RF transmitter for electrically short antenna

• US 9,203,446 B2
• Filed: 11/21/2012
• Issued: 12/01/2015
• Est. Priority Date: 11/25/2011
• Status: Expired due to Fees

• Current Assignee: Oticon A/S (William Demant Fonden)
• Original Assignee: Oticon A/S (William Demant Fonden)
• Inventors: Christensen, Kre Tais
• Primary Examiner(s): Nguyen, Simon
• Application Number:
  US13/683,132
• Publication Number:
  US 20130137387A1
• Time in Patent Office: 1,105 Days
• Field of Search: 455/41.1, 455/91, 455/121, 455/127.1, 340/10.1
• US Class Current: 1/1
• CPC Class Codes: H03J 5/00 Discontinuous tuning; Selecting predetermined frequencies; Selecting frequency bands with or without continuous tuning in one or more of the bands, e.g. push-button tuning, turret tuner H03J7/00, H03J9/00 take precedence; for bandspreading H03J3/10
• CPC Class Codes: H04B 1/04 Circuits

Abstract

An RF transmitter comprises a capacitive energy storage, an output stage and a switching circuit with an open state and a closed state. The capacitive energy storage forms with the antenna when connected thereto a resonance circuit with a resonance frequency and a quality factor. The output stage provides an electric transmission signal to the resonance circuit. The switching circuit comprises a first transistor for switching between the open state and the closed state and is connected to an antenna output terminal through a capacitance formed by the capacitive energy storage such that the maximum signal voltage occurring across the switching circuit in its open state is lower than the maximum signal voltage occurring across the antenna output terminals. The transmitter is adapted to alter the quality factor by changing the series resistance of the switching circuit in its closed state.

Claims

1. A transmitter, comprising:

an antenna output terminal for connecting to a corresponding antenna terminal of an electrically short antenna for transmission of an electromagnetic signal;

a plurality of capacitive energy storage circuits configured to form with the antenna when connected to the antenna a resonance circuit with a resonance frequency and a quality factor;

an output stage configured to provide an electric transmission signal to the resonance circuit;

a switching circuit having an open state and a closed state, the switching circuit switching between the open state and the closed state and being connected to the antenna output terminal through a capacitance formed by the plurality of capacitive energy storage circuits such that a maximum signal voltage occurring across the switching circuit in its open state is lower than a maximum signal voltage occurring at the antenna output terminal; and

a battery supplying power to the transmitter via a wired connection, wherein

the transmitter is adapted to alter the quality factor by changing a series resistance of the switching circuit by changing the switching circuit to its alternate state, and

each capacitive energy storage circuit of said plurality of capacitive energy storage circuits comprises

a first capacitor,

a second capacitor, and

a third capacitor, and

said plurality of capacitive energy storage circuits comprises

a first capacitive energy storage circuit having its respective first capacitor having a first capacitance C1, its respective second capacitor having a second capacitance C2, and its respective third capacitor having a third capacitance C3;

a second capacitive energy storage circuit having its respective first capacitor having a capacitance one half of C1, its respective second capacitor having a capacitance one half of C2, and its respective third capacitor having a capacitance one half of C3; and

a third capacitive energy storage circuit having its respective first capacitor having a capacitance one quarter of C1, its respective second capacitor having a capacitance one quarter of C2, and its respective third capacitor having a capacitance one quarter of C3.

2. The transmitter according to claim 1, wherein

the transmitter further is adapted to alter the resonance frequency by switching between the open and closed states of the switching circuit.

3. The transmitter according to claim 1, wherein

the switching circuit is comprised in the output stage.

4. The transmitter according to claim 1, further comprising:

a bandwidth controller adapted to control the series resistance of the switching circuit in its closed state.

5. The transmitter according to claim 4, further comprising:

a calibration unit adapted to calibrate the bandwidth controller.

6. The transmitter according to claim 5, wherein

the calibration unit is configured to have the output stage provide an electric transmission signal to the resonance circuit and to determine the quality factor in dependence on the electric transmission signal.

7. The transmitter according to claim 6, wherein

the calibration unit comprises a current measurement device for determining a current through an output transistor of the output stage and is adapted to determine the quality factor in dependence on the determined current.

8. The transmitter according to claim 1, further comprising:

a receiver circuit connected to receive signals from the resonance circuit.

9. The transmitter according to claim 1, wherein the switching circuit includes a first transistor for switching between the open state and the closed state.

10. The transmitter according to claim 9 and further adapted to change the series resistance of the switching circuit in its closed state by controlling a gate voltage applied to the first transistor thereby changing the intrinsic on-resistance of the first transistor.

11. The transmitter according to claim 9 or 10 wherein the switching circuit further comprises a second transistor arranged in series with the first transistor and wherein the transmitter is further adapted to change the series resistance of the switching circuit in its closed state by controlling a gate voltage applied to the second transistor.

12. The transmitter according to claim 11, wherein

the switching circuit further comprises a resistor connected in parallel with the second transistor.

13. The transmitter according to claim 9, wherein the first transistor is an output transistor of the output stage.

14. The transmitter according to claim 1, wherein the switching circuit includes an electronic switch for switching between the open state and the closed state.

15. The transmitter according to claim 1, wherein

each capacitive energy storage circuit of the plurality of capacitive energy storage circuits further comprises

a first switch, and

a second switch,

the first capacitor has one terminal electrically to one terminal of the second capacitor and to one terminal of the first switch,

the third capacitor has one terminal electrically connected to another terminal of the second capacitor and to one terminal of the second switch,

each of the first and second switches has another terminal electrically grounded, and

switching of the first and second switches controls effective capacitance of the capacitive energy storage circuit.

16. A portable apparatus, comprising:

an electrically short antenna; and

a transmitter connected to the electrically short antenna, the transmitter including

an antenna output terminal for connecting to a corresponding antenna terminal of the electrically short antenna for transmission of an electromagnetic signal;

a plurality of capacitive energy storage circuits configured to form with the antenna when connected to the antenna a resonance circuit with a resonance frequency and a quality factor;

an output stage configured to provide an electric transmission signal to the resonance circuit; and

a switching circuit having an open state and a closed state, the switching circuit switching between the open state and the closed state and being connected to the antenna output terminal through a capacitance formed by the plurality of the capacitive energy storage circuits such that a maximum signal voltage occurring across the switching circuit in its open state is lower than a maximum signal voltage occurring at the antenna output terminal; and

a battery within the portable apparatus supplying power to the transmitter, wherein the transmitter is adapted to alter the quality factor by changing a series resistance of the switching circuit by changing the switching circuit to its alternate state, and

each capacitive energy storage circuit of said plurality of capacitive energy storage circuits comprises

a first capacitor,

a second capacitor, and

a third capacitor, and

said plurality of capacitive energy storage circuits comprises

a first capacitive energy storage circuit having its respective first capacitor having a first capacitance C1, its respective second capacitor having a second capacitance C2, and its respective third capacitor having a third capacitance C3;

a second capacitive energy storage circuit having its respective first capacitor having a capacitance one half of C1, its respective second capacitor having a capacitance one half of C2, and its respective third capacitor having a capacitance one half of C3; and

a third capacitive energy storage circuit having its respective first capacitor having a capacitance one quarter of C1, its respective second capacitor having a capacitance one quarter of C2, and its respective third capacitor having a capacitance one quarter of C3.

17. A hearing device, comprising:

an electrically short antenna; and

a transmitter connected to the electrically short antenna, the transmitter including

an antenna output terminal for connecting to a corresponding antenna terminal of the electrically short antenna for transmission of an electromagnetic signal;

a plurality of capacitive energy storage circuits configured to form with the antenna when connected thereto a resonance circuit with a resonance frequency and a quality factor;

an output stage configured to provide an electric transmission signal to the resonance circuit; and

a switching circuit having an open state and a closed state, the switching circuit switching between the open state and the closed state and being connected to the antenna output terminal through a capacitance formed by the plurality of the capacitive energy storage circuits such that a maximum signal voltage occurring across the switching circuit in its open state is lower than a maximum signal voltage occurring at the antenna output terminal; and

a battery within the hearing device supplying power to the transmitter, wherein

the transmitter is adapted to alter the quality factor by changing a series resistance of the switching circuit by changing the switching circuit to its alternate state, and

each capacitive energy storage circuit of said plurality of capacitive energy storage circuits comprises

a first capacitor,

a second capacitor, and

a third capacitor, and

said plurality of capacitive energy storage circuits comprises

a first capacitive energy storage circuit having its respective first capacitor having a first capacitance C1, its respective second capacitor having a second capacitance C2, and its respective third capacitor having a third capacitance C3;

a second capacitive energy storage circuit having its respective first capacitor having a capacitance one half of C1, its respective second capacitor having a capacitance one half of C2, and its respective third capacitor having a capacitance one half of C3; and

a third capacitive energy storage circuit having its respective first capacitor having a capacitance one quarter of C1, its respective second capacitor having a capacitance one quarter of C2, and its respective third capacitor having a capacitance one quarter of C3.