Abstract:
An AM radio antenna circuit having a ferrite bar loop antenna comprises a resonating structure forming a balanced antenna circuit, a varactor diode tuning structure presenting a controllable capacitance to said winding structure, a DC path including the winding structure coupled to the varactor constructed and arranged to deliver a tuning signal to the varactor, and means for connecting the antenna circuit to the input of an external detector integrated circuit.

Description:
[0001]    The present invention relates in general to radio antenna noise reducing and more particularly concerns novel apparatus and techniques for reducing interfering noise in the AM band with an AM antenna.  
         BACKGROUND OF THE INVENTION  
         [0002]    Operation of electronic power controllers, such as a triac light dimmer, can create severe interfering noise in the AM radio band. The interfering noise may enter the radio through any of the mechanisms of capacitive coupling to the antenna, conduction through the AC mains, or magnetic coupling to the antenna. In home use, a major mode is through the AC mains.  
           [0003]    Typical antennas for AM radios are external loop or internal loop types, such as ferrite rod loop AM antennas. External loop antennas typically use twisted pair lead-ins connected to a balanced input. Internal ferrite rod loop antennas are typically unbalanced, with one side of the loop at RF ground while the other side is connected to a varactor diode. An unbalanced pickup coil is typically used to drive the detector integrated circuit (IC).  
           [0004]    It is an important object of the invention to reduce electrical interference in an AM radio with an improved antenna.  
         SUMMARY OF THE INVENTION  
         [0005]    According to the invention, there is a loop antenna having winding structure with ends coupled to the input of the radiofrequency amplifying circuit and a varactor tuning diode structure coupled to the winding structure.  
           [0006]    Other features, objects and advantages will become apparent from the following description when read in connection with the accompanying drawing in which: 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0007]    [0007]FIG. 1 is a schematic circuit diagram of an center grounded ferrite bar loop antenna according to the invention; and  
         [0008]    [0008]FIG. 2 is a schematic circuit diagram of a end grounded ferrite bar loop antenna according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]    Referring to FIG. 1, there is shown a schematic circuit diagram of an embodiment of the invention incorporating a center grounded ferrite bar loop antenna. The circuit includes a ferrite bar  11  having a resonant circuit winding  12  and a pickup winding  13 . One end of resonant circuit winding  12  is directly coupled to varactor tuning diode  14 , the other end of winding  12  is coupled to varactor diode  14  through low impedance coupling capacitor  15 . An intermediate tap  16  of resonant circuit winding  12  is coupled to a reference potential through a low impedance coupling capacitor  17 . The reference potential is assumed to be ground for the rest of this disclosure, but it should be noted that the reference can be set to be any desired potential Intermediate tap  16  also receives a tuning voltage through resistor  18  for controlling the effective capacity of varactor diode  14  to tune the resonant circuitry to the frequency of the desired AM carrier. The junction of varactor diode  14  and low impedance capacitor  15  is connected to ground through resistor  21 . Representative parameter values are set forth in FIG. 1. A low impedance bypass capacitor  22  couples the end of pickup winding  13  that receives a bias voltage for the detector integrated circuit input to ground. The other end of pickup winding  13  is connected to the input of the detector integrated circuit.  
         [0010]    The embodiment of FIG. 1 balances the antenna circuit by placing the RF ground near the center of the resonant circuit winding  12 . The intermediate tap  16  is preferably displaced from the physical center of winding  12  to account for the effects of unbalanced pickup coil  13  and the capacitance to the external environment of the conductors attached to the detector integrated circuit input. The position of intermediate tap  16  should be offset from the center of the winding coil and may be experimentally determined for maximum interference reduction. In this example, intermediate tap  16  was located  16  turns from the capacitor end and  31  turns from the varactor end of winding  12  in a 220 microhenry inductance with winding  13  having 24 turns and an inductance of 55 microhenries to provide at least 27 dB improvement in line conducted interference rejection.  
         [0011]    It is possible to eliminate coil  13  of FIG. 1. In this case, an appropriate intermediate point along coil  12  is located where an RF signal can be tapped off. This point is chosen such that the coil impedance matches the input impedance requirements of the circuitry coupled to this intermediate tap, which would typically be the RF input of the detector IC.  
         [0012]    Referring to FIG. 2, there is shown another embodiment of the invention comprising coils  12 A and  12 B forming the resonating winding with the opposed ends maintained at RF ground through capacitors  15 A and  15 B, respectively to balance the antenna. Either winding provides the correct driving point impedance for the detector integrated circuit so pickup coil  13  is unnecessary. Therefore, the input to the detector chip is now taken directly from the junction of windings  12 A and  12 B maintained at the same RF potential through capacitor  22 A. The negative effects of stray capacitance can be reduced by adding an electrically conductive structure, such as a geometric structure formed in the printed circuit board (PCB) copper, to the circuit. As shown in FIG. 2, an additional trace wire  23  is added to the hot side of winding  12 B and is routed as close as practical to the lead connected to the RF input of the detector IC, along its entire length. The minimum spacing between the lead the added structure is determined by the PCB design rules used to design and manufacture the PCB. The rules are chosen based on cost and performance requirements. Smaller trace spacing typically provides better system performance in terms of reducing stray effects, at a higher cost. In the present invention, a trace spacing of 0.006 inches was implemented.  
         [0013]    Additional copper structure  23 A at the end of this wire further compensates the negative effect created by the capacitance of the conductors connected to the detector integrated circuit input. In a specific form of this embodiment, each of windings  12 A and  12 B has  24  turns.  
         [0014]    There has been described novel apparatus and techniques for significantly reducing undesired noise entering the antenna circuit of an AM radio. It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific apparatus and techniques herein disclosed without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.