Abstract:
A transmitter which comprises a rf carrier signal generator for generating a rf carrier, a Class C amplifier arranged to amplify the rf carrier, a power supply for the Class C amplifier, a detector for detecting the modulated rf carrier and establishing a detected signal, a controller for controlling the power supply, means for supplying an audio signal to the controller for the purpose of amplitude modulating the rf carrier, and wherein the audio signal and the detected signal are applied to the controller to compensate for the non-linearily of the Class C amplifier.

Description:
FIELD OF THE INVENTION 
     This invention relates to a transmitter for use in transmitting an amplitude modulated radio frequency (rf) signal. The invention has been developed for use in a locator beacon which is being made the subject of an independent patent application and the invention is hereinafter described in the context of the locator beacon. However, it will be understood that the invention does have broader applications, to a transmitter for use in any rf communication system. 
     BACKGROUND OF THE INVENTION 
     The transmitter has been developed to meet the requirements for a relatively low power demand and this virtually dictates the use of a Class C amplifier stage. However, the traditional problem with a Class C amplifier is that its modulation characteristic is non-linear and introduces distortion and this is incompatible with a requirement for low distortion transmission of audio information. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to reconcile these conflicting requirements by providing a transmitter which comprises: 
     (a) an rf carrier signal generator for generating a rf carrier; 
     (b) a Class C amplifier arranged to amplify the rf carrier; 
     (c) a power supply for the Class C amplifier; 
     (d) a detector for detecting the modulated rf carrier and establishing a detected signal; 
     (e) a controller for controlling the power supply; 
     (f) means for supplying an audio signal to the controller for the purpose of amplitude modulating the rf carrier; 
     and wherein the audio signal and the detected signal are applied to the controller to compensate for the non-linearity of the Class C amplifier. 
     In a preferred embodiment, amplitude modulation sufficient for transmitting the audio form signal on the rf carrier can be achieved with reduced distortion while maintaining the low power demand advantage of the Class C amplifier. 
     Preferably, the detector comprises a directional coupler and/or a power detector. 
     Preferably, the power supply comprises a switch mode power supply. 
     Advantageously, the controller comprises an amplitude comparison means for performing an amplitude comparison between the audio signal and the detected signal. 
     In one embodiment, the transmitter further comprises a preamplification means between the rf carrier signal generator and the Class C amplifier for preamplifying the rf carrier. 
     Preferably, the transmitter further comprises an antenna for transmitting the amplitude modulating rf carrier, the antenna comprising a circuit board which constitutes an active portion of the length of the antenna and on which are mounted electrical components for tuning the antenna. 
    
    
     The invention may be more fully understood from the description of a preferred form of the transmitter provided below in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings 
     FIG. 1 is a block diagram of a locator beacon. 
     FIG. 2 is a block diagram of a transmitter for use in the locator beacon of FIG.  1 . 
     FIG. 3 is a top view of an antenna for use in the locator beacon of FIG.  1 . 
     FIG. 4 is an exploded side view of the antenna of FIG.  3 . 
     FIG. 5 is a schematic top view of a circuit board for use in the antenna of FIG.  3 . 
     FIG. 6 is a perspective view of the assembled, folded antenna of FIG.  3 . 
     FIG. 7 is a front view of an assembled locator beacon. 
     FIG. 8 is a cross-sectional view of a battery compartment cap for use with the locator beacon. 
     FIG. 9 is a perspective top view of the battery compartment cap of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 the locator beacon  20  includes a receiver  22  for receiving a global positioning system signal via antenna  24 . The receiver  22  is connected to a microprocessor/digital signal processor  26  in which geographical position information contained on the global positioning system signal is converted into a digital signal. The digital signal produced by the microprocessor/digital signal processor  26  is processed by an audio signal processor  28 . In the audio signal processor  28  an audio signal corresponding to the geographical position information is generated and send to a transmitter  30  of the locator beacon  20 . The transmitter  30  transmits amplified rf signals some of which carry the audio signal (amplification unit  31 ) via a VHF/UHF antenna  32 . 
     In an emergency situation the locator beacon  20  described above can transmit an audio signal which can communicate in a computer generated voice the geographical position of the locator beacon via radio. The information can be received by conventional rf receivers. This means that a large number of potential searchers is ‘accessible’ and that those searchers are provided with the geographical position of the individual ‘automatically’, independent on whether the individual is capable of active communication or not. 
     In FIG. 2 the transmitter  30  includes functional blocks  120  and  122  which transmit the audio signal  135 . The functional block  120  includes a rf generator stage  124  which generates a fixed amplitude rf signal which is preamplified in a Class C amplifier  126 . The pre-amplified, fixed amplitude rf signal is then sent into a second Class C amplifier  134 , which is one component of an amplitude modulation stage  128 . The amplitude modulation stage further includes a controller  130  and a switch mode power supply  132 . The controller  130  controls the switch mode power supply  132  in a manner so as to vary a driving power supplied to the second Class C amplifier  134  based on the audio signal  135  and a detected signal sent to the controller  130  via feed-back  136 . The detected signal is derived from a power detector stage  138  which includes a directional coupler  140  and a power detector  142 . The power detector  142  detects the forward power (via directional coupler  140 ) from the second Class C amplifier  134  and derives the detected signal which is sent to the controller  130  via feed-back  136 . Feed-back  136  enables linear amplitude modulation of the rf carrier. Any non-linearity of the modulation characteristic of the second Class C amplifier  134  is compensated for by driving it non-linearly through the switch mode power supply  132  on the basis of an amplitude comparison between the audio signal  135  and the detected signal. Thus, distortion between the audio signal  135  and the amplitude modulated rf signal  144  is reduced. 
     Functional block  122  is substantially equivalent to functional block  120  only that a different rf carrier is generated and amplitude modulated and the rf output signals of both functional blocks  120  and  122  are transmitted via a diplexer  146  through antenna  40 . 
     In FIG. 3 the antenna  40  includes conducting portions  42 ,  44  and  46  and a non-conductive protection  48  formed over a portion  50  of the antenna  40 . The portion  42 ,  44 ,  46  and  50  are connected via hinge points  43 ,  45  and  47 . 
     In FIG. 4 the non-conductive protection  48  includes two portions  52  and  54  between which a flexible circuit board  56  is enclosed. The flexible board  56  is arranged to make electrical contacts between traces of the circuit (not shown)and two end portions  58  and  60  of the antenna portion  50 , both made from the same conducting material as portions  42 ,  44  and  46  of antenna  40 . Pins e.g.  49  are used to connect the portions of the antenna at hinge points  43 ,  45  and  47 . Antenna  40  can be folded as illustrated in FIG.  5 . 
     In FIG. 6 the circuit board  56  includes electrically conducting traces  61 - 64  which are electrically interconnected by LC components  65 - 67 . The LC components  65 - 67  include in parallel inductances  08 - 70  and capacitors  71 - 73 . The LC components  65 - 67  can be formed from circuit elements mounted on substrates which in turn are mounted to the circuit board  56 . Alternatively, the circuit elements can be mounted directly to the circuit board  56 . It will be appreciated by a person skilled in the art that various other components/traces can be used to tune the antenna  40 . 
     FIG. 7 shows a front view of a locator beacon  80  including a battery compartment  82  and antenna  40  (folded). The battery compartment  82  has a cap  84  which sealingly closes the battery compartment  82  when screwed over a battery insertion port of the battery compartment. 
     In FIG. 8 the metal cap  84  includes a cylindrical wall  86  and a bottom portion  88  which is shaped to form a recess  90  on the outside bottom surface of the cap  84 . On the inner surface of the bottom portion  88  a second recess  92  is formed over which sits a resilient metal spring  94 . In the centre of the bottom portion  88  there is located a conically shaped hole  96 . The wall  98  surrounding the hole  96  tapers towards the outer surface of the bottom portion  88 . An insert  100  is inserted through the hole  96  and mounted to the spring  94  utilising clip  112  and in a manner so that when the spring  94  is in a relaxed state a conically shaped portion  102  of the insert  100  abuts the wall  98  and an O-ring  104  disposed between a bottom portion  106  of the insert  100  and the bottom portion  88  of the cap  84 . Thereby the hole  96  is sealed. 
     In use, the upper portion  106  of spring  94  is in electrical contact with one pole of a battery in the battery compartment and forms an electrical connection of the pole to the metal casing  83  of the locator beacon  80 . This is achieved through the bottom portion  88  and wall  86  of the battery compartment cap  84 , which in use is screwed onto the battery compartment  82  which forms part of the casing  83  (see FIG.  7 ). 
     Any gas pressure  108  within the battery compartment  82  acts on the inside walls of the battery compartment, including the top portion  110  of the insert  100 . If the internal pressure  108  exceeds a predetermined value, the force on the top portion  110  of insert  100  will result in a deformation of spring  94  into the recess  92  within the bottom portion  88 . Thereby, gas contained within the compartment can be released through hole  96  and pass O-ring  104 . 
     It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiment is, therefore, to be considered in all respects to be illustrative and not restrictive.