WO2001011774A1

WO2001011774A1 - Am demodulator using carrier phase correction

Info

Publication number: WO2001011774A1
Application number: PCT/US2000/021428
Authority: WO
Inventors: Gordon Edward Hardman; John W. Pyne
Original assignee: Crosslink, Inc.
Priority date: 1999-08-05
Filing date: 2000-08-04
Publication date: 2001-02-15
Also published as: AU6759500A

Abstract

A Radio Frequency (RF) receiver (300) that receives (401) RF signals and mixes (403) signals from a local oscillator with the signals. The signal strength of the desire demodulated signals is maximized by adjusting (417) the phase of the signal from the local oscillator in the following manner. The RF signals including the desired demodulated signals are received (401). Signals from a local oscillator are applied to the RF signals (403). The strength of the desired demodulated signals are then determined (405) and the phase of the signal from the local oscillator is adjusted tomaximize signal strength of the desired demodulated signals (417).

Description

AM DEMODULATOR USING CARRIER PHASE CORRECTION

Field of the Invention

This invention relates to a receiver of Radio Frequency (RF) signals. More particularly, this invention relates to a receiver that receives RF signals transmitting data at a carrier frequency. Still more particularly, this invention relates to a receiver that improves signal strength of RF signals received by mixing the received RF signal with a signal from an oscillator having an optimal phase to increase signal strength.

Problem Many technologies use RF signals to transmit data. One example of such a technology is Radio Frequency Identification (RFID). In a RFID system, RF signals identifying an object or giving information about an object are transmitted from a tag affixed to the object. The tag is a device containing a RF transmitter. The RF signals are received by a reader which is an RF receiver that processes the RF signals to retrieve data from the signals.

A common technique used in RFID is called a modulated backscatter signal. In modulated backscatter signals, a reader has an RF transmitter that transmits strong, unmodulated RF signals in a carrier frequency. These RF signals illuminate an antenna in an RFID tag. To insert data into the RF signals, circuitry in the tag modulates the carrier by changing the feedpoint impedance of the antenna. A weak backscatter RF signal is then transmitted from the antenna to the reader.

It is a problem to retrieve the data from the received RF signals in an inexpensive, reliable, and simple manner. One type of receiver for retrieving the data is a superheterodyne or multi-conversion receiver. In superheterodyne receiver, the receiver signal is translated to one or more intermediate frequencies before the data is retrieved. These receivers are used when the greatest possible selectivity and sensitivity are required.

When high data rates and large dynamic ranges are required, direct conversion receivers are typically used. Direct conversion receivers are typically inexpensive and are not very channel selective. One example of a direct conversion receiver is a crystal set built by many radio novices which is a diode envelope detector. A crystal set typically suffers from poor selectivity, sensitivity and dynamic range. To overcome the problems of poor selectivity, sensitivity, and dynamic range, several form of synchronous and semi-synchronous direct conversion receivers have been developed such as a zero IF receiver. These zero IF receivers use a local oscillator and mixers to convert a carrier frequency to baseband and retrieve the data. These zero IF receivers operate in the following manner.

An antenna receives the RF signals. A local oscillator generates two signals, an I and a Q signal, separated 90 degrees in phase. A first mixer combines the I signal and the receiver RF signal, and a second mixer combines the Q signal and the RF signals. The resulting signals contain the original modulating signal frequency. However, the resulting signals are rotated in phase due to imperfect alignment of the local oscillator frequency and the carrier frequency. The use of two mixing signals assures that the resulting signal never are both zero. However, strength of the resulting signals vary as phase rotation proceeds.

Further processing is required to recover a constant amplitude of a demodulated signals. First, rectangular to polar conversion is performed on the signals. The phase rotation is removed. These are performed by conventional analog or digital signal processing techniques. In either digital or analog signal processing, linearity of the received signals must be maintained. If signal strength varies, automatic gain control may be used to maintain linearity. This requires several additional components and increases the complexity of the circuitry used to recover data.

Another direct conversion receiver is a homodyne receiver. A homodyne receiver phase-locks a local oscillator to a carrier signal. Homodyne receivers are not practical because receivers cannot adequately filter the carrier signal from accompanying modulation and interfering signals to perform the phase lock.

From the above descriptions of various receivers, it can be seen that it is a problem to design an inexpensive and simple receiver to retrieve data from RF signals.

Solution The above and other problems are solved and an advance in the arts are made by the RF receiver of this invention. In accordance with this invention, a phase of the signal generated by a local oscillator and applied to received RF signals by a mixer is adjusted to optimize signal strength of the desired received RF signals. A first advantage of this invention is elimination of one mixer and a phase splitter. A second advantage of this invention is that post-detection signal processing is eliminated. A third advantage of this invention is that linearity of the RF signals is not required. A fourth advantage of this invention is that automatic gain control can be eliminated in most systems. A fifth advantage of this invention is the ease of post-detection signal processing circuitry design. A sixth possible advantage of this invention is a reduction in power consumption. The above and other advantages are provided in the following manner.

RF signals are received by antenna. A local oscillator generates an oscillator signal. A mixer applies the oscillator signal to the received RF signals. The strength of the wanted demodulated signals is then converted to a digital signal by an Analog to Digital convertor. A processor receives the digital signal. The strength of the modulation signal is then determined. After signal strength is determined, the processor determines a direction in which to adjust the phase of the oscillator signal. A signal that directs the oscillator to adjust the phase of the oscillator signal in the desired direction is then generated and transmitted to the local oscillator.

The local oscillator adjusts the phase of the oscillator signal in response to receiving the signal from the processor. This processes may continually be repeated until signal strength of the received signals is optimized. Description of the Drawings

The above and other advantages and features of this invention are described in the Detailed Description below and the following drawings:

FIG. 1 illustrating a block diagram a first prior art receiver; FIG. 2 illustrating a block diagram of a second prior art receiver; FIG. 3 illustrating a block diagram of a receiver in accordance with this invention;

FIG. 4 illustrating a flow diagram of a method for adjusting a phase of a signal from an oscillator in accordance with this invention; and

FIG. 5 illustrating a flow diagram of a process performed by a processor in a receiver in accordance with this invention.

Detailed Description This invention relate to an RF signal receiver that adjust the phase of signals from a local oscillator to maximize the strength of received RF signals. FIGS. 1 and 2 illustrate block diagrams of prior art RF receiver to give a better understanding of the present invention.

FIG. 1 illustrates a block diagram of a zero IF receiver 100. In a zero IF receiver 100, antenna 102 receives RF signals and applies the RF signals to splitter 105 via path 103. Splitter 105 applies the RF signals to paths 107 and 109. A local oscillator generates an I signal and a Q signal which are 90 degrees apart in phase.

The I and Q signals have are the same frequency as the carrier frequency of the received RF signals.

Mixer 111 receives RF signals via path 107 and an I signal via path125. Mixer 111 adds the I signal to the RF signals and applies the mixed RF signals to path 115.

Mixer 113 receives RF signals via path 109 and the Q signal via path227. Mixer 113 adds the Q signal to the RF signals and applies the RF signals to path 117. The RF signals from mixers 111 and 113 are approximate an original. However, the RF signals from mixers 111 and 113 are rotated in phase due to an imperfect alignment of the local oscillator frequency and the frequency of the carrier frequency. The use of two channels guarantees that there is no phase of the RF signals that can produce a zero output on both channels.

It is a problem that the strength RF signals from mixers 111 and 113 vary as phase rotation proceeds. Therefore, processing circuitry 121 performs a rectangular to polar conversion of the RF signals from mixers 111 and 113 received via paths 115 and 117. Phase rotation is then removed from the RF signals by processing circuitry

121. One skilled in the art will recognize that circuitry 121 may be conventional analog circuitry or a digital signal processor. To process these RF signals, linearity of the RF signals must be maintained. Automatic gain control circuitry 129 may be needed to maintain the linearity. After the signals are processed the output is applied to path

123.

FIG. 2 illustrates RF receiver 200 which is an RF receiver that operates explicitly at the frequency to down convert RF signals without suffering phase rotation.

An example of such a receiver is an RFID reader that receives back scatter signals from an antenna of an RFID tag that is illuminated by a transmitted carrier signal. In receiver 200, antenna 201 receives RF signals and applies the RF signals to splitter

205. Splitter 205 applies RF signals to mixers 209 and 215 via paths 207 and 213. Mixer 209 receives RF signals via path 207 and an I signal from a local oscillator via path 223. Mixer 209 add the I signal to the RF signals and applies the Rf signals to path 211. Mixer 215 receives RF signals via path 213 and the Q signals via path 225. The I signals and the Q signals are the same frequency as the carrier signal and phase shifted 90 degrees from one another. Signal processing circuitry 225 then receives the RF signals via path 211 and 217 and processes the signals to generate and output applied to path 227.

Although phase rotation is not a problem in receiver 200, two channels are still required because the phase of the RF signals is unknown and time varying. Because the phase is unknown and time varying, the linearity and signal processing constraints of zero IF receiver 100 (FIG.1 ) still apply.

FIG. 3 illustrates a block diagram of receiver 300 designed in accordance with this invention. In receiver 300, antenna 301 receives transmitted RF signals. The RF signals received are of a known carrier frequency. For example, the RF signals may be from an illuminated antenna of a RFID tag in a back scatter RFID system. Antenna 301 applies the RF signals to mixer 305 via path 303. Local oscillator 339 generates an oscillator signal that has an identical frequency of the carrier frequency of the RF signals received. In a backscatter system, the carrier and the oscillator signal may be generated by the same oscillator.

Mixer 305 receives RF signals via path 303 and local oscillator signals via path 341. Mixer 305 mixes the local oscillator signal with the RF signals and applies the demodulated signals to path 309. Processing circuitry 313 receives the RF signals from mixer 305 via path 309 and creates DC signal proportional to the strength of the desired demodulated signals and applies the signals to path 315. Processing circuitry 313 and applies the signals to produce the output signals applied to path 343.

The only requirement need to produce the desired output signals in this system is that the desired demodulated signal be monotonically related to the received signal strength of the received RF signals. This can be accomplished in the following manner.

Path 315 picks off signals related to the strength of the desired signals from path 309 and applies a DC signal proportional to the RF signals to A/D convertor 317. Processor 319 receives the digital RF signals via path 318. Processor 319 is a processor, microprocessor or group of processors and/or microprocessors that execute instructions stored in a memory. Random Access Memory (RAM) 323 connects to processor 319 via path 321. RAM 323 is an example of a volatile memory that stores instructions and memories needed by processor 319 to perform an application. Read Only Memory (ROM) 327 connects to processor 319 via path 325. Processor 325 receives the digital RF signals and determines the strength of the RF signals. The processor then determines the amount by which to adjust the phase of the local oscillator signal. This may be done by simply adjusting the phase in a first direction. The processor then takes a second sample and if the RF signals are stronger in the second sample the signal is adjusted in the first direction again. Otherwise, the phase is changed in a second, opposite direction. This is process is repeated until an optimal strength is reached or is just constantly repeated to maintain optimal strength. Another method maybe to simply rotate and sample the Received Signal Strength Indicator until a threshold level is reached. This method can increase the speed of the processes.

To adjust the phase of the local oscillator, signal processor 319 generates a signal and transmits the signal via path 329. Digital to Analog (D/A) convertor 331 receives the signal via path 329 and converts the signal to analog. Phase shifter 335 receives the signals from D/A convertor 331 via path 333 and generates a phase shifted signal that is applied to local oscillator 339 via path 337. The phase shifted signal is then used to generate the local oscillator signal.

FIG. 4 illustrates a process 400 performed by receiver 300 to optimize the strength of received RF signals. Process 400 begins in step 401 with antenna 301 receiving RF signals. In step 403, a local oscillator signal is applied to the RF signals by mixer 305. In step 404, a signal predominately proportional to the strength of the desired modulation termed "Recied Signal Strength Indicator" (RSSI) is developed A/D convertor 317 converts the RF signals into digital signals in step 405. The digital RF signals are transmitted to processor 319 in step 407. Processor 319 then determines the strength of the RF signals in step 409. In optional step 411 , processor 319 may determine whether the signal strength of the RF signals is maximized. If the signal strength is optimized process 400 ends in step 420. If processor 319 determines that the signal strength is not maximized of if step 411 is not executed, processor 319 determines how to adjust the phase of the local oscillator signal and generates a signal indicating the adjustment of the local oscillator in step 413.

The determination of how to adjust the local oscillator is done in the following manner. The processor may adjust phase of the local oscillator signal in a first direction in a first iteration. If the signal strength increases in a second iteration, the phase is changed in the first direction again. If the signal strength decrease in the second iteration, the phase is changed in a second opposite direction. The amount of change in the direction may be changed in steady increments or the increments may be reduced as the direction changes back and forth in following iterations center on a phase the maximizes the RF signal strength.

In step 415, the signal is transmitted from processor 319 to local oscillator 339. Local oscillator 339 adjusts the phase of the local oscillator signal being produced in step 415. Process 400 is then repeated from step 401. This allows the ideal phase to be quickly determined. Process 500 illustrates a flow diagram of a process 500 executed by processor

319 to adjust the phase of the local oscillator signal in accordance with this invention. Process 600 begins in step 601 with processor 319 receiving digital signals. In step 603, processor 319 determines the signal strength of the RF signals. Processor 319 may then perform an optional step of determining the signal strength is maximized. If the signal strength is maximized, process 500 may end. Otherwise, processor 319 then determines how much the phase of the local oscillator signal must be adjusted in step 505.

Process 500 can also be used with a small number of coarse phase steps and can be terminated by rapidly jumping to the phase at which the strongest RSSI is observed. This minimizes the number of iterations and increases the speed of the process by finding a near optimum phase.

Processor 319 then generates a signal that indicates the adjustment needed to the phase of the local oscillator signal in step 507. The signal is then transmitted to the local oscillator in step 509. Process 500 then is repeated from the beginning at step 501. This allows processor 319 to close in on an optimal phase to maximize signal strength by performing successive iterations of process 500.

The above is a description of a radio receiver incorporating phase seeking for local oscillator in accordance with this invention. It is expected that those skilled in the art can and will design alternative radio receivers that infringe on the radio receivers of this invention as set forth in the claims below either literally or through the Doctrine of Equivalents.

Claims

What is Claimed is:

1. A method (400) for providing maximizing signal strength of Radio Frequency (RF) signals comprising the steps of: receiving (401 ) said RF signals including a desired modulation signals; applying (403) a signal from a local oscillator to said RF signals; determining (404) a strength of said desired modulation signals; and adjusting (417) a phase of said signal from said local oscillator being applied to said RF signals to maximize signal strength of said desired modulation signals.

2. The method (400) of claim 1 wherein said step (404) of determining said strength of said RF signals comprises the steps of: converting (405) said desired modulation signals to digital signal strength signals responsive to applying said signal from said local oscillator to said RF signals; transmitting (407) said digital RF signals to a processor; and determining (404) said strength of said RF signals with said processor.

3. The method (400) of claim 1 wherein said step (417) of adjusting said phase of said signal from said local oscillator comprises the steps of: generating (413) a signal indicating an adjustment to said phase of said signal from said local oscillator; and transmitting (415) said signal indicating said adjustment to said local oscillator; and adjusting (417)said phase of said signal in accordance with said signal.

4. The method (400) of claim 3 wherein said step (415) of transmitting said signal indicating said adjustment comprises the step of: converting said signal from a digital signal to an analog signal.

5. The method (400) of claim 1 further comprising the step of: determining (411 ) whether said strength of said desired modulation signals is maximized responsive to determining said strength of said desired modulation signals.

6. The method (400) of claim 1 further comprising the step of: repeating said method responsive to adjusting said phase of said signal from said local oscillator.

7. The method (400) of claim 6 wherein said step (417) of adjusting said phase of said signal from said local oscillator comprises the step of: adjusting said phase of said signal in a first direction.

8. The method (400) of claim 7 wherein said step (417) of adjusting comprises the step of: determining whether said signal strength increases compared to a prior iteration of said method in response to adjusting said phase in said first direction; and adjusting said phase in a second direction responsive to a determination said signal strength did not increase.

9. The method (400) of claim 8 wherein said step of adjusting comprises the step of: adjusting said phase in a first direction in response to a determination of an increase in strength of said desired modulation signals.

10. The method (400) of claim 1 wherein said step of determining said signal strength further comprises the step of: developing (404) a signal proportional to said desired modulation signals.

11. Apparatus (300)for maximizing strength of received RF signals comprising: a local oscillator(339) that generates a signal; a mixer (305) that mixes said signal from said local oscillator with said RF signals; an analog to digital convertor (317) that converts said RF signals into digital signals; a processor (319); instructions that direct said processor to: receive (501 )said digital signals, determine (503) a strength of said signals, and adjust (505) a phase of said signal from said local oscillator being applied to said signals to maximize signal strength of said signals; a media readable by said processor for storing said instructions.

12. The apparatus of claim 11 wherein said instructions further comprise: instructions for directing said processor to: generate (507) a signal indicating an adjustment to said phase of said signal from said local oscillator; and transmit (509) said signal indicating said adjustment to said local oscillator.

13. The apparatus of claim 12 further comprising: a digital to analog convertor (331 ) that converts said signal from a digital signal to an analog signal.

14. The apparatus of claim 11 wherein said instructions further comprise: instruction that direct said processor to determine whether said strength of said

RF signal is maximized responsive to determining said strength of said RF signals.

15. The apparatus of claim 11 wherein said instructions further comprise: instructions that direct said processor to repeat said instructions responsive to adjusting said phase of said signal from said local oscillator.

16. The apparatus of claim 15 wherein said instructions to adjust said phase of said signal from said local oscillator comprise: instructions that direct said processor to adjust said phase of said signal in a first direction.

17. The apparatus of claim 16 wherein said instructions to adjust said phase of said signal comprise: instructions that direct said processor to: determine whether said signal strength increases compared to a prior iteration of said method in response to adjusting said phase in said first direction, and adjust said phase in a second direction responsive to a determination said signal strength did not increase.

18. The apparatus of claim 17 wherein said instructions to adjust said phase comprise: instructions that direct said processor to adjust said phase in a first direction in response to a determination of an increase in strength of said signals.

19. The apparatus of claim 18 wherein said instructions cause said phase to be set to a value proximate said optimum.