Abstract:
A hand-held radio frequency transmitter for use in generating a coded radio frequency transmission has a switched mode power supply energizable by a battery. A radio frequency oscillator is driven by the switched mode supply. A modulator controls operation of the oscillator which produces a coded radio frequency signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates in general to radio frequency transmitters and, in particular, to a switched mode power supply for a radio frequency transmitter.  
           [0002]    Garage door operators commonly may be controlled from wired switches as well as radio frequency transmitting switches such as hand-held radio transmitters. Hand-held radio transmitters are energized by a battery and it is desirable to maintain as long a battery life as possible. However, it is also important for hand-held radio transmitters to be inexpensive, while still conforming with requirements of the Federal Communications Commission for center frequencies, bandwidth limitations, power limitations and the like. In the past hand-held radio transmitters, particularly of the type disclosed in U.S. Pat. No. 4,806,930 to Wojciak, Jr., were directed to battery operated systems wherein a radio transmitter might include a code generator as well as an oscillator having an output modulated in some fashion by a code generator for producing a modulated radio frequency output. The transmitter was energized by a battery E.  
           [0003]    Such a transmitter, however, was relatively bulky, in part because relatively large batteries, such as 9-volt and 12-volt batteries were used to energize the transmitters. Today, however, people have come to enjoy keyfob-type transmitters which have very small volumes and, accordingly, in many cases are now using 3-volt lithium batteries. Unfortunately, 3-volt lithium batteries do suffer from voltage drop-off with battery life and the initial voltage of the batteries as received from the supplier varies to some extent. As a result, the transmitter radio frequency generating section will not always receive the same energizing voltage and this may lead to the transmitter drifting in frequency and possibly having its electrical characteristics affected greatly by the change in the battery voltage that it will be unable to oscillate at all and, hence, will be unable to generate a radio frequency signal.  
           [0004]    What is needed, then, is a hand-held radio frequency transmitter including a voltage stabilized power supply which is compact and has a relatively small number of components.  
         SUMMARY OF THE INVENTION  
         [0005]    A hand-held radio frequency transmitter includes a switch mode power supply for delivering a pre-selected potential to a radio-frequency oscillator. The switched mode supply is controlled by a microprocessor or other digital logic device, such as an application-specific integrated circuit or a custom integrated circuit and includes a switch controlled by the microprocessor, which switch enables or disables current from flowing through an inductor coupled to a storage capacitor. A feedback loop is coupled to the storage capacitor for signalling the microcontroller when the storage capacitor has reached a preselected voltage. A radio frequency oscillator is connected to be energized from the storage capacitor and includes an input for receiving a modulating code. The modulating code causes the oscillator to be switched on and off, thereby producing a pulsed carrier wave which may be supplied to a garage door operator or other device to be operated.  
           [0006]    It is a principal aspect of the present invention to provide a hand-held radio frequency transmitter including a switch mode power supply driven from a battery for supplying uniform potential electrical energy to a radio frequency oscillator.  
           [0007]    Other advantages of the present invention will become apparent to one of ordinary skill in the art, upon a perusal of the following specification and claims in light of the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic diagram of a hand-held radio frequency transmitter embodying the present invention; and  
         [0009]    [0009]FIG. 2 is a flow chart showing operation of a microcontroller of the radio frequency transmitter shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    Referring now the drawings and especially to FIG. 1, a hand-held radio frequency transmitter embodying the present invention is generally shown therein and is identified by numeral  10 . The transmitter  10  includes a switch mode power supply  12  coupled to a radio frequency oscillator  14 . A microcontroller  16  produces a modulating code on an output line  18  which is supplied to the oscillator  14  for causing the oscillator  14  to generate radio frequency energy which is emitted by an antenna  20  and which may be received by a garage door operator or other device to be operated.  
         [0011]    A plurality of switches, respectively numbered  30 ,  32  and  34 , are connected in parallel to a ground  36  and to three inputs  40 ,  42  and  44  of the microcontroller  16 . Closure of switches  30 ,  32  or  34  each will respectively cause a different modulation code stream to be produced by the microcontroller  16  on the line  18 . For instance, each of the switches may be dedicated to a particular garage door so that three garage doors could be operated separately from the three switches. In the alternative, one of the switches might be dedicated to sending the code, which would cause a light to be illuminated within the garage even when a garage door is not being operated. In response to closure of one of the three switches  30 ,  32  and  34 , the modulation output is supplied on line  18  to a resistor  50  which ultimately feeds to the oscillator  14 . The oscillator  14 , however, must be energized in order to provide an output. It should be appreciated that a battery  52  is coupled to VCC pin of the microcontroller  16  to energize it at all times. The microcontroller  16  may, for instance, be a CMOS 8-bit microcontroller such as a Zilog Z86C03. One of the pins of the microcontroller  16  provides a pulsed output at pin  54  which is fed through a 470-ohm resistor  56  to an NPN transistor  58  which receives the pulses at its base  60 , transistor emitter  62  is connected to ground and its collector  64 . Current from the battery  52  may be supplied to a one millihenry inductor  70  coupled to the battery  52  and to the collector  64 . A pulse on the line  54  switches the transistor  58  off uncoupling the inductor  70  from ground and causing current to flow through a diode  80  to a 10 microfarad electrolytic capacitor  82  and charging the 10 microfarad electrolytic capacitor  82  until the point that it reaches a voltage which is equal to the breakdown voltage of a Zener diode  84  connected to the capacitor  82 . A 100 kilohm resistor  86  is connected between the Zener diode  84  and ground and a feedback line  88  couples the junction of the Zener diode  84  and resistor  86  to a feedback input terminal on the microcontroller. Thus, when the Zener diode goes into avalanche, driving the line  88  high, the microcontroller drives the pulsed output  54  high, biasing the transistor  58  on and interrupting current flow to the capacitor  82 . The capacitor  82 , however, has received sufficient potential to energize the oscillator  14  as will be seen hereinafter.  
         [0012]    Referring now to FIG. 2, operation of the microcontroller  16  is shown therein. In a step  100  an interrupt occurs every  50  milliseconds. In a step  102 , a test is made to determine whether the pulsed output pin connected to the line  54  is high. If it is high, the switched mode output on the line  54  is set low in a step  104 . If the test of step  102  is negative, a test is made in a step  106  to determine whether the feedback input on line  88  is high. If it is, control is transferred to the step  104 , causing the pin connected to line  54  to switch low. If it is not, control is transferred to a step  110 , causing the switch mode output line connected to line  54  to switch high, turning on transistor  58 . In a step  112 , a test is made to determine whether the period for the modulation bit timing for the modulation generated on line  18  has expired. If the period for modulation bit timing has expired, the routine is exited in a step  114 . If it has not expired, the next bit position is obtained in a step  116  and it is outputted, following which the routine is exited in a step  118 .  
         [0013]    The modulation bits supplied to resistor  50  are fed to a line  130  after having been reduced in potential by the voltage divider, including resistor  50  and a resistor  132 , connected thereto. A capacitor  140 , connected to ground, comprises a stabilizing filter capacitor and is connected to a series inductor  142  which blocks radiation of stray harmonics of the radio frequency in the range of 300 to 400 MHz. The modulated signal is then fed to a base  144  of an NPN transistor  146 , also having a collector  148  and an emitter  150 . The emitter  150  is connected through an inductor  152  for harmonics suppression and through a parallel resistor  154  and capacitor  156 . The resistor  154  and capacitor  156  which are connected to ground. The resistor  154  supplies control over the DC bias and the capacitor  156  controls the radio frequency gain of the oscillator of transistor  146 . The emitter  148  of the transistor is connected via a line  160  to a grounded capacitor  162 , having an antenna coil connected thereto. A second grounded capacitor  164  is connected to the line  81  as well as to the antenna coil  20 . The coded transmission is transmitted by the antenna  20  to a garage door operator or other device to be operated as appropriate.  
         [0014]    While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.