Abstract:
A logic gate includes a low noise current source coupled between a first terminal of a voltage supply and an output terminal. The low noise current source is capable of delivering a preselected voltage signal to the output terminal having a magnitude responsive to a first control signal relatively independent of the magnitude of the voltage on said first terminal of said voltage supply. At least one switching element is coupled between the output terminal and a second terminal of the voltage supply. The switching element is capable of coupling the output terminal to the second terminal of the voltage supply in response to receiving a second control signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to a logic gate, and, more particularly, to a low-noise, logic gate used, for example, in a voltage controlled oscillator.  
           [0003]    2. Description of the Related Art  
           [0004]    In the field of wireless telecommunications, noise problems are a significant problem. For example, in cordless telephone sets, a handset and base communicate over a radio link instead of through a wire. Noise may reduce the useable range, or more problematic, even eliminate proper operation by interfering with the radio link between the handset and the base of the telephone set.  
           [0005]    Some noise is environmental in nature. That is, some noise comes from the environment in which the telephone set is located. For example, other electronic equipment operating near the telephone set can produce EMI (electromagnetic interference) that causes noise to be impressed onto the circuitry of the telephone set. Shielding the telephone set can reduce some environmental noise, however, in applications such as cordless telephones, shielding may not be entirely effective. In fact, cordless telephones have been recently designed to operate in a frequency range (˜900 MHz) that should normally be outside the range of noise produced by many home appliances. That is, the radio signals delivered between the handset and the telephone base are carried by a 900 MHz electromagnetic signal. Most home appliances do not produce EMI in this frequency range.  
           [0006]    Another type of noise that can impair voice signal clarity arises from inside the telephone set. That is, the telephone set may produce noise internally that is impressed on the radio signal. Two examples are device noise and switching noise. Oscillators are commonly used to produce the ˜900 MHz carrier signal used in cordless telephones. Any noise introduced by the oscillator will ultimately be delivered over the radio link, interfering with the proper operation.  
           [0007]    The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.  
         SUMMARY OF THE INVENTION  
         [0008]    In one aspect of the present invention, a logic gate is provided. A low noise current source is coupled between a first terminal of a voltage supply and an output terminal. The low noise current source is capable of delivering a preselected voltage signal to the output terminal having a magnitude responsive to a first control signal relatively independent of the magnitude of the voltage on said first terminal of said voltage supply. At least one switching element is coupled between the output terminal and a second terminal of the voltage supply. The switching element is capable of coupling the output terminal to the second terminal of the voltage supply in response to receiving a second control signal.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:  
         [0010]    [0010]FIG. 1 illustrates an electrical schematic of one embodiment of a voltage controlled oscillator;  
         [0011]    [0011]FIG. 2 illustrates an electrical schematic of one embodiment of a comparator circuit of the voltage controlled oscillator of FIG. 1;  
         [0012]    [0012]FIG. 3 illustrates an electrical schematic of one embodiment of a delay circuit of the voltage controlled oscillator of FIG. 1; and  
         [0013]    [0013]FIG. 4 illustrates a timing diagram of waveforms corresponding to various nodes of the voltage controlled oscillator of FIG. 1. 
     
    
       [0014]    While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0016]    Turning now to the drawings, and in particular, to FIG. 1, a schematic of one embodiment of a voltage controlled oscillator  10  is shown. The voltage controlled oscillator  10  includes first and second edge delay circuits  12 ,  14 . The edge delay circuits  12 ,  14  have output terminals respectively connected to first and second comparator circuits  16 ,  18 . The comparator circuits  16 ,  18  are respectively formed from an AND gate  20 ,  24  having its output terminal coupled to an input terminal of a NOR gate  22 ,  26 . The output terminals of the comparator circuits  16 ,  18  are coupled to complementary phase output terminals  30 ,  28 . The complementary phase output terminals  28 ,  30  are also coupled to the input terminals of the edge delay circuits  12 ,  14  and to one of the input terminals of the NOR gates  22 ,  26 .  
         [0017]    Oscillation of the VCO  10  is ensured by a pair of NOR gates  34 ,  36 . Each of the NOR gates  34 ,  36  has a first input terminal coupled to the complementary phase output terminals  28 ,  30 . Output terminals of the NOR gates  34 ,  36  are coupled to a second input terminal of the AND gates  20 ,  24 , respectively. The output terminals of the NOR gates  34 ,  36  are also cross coupled to a second input terminal of the NOR gates  36 ,  34 , respectively.  
         [0018]    The frequency at which the VCO  10  oscillates is effected by a control input terminal  32  coupled to a control input terminal of the NOR gates  22 ,  26  and the edge delay circuits  12 ,  14 . An analog voltage placed on the control input terminal  32  affects the rate at which the edge delay circuits  12 ,  14  charge to a logically high voltage level and the level at which the comparator circuits  16 ,  18  switch between a logically low level and a logically high level. Thus, a single control voltage is advantageously used in multiple components to control the frequency of the VCO  10 .  
         [0019]    The edge delay circuits  12 ,  14  are capable of pulling their output terminals to a logically low level relatively quickly, whereas the transition to a logically high level occurs substantially slower. Operation of the VCO  1   0  may be understood by reference to the timing diagram of FIG. 4. The timing diagram of FIG. 4 includes representations of various terminals of FIG. 1. For example, the waveforms at the output terminals of the edge delay circuits  12 ,  14  are represented by the lines labeled  50  and  51 , respectively. Likewise, the waveforms at the output terminals of the NOR gates  34 ,  36  are represented by the lines labeled  52  and  53 , respectively. The waveforms present at the complementary phase output terminals  30 ,  28  are represented by the lines labeled  55 ,  54 , respectively.  
         [0020]    For purposes of describing the operation of the VCO  10 , assume an initial condition where the output  54  of the complementary phase output terminal  28  is at a logically high level and just beginning a transition to a logically low level (point A on line  54 ). As the output  54  transitions to a logically low level, it causes the output  50  of the edge delay circuit  12  to begin a relatively slow transition to a logically high level. At the same time, the logically low level at the output  54 , through the comparator circuit  16  causes the output  55  to rapidly transition to a logically high level. The output  55  will remain at the logically high level until the relatively slow charging output  50  reaches the logically high level. Once the output  50  reaches the logically high level, the output  55  of the comparator circuit  16  rapidly transitions to a logically low level. The logically low level of the output  55  causes the output  51  of the edge delay circuit  14  to begin a relatively slow transition to a logically high level, and to immediately transition the output  54  back to a logically high level. Once the output  51  reaches the logically high level, the output  54  of the comparator circuit  18  rapidly transitions to a logically low level, repeating the operation.  
         [0021]    It should be appreciated that the VCO  10  does not exhibit a hysteresis effect, which eliminates regenerative feedback, and thus, noise sensitivity. It should also be appreciated that the logic circuitry used in the VCO  10  could be readily modified to provide a relatively fast high-to-low transition and a relatively slow low-to-high transition by, inter alia, replacing the NOR gates  22 ,  26  with NAND gates (not shown) and the AND gates  20 ,  24  with OR gates (not shown).  
         [0022]    Turning now to FIG. 2, an electrical schematic of one embodiment of the comparator circuit  16  of FIG. 1 is shown. A low noise current source  60  is coupled to a power supply  62 . The current source  60  is formed from a serially connected resistor  64  and a P-intrinsic transistor  66 . The gate of the transistor  66  is coupled to the control input terminal  32 , such that an analog voltage placed on the control input terminal  32  affects the magnitude of the current supplied. A pair of transistors  68 ,  70  are gate-to-drain coupled in series between system ground and the complementary phase output terminal  30 . The transistors  68 ,  70  clamp the voltage appearing at the complementary phase output terminal  30  to about a two threshold voltage drop. This keeps the current source  60  working despite variations in the supply voltage.  
         [0023]    The AND gate  20  of the comparator circuit  16  is formed by a pair of transistors  72 ,  74  serially coupled between the complementary phase output terminal  30  and system ground. The gates of the transistors  72 ,  74  are coupled to the output terminals  71   a ,  71   b  of the edge delay circuit  12  and the NOR gate  34 . Thus, when the gates of the transistors  72 ,  74  are both logically high, then both conduct current, pulling the complementary phase output terminal  30  toward system ground. When neither, or only one, of the gates of the transistors  72 ,  74  are logically high, then the current source  60  supplies current to charge the complementary output terminal  30 .  
         [0024]    The NOR gate  22  of the comparator circuit  16  is formed by the AND gate  20  connected in parallel with a transistor  76 . The gate of the transistor  76  is coupled to the complementary phase output terminal  28  through a terminal  77 . Thus, when the complementary phase output terminal  28  reaches a logically high level, the transistor  76  conducts, pulling the complementary phase output terminal  30  to a logically low level. Likewise, when both input terminals of the AND gate  20  reaches a logically high level, the transistors  72 ,  74  conduct, pulling the complementary phase output terminal  30  to a logically low level.  
         [0025]    The current source  60  is resistant to noise for at least two reasons. First, current sources in general are inherently resistant to variations in their supply voltage. Thus, variations in the magnitude of the voltage delivered by the power supply  62  has little or no impact on the current level delivered to the output terminal  30  as long as the input terminal  32  is varying with the voltage supply  62 . Second, the resistor  64  and P-intrinsic transistor  66  are low noise devices as compared to standard MOS devices. The P-intrinsic transistor  66  is formed using only a single injected dopant. Ordinarily, MOS transistors (non-intrinsic) are formed using multiple dopings. Often, three doping steps are used to form a conventional transistor. As doping levels increase in a transistor, a phenomena known as flicker noise becomes more significant. Flicker noise, induced by a conventional transistor, can show up in the oscillator signal and ultimately reduce the performance of the telephone set. In the instant invention, less doping means less noise and better performance.  
         [0026]    The low noise current source  60  generally performs well throughout most of its expected range of operation. However, as the voltage appearing at the terminal  30  approaches a power supply rail, such as system voltage, performance lags. Accordingly, the clamping transistors  68 ,  70  have been introduced to clamp the voltage to a stable level as the voltage at the terminal  30  approaches the rail.  
         [0027]    Turning now to FIG. 3, an electrical schematic of one embodiment of the edge delay circuit  12  of FIG. 1 is shown. A low noise current source  80  (similar to the current source  60  of FIG. 2) is coupled to the power supply  62 . The current source  80  is formed from a serially connected resistor  82  and a P-intrinsic transistor  84 . The gate of the transistor  84  is coupled to the control input terminal  32 , such that an analog voltage placed on the control input terminal  32  affects the magnitude of the current supplied. A capacitor  86  is serially coupled between the current source  80  and system ground, such that the current source  80  is capable of charging the capacitor  86  at a rate determined by the magnitude of the current supplied by the current source  80 . That is, the higher the voltage delta at the control input terminal  32  relative to Vcc, the more current that is supplied, and the faster the capacitor  86  charges.  
         [0028]    A transistor  88  is coupled in parallel with the capacitor  86 , and has its gate coupled to the complementary phase output terminal  28 . Thus, when the complementary phase output terminal  28  is at a logically high level, the transistor  88  conducts, quickly pulling the complementary phase output terminal  28  to a logically low voltage level. On the other hand, when the complementary phase output terminal  28  is at a logically low level, the transistor  88  does not conduct, and the capacitor is relatively slowly charged toward a logically high voltage level by the current source  80 .  
         [0029]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.