Fast frequency switching for RFID applications
RFID readers may be configured to supply power to tags during frequency hops. When a reader is supplying power to a passive RFID tag via a first RF waveform having a first radio frequency and determines that it is to frequency-hop, the reader may determine whether the tag requires power during the hop. If so, the reader begins (or continues) to synthesize a second RF waveform with a second radio frequency while also synthesizing the first RF waveform, and frequency-hops by transitioning from transmitting the first RF waveform to transmitting the second RF waveform such that the power transmitted during the transition is sufficient for the tag to operate.
1. A method for a Radio-Frequency Identification (RFID) reader that transmits commands to an RFID tag using amplitude modulation (AM) of a radio frequency (RF) waveform to avoid powering-down the RFID tag when changing a frequency of the RF waveform, the method comprising:
transmitting the RF waveform at a first frequency and with a first average power sufficient for the tag to receive a command modulated onto the RF waveform;
modulating the RF waveform with a low-amplitude AM pulse whose duration is compatible with the command;
switching the frequency of the RF waveform to a second frequency different from the first frequency during the low-amplitude pulse; and
at the end of the low-amplitude pulse, transmitting the RF waveform at the second frequency with a second average power sufficient for the tag to receive the command.
2. The method of claim 1 , wherein the amplitude modulation is amplitude-shift keying.
3. The method of claim 1 , wherein:
the low-amplitude pulse is part of the command; and
switching the frequency of the RF waveform includes switching the frequency while sending the command, the method further comprising:
transmitting the RF waveform at the second RF frequency with no modulation after sending the command; and
receiving a tag response modulated onto the second-frequency RF waveform.
4. The method of claim 1 , further comprising:
generating the first frequency using a first RF synthesizer; and
generating the second frequency using a second RF synthesizer different from the first RF synthesizer.
5. The method of claim 4 , further comprising tuning the second RF synthesizer to the second frequency while transmitting the RF waveform at the first frequency.
6. The method of claim 1 , further comprising generating the first and second frequencies using a single frequency synthesizer.
7. The method of claim 1 , wherein:
the command is according to the Gen2 Specification; and
the low-amplitude pulse is one of a delimiter, a Tari symbol, a TRcal symbol and an RTcal symbol, all according to the Gen2 Specification.
8. A Radio-Frequency Identification (RFID) reader that transmits commands to an RFID tag using amplitude modulation (AM) of a radio frequency (RF) waveform and is configured to avoid powering-down the RFID tag when changing a frequency of the RF waveform, the reader comprising:
a transceiver configured to transmit RF waveforms; and
a processor coupled to the transceiver and configured to:
amplitude-modulate the RF waveform with a command, wherein the modulation includes a low-amplitude pulse whose duration is compatible with the command;
cause the transceiver to transmit the modulated RF waveform at a first frequency and with a first average power sufficient for the tag to receive the command;
during the low-amplitude pulse, switch the frequency of the RF waveform from the first frequency to a second frequency different from the first frequency; and
at the end of the pulse, cause the transceiver to transmit the RF waveform at the second frequency with a second average power sufficient for the tag to receive the command.
9. The reader of claim 8 , wherein the processor is configured to amplitude-modulate the RF waveform using amplitude-shift keying.
10. The reader of claim 8 , wherein the processor is further configured to:
switch the frequency of the RF waveform while sending the command;
cause the transceiver to transmit the RF waveform at the second RF frequency with no modulation after sending the command; and
receive, via the transceiver, a tag response modulated onto the second-frequency RF waveform.
11. The reader of claim 8 , further comprising:
a first RF synthesizer configured to generate the first frequency;
a second RF synthesizer configured to generate the second frequency while the first RF synthesizer is generating the first frequency and the transceiver is transmitting the RF waveform at the first frequency; and
an RF switch coupled to the first and second RF synthesizers, wherein the processor is configured to switch the frequency of the RF waveform using the RF switch.
12. The reader of claim 8 , further comprising a single frequency synthesizer configured to:
generate the first frequency prior to the low-amplitude pulse; and
cease generating the first frequency during the low-amplitude pulse and instead begin generating the second frequency by at least the end of the low-amplitude pulse.
13. The reader of claim 12 , wherein the single frequency synthesizer is one of a direct-digital frequency synthesizer and a fast-tuning synthesizer.
14. The reader of claim 8 , wherein:
the command is according to the Gen2 Specification; and
the low-amplitude pulse is one of a delimiter, a Tari symbol, a TRcal symbol, and an RTcal symbol, all according to the Gen2 Specification.
15. A method for a Radio-Frequency Identification (RFID) reader that transmits commands to an RFID tag using amplitude modulation (AM) of a radio frequency (RF) waveform to avoid powering-down the RFID tag when changing a frequency of the RF waveform, the method comprising:
transmitting an RF waveform at a first frequency, the RF waveform amplitude-modulated with a command;
identifying a modulated portion of the RF waveform whose amplitude and duration are suitable for frequency switching;
during the identified modulated portion, switching the frequency of the RF waveform to a second frequency different from the first frequency; and
transmitting the RF waveform at the second frequency, wherein an average power of the entire modulated RF waveform is sufficient for the tag to receive the command.
16. The method of claim 15 , wherein the RF waveform is amplitude-modulated using amplitude-shift keying.
17. The method of claim 15 , wherein:
the identified modulated portion is part of the command, and
switching the frequency of the RF waveform includes switching the frequency while sending the command; the method further comprising:
transmitting the RF waveform at the second frequency with no modulation after sending the command; and
receiving a tag response modulated onto the second-frequency RF waveform.
18. The method of claim 15 , further comprising:
generating the carrier frequency using a first RF synthesizer; and
generating the second frequency using a second RF synthesizer different from the first RF synthesizer.
19. The method of claim 18 , further comprising tuning the second RF synthesizer to the second frequency while transmitting the RF waveform at the first frequency.
20. The method of claim 15 , wherein:
the command signaling scheme is according to the Gen2 Specification; and
the low-amplitude pulse is one of a delimiter, a Tari symbol, a TRcal symbol, and an RTcal symbol, all according to the Gen2 Specification.