Abstract:
The present invention relates to a distributed oscillator, and more particularly, to a high frequency distributed oscillator using coupled transmission lines. According to the present invention, coupled transmission lines capable of increasing the frequency selectivity are used as transmission lines. Signal interference occurs among the coupled transmission lines because of mutual approximation. The coupled transmission lines serve as a filter due to the signal interference. A phase noise characteristic can be thus increased.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a distributed oscillator, and more particularly, to a high frequency distributed oscillator using coupled transmission lines. 
   2. Background of the Related Art 
   In an oscillation circuit, an oscillator using an inductor and a capacitor as a resonator has been widely used. 
   In a several GHz bandwidth, however, if a circuit in which an application frequency operates 10 GHz or more is used although not becoming a significant problem in analyzing the circuit, parasitic component or parasitic parameters are generated in the inductor and the capacitor. 
   That is, capacitance of the capacitor exists in the conductor of the inductor, and inductance component also exists in the plate of the capacitor. 
   An oscillator is difficult to design so that it operates at a desired frequency and a frequency tuning characteristic is also significantly lowered, because of the parasitic component or parasitic parameters. 
   There is a disadvantage in that it is difficult to design and implement a passive element due to the parasitic parameters. 
   Recently, in order to overcome this disadvantage, technology for a distributed oscillator has been proposed. 
     FIG. 1  is a circuit diagram illustrating the operational principle of a conventional distributed oscillator. 
   Referring to  FIG. 1 , the conventional distributed oscillator includes active elements M 11  to M 15  respectively having first to third terminals, which are responsible for signal amplification, and transmission lines  111  to  120 . 
   A DC voltage is applied to the transmission line  119 , and an output signal is transmitted through the transmission line  120 . 
   The first terminals of the active elements M 11  to M 15  are connected between the transmission lines  111 ,  112 ,  113  and  114 , so that signals are amplified due to the active elements M 11  to M 15 . 
   In this time, signals inputted to the second terminals of the active elements M 11  to M 15  are transmitted through the transmission lines  115 ,  116 ,  117  and  118 . 
   Furthermore, if amplified signals generated from the active elements M 11  to M 15  have the same phase, it can be represented as the sum of those amplified signals. 
   That is, if a signal  131  amplified in the active element M 11  has the same phase as that amplified in the active element M 12 , they are added to become a summed and amplified signal  132 . 
   As such, if the above process is repeatedly performed between the active elements M 11  to M 15 , an amplified signal  134  becoming the sum of the signals amplified in the active elements M 11  to M 15  is obtained. Consequently, the summed signal  134  is feedbacked to the second terminals of the active elements M 11  to M 15 , thus forming a distributed oscillator. 
   Through the above-described construction and operation, the distribution type oscillator is advantageous in that it can operate up to the maximum frequency of oscillation (fmax) of an active element in such a manner that the distributed amplifier is oscillated by feedbacking its output terminal to its input terminal. 
   However, the distributed oscillator is disadvantageous in that the chip size increases because a ground line has to be formed at both sides of a signal line when designing the distributed oscillator. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made in view of the above problems occurring in the prior art, and it is an object of the present invention to provide a distributed oscillator in which coupled transmission lines, which can increase the frequency selectivity, are used as transmission lines, wherein signal interference occurs among the coupled transmission lines because of mutual approximation, and the coupled transmission lines serve as a filter at a predetermined frequency due to the signal interference, whereby a phase noise characteristic can be improved. 
   Another object of the present invention is to reduce the chip size in such a manner that a ground line is formed only at one end of a signal line in designing the coupled transmission lines. 
   To achieve the above object, according to the present invention, there is provided a high frequency distributed oscillator including transmission lines responsible for frequency selection and signal transmission, transistors for amplifying the signal, and a feedback line for feedbacking the signal that is amplified through the transmission lines and the transistors, wherein the transmission lines use coupled transmission lines capable of increasing the frequency selectivity, signal interference occurs among the coupled transmission lines because of mutual approximation, and the coupled transmission lines serve as a filter at a predetermined frequency due to the signal interference, whereby a phase noise characteristic is improved. 
   In the above, the coupled transmission lines have two coupling signal lines at their center, and ground lines around it. 
   Through the above construction, the coupled transmission lines can serve as a filter due to signal interference, and only a predetermined frequency can be selected/amplified. Accordingly, a phase noise characteristic can be improved. 
   Furthermore, the coupled transmission lines are used, and a ground line is formed only at one end of the coupled transmission line. Thus, it is possible to reduce the chip size compared to a case where the ground lines are formed at both sides of the coupled transmission line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a circuit diagram illustrating the operational principle of a conventional distributed oscillator; 
       FIG. 2  is a circuit diagram illustrating the operational principle of a coupled type distributed oscillator according to the present invention; 
       FIG. 3  shows a photography of an actually fabricated chip of a distributed oscillator shown in  FIG. 1  and a coupled type distributed oscillator shown in  FIG. 2 ; 
       FIG. 4  is a graph showing a phase characteristic depending on a frequency of a coupled transmission line; 
       FIG. 5  is a graph showing a phase noise characteristic of the coupled type distributed oscillator according to the present invention; and 
       FIG. 6  shows scanning electron microscope (SEM) photography of a coupled type distributed oscillator fabricated by means of micro electro mechanical systems (MEMS). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will now be described in detail in connection with preferred embodiments with reference to the accompanying drawings. 
     FIG. 2  is a circuit diagram illustrating the operational principle of a coupled type distributed oscillator according to the present invention. 
   The construction and operation of the coupled type distributed oscillator will now be described with reference to  FIG. 2 . 
   Referring to  FIG. 2 , the coupled type distributed oscillator includes active elements M 21  to M 25  respectively having first to third terminals, which are responsible for signal amplification, and coupled transmission lines  211  to  220 . 
   In this structure, the coupled type distributed oscillator according to the present invention employs the coupled transmission lines  211  to  220  so as to increase the frequency selectivity. 
   Furthermore, the coupled transmission lines  215 ,  216 ,  217  and  218  are connected to the input terminals of the active elements M 21  to M 25 , and the coupled transmission lines  211 ,  212 ,  213  and  214  are connected to the output terminals of the active elements M 21  to M 25 . The coupled transmission lines  211  to  220  and the active elements M 21  to M 25  constitute amplifiers, which are in turn connected in series. 
   A voltage is applied to the transmission line  219 , and an output signal is transmitted through the transmission line  220 . 
   The first terminals of the active elements M 21  to M 25  are connected between the transmission lines  211  to  214 , so that signals are amplified by means of the active elements M 21  to M 25 . 
   In this time, signals inputted to the second terminals of the active elements M 21  to M 25  are transmitted through the transmission lines  215  to  218 . 
   Furthermore, if amplified signals generated from the active elements M 21  to M 25  have the same phase, it can be represented as the sum of those amplified signals. 
   That is, if a signal amplified in the active element M 21  has the same phase as that amplified in the active element M 22 , they are added to become a summed and amplified signal. 
   If this process is repeatedly performed between the active elements M 21  to M 25 , an amplified signal becoming the sum of the signals amplified in the active elements M 21  to M 25  is obtained. 
   In this time, the summed signal is feedbacked to the second terminals of the active elements M 21  to M 25 , thus forming the coupled type distributed oscillator. 
   Through this construction, the coupled type distributed oscillator has characteristics in which the transfer of a signal is prohibited in a range exceeding a resonant frequency and a phase is rapidly changed in the resonant frequency, in such a way that it is oscillated by feedbacking its output terminal to its input terminal. 
   Furthermore, in a common circuit, in order to reduce mutual interference among neighboring signals, the signal lines are sufficiently spaced apart from one another. If the mutual interference is employed in coupled type distributed oscillators, however, it is possible to increase the selectivity in a particular frequency. 
   In this case, if the coupled transmission lines are used in the distributed oscillator, phase noise can be reduced due to the characteristic. 
   Moreover, when the coupled type distributed oscillator is fabricated, the ground line is formed only at one end of the signal line. Thus, the coupled type distributed oscillator of the present invention has the chip size, which is 30% smaller than that of the conventional oscillator assuming that they operate in the same frequency. 
   In addition, the coupled type distributed oscillator according to the present invention is fabricated by using low-temperature surface micro machining technology at a low temperature of 120° C. through the introduction of micro electro mechanical systems (MEMS) technology. Accordingly, the present invention can present a good device manufacturing process, which can be compatible with a conventional CMOS process. 
     FIG. 3  shows two distributed oscillators fabricated by means of the same process. 
   Referring to  FIG. 3 , a circuit at the top is a conventional distributed oscillator, and a circuit at the bottom is a coupled type distributed oscillator according to the preset invention. 
   From  FIG. 3 , it can be seen that the circuit at the top has the size of 0.3 mm 2  (excluding a 1.5 mm×0.2 mm pad), and the circuit at the bottom has the size of 0.2 mm 2  (excluding a 1.5 mm×0.13 mm pad). 
   Moreover, it can be seen that the chip size of the coupled type distributed oscillator according to the present invention is about 30% smaller than that of a conventional distributed oscillator. 
     FIG. 4  is a graph showing a frequency characteristic of the coupled transmission line used in the present invention. 
   Referring to  FIG. 4 , solid lines indicate a frequency response of the coupled transmission lines according to the present invention, and dotted lines designate a frequency response of a conventional transmission line. 
   From  FIG. 4 , it can be seen that the conventional transmission line has the same signal output over the whole frequency band, whereas the coupled transmission line of the present invention has a maximum signal output at an oscillating frequency. 
   This means that oscillation other than the oscillating frequency is prohibited. 
   It can be also seen that at the oscillating frequency, a phase shift characteristic more abruptly occurs in the coupled transmission line of the present invention than in the conventional transmission line. 
   This means that the coupled type distributed oscillator according to the present invention has the frequency selectivity, which is higher than that of the conventional transmission line. 
     FIG. 5  is a graph showing a phase noise characteristic of the coupled type distributed oscillator according to the present invention. 
   Referring to  FIG. 5 , it can be seen that in the case where the oscillating frequency is about 13 GHz, a conventional distributed oscillator has a phase noise of −117 dBc/Hz at an offset frequency of 1 MHz, whereas the coupled type distributed oscillator of the present invention has a phase noise of −125.67 dBc/Hz at an offset frequency of 1 MHz. 
   From the above, it can be known that the coupled type distributed oscillator according to the present invention has phase noise of −125.67 dBc/Hz at the 1 MHz offset frequency. 
   In  FIG. 5 , dotted lines indicate a noise characteristic curve of the coupled type distributed oscillator according to the present invention. 
     FIG. 6  shows scanning electron microscope (SEM) photography of a coupled type distributed oscillator fabricated by means of micro electro mechanical systems (MEMS). 
   In this time, a transmission line of the coupled type distributed oscillator fabricated by means of the MEMS is formed by using a copper layer of 10 μm in thickness, and is spaced from the top of a complementary metal oxide semiconductor (CMOS) circuit by 25 μm. 
   The coupled type distributed oscillator constructed above can reduce resistance and loss in a substrate. 
   Table 1 below shows comparison between the properties of the coupled type distributed oscillator according to the present invention and those of the conventional distributed oscillator. 
   
     
       
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Comparison between common and coupled 
             
             
               type distributed oscillator 
             
           
        
         
             
                 
               Oscillating 
                 
               Phase 
                 
               Power 
             
             
                 
               frequency 
               Output 
               noise 
               Harmonics 
               consumption 
             
             
               Type 
               (GHz) 
               (dBm) 
               @ 1 MHz 
               (dB) 
               (mW) 
             
             
                 
             
           
        
         
             
               Coupled type 
               15.6 
               0.3 
               −124 
               −16 
               26 
             
             
               distributed 
             
             
               oscillator 
             
             
               Common 
               13 
               −0.8 
               −117 
               −25 
               26 
             
             
               distributed 
                 
                 
                 (−96.6) 
             
             
               oscillator 
             
             
                 
             
           
        
       
     
   
   As listed in Table 1, assuming that power consumption is the same, the oscillating frequency of the conventional distributed oscillator is 13 GHz, whereas the oscillating frequency of the coupled type distributed oscillator according to the present invention is 15.6 GHz. Accordingly, it can be seen that the coupled type distributed oscillator according to the present invention is useful in designing a circuit that requires a high frequency. 
   As described above, according to the present invention, the high frequency coupled type distributed oscillator has an improved frequency tuning characteristic by using the coupled transmission lines. Therefore, the present invention is advantageous in that it can increase the frequency selectivity. 
   Furthermore, high frequency phase noise can be reduced. 
   Also, when designing a coupled type distributed oscillator, it is not required that ground lines be formed at both sides of a signal line. It is thus possible to reduce the chip size. 
   While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.