US20070249314A1 - Adjusting parameters associated with transmitter leakage - Google Patents

Abstract

An apparatus comprises a transmitter, a receiver, an antenna and a signal cancellation circuit. The transmitter is configured to send a transmitter signal associated with a frequency. The receiver is associated with the frequency. The antenna is coupled to the transmitter and the receiver. The signal cancellation circuit is coupled to the transmitter, the receiver and the antenna. The signal cancellation circuit is configured to phase shift a first portion of the transmitter signal to produce a phase-shifted signal. The signal cancellation circuit is configured to combine the phase-shifted signal with a second portion of the transmitter signal to produce a combined signal. The second portion of the transmitter signal is associated with a reflection of a third portion of the transmitter signal from the antenna. The first portion, the second portion and the third portion of the transmitter signal are different from each other.

Description

CLAIM OF PRIORITY
• This application claims priority under 35 USC §119(e) to U.S. patent application Ser. No. 10/804,198, filed on Mar. 19, 2004, the entire contents of which are hereby incorporated by reference.
TECHNICAL FIELD
• This invention relates to Radio Frequency (RF) signals and, more particularly, to adjusting parameters associated with transmitter leakage.
BACKGROUND
• In a known transceiver without signal cancellation, a single antenna is shared by the transmitter portion and receiver portion while simultaneously transmitting and receiving on the same frequency. SeeFIG. 1. The knowntransceiver100 includes acontroller110, afrequency source120, atransmitter modulator130, a variable gain amplifier (VGA)140, a power amplifier (PA)150, adetector160, acirculator170, anantenna180, anantenna connector185 and areceiver190.Controller110 is a microprocessor. Thefrequency source120 is a frequency agile synthesizer.Detector160 can measure the power output by thetransmitter modulator130.
• The output oftransmitter modulator130 is calibrated using an accurate power sensor (not shown) at theantenna connector185 by adjusting the gain setting ofVGA140, and then storing the gain setting ofVGA140 and the detector reading that produced the desired output power level(s). Thetransmitter modulator130 andreceiver190 operate on the same frequency, and thus the performance ofreceiver190 is adversely affected by the energy fromtransmitter modulator130 that is reflected back fromantenna180, which is non-ideal in its implementation. If all radio frequency (RF) components in thetransceiver modulator130 are precisely 50 ohms, for example, then all energy fromtransmitter modulator130 applied toantenna180 is radiated, and no energy reflects back towardsreceiver190. Because an ideal 150-ohm implementation is not realistically achievable,receiver190 will experience degraded performance due to the energy reflected from thetransmitter modulator130 byantenna180, relative to the weaker signal sent toreceiver190 detected by the detector.
• The signal level reflected back fromantenna180 is, for example, typically between 15 to 25 dB below the signal from thetransmitter modulator130 and radiated byantenna180. The energy level of the signal reflected back toreceiver190 can be, for example, as high as 100 m W. This can cause signal overload of the sensitive components ofreceiver190, resulting in degradation of the sensitivity and range ofreceiver190. For the case of a homodyne receiver, this can cause a large direct current (DC) (i.e., 0 Hz) component.
SUMMARY
• An apparatus comprises a transmitter, a receiver, an antenna and a signal cancellation circuit. The transmitter is configured to send a transmitter signal associated with a frequency. The receiver is associated with the frequency. The antenna is coupled to the transmitter and the receiver. The signal cancellation circuit is coupled to the transmitter, the receiver and the antenna. The signal cancellation circuit is configured to phase shift a first portion of the transmitter signal to produce a phase-shifted signal. The signal cancellation circuit is configured to combine the phase-shifted signal with a second portion of the transmitter signal to produce a combined signal. The second portion of the transmitter signal is associated with a reflection of a third portion of the transmitter signal from the antenna. The first portion, the second portion and the third portion of the transmitter signal are different from each other.
• The details of one of more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
DESCRIPTION OF DRAWINGS
• FIG. 1 shows a block diagram of a known transceiver.
• FIG. 2 shows a block diagram of a transceiver having signal cancellation circuitry, according to an embodiment of the invention.
• Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
• In an embodiment of the invention, ′″ transceiver having a signal cancellation circuit simultaneously transmits and receives on the same frequency while sharing a single antenna. In particular, a reflected signal from an antenna can be reduced significantly (for example, by 30 dB or more) via a signal cancellation circuit that takes a small amount of the transmitter signal (adjusted in amplitude to be substantially equal in amplitude to the reflected signal), and shifts the phase of the signal such that the phase-shifted signal is 1800 out of phase with the reflected signal. Consequently, when the two signals are combined, a signal having reduced amplitude is produced. Such signal can also be referred to as a “cancelled” signal or can have, for example, significantly reduced amplitude.
• FIG. 2 shows a block diagram of a transceiver having a signal cancellation circuit, according to an embodiment of the invention. As shown inFIG. 2, thetransceiver200 includes acontroller210, frequency source220,transmitter modulator230, VGA240, PA250, detector260, circulator270,antenna coupler285,antenna280 and receiver290.Transceiver200 also includessignal cancellation circuit300 havingcoupler310,variable attenuator320,phase shifter330, coupler/combiner340,detector350,controller360, limiter370 and low noise amplifier (LNA)380. Each of the components of thesignal cancellation circuit300 is discussed below.
• Coupler310 can be, for example, a directional coupler inserted between the output of PA250 and circulator270.Coupler310 receivessignal402 and sendssignals404 and406 wheresignal404 has a smaller amplitude than the amplitude of signal406.Signal404 can used to cancel the reflected signal from theantenna280 as described below in more detail.
• Variable attenuator320 can be, for example, a variable attenuator used to adjust the amplitude ofsignal404 so that the amplitude of thesignal412 substantially corresponds to the amplitude ofsignal408 atcombiner340, wheresignal408 is reflected fromantenna280.Phase shifter330 can be, for example, a phase shifter configured to adjust the phase of the signal 41 0 1800 relative to the phase of thesignal408 reflected fromantenna280 and received bycoupler340.
• Coupler340 can be, for example, a signal coupler configured to combine signal.412 withsignal408 reflected from the antenna. In other words, aftervariable attenuator320 adjusts the phase ofsignal404 andphase shifter330 adjusts the phase ofsignal410,coupler340 combinessignal412 withsignal408 received as reflection fromantenna280.
• Detector350 can be, for example, a power detector configured to measure the power ofsignal414.Detector350 provides the detected power ofsignal414 tocontroller360.Controller360 is configured to adjustvariable attenuator320 based on the detected power ofsignal414. More specifically,controller360 provides a control signal tovariable attenuator320 so thatvariable attenuator320 modifies the amplitude ofsignal404 to substantially correspond to t4e amplitude ofsignal408.
• Controller360 is also configured to adjustphase shifter330 based on the detected power ofsignal414. More specifically,controller360 provides a control signal tophase shifter330 so that the phase ofsignal412 is shifted substantially1800 fromsignal408 reflected from the antenna. The output ofdetector350 can be minimized, for example, when the amplitude ofsignals412 and408 are substantially equal, and the phase ofsignals412 and408 are substantially 180° relative to each other.
• Detector350 can also be coupled elsewhere withinsignal cancellation circuit300. Such alternative locations ofdetector350 withinsignal cancellation circuit300 can provide an alternative measure of selectivity and sensitivity. Such alternative locations can be, for example, between LNA380 and receiver290, or after the mixer (not shown) of the receiver290.
• Controller360 can be configured, for example, as a control loop used to adjust the amplitude and phase ofsignal404 so thatsignal414 output bycoupler340 as detected by detector bydetector350 is minimized. As described above,controller360 provides control signals tovariable attenuator320 andphase shifter330, which adjust the amplitude and phase, respectively, ofsignal404 based on these control signals.
• Limiter370 receivessignal414 and outputs signal416. Limiter370 is configured to limit the amplitude ofsignal414 to produce signal416 thereby protectingLNA380. More specifically, LNA8 is configured to amplify signal416 to improve the sensitivity performance and range of the receiver290. Before such amplification, however, the amplitude ofsignal416 provided toLNA380 is limited by limiter370 to protectLNA380 from damage by a high signal level reflected from the antenna, prior to the cancellation adjustment I refinement viacontroller360.
• Various alternative embodiments are possible. For example, in one embodiment, the transceiver includes an optional memory device (not shown). In such an embodiment, the transceiver detectors (e.g., detectors260 and350) can be calibrated based on the calibrated data stored in the memory device of the transceiver. By calibrating the transceiver detectors based on the calibration data, the time it takes to minimize the cancellation by the signal cancellation circuit can be minimized.
• Similarly,transmitter modulator230 can be calibrated by using a power meter (not shown) atantenna connector285, and by adjustingVGA240 until the desired power output fromtransmitter modulator230 is achieved. The setting ofVGA240 and the detected power of PA250 can be stored in the optional memory device for each possible output power setting oftransmitter modulator230. Finally, withvariable attenuator320 set for maximum attenuation, a signal can be applied toantenna connector285 while calibratingdetector350 over its usable range.
• In another embodiment, a “training” sequence can be implemented in whichvariable attenuator320 is set for maximum attenuator and detector350 (previously calibrated) measures the power level of the reflected signal (e.g., signal414). Based on the current power level detected by detector260, the initial value ofvariable attenuator320 can be approximated to achieve a best guess of “equal amplitude,” and thenphase shifter330 can be adjusted to minimize the power level of thesignal414 detected bydetector350. Subsequently,variable attenuator320 can be fine tuned to produce a minimum power level ofsignal414 detected bydetector350, and thenphase shifter330 can be fine tuned to produce a minimum power level ofsignal414 detected bydetector350.
• While the invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changed in form and details may be made therein without departing from the spirit and scope of the invention.

Claims (22)

1. A method for reducing noise, comprising:

sampling, on a receiver path, Radio Frequency (RF) signal including a signal received from a transponder and an interference signal, the interference signal based, at least in part, on an RF transmission signal;

dynamically adjusting a portion of the RF transmission signal based, at least in part, on the sampled signal; and

combining the adjusted portion of the RF transmission signal into receive path to reduce the interference signal.

2. The method ofclaim 1, further comprising splitting, from the RF transmission signal, the portion of the RF transmission signal.

3. The method ofclaim 1, wherein dynamically adjusting the portion of the transmission signal comprises:

dynamically adjusting the amplitude of the portion of the transmission signal based, at least in part, on the sampled signal; and

dynamically adjusting the phase of the portion of the transmission signal based, at least in part, on the sampled signal.

4. The method ofclaim 1, wherein the interference signal includes a portion of the RF transmission signal reflected by an antenna.

5. The method ofclaim 1, a frequency of the RF transmission signal is substantially the same frequency of the signal received from the transponder.

6. The method ofclaim 1, wherein the receive-path signal is sampled as an RF signal.

7. The method ofclaim 1, wherein the interference signal is reduced by at least 30 dB by the adjusted portion of the transmission signal.

8. The method ofclaim 1, wherein sampling the RF signal from the received path comprises determining an amplitude of the receive-path signal.

9. Method ofclaim 1, further comprising:

dynamically generating control signals for adjusting the amplitude of the portion of the RF transmission signal based, at least in part, on the sampled signal;

dynamically generating control signals for adjusting the phase of the portion of the RF transmission signal based, at least in part, on the sampled signal; and

wherein the amplitude and the phase of the portion of the RF transmission signal are adjusted in response to at least the control signals.

10. The method ofclaim 1, wherein the portion of the RF transmission signal is dynamically adjusted using a control loop.

11. An RF reader, comprising:

a detector configured to sample, on a receive path, an Radio Frequency (RF) signal including a signal received from a transponder and an interference signal, the interference signal based, at least in part, on a RF transmission signal;

a control element configured to dynamically adjust a portion of the RF transmission signal based, at least in part, on the sampled signal; and

a coupler configured to combine the adjusted portion of the RF transmission signal to the receive path to reduce the interference signal.

12. The RF reader ofclaim 11, further comprising a coupler configured to split, from the RF transmission signal, the portion of the RF transmission signal.

13. The RF reader ofclaim 11, wherein the control element comprises:

a amplitude element configured to dynamically adjust an amplitude of the portion of the transmission signal based, at least in part, on the sampled signal; and

a phase element configured to dynamically adjust a phase of the portion of the transmission signal based, at least in part, on the sampled signal.

14. The RF reader ofclaim 11, wherein the amplitude element comprises a variable attenuator.

15. The RF reader ofclaim 11, wherein the phase element comprises a phase shifter.

16. The RF reader ofclaim 11, wherein the interference signal includes a portion of the RF transmission signal reflected by an antenna.

17. The RF reader ofclaim 11, the frequency of the RF transmission signal is substantially the same frequency of the signal received from the transponder.

18. The RF reader ofclaim 11, wherein the receive-path signal is sampled as an RF signal.

19. The RF reader ofclaim 11, wherein the interference signal is reduced by at least 30 dB by the adjusted portion of the transmission signal.

20. The RF reader ofclaim 11, wherein the detector is configured to determine an amplitude of the receive-path signal.

21. The RF reader ofclaim 11, further comprising wherein the control element includes a controller configured to dynamically generate control signals for adjusting an amplitude and a phase of the portion of the RF transmission signal based, at least in part, on the sampled signal, wherein an amplitude and a phase of the portion of the RF transmission signal are adjusted in response to at least the control signals.

22. The RF reader ofclaim 1, wherein the portion of the RF transmission signal is dynamically adjusted using a control loop.

Images