Linearized power amplifier modulator in an RFID reader

• US 20050140457A1
• Filed: 12/23/2004
• Published: 06/30/2005
• Est. Priority Date: 12/31/2003
• Status: Abandoned Application

• Current Assignee: WJ Communications, Inc. (Qorvo, Inc.)
• Original Assignee: WJ Communications, Inc. (Qorvo, Inc.)
• Inventors: Bellantoni, John Vincent
• Application Number:
  US11/021,946
• Publication Number:
  US 20050140457A1
• Time in Patent Office: Days
• US Class Current: 332/106
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer

Abstract

An RFID reader accessible thorough a personal computer and includes a PC card interface and a controller both operating according to clock signals from a crystal oscillator. The RFID reader further includes a linearized power amplifier modulator in a transmit path, a receive chain capable of demodulating either class_1 or class_0 signals from RFID tags, and an integrated switching device for selecting one of a plurality of antenna for transmitting or receiving RF signals.

Claims

1. A linearized power amplifier modulator for modulating an input signal according to a control signal, comprising:

a pulse-shaping filter configured to receive the control signal and to generate a ramp output based on the control signal;

a current mirror configured to receive the ramp output and to produce a reference current based on the ramp output; and

a power amplifier configured to receive the reference current and the input signal and configured to modulate the input signal according to the reference current.

2. The modulator of claim 1 wherein the ramp output is a ramp voltage output and the reference current is linearly proportional to the ramp output.

3. The modulator of claim 1 wherein the control signal comprises step transitions and the ramp output comprises a linear ramp over a ramp time period corresponding to each step transition in the control signal.

4. The modulator of claim 3 wherein the pulse-shaping filter comprises an operational amplifier having a first input connected to a ground potential via a first resistor and to a supply voltage via a second resistor, a second input connected to a control voltage associated with the control signal via a third resistor, and an output coupled to the second input via a capacitor.

5. The modulator of claim 3 wherein the operational amplifier has a slew rate that is very fast compared to the ramp time period.

6. The modulator of claim 1 further comprising a low-pass filter coupled to the output of the pulse-shaping filter and configured to smooth the ramp output.

7. The modulator of claim 1 wherein the current mirror includes a first transistor configured as a diode between a supply voltage and an output of the pulse-shaping filter, and a second transistor coupled with the first transistor in a current mirror configuration, wherein the reference current is generated in the second transistor.

8. The modulator of claim 1 wherein the power amplifier includes a reference transistor coupled between a supply voltage and a ground potential and configured to receive the reference current from the current mirror, and a plurality of power transistor cells each producing a bias current based on the reference current and the input signal.

9. The modulator of claim 1 in a transmit signal path of an RFID reader.

10. The modulator of claim 9 wherein the input signal is a continuous wave signal generated by a frequency synthesizer in the RFID reader.

11. A method for modulating an input signal comprising:

receiving a control signal, the control signal comprising step transitions;

generating a ramp signal according to the control signal, the ramp signal ramping each step transition in the control signal over a ramp time period;

generating a reference current signal according to the ramp signal using a current mirror;

supplying the reference current signal to a power amplifier receiving the input signal; and

modulating the input signal according to the reference current signal using the power amplifier.

12. The method of claim 11 wherein the reference current signal is linearly proportional to the ramp signal.

13. The method of claim 11 wherein receiving the control signal comprises receiving the control signal at a first input of an operational amplifier, a second input of the operational amplifier being coupled to ground through a first resistor and to a supply voltage through a second resistor, and an output of the operational amplifier is coupled to the first input via a capacitor.

14. The method of claim 13 wherein generating the ramp signal comprises filtering an output signal from the operational amplifier through a low-pass filter.

15. The method of claim 11 wherein supplying the reference current signal to the power amplifier comprises supplying the reference current to a reference transistor in the power amplifier.

16. The method of claim 15 wherein modulating the input signal according to the reference current signal comprises generating a bias current based on the reference current in each of a plurality of power transistors.