator for generating four quadrature signals. The quadrature oscillator of the present invention can be advantageously applied in electronic circuit and communication system. 
     2. Related Art 
     Quadrature signals play an important role in the modulation mechanism of advance communication system. For example, the image signals will automatically disappear by mixing with quadrature signals. Application of this technique for communication circuit will lower the cost of having to use filters. However, quadrature signals are usually generated from RC phase shifters and quadrature oscillators. 
     Oscillator is an electronic apparatus, which can generate continuous (or discrete) sinusoidal or impulse waveforms. Oscillators of sinusoidal generator usually includes LC tanks, amplify circuit and feedback circuit, and which are widely used in circuit and communication system. 
     The conventional quadrature generators are formed by coupling RC phase shifters with quadrature oscillators. The RC phase shifter inputs a signal from single-phase oscillator to two RC-CR circuits, which are connected in series, for generating two continuous quadrature signals which are 90 degrees out of phase relative to each other. Poly-phase filters are formed by suitable connection of resistors and capacitors in series. We can obtain quadrature signals from poly-phase filters by inputting differential frequency. 
     FIG.  1 (A) is the conventional quadrature signal generator formed by RC phase shifters. FIG.  1 (B) is the conventional quadrature signal generator formed using poly-phase filters. The quadrature signal generators, which are formed by the RC phase shifters or the poly-phase filters, contain resistors and capacitors. However, the signal&#39;s amplitude will be attenuated due to these passive devices (resistors and capacitors). Therefore, additional power compensation circuit will be needed. The degree of device matching can affect the central frequency, amplitude and phase-error. 
     The quadrature generator, which is formed from differential oscillators, is composed of active devices. For example, the feedback circuit of two differential oscillators, which are formed by BJT and CMOS, can generate four quadrature signals. 
     FIG. 2 is a well-known differential oscillator formed from quadrature generators. Basically, this quadrature generator, which is formed from differential oscillators, needs four inductors and four branch constant-current sources. Thus, it requires a larger area for circuit layout and involves more power dissipation. 
     SUMMARY OF THE INVENTION 
     This invention relates to a quadrature oscillator for generating four quadrature signals and differential double-frequency signals. It has the advantages of requiring less circuit area and less power dissipation, and reduces the phase-error of the output signal by coupling the two output terminals having a constant-current source. 
     Even though the examples of present invention as disclosed above are very descriptive, however, they are not limitative of the present invention. Any person with skills in these techniques can make certain changes and modifications, within the claims and spirit of this invention, for more applications. Therefore, the protections of this invention are according to the claims provided at the end of this disclosure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed descriptions given below are for illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG.  1 (A) is the conventional quadrature generator formed by the RC phase shifters;(B) is the conventional quadrature generator formed by the poly-phase filters; 
     FIG. 2 is a conventional generator formed by the differential oscillators; 
     FIG. 3 is the first example of the quadrature oscillator according to the disclosed invention; 
     FIG. 4 shows the output quadrature signals of the quadrature oscillator from the disclosed invention; 
     FIG. 5 shows the output double-frequency signals of the quadrature oscillator from the disclosed invention; 
     FIG. 6 is the second example of the quadrature oscillator according to the disclosed invention; 
     FIG. 7 is the third example of the quadrature oscillator according to the disclosed invention; and 
     FIG. 8 is the forth example of the quadrature oscillator according to the disclosed invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 3, the first example of the quadrature oscillator of the present invention includes a first constant-current source  301 , a first common-sourced P-type metal oxide semiconductor (PMOS) transistor set  302 , a second constant-current source  303 , a first common-sourced N-type metal oxide semiconductor (NMOS) transistor set  304 , a first LC tank  305 , a third current  311 , a second common-sourced PMOS transistor set  312 , a forth current source  313 , a second common-sourced NMOS transistor set  314 , and a second LC tank  315 . 
     Therein, the first current source  301  and the third constant-current source  311  each are formed by a PMOS transistor, and each source terminal is connected to a constant current-source voltage VDD, and each gate is connected to a P-type constant current-controlled voltage, then each drain produce a constant current. 
     The source terminals of the first common-sourced PMOS transistor set  302  and the second common-sourced PMOS transistor set  312  are coupled to the first constant current-source  301  and the third constant current-source  311 , respectively, the gates and drains are cross-coupled to form a feedback circuit. For example, the first drain of  302  is coupled to the first gate of  312 , and the first drain of  312  is coupled to the second gate of  302 ; on the other hand, the second drain of  302  is coupled to the second gate of  312 , and the second drain is coupled to the first gate of  302 . 
     The second current source  303  and the fourth current source  313  each are formed by a NMOS transistor, which sources are connected to ground, and the gates are connected to a N-type constant-current control-voltage VNG, then the drains produce a constant current. 
     The drains of the first common-sourced NMOS transistor set  304  and the second common-sourced NMOS transistor set  314  are coupled to the second constant-current source  303  and the fourth constant-current source, respectively. The gates and the drains are self cross-coupled, i.e., the gate of  304  is coupled to the drain of  314 , and the gate of  314  is coupled to the drain of  304 . 
     The first LC tanks  305  and second LC tanks  315  are both for generating constant-frequency signals. The first LC tank  305  is connected to the fist common-sourced PMOS transistor set  302  and the first common-sourced NMOS transistor set  304  in series. The second LC tank  315  is connected to the fist common-sourced PMOS transistor set  312  and the first common-sourced NMOS transistor set  314  in series. These circuits of the two LC tanks can be obtained from combination of the inductor-coils and capacitors in series. Signals (or noises) of specific frequency are first selected by the LC tanks and then by amplify effect of the above connection circuits, and these form oscillation circuit. Four quadrature signals are generated by the symmetrization of the circuit. 
     Referring to FIG. 4, the output quadrature signals of the quadrature oscillator from the technique disclosed by this invention. The first LC tank  305  and the second LC tank  315  can both generate signals of constant frequency f. Inputting the signal to the feedback circuit, which is formed by the first common-sourced PMOS transistor set  302  and the second common-sourced PMOS transistor set  312 , then we get four output signals at the first output terminal  306 , the second output terminal  307 , the third output terminal  316  and the fourth output terminal  317 , i.e. quadrature signals. 
     By the additive property of inputting signal at the common-sourced terminal of the differential transistor set, we thus can get the double-frequency output. This invention can generate quadrature signals, in addition, can also generate signal of frequency 2f at the output terminal  308  and  318 . The frequency is double to the constant frequency generated from the first LC tank  305  and the second LC tank  315 , i.e. the double-frequency signal. 
     The difference between the conventional quadrature generator (as showed in FIG. 2) formed by conventional differential oscillators and the present invention is that the conventional one requires four inductors, however, the present one needs only two inductors. Under not to be the optimization consideration for inductor design, it can always reduce the area for circuit design. 
     This invention use the current-reuse technique, first input current from the first constant-current source  301  to the first common-sourced PMOS transistor set  302  and the first common-sourced NMOS transistor set  304 , then the current goes through the second constant-current source  303 ; similarly, we first input current from the third constant-current source  311  to the second common-sourced PMOS transistor set  312  and the second common-sourced NMOS transistor set  314 , then the current goes through the fourth constant-current source  313 . Because the current reuse for many devices, therefor, the present invention has lower power dissipation. 
     Referring now to FIG. 6, the second example of the quadrature oscillator, which is disclosed by this invention, includes same device as described in the first example, i.e. FIG.  3 . The differences, between the first and second examples, are that the gate and drain terminals of the first common-sourced PMOS transistor set  604  and second common-sourced PMOS transistor set  614  are self cross-coupled; similarly, the gate and drain terminals of the first common-sourced PMOS transistor set  602  and second common-sourced PMOS transistor set  612  are self cross-coupled, and thus form the feedback circuit. The mechanism, purpose and efficiency are as same as those in the first example. 
     Referring now to FIG. 7, the third example of the quadrature oscillator, which is disclosed by this invention, the components are same as those of the first example, i.e. FIG.  3 . The difference is that the drain of the second constant-current source  703  is coupled to the drain of the fourth constant-current source  713 . 
     Contrast to the drain of the second constant-current source  703 , the drain of the fourth constant-current source  713  is similar to an equivalent capacitor, vise versa. From the above outputs of double-frequency are differential, we thus know that the voltage of the drain of the fourth constant-current source will drop as the voltage of the drain of the second constant-current source raised. By coupling the drains of the second constant-current source  703  and the fourth constant-current source  713 , the (electric) potential of these two terminals tends to be equilibrium, it will help to improve the phase noise. However, differing to the conventional techniques of quadrature differential oscillators, the present can also get signals of 2f frequency from the positive double-frequency output terminal  708  and the negative double-frequency output terminal  718 . 
     Referring now to FIG. 8, the fourth example of the quadrature oscillator, which is disclosed by this invention, the components are with those as described in the first example, i.e. FIG.  3 . The differences, between the first and fourth examples, are that the capacitors of wherein the first LC tank  805  and second LC tank  815  are formed by two variable capacitors in series. 
     By changing the voltage between the capacitors of the first and second LC tanks, we can change the equivalent value of any LC tank, and further change the frequency of the output signals, and this is one of the advantages of this invention by providing variable oscillation frequency.