Abstract:
A transmitter for generating and transmitting frequency shift keying signals. The transmitter comprises a resonant circuit, a capacitor, a switch for coupling and decoupling the capacitor to the resonant circuit, a sensor and a controller. The controller includes an input for receiving a data signal. The controller actuates the switch to selectively couple the capacitor to the resonant circuit based on the state of the data signal. The sensor determines when the energy level in the capacitor is substantially zero and the controller synchronizes actuation of the switch with the substantially zero energy state of the capacitor. The transmitter also includes a circuit for efficiently supplying power to the resonant circuit in form the power pulses.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for generating electromagnetic frequency shift keying signals. 
     BACKGROUND OF THE INVENTION 
     Frequency shift keying (FSK) transmitters convert incoming baseband binary data signals into corresponding frequency changes of an electromagnetic carrier signal. The modulated carrier signal is then decoded by a receiver into the original baseband binary data signals. 
     FSK transmitters typically include a resonant circuit which oscillates at the carrier frequency. Inductive reactive or capacitive reactive circuit elements are coupled to the resonant circuit through electronic switches. To achieve the frequency shifts of the carrier signal, the electronic switches selectively connect the reactive circuit elements to, and disconnect the reactive circuit elements from, the resonant circuit in response to the baseband binary data signals. 
     Conventional FSK transmitters introduce voltage transients, or “jitter”, into the modulated carrier signal when the carrier frequency is shifted. These transients are attributable to the asynchronous nature of the baseband binary data signals with respect to the carrier signal. Since the transmitter can be required to shift frequencies at any given moment, transients are introduced into the carrier signal if the initial conditions of the switched reactive circuit elements do not match the circuit conditions existing in the resonant circuit at the instant of the frequency shift. 
     Such voltage transients can be troublesome, particularly at high data transmission rates, because the transients can cause the receiver to produce bit errors in decoding the baseband data. Further errors can be introduced from the power source driving the resonant circuit if the magnitude of the carrier signal is allowed to fluctuate. Accordingly, it is desirable to eliminate both transients introduced into the carrier signal, and variations in magnitude of the carrier signal. It is also desirable, particularly where energy sources are limited, for the transfer of energy between the power source and the electromagnetic carrier signal to be as efficient as possible. 
     Various attempts have been made to eliminate frequency shift distortion from transients. For example, Baker (U.S. Pat. No. 3,249,896), Spiro (U.S. Pat. No. 3,451,012) and Andersen (U.S. Pat. No. 5,300,904) teach FSK transmitters each using a pair of magnetically-coupled inductors which are alternately coupled to a capacitor through a switch. Distortion introduced when the capacitor is switched from one inductor to the other is minimized since the magnetic fields in both inductors are continuously in phase. However, the cost of magnetically-coupled inductors unnecessarily increases the cost of the circuits. Furthermore, the circuits are inefficient since two magnetic fields must be continually maintained, even though only one is used to generate the modulated carrier signal at any given time. 
     Hekimian (U.S. Pat. No. 3,222,619) teaches a frequency shift keying generator comprising a resonant circuit coupled to a capacitor through the parallel combination of a switch and an amplifier. When the switch is open, the capacitance C of the capacitor appears to the resonant circuit as C/B, B being the gain of the amplifier. When the switch is closed, the capacitance C appears to the resonant circuit as C. Distortion introduced when the capacitor is switched is minimized by selecting an amplifier having unity voltage gain. However, as voltage gain can fluctuate with temperature and frequency, in practice distortion may be introduced nevertheless. 
     Kageyama (U.S. Pat. No. 3,363,204) teaches a frequency shift oscillator having a switch which selectively connects an inductor and a capacitor to a resonant circuit. Distortion introduced when the inductor and capacitor are switched is minimized by selecting the values of the components such that the characteristic impedance of the circuit in both modes is identical. However, as the reactance of the components can vary with temperature and the age of the components, in practice the characteristic impedance may not always be identical. 
     Accordingly, there remains a need for a frequency shift keying signal transmitter which allows for frequency shifts to occur in the carrier signal without inducing transients from the switching action, and which also utilizes a highly efficient energy transfer between the power source and the modulated magnetic fields. 
     SUMMARY OF THE INVENTION 
     The present invention provides a frequency shift keying signal transmitter which allows for frequency shifts to occur in the carrier signal without inducing voltage transients in the carrier signal from the switching action. The frequency shift keying transmitter also features a highly efficient energy transfer between the power source and the modulated magnetic fields. 
     In a first aspect the present invention provides a frequency shift keying generator comprising: (a) a resonant circuit; (b) a reactive element; (c) a switch for coupling and decoupling said reactive element to and from said resonant circuit, said resonant circuit oscillating at a first frequency when said reactive element is coupled and said resonant circuit oscillating at a second frequency when said reactive element is uncoupled; (d) a power supply having an output port coupled to said resonant circuit, said power supply including means responsive to a control signal for enabling said output port for energizing said resonant circuit; (e) a detector coupled to said resonant circuit for detecting an energy level for said reactive element and producing an output signal indicative of said energy level; and (f) a controller for controlling said switch, said controller including an output port for actuating said switch and an input port coupled to said detector for receiving said energy level output signal, said controller also having a data input for receiving a data signal and means for selectively actuating said switch to shift the oscillation of said resonant circuit between said first and second frequencies in response to a change of state in the data signal, and said controller including means responsive to said energy level for synchronizing the actuation of said switch with the energy level in said reactive element. 
     In a second aspect, the present invention provides a method for producing frequency shift keying in an oscillating carrier signal, the oscillating carrier signal being generated by a resonant circuit having a modulating capacitor and a switch for selectively coupling and decoupling the capacitor to the resonant circuit, said method comprising the steps of: (a) generating the oscillating carrier signal at a given frequency; (b) receiving a binary data signal comprising a sequence of binary states; (c) monitoring the energy level in the modulating capacitor; (d) introducing a shift in the frequency of said oscillating carrier signal when there is a change in the binary state of the data signal; (e) wherein said step of introducing a frequency shift comprises selectively coupling the modulating capacitor to the resonant circuit and wherein said step of coupling is synchronized to the monitored energy level in the modulating capacitor. 
     In another aspect, the present invention provides a frequency shift keying generator comprising: (a) a resonant circuit; (b) a reactive element; (c) a switch for coupling and decoupling said reactive element to and from said resonant circuit, said resonant circuit oscillating at a first frequency when said reactive element is coupled and said resonant circuit oscillating at a second frequency when said reactive element is uncoupled; (d) a power supply having an output port coupled to said resonant circuit, said power supply including means responsive to a control signal for enabling said output port for energizing said resonant circuit; (e) a sensor coupled to said resonant circuit, said sensor including means for sensing current flow in said resonant circuit and means for generating an output signal indicative of the current flow; (f) a controller for controlling said switch, said controller including an output port for actuating said switch and a data input for receiving a data signal and means for selectively actuating said switch to shirt the oscillation of said resonant circuit between said first and second frequencies in response to a change of state in the data signal, and said controller including means responsive to said current flow signal for generating said control signal for enabling said output port and coupling said power supply to said resonant circuit for a predetermined duration when the current flow is substantially zero, so that energy is injected into said resonant circuit when said output port is enabled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference will now be made to the accompanying drawings which show, by way of example, preferred embodiments of the invention, and in which: 
     FIG. 1 shows in block diagram a frequency shift keying transmitter according to the present invention; 
     FIG. 2 shows in block diagram another embodiment of the frequency shift keying transmitter according to the present invention; 
     FIG. 3 is a schematic representation of an implementation of the frequency shift keying transmitter of FIG. 1; and 
     FIG. 4 is a timing diagram showing the timing of selected signals in the transmitter of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is first made to FIG. 1, which shows a frequency shift keying (FSK) transmitter  10  according to the present invention. The FSK transmitter  10  comprises an antenna circuit  12 , a power supply circuit  14 , a modulation control switch  16 , and control logic  18 . 
     The antenna circuit  12  comprises a coil  20 , a main capacitor  21 , a modulation capacitor  22 , and a pump capacitor  23 . The coil  20  comprises an antenna and provides the radio signal output for the transmitter  10 . As shown in FIG. 1, the coil is connected in series to the main capacitor  21  and the coil  20  together with the capacitor  21  are connected in parallel to the two capacitors  22  and  23  which are connected in series. The antenna coil  20  together with the three capacitors  21  to  23  form a parallel resonant LC circuit. The capacitors  21  to  23  are selected to provide the necessary capacitance, voltage and current ratings as mandated by the application for the transmitter  10 . Preferably, the antenna circuit  12  will have a high Q value. 
     The power supply circuit  14  comprises a voltage source  24 , a bridge network  26 , and an output port  28 . The voltage source  24  preferably comprises a high voltage DC source. The bridge  26  comprises four switching elements  30 , indicated individually as  30   a ,  30   b ,  30   c  and  30   d . The switching elements  30  are connected to form a H-bridge. The switching elements  30  are actuated in pairs  30   a / 30   c  and  30   b / 30   d  by a control output  31  from the control logic  18 . The output port  28  is coupled to the pump capacitor  23  and driven by the output from the H-bridge  26 . The control logic  18  actuates the switching element pairs  30   a / 30   c ,  30   b / 30   d  in the H-bridge  26  to alternate the polarity of the output from the DC source  24  being applied to the pump capacitor  23 . 
     The antenna circuit  12  being a resonant circuit will oscillate. The circuit  12  will continue to oscillate as long as the energy which is lost is replenished. The energy loss is primarily due to the power dissipated in the coil of the antenna  20 . According to this aspect of the invention, the lost energy is replenished by supplying short pulses of high voltage to charge the pump capacitor  23 . The pulses are supplied at precise intervals in the oscillatory cycle by controlling the switching elements  30 . As will be described in more detail below, the pulses are applied to the pump capacitor  23  for a very short interval of time and at an instant when the voltage on the pump capacitor  23  is at or near its maximum value. This maintains the maximum positive or negative peak value of the voltage on the pump capacitor  23  at a constant value. As a result, the AC current in the pump capacitor  23  and therefore the antenna circuit  12  is forced to remain essentially constant. The energy delivered to the pump capacitor  23  in each switching period will be equal to the amount dissipated in the antenna circuit  23  over the interval between two successive switching periods. 
     As shown in FIG. 1, the modulation control switch  16  is connected across the modulating capacitor  22 . The control logic  18  controls the actuation of the modulation control switch  16  with a control output  32 . When the switch  16  is closed, the modulating capacitor  22  is shorted and removed from the resonant LC circuit  12 . This effectively increases the amount of capacitance in the circuit  12 , which in turn decreases the resonant frequency of the output signal from the antenna  20 . Conversely, when the switch  16  is opened, the modulating capacitor  22  is coupled to the other two capacitors  21  and  23  and the capacitance in the circuit  12  decreases. The decrease in capacitance increases the resonant frequency of the output signal from the antenna  20 . The changes in the frequency of the output signal from the antenna  20  provide the frequency key shifting. Modulation for frequency shift keying is accomplished by controlling the opening and closing of the modulating switch  16  based on the state or value of a binary data signal indicated generally by  99  in FIG.  3 . 
     Reference is next made to FIG. 2 which shows a variation of the FSK transmitter indicated generally by  11 . In FIGS. 1 and 2, like references indicate like elements. The FSK transmitter  11  includes a modified power supply circuit  15 . The power supply circuit  15  includes two DC sources  25   a ,  25   b  and two switches  33   a  and  33   b . The polarity of the voltage pulses applied to the pump capacitor  23  is controlled by closing one of the switches  33   a ,  33   b . The actuation of the switches  33   a ,  33   b  is controlled by respective output lines  34   a  and  34   b  from the control logic  18 . 
     Reference is next made to FIG. 3 which shows an implementation of a FSK transmitter  100  according to the present invention. The FSK transmitter  100  has a topology similar to the transmitter  10  shown in FIG.  1 . The FSK transmitter  100  comprises an antenna circuit  112 , a power supply circuit  114 , a modulation control switch  116 , and a control circuit  118 . 
     The antenna circuit  112  comprises a coil  20 , a main capacitor  121 , a modulation capacitor  122 , and a pump capacitor  123 . The coil  120  comprises an antenna and provides the radio signal output for the transmitter  110 . The coil or antenna  120  is connected in series to the main capacitor  121 , and the coil  120  and capacitor  121  are connected in parallel to the capacitor  122  and  123  which are connected in series. The antenna coil  120  together with the three connected capacitors  121  to  123  from a parallel resonant LC cicuit. As shown in FIG. 3, the main capacitor  121  may be formed from multiple capacitors shown individually as  121   a ,  121   b , . . .  121   k  (not shown). The modulation capacitor  122  may comprise a bank of capacitors shown individually as  122   a ,  122   b , . . .  122   n  (not shown). The pump capacitor  123  may also comprise a bank of capacitors shown individually as  123   a ,  123   b , . . .  123   m  (not shown). By grouping banks of capacitors, greater flexibility is provided for achieving the necessary capacitance, voltage and current ratings for the particular application. 
     The power supply circuit  114  comprises a high voltage source  124 , a switching network  126 , and an output port  128  formed by terminals  128   a  and  128   b . The voltage source  24  comprises a high voltage DC source, for example, 125 VDC. The switching network  126  comprises four switching elements  130 , indicated individually as  130   a ,  130   b ,  130   c  and  130   d . The switching elements  130  comprise MOSFET transistors and are connected to form a H-bridge. The pump capacitor bank  123  is coupled to the output port at terminals  128   a  and  128   b . The switching elements  130  may utilize other types of devices such as insulated-gate field-effect transistors or IGFET&#39;s. The selection of the particular device depends on the power and voltage levels for the design of the transmitter  10 . 
     As shown in FIG. 3, the modulation control switch  116  comprises first and second enhancement-mode MOSFET transistors  117   a  and  117   b . The gates of the MOSFET&#39;s  117   a ,  117   b  are tied together and coupled to a control output line  151  from the control circuit  118 . The sources of the MOSFET&#39;s  117   a ,  117   b  are also tied together and coupled to another control output line  152  from the control circuit  118 . The modulation control switch  116  is coupled across the modulating capacitor bank  122 , with one terminal of the capacitor  122  being connected to the drain of the first MOSFET  117   a  and other terminal being connected to the drain of the second MOSFET  117   b , The function of the modulation control switch  116  is to couple and decouple the modulating capacitor bank  122  from the antenna circuit  112  in response to control signals from the control circuit  118 . When the two MOSFET&#39;s  117   a ,  117   b  are conducting or ON, the modulating capacitor bank  122  is effectively “shorted out”. 
     As shown in FIG. 3, the control circuit  118  comprises a controller  140 , MOSFET driver circuits  142 ,  144 ,  146 , a zero-crossing current detector  148 , and a zero-crossing voltage detector  150 . 
     The controller  140  preferably comprises a micro controller which has been suitably programmed to execute the processing steps for controlling the FSK transmitter  100  according to the invention. A suitable device for the controller  140  is the S80C652 available from Signetics Corp. Advantageously, the controller  140  can be programmed to provide variable modulation timing. 
     The MOSFET driver circuits  142 ,  144 ,  146  interface the power output elements, i.e. the MOSFET&#39;s, which operate at high voltage and current levels to the controller  140 . As shown in FIG. 3, the MOSFET driver circuit  142  interfaces the MOSFET&#39;s  130   a  and  130   c  to the controller  140 . The driver circuit  142  comprises level translation and driver circuits, the implementation of which are within the knowledge of one skilled in the art. The driver circuit  142  includes an output  161  connected to the gate of the MOSFET  130   a , an output  162  connected to the gate of the MOSFET  130   c  and an output  163  connected to the tied-together sources of the two MOSFET&#39;s  130   a ,  130   c . The driver circuit  142  controls the actuation of the MOSFET&#39;s  130   a  and  130   c  in response to signals issued by the controller  140 . 
     Similarly, the MOSFET driver circuit  144  interfaces the MOSFET&#39;s  130   b  and  130   d  to the controller  140 . The driver circuit  144  also comprises level translation and driver circuits, and includes an output  164  connected to the gate of the MOSFET  130   b , an output  165  connected to the gate of the MOSFET  130   d  and an output  166  connected to the tied-together sources of the two MOSFET&#39;s  130   b ,  130   d . The driver circuit  144  controls the actuation of the MOSFET&#39;s  130   b  and  130   d  an output lines  164  to  166  in response to signals issued by the controller  140 . 
     The MOSFET driver  146  interfaces the modulation control switch  116  to the controller  140 . The driver circuit  146  comprises circuits for isolating and biasing the MOSFET&#39;s  117   a  and  117   b  which are floating at a high voltage. The implementation such circuits is conventional and within the knowledge of those skilled in the art. The driver circuit  146  includes the control output  151  which is connected to the tied-together gates of the two MOSFET&#39;s  117   a  and  117   b , and the control output  152  which is connected to the tied-together source terminals. The driver circuit  146  controls the actuation of the modulation control switch  116 , i.e. the MOSFET&#39;s  117   a  and  117   b , in response to signals issued by the controller  140 . When the MOSFET&#39;s  117   a  and  117   b  are turned ON, i.e. conducting, the modulating capacitor bank  122  is shorted out, and the total capacitance in the antenna circuit  112  increases. This, in turn, causes the resonant frequency of the output signal from the antenna  120  to decrease. When the MOSFET&#39;s  117   a  and  117   b  are turned OFF, i.e. non-conducting, the modulating capacitor bank  122  is connected in series with the main capacitor  121  and the pump capacitor bank  123  thereby decreasing the total capacitance in the antenna circuit  112 . This, in turn, causes the resonant frequency in the circuit  112  to increase and results in a frequency key shift. The controller  140  controls the actuation of the modulating control switch  116  to convert a data signal  99  into frequency changes or shifts in the carrier signal emitted by the antenna. 
     The zero-crossing current detector  148  comprises a toroidal current transformer  153  and a sensing circuit  154 . The current transformer  153  is magnetically coupled to the antenna coil  120 . The sensing circuit  154  uses the output from the transformer  153  to determine when the current in the antenna circuit  112  is zero. This information is passed to the controller  140  and used in the timing control of the switching elements. The zero-crossing detector  148  is implemented using conventional techniques as will be within the knowledge of one skilled in the art. 
     The zero-crossing voltage detector  150  comprises a voltage transformer  155  and a zero-crossing voltage sensing circuit  156 . The transformer  155  provides isolation between the high voltage antenna circuit  112  and the low voltage control circuit  118 . The secondary winding of the transformer  155  is coupled across the pump capacitor bank  123  and the output port  128  from the power supply circuit  114 . The voltage across the pump capacitor  123  is induced in the primary winding of the transformer  155  and the sensing circuit  156  uses the output from the primary to determine when the voltage across the pump capacitor bank  123  is zero. This information is utilized by the controller  140  to time the conduction of the MOSFET&#39;s  117   a  and  117   b  in the modulating control switch  116  in order to eliminate the development of a DC bias across the modulating capacitor bank  122 . The timing also ensures that no transients are caused in the circuit  112  by the switching of the capacitor bank  122 . 
     The control circuit  118  also includes timing logic  141 . The timing logic provides the signals required by the various digital and analog circuits, typically as phase references and pulse width timing. 
     Next the operation of the FSK transmitter  100  is described. Reference is also made to FIG. 4, which shows the relationship between selected signals in the FSK transmitter  100 . 
     When the binary data signal  99  received by the controller  140  is in logic state ONE or HIGH, the corresponding voltage applied between the gates and source terminals (i.e. on output  151  and  152 ) of the MOSFET&#39;s  117   a  and  117   b  will be below the conduction voltage. As a result, the modulating capacitor bank  122  is not shorted and remains connected to the main capacitor  121  and the pump capacitor  123 , and the antenna circuit  12  will oscillate at a radian frequency of approximately        ω1              =     1       L        (       1     C   pump       +     1     C   main       +     1     C   modulation         )                                  
     which is indicated by reference  201  in FIG.  4 . While the antenna circuit  12  oscillates, the zero-crossing voltage detector  150  continuously senses the voltage drop across the pump capacitor  123 . When the voltage drop across the pump capacitor  123  is zero, the detector  150  notifies the controller  140 . 
     Since the modulating capacitor bank  122  is in series with the pump capacitor bank  123 , the voltage drop measured by the voltage detector  150  will be proportional to the voltage drop across the modulating capacitor bank  122 . As the energy stored in a capacitor is related to the voltage drop across the capacitor by the equation          Ec              =       1   2          CV   2         ,                          
     it will be appreciated that the voltage drop measured by the voltage detector  150  will be indicative of the energy stored in the modulating capacitor bank  122 . Therefore, in effect, the voltage detector  150  signals the controller  140  when the energy stored in the modulating capacitor  122  is zero. 
     When the data signal  99  changes from a logic HIGH to a logic ZERO, and the voltage detector  150  signals that the energy stored in the modulating capacitor bank  122  is zero, then the controller  140  issues control signals on outputs  151  and  152  to turn ON the two MOSFET&#39;s  117   a  and  117   b  in the modulation control switch  116 . When the MOSFET&#39;s  117   a  and  117   b  are turned ON or conducting the modulating capacitor bank  122  is “shorted out” and effectively removed from the antenna circuit  112 . As the antenna circuit  112  is oscillating at resonance, at this point substantially all the energy in the circuit  112  will be stored in the magnetic field of the antenna coil  120 . With the modulating capacitor bank  122  shorted out, the antenna circuit  112  will oscillate at a radian frequency of approximately        ω2              =     1       L        (       1     C   pump       +     1     C   main         )                                  
     which is indicated by reference  202  in FIG.  4 . Because the frequency change, i.e. from ω 1  to ω 2 , occurs when the energy stored in the modulating capacitor bank  122  is substantially equal to zero, and substantially all the energy in the antenna circuit  112  is stored in the magnetic field of the antenna coil  120 , it will appreciated that virtually no voltage transients will be introduced into the antenna circuit  112 . As a result, the electromagnetic field emitted by the antenna coil  120  will be substantially jitter-free. 
     When the state of the data signal  99  received by the controller  140  changes from logic LOW to a logic HIGH, and the voltage detector  150  signals that the energy stored in the pump capacitor bank  123  is zero, the controller  140  issues control signals to outputs  151  and  152  to turn OFF the two MOSFET&#39;s  117   a  and  117   b  and thereby reconnect the modulating capacitor bank  122  to the antenna circuit  112 . Again, at this point substantially all the energy in the antenna circuit  112  will be stored in the magnetic field of the antenna coil  120 , 
     Shortly after the modulating capacitor bank  122  is re-coupled, the resonant frequency of the antenna circuit  112  returns to ω 1 , As the re-coupled modulation capacitor  16  initially has no stored energy, and is coupled to the antenna circuit  112  when substantially all the energy is retained in the magnetic field of the antenna coil  120 , it will be appreciated that virtually no voltage transients will be introduced into the antenna circuit  112  during this latter frequency change. 
     To replenish the energy loss in the antenna circuit  112  which occurs primarily due to power dissipation in the coil  120 , the controller  140  intermittently couples the DC source  124  to the pump capacitor bank  123 . It is a feature of this aspect of the invention that the DC source  124  is only coupled to the pump capacitor bank  123  in the antenna circuit  112  when the voltage drop across the pump capacitor  123  is at or near its maximum. This ensures that the energy transfer occurs as efficiently as possible. 
     The controller  140  uses the zero-crossing current detector  148  to monitor the pump capacitor  123 . The current detector  34  continuously measures the current flowing in the antenna circuit  112 . When the current is zero, the current detector  148  notifies the controller  140 . In response, the controller  140  issues command signals to the respective driver circuits  142 ,  144  to couple the voltage source  24  to the pump capacitor bank  123 . At this point, the voltage drop across the pump capacitor  123  will be at or near its maximum, and the energy stored in the pump capacitor  123  will also be at or near maximum. 
     According to this aspect of the invention, energy is supplied to the pump capacitor bank  123  in short pulses as follows. In response to a signal from the zero-current crossing detector  154 , the controller  140  issues command signals on outputs  161  to  163  to turn ON the two MOSFET&#39;s  130   a  and  130   c . In the conduction state, the MOSFET&#39;s  130   a  and  130   c  couple the DC source  124  across the pump capacitor bank  123  so that energy is transferred to the pump capacitor bank  123 . The anode of the DC source  124  is coupled to terminal  129  of the pump capacitor  123  and the cathode is coupled to the other terminal  127  of the capacitor bank  123 . 
     After a short predetermined interval (described below), the controller  140  turns OFF the two MOSFET&#39;s  130   a  and  130   c  thereby disconnecting the DC source  124  from the pump capacitor  123 . At the next half oscillation of the current flowing in the antenna circuit  112 , the voltage at terminal  127  in the pump capacitor bank  123  will be positive with respect to terminal  129 . Accordingly, when the detector  148  detects that the current in the antenna circuit  112  is zero and notifies the controller  140 , the controller  140  issues command signals on outputs  164  to  166  to turn ON the two MOSFET&#39;s  130   b  and  130   d . Conduction of the MOSFET&#39;s  130   b  and  130   d  couples the anode of the DC source  124  to terminal  127  of the pump capacitor bank  123  and the cathode of the DC source  124  to the other terminal  129  of the bank  123 . This causes energy to be transferred from the DC source  124  to the pump capacitor bank  123 . Shortly thereafter, the controller  140  turns OFF the two MOSFET&#39;s  130   b  and  130   d  to isolate the DC source  124  from the antenna circuit  112 . 
     It will be understood that the amount of energy transferred from the DC source  124  to the pump capacitor bank  123  during each half-cycle is proportional to the duration over which the DC source  124  is coupled to the pump capacitor bank  123 . For the magnitude of the current flowing in the antenna circuit  112  to remain constant, the duration which the DC source  124  is coupled to the pump capacitor  123  is adjusted so that the energy delivered to the antenna circuit  12  over each interval is substantially equal to the energy lost by the antenna circuit  112  during. each half-cycle oscillation of the resonant current in the circuit  112 . In other words, the DC source  124  is coupled to the antenna circuit  112  only long enough to replace the energy lost by the circuit  112 . By adjusting the-pulse width (i.e. duration) of the pulses supplied to the pump capacitor  123 , optimal operating efficiency can be achieved. Accordingly, the controller  140  preferably includes the capability to adjust the width of the pulses. For the example implementation described below, a pulse width adjustable over a range of 5 to 15 microseconds provides optimal operating efficiency for the transmitter. 
     Accordingly, the magnitude of the current i coil  in the antenna circuit  112  can be controlled as follows: (1) by varying the voltage output of the DC source  124 ; (2) by varying the duration (i.e. pulse width) of the charging pulses applied to the pump capacitor bank  123 ; or (3) by changing the capacitance value of the pump capacitor bank  123 . The selection of one of these techniques is a design choice and will depend on the specifications of the particular application as will be within the knowledge of one skilled in the art. 
     In one application of the FSK transmitter  100  according to the present invention, the antenna is 3.5 m square and approximately 10 cm deep. The windings for the antenna are made formed from 25 turns of #8 AWG wire (RW-90 or equivalent)which is wound onto a form in two layers. The resulting inductance of the antenna coil is 7.885 mH. The main tuning capacitor ( 121  in FIG. 3) comprises eight 3 uF/660 VAC metallized polypropylene capacitors connected in series, for a nominal value of 0.375 uF/5820 VAC (specified at 60 Hz). For reasons of efficiency and safety, the main capacitor is mounted on the antenna form. The modulating control or switching capacitor bank ( 122  in FIG. 3) comprises two 3 uF/660 VAC capacitors connected in parallel and yielding 6 uF/660 VAC. The pump capacitor bank ( 123  in FIG. 3) comprises two 8 uF/660 VAC capacitors connected in parallel and yielding 16 uF/660 VAC. The net capacitance for the antenna circuit is 0.3453 uF total and 0.3664 uF shifted (i.e. with modulating capacitor bank shorted out). The resonant frequency is 3050 Hz and the shifted frequency is 2950 Hz. During testing with 110 VDC power source ( 124  in FIG.  3 ), the antenna coil developed 24.5 Aac at about 3500 VAC, and a moment of 7500 Am 2 . The dissipation was estimated to be 600 W. In comparison, the total power consumption was measured at 616 W. It will be appreciated that this represents an extremely high efficiency for these types of transmitters. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Certain adaptations and modifications of the invention will be obvious to those skilled in the art. Therefore, the presently discussed embodiments are considered to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.