Circuit for a loop antenna and method for tuning

• US 8,306,489 B2
• Filed: 10/14/2009
• Issued: 11/06/2012
• Est. Priority Date: 10/27/2008
• Status: Active Grant

• Current Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Original Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Inventors: Schwarzmueller, Marco, Schulz, Holger, Blatz, Werner, Erben, Uwe
• Primary Examiner(s): Jackson, Blane J
• Application Number:
  US12/578,919
• Publication Number:
  US 20100124890A1
• Time in Patent Office: 1,119 Days
• Field of Search: 455/107, 455/121, 455/123, 455/129, 455/120, 455/193.1, 455/193.3
• US Class Current: 455/123
• CPC Class Codes: H01Q 7/00 Loop antennas with a substantially uniform current distribution around the loop and having a directional radiation pattern in a plane perpendicular to the plane of the loop
• CPC Class Codes: H04B 1/0458 Arrangements for matching and coupling between power amplifier and antenna or between amplifying stages matching circuits in general H03H

Abstract

A circuit for a loop antenna having a first antenna terminal and a second antenna terminal with an antenna impedance, and method for tuning an overall impedance that has an antenna impedance of a loop antenna and a tuning impedance, with an output amplifier for amplifying a transmit signal that has an output for connection to the first antenna terminal of the loop antenna, with a tuning device designed for automatic tuning that has a terminal, which is separated from the output of the output amplifier for connection to the second antenna terminal, in which the tuning device has an adjustable tuning impedance that is connected to the terminal, in which the tuning device has a measurement device that is connected to the tuning impedance in order to measure a voltage amplitude across the tuning impedance, in which the tuning device has a computing unit that is connected to the measurement device and the adjustable tuning impedance, and in which the computing unit is designed for automatic adjustment of the tuning impedance based on evaluation of the voltage amplitude and the tuning impedance.

Claims

1. A circuit comprising:

an output amplifier configured to amplify a transmit signal, the output amplifier having an output connectable to a first antenna terminal of a loop antenna; and

a tuning device for automatic tuning, the tuning device comprising;

a terminal that is separated from the output of the output amplifier and that is connectable to a second antenna terminal of the loop antenna;

an adjustable tuning impedance that is connectable to the second terminal;

a measurement device that is connectable to the adjustable tuning impedance, the measurement device operable to measure a voltage amplitude across the adjustable tuning impedance, the measurement device operable to be switched off; and

a computing unit that is connectable to the measurement device and the adjustable tuning impedance, the computing unit configured to;

automatically adjust the adjustable tuning impedance based on an evaluation of the voltage amplitude and the adjustable tuning impedance; and

switch off the measurement device during a transmit operation.

2. The circuit of claim 1, wherein the evaluation comprises multiplying, by the computing unit, a value associated with a current tuning impedance by the voltage amplitude.

3. The circuit of claim 2, wherein the computing unit is further configured to determine a maximum of a product of the multiplication.

4. The circuit of claim 1, wherein the computing unit comprises a table of values associated with the adjustable tuning impedance for the evaluation.

5. The circuit of claim 4, wherein the values are multiplication factors for multiplication with the voltage amplitude.

6. The circuit of claim 1, wherein:

a sensitivity of the measurement device is settable; and

the computing unit is operable to set the sensitivity of the measurement device.

7. The circuit of claim 1, wherein:

the tuning device is connectable to the output amplifier; and

the tuning device is configured to control the output amplifier, the controlling comprising adjusting an output amplitude or an output frequency of the transmit signal at the output of the output amplifier.

8. The circuit of claim 1, wherein:

the tuning device is connectable to a transmitter circuit; and

the tuning device is operable to control the transmitter circuit in order to adjust an output amplitude and an output frequency of the transmit signal at the output of the output amplifier.

9. A radio system comprising:

a loop antenna having a first antenna terminal and a second antenna terminal; and

a circuit comprising;

an output amplifier configured to amplify a transmit signal, the output amplifier having an output connectable to the first antenna terminal of the loop antenna; and

a tuning device for automatic tuning, the tuning device comprising;

a terminal that is separated from the output of the output amplifier and that is connectable to the second antenna terminal;

an adjustable tuning impedance that is connectable to the second antenna terminal;

a measurement device that is connectable to the adjustable tuning impedance, the measurement device operable to measure a voltage amplitude across the adjustable tuning impedance, the measurement device operable to be switched off;

a computing unit that is connectable to the measurement device and the adjustable tuning impedance, the computing unit is configured to;

automatically adjust the adjustable tuning impedance based on an evaluation of the voltage amplitude and the adjustable tuning impedance; and

switch off the measurement device during a transmit operation.

10. The radio system of claim 9, wherein the evaluation comprises multiplying, by the computing unit, a value associated with a current tuning impedance by the voltage amplitude.

11. The radio system of claim 9, wherein the computing unit comprises a table of values associated with the adjustable tuning impedance for the evaluation.

12. The radio system of claim 11, wherein the values are multiplication factors for multiplication with the voltage amplitude.

13. The radio system of claim 9, wherein:

a sensitivity of the measurement device is settable; and

the computing unit is operable to set the sensitivity of the measurement device.

14. The radio system of claim 9, wherein:

the tuning device is connectable to the output amplifier; and

the tuning device is configured to control the output amplifier, the controlling comprising adjusting an output amplitude or an output frequency of the transmit signal at the output of the output amplifier.

15. The radio system of claim 9, wherein:

the tuning device is connectable to a transmitter circuit; and

the tuning device is operable to control the transmitter circuit in order to adjust an output amplitude and an output frequency of the transmit signal at the output of the output amplifier.

16. A method comprising:

outputting an amplified signal via an output amplifier that is connectable to a first antenna terminal of a loop antenna; and

determining a maximum of a radiated power by performing the following steps repeatedly;

detecting, by a measurement device, a voltage amplitude across an adjustable tuning impedance, wherein the adjustable tuning impedance is connectable to a second antenna terminal of the loop antenna, the measurement device operable to be switched off;

evaluating, by a computing unit coupled to the measurement device, the detected voltage amplitude and the adjustable tuning impedance;

switching off, by the computing unit, the measurement device during a transmit operation; and

adjusting, by the computing unit, the adjustable tuning impedance based on the evaluation.

17. The method of claim 16, wherein the voltage amplitude is multiplied by a multiplication factor associated with a current tuning impedance for the evaluation.

18. The method of claim 17, wherein a maximum of a product of the multiplication is determined for the evaluation.

19. The method of claim 16, wherein, prior to the steps for determining the maximum of the radiated power, an output power of the output amplifier is reduced or minimized, the voltage amplitude across the adjustable tuning impedance is determined, and a maximum permissible gain value of the output amplifier is established for a subsequent determination of the maximum of the radiated power.

20. The radio system of claim 16, wherein the computing unit is further configured to determine a maximum of a product of the multiplication.