RF TRANSMITTER FOR ELECTRICALLY SHORT ANTENNA

• US 20130137387A1
• Filed: 11/21/2012
• Published: 05/30/2013
• Est. Priority Date: 11/25/2011
• Status: Active Grant

• Current Assignee: Oticon A/S (William Demant Fonden)
• Original Assignee: Oticon A/S (William Demant Fonden)
• Inventors: CHRISTENSEN, Kre Tais
• Granted Patent: US 9,203,446 B2
• Time in Patent Office: Days
• US Class Current: 455/101
• 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. Transmitter with two antenna output terminals for connecting to corresponding antenna terminals of an electrically short antenna for transmission of an electromagnetic signal, the transmitter comprising a capacitive energy storage, an output stage and a switching circuit having an open state and a closed state, the capacitive energy storage being adapted to form with the antenna when connected thereto a resonance circuit with a resonance frequency and a quality factor, the output stage being adapted to provide an electric transmission signal to the resonance circuit, the switching circuit comprising a first transistor for switching between the open state and the closed state and being 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, characterised in that the transmitter is adapted to temporarily alter the quality factor by changing the series resistance of the switching circuit in its closed state.

2. Transmitter according to claim 1 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.

3. Transmitter according to claim 1 or 2 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.

4. Transmitter according to claim 3 wherein the switching circuit further comprises a resistor connected in parallel with the second transistor.

5. Transmitter according to claim 1 wherein the transmitter further is adapted to temporarily alter the resonance frequency by switching between the open and closed states of the switching circuit.

6. Transmitter according to claim 1 wherein the switching circuit is comprised in the output stage.

7. Transmitter according to claim 6 wherein the first transistor is an output transistor of the output stage.

8. Transmitter according to claim 1 and further comprising a bandwidth controller adapted to control the series resistance of the switching circuit in its closed state.

9. Transmitter according to claim 8 and further comprising calibration means adapted to calibrate the bandwidth controller.

10. Transmitter according to claim 9, wherein the calibration means are adapted 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.

11. Transmitter according to claim 10, wherein the calibration means comprises current measurement means 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.

12. Transmitter according to claim 1 and further comprising a receiver circuit connected to receive signals from the resonance circuit.

13. Portable apparatus comprising a transmitter according to claim 1 connected to an electrically short antenna.

14. Hearing device comprising a transmitter according to claim 1 connected to an electrically short antenna.

15. Use of a transmitter according to claim 1 for transmitting an electromagnetic signal via an electrically short antenna.