Fully integrated All-CMOS AM transmitter with automatic antenna tuning

Abstract

A monolithic AM transmitter is disclosed. An external antenna forms part of a resonance network so that the antenna resonance point is automatically tuned to the transmit frequency. This provides flexibility with no added cost to the transmitter. Additionally, components of the transmitter can be formed on a single monolithic integrated circuit.

1. A transmitter comprising:
 a voltage-controlled oscillator having an operating frequency;  
 an antenna, the antenna forming part of a tuned circuit coupled to the voltage-controlled oscillator,  
 a differential structure of varactor diodes;  
 a varactor charge pump to provide a bias charge for varactor diodes in the differential structure of varactor diodes; and  
 a tuned circuit including the antenna and the differential structure of varactor diodes, wherein the differential structure of varactor diodes tunes the resonance point of the antenna to the frequency of the voltage-controlled oscillator.  
 
     
     
       2. The transmitter of  claim 1 , wherein the voltage-controlled oscillator is coupled serially with a phase detector and a loop filter to form a phase-locked loop. 
     
     
       3. The transmitter of  claim 2 , wherein the phase detector is further coupled to a reference signal so that the operating frequency of the voltage controlled oscillator is related to the frequency of the reference signal. 
     
     
       4. The transmitter of  claim 2 , wherein the phase-locked loop further includes a prescalar and a divide-by M circuit coupled between the voltage-controlled oscillator and the phase detector. 
     
     
       5. The transmitter of  claim 4 , wherein the voltage controlled oscillator and the phase-locked loop are formed on a single integrated circuit. 
     
     
       6. The transmitter of  claim 2 , further including a reference oscillator supplying a signal of a reference frequency to the phase detector, wherein the reference oscillator, the voltage controlled oscillator, and the phase-locked loop are formed on a single integrated circuit. 
     
     
       7. The transmitter of  claim 6 , wherein the reference oscillator is of the Colpitts variety and is coupled to receive a signal from a timing device external to the single integrated circuit. 
     
     
       8. The transmitter of  claim 1 , wherein a power amplifier is coupled between the voltage controlled oscillator and the antenna. 
     
     
       9. The transmitter of  claim 8 , wherein the gain of the power amplifier is controlled by a power controller. 
     
     
       10. The transmitter of  claim 9 , wherein the power amplifier, the voltage-controlled oscillator, and the power controller are formed on a single integrated circuit. 
     
     
       11. The transmitter of  claim 1 , wherein the differential structure of varactor diodes, the voltage-controlled oscillator, and the varactor charge pump are formed on a single integrated circuit. 
     
     
       12. The transmitter of  claim 1 , wherein the veractor diodes include an array of capacitors that can be switched in and out of the tuned circuit. 
     
     
       13. The transmitter of  claim 1 , wherein the differential structure of varactor diodes and the voltage-contrlled oscillator are formed on a single integrated circuit. 
     
     
       14. The transmitter of  claim 1 , further including a charge pump supplying a voltage to the voltage-controlled oscillator, wherein the charge pump and the voltage-controlled oscillator are formed on a single integrated circuit. 
     
     
       15. The transmitter of  claim 1 , further including a bandgap reference circuit generating reference voltages that are temperature and supply voltage stable, the bandgap reference circuit and the voltage-controlled oscillator formed on a single integrated circuit. 
     
     
       16. The transmitter of  claim 1 , further including a shutdown mode circuit coupled to the voltage-controlled oscillator, the shutdown mode circuit and the voltage-controlled oscillator being formed on a single integrated circuit. 
     
     
       17. The transmitter of  claim 1 , further including a data encoder coupled between a data input pad and the voltage-controlled oscillator, the data encoder and the voltage-controlled oscillator being formed on a single integrated circuit. 
     
     
       18. A transmitter having an automatic antenna tuning circuit, the transmitter comprising:
   a transmit amplifier for being driven at a carrier frequency, the transmit amplifier for being connected to an antenna;        a variable capacitance, the variable capacitance and antenna forming at least part of a tuned circuit having a resonant frequency, wherein varying the variable capacitance tunes the resonance frequency of the tuned circuit; and        a tuning circuit connected to control the variable capacitance in response to a feedback control signal, the feedback control signal indicating whether the resonance frequency of the tuned circuit is matched to the carrier frequency, the tuning circuit for varying the variable capacitance based on the feedback control signal to cause the resonance frequency of the tuned circuit to match the carrier frequency,        at least the amplifier, variable capacitance, and tuning circuit being formed on a single integrated circuit.     
     
     
       19. The transmitter of  claim 18  wherein the amplifier contains an oscillator for generating the carrier frequency. 
     
     
       20. The transmitter of  claim 18  further comprising an oscillator providing a drive signal to the amplifier at the carrier frequency. 
     
     
       21. The transmitter of  claim 18  further comprising:
   an oscillator providing a drive signal to the amplifier at the carrier frequency;        a phase difference circuit receiving a first signal corresponding to a phase of a signal generated by the amplifier and receiving a second signal corresponding to a phase of a signal generated by the oscillator, the phase difference circuit generating the feedback control signal, the feedback control signal being related to a difference in phase between the first signal and the second signal.     
     
     
       22. The transmitter of  claim 21  wherein the phase difference circuit is a phase detector. 
     
     
       23. The transmitter of  claim 18  wherein the second signal is a quadrature signal  90  degrees out of phase with the drive signal. 
     
     
       24. The transmitter of  claim 18  wherein the second signal is  90  degrees out of phase with the drive signal. 
     
     
       25. The transmitter of  claim 18  wherein the variable capacitance comprises an array of capacitors that are selectively connected, by the tuning circuit, to the tuned circuit to adjust the resonance frequency of the tuned circuit. 
     
     
       26. The transmitter of  claim 18  wherein the variable capacitance comprises MOS capacitors. 
     
     
       27. The transmitter of  claim 18  wherein the variable capacitance comprises varactors. 
     
     
       28. The transmitter of  claim 18  wherein the tuning circuit comprises a logic circuit whose digital output signal controls the variable capacitance, the feedback control signal causing the logic circuit to change the digital output signal to adjust the resonance frequency of the tuned circuit. 
     
     
       29. The transmitter of  claim 28  wherein the logic circuit is a counter. 
     
     
       30. The transmitter of  claim 29  wherein the counter also receives a clock signal for clocking transitions of the counter. 
     
     
       31. The transmitter of  claim 18  wherein the variable capacitance is in parallel with the antenna. 
     
     
       32. The transmitter of  claim 18  wherein the antenna is a loop antenna. 
     
     
       33. The transmitter of  claim 18  wherein the tuning circuit varies the variable capacitance based on the feedback control signal to cause the resonance frequency of the tuned circuit to precisely match the carrier frequency. 
     
     
       34. The transmitter of  claim 18  wherein the variable capacitance comprises an array of capacitors that are selectively connected, by the tuning circuit, to the tuned circuit to adjust the resonance frequency of the tuned circuit, each capacitor in the array having a different size, wherein sizes of the capacitors are exponentially related. 
     
     
       35. The transmitter of  claim 18  wherein the amplifier is a differential amplifier. 
     
     
       36. The transmitter of  claim 18  further comprising the antenna. 
     
     
       37. A method for performing automatic tuning in a transmitter comprising:
   driving a transmit amplifier at a carrier frequency, the amplifier being connected to an antenna;        varying a variable capacitance, the variable capacitance and antenna forming at least part of a tuned circuit having a resonant frequency, wherein varying the variable capacitance tunes the resonance frequency of the tuned circuit;        applying a feedback control signal to a tuning circuit connected to control the variable capacitance, the feedback control signal indicating whether the resonance frequency of the tuned circuit is matched to the carrier frequency; and        varying the variable capacitance by the tuning circuit based on the feedback control signal to cause the resonance frequency of the tuned circuit to match the carrier frequency,        at least the amplifier, variable capacitance, and tuning circuit being formed on a same integrated circuit.     
     
     
       38. The method of  claim 37  further comprising:
   providing a drive signal to the amplifier by an oscillator at the carrier frequency;        receiving by a phase difference circuit a first signal corresponding to a phase of a signal generated by the amplifier and receiving by the phase difference circuit a second signal corresponding to a phase of a signal generated by the oscillator; and        generating by the phase difference circuit the feedback control signal, the feedback control signal being related to a difference in phase between the first signal and the second signal.     
     
     
       39. The method of  claim 38  wherein the phase difference circuit is a phase detector. 
     
     
       40. The method of  claim 38  wherein the second signal is  90  degrees out of phase with the drive signal. 
     
     
       41. The method of  claim 37  wherein varying the variable capacitance comprises selectively connecting an array of capacitors to the tuned circuit to adjust the resonance frequency of the tuned circuit. 
     
     
       42. The method of  claim 37  wherein the variable capacitance comprises one or more varactors, and wherein varying the variable capacitance comprises applying a signal to the one or more varactors to adjust the resonance frequency of the tuned circuit. 
     
     
       43. The method of  claim 37  wherein the tuning circuit comprises a logic circuit generating a digital output signal, and wherein varying the variable capacitance comprises selectively controlling the coupling of capacitors to the tuned circuit based on the digital output signal, the feedback control signal causing the logic circuit to change the digital output signal to adjust the resonance frequency of the tuned circuit. 
     
     
       44. The method of  claim 37  wherein the variable capacitance is in parallel with the antenna. 
     
     
       45. The method of  claim 37  wherein the antenna is a loop antenna. 
     
     
       46. The method of  claim 37  wherein varying the variable capacitance based on the feedback control signal comprises varying the variable capacitance to cause the resonance frequency of the tuned circuit to precisely match the carrier frequency. 
     
     
       47. A transmitter comprising:
   a voltage - controlled oscillator having an operating frequency;        an antenna, the antenna forming part of a tuned circuit coupled to the voltage - controlled oscillator,        a variable capacitance forming part of the tuned circuit; and        a tuning circuit connected to control the variable capacitance in response to a feedback control signal, the feedback control signal indicating whether the resonance frequency of the tuned circuit is matched to the carrier frequency, the tuning circuit varying the variable capacitance based on the feedback control signal to cause the resonance frequency of the tuned circuit to match the carrier frequency,        at least the oscillator, variable capacitance, and tuning circuit being formed on a same integrated circuit.     
     
     
       48. The transmitter of  claim 47  wherein the variable capacitance comprises an array of capacitors that are selectively connected, by the tuning circuit, to the tuned circuit to adjust the resonance frequency of the tuned circuit. 
     
     
       49. The transmitter of  claim 47  wherein the variable capacitance comprises varactors. 
     
     
       50. The transmitter of  claim 47  wherein the tuning circuit comprises a logic circuit whose digital output signal controls the variable capacitance, the feedback control signal causing the logic circuit to change the digital output signal to adjust the resonance frequency of the tuned circuit. 
     
     
       51. The transmitter of  claim 47  further comprising a phase difference circuit receiving a first signal corresponding to a phase of a signal generated by the amplifier and receiving a second signal corresponding to a phase of a signal generated by the oscillator, the phase difference circuit generating the feedback control signal, the feedback control signal being related to a difference in phase between the first signal and the second signal. 
     
     
       52. The transmitter of  claim 47  further comprising a reference generator, external to the integrated circuit, providing a reference frequency for controlling a frequency of the voltage oscillator.