Abstract:
A method and apparatus for providing RF-photonic filtering link. Specifically, one embodiment is an apparatus comprising a radio frequency (RF)-photonic filter for filtering an RF signal, where the RF-photonic filter comprises a loop comprising an electro-optical modulator, an optical fiber, a photo detector. Another embodiment is a method of operating an RF-photonic filter comprising applying a reference signal to the RF-photonic filter; selecting a reference frequency for the RF-photonic filter upon the RF-photonic filter locking to the reference frequency, disconnecting the reference signal; and applying an RF input signal to the RF-photonic filter to lock the RF input signal to the RF-photonic filter.

Description:
GOVERNMENT INTEREST 
     Governmental Interest—The invention described herein may be manufactured, used and licensed by or for the U.S. Government. 
    
    
     FIELD OF INVENTION 
     Embodiments of the present invention generally relate to radio frequency signal receivers and, more particularly, to a method and apparatus for providing radio frequency (RF) photonic filtering within an RF receiver. 
     BACKGROUND OF THE INVENTION 
     Current radio frequency (RF) receivers operate over large bandwidths and utilize frequency agile techniques to suppress noise and provide high spur-free dynamic range (SFDR) to improve signal reception. Such low noise, high SFDR receivers find use in communication systems as well as radar systems. To operate over large bandwidths and provide high SFDR, these systems utilize multiple receivers and one or more associated antennas in a conjoined, synchronous manner. Each individual receiver with the system is designed to accurately operate over a small bandwidth. Through parallel operation of a plurality of receivers, the system achieves a large reception bandwidth. However, using multiple receivers requires sophisticated command and control techniques to ensure the receivers operate in a synchronous manner. 
     Furthermore, in high-frequency receiver applications, it is important that every component within the receiver have very low loss to facilitate the accurate reception of signals with very low signal strength. In some situations, a front end of the receiver (high-frequency components) may be co-located with an antenna that is remote from a back end of the receiver (low frequency components). Expensive, low loss RF cables are required to interconnect the front-end and backend of each receiver. 
     Therefore, there is a need in the art for a method and apparatus for wideband, low loss signal processing of RF signals within RF receivers. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate to a method and apparatus for providing RF-photonic filtering. Specifically, one embodiment of the invention is an apparatus comprising a radio frequency (RF)-photonic filter for filtering an RF signal, where the RF-photonic filter comprises a loop comprising an electro-optical modulator, an optical fiber, a photo detector. 
     Another embodiment of the invention is a method of operating an RF-photonic filter comprising applying a reference signal to the RF-photonic filter; selecting a reference frequency for the RF-photonic filter; upon the RF-photonic filter locking to the reference frequency, disconnecting the reference signal; and applying an RF input signal to the RF-photonic filter to lock the RF input signal to the RF-photonic filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a block diagram of an RF signal processing system in accordance with exemplary embodiments of the present invention; 
         FIG. 2  is a block diagram of an RF-photonic filter in accordance with exemplary embodiments of the present invention; 
         FIG. 3  is a detailed functional block diagram of the RF-photonic filter in accordance with exemplary embodiments of the present invention; 
         FIG. 4  is a flow diagram of a method of operating the RF-photonic filter in accordance with exemplary embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention comprise a method and apparatus for receiving an RF signal from an antenna and using a RF-photonic filter to provide broadband tuning and extended spur-free dynamic range (SFDR). The RF-photonic filter, operating as a single mode resonator, amplifies the desired RF signal and substantially suppresses other RF input signals, causing signal spurs to be substantially reduced. Additionally, in one embodiment, the RF-photonic filter provides an optical output for coupling RF signals to a remotely located signal processor using a low loss, low cost optical fiber. 
       FIG. 1  is a block diagram of an RF signal processing system  100  in accordance with exemplary embodiments of the present invention. The system  100  comprises an antenna  108 , a transceiver  102 , and a signal processor  110 . The transceiver  102  comprises a circulator  112  (or other type of directional coupler), a transmitter  104 , and a receiver  106 . In operation, RF signals from the local oscillator (LO)  118 , may be modulated or processed by the signal processor  110  and coupled to the transmitter  104 . Signals from the transmitter  104  are amplified and coupled to the antenna  108  through the circulator  112 . In a radar embodiment, the signals are reflected from an object (target) and return to the antenna  108 . The received signals are coupled from the antenna  108  through the circulator  112  to the receiver  106 . The received signals are coupled through the receiver to the signal processor  110 . Information carried by the received signals is provided as an output signal processor. 
     In one embodiment of the receiver  106 , the receiver  106  comprises an RF-photonic filter  114 , a mixer  116  and a local oscillator  118 . The mixer  116  and the local oscillator  118  operate together as a frequency converter  120 . The RF-photonic filter  114  operates as a single mode resonator that amplifies the desired RF signal and suppresses undesired RF input signals. The desired RF signal is coupled to the mixer  106 . Through mixing a local oscillator signal from the local oscillator  118  with the RF signal, an intermediate frequency (IF) signal is generated. The IF signal is coupled to the signal processor to facilitate extraction of Information from the received signal. 
     In other embodiments of the receiver  106 , an optical output signal from the RF-photonic filter may be coupled through fiber-optic cable  122  to a remotely located frequency converter and signal processor (not shown). In other embodiments, the receiver  106  may not be co-located with a transmitter to form a transceiver. In such embodiments, the receiver operates autonomously. Any RF receiver utilizing a RF-photonic filter as described herein is considered within the scope of the present invention. 
       FIG. 2  is a block diagram the RF-photonic filter  114  in accordance with exemplary embodiment of the present invention. The filter  114  uses photonic technology in combination with RF technology to form a high Q, high SFDR filter for filtering RF signals within the front end of an RF receiver. The RF-photonic filter  114  comprises a first RF switch  200 , and RF reference source  202 , a second RF switch  204 , a high Q filter  206 , a reference block module  208 , a signal lock module  210 , a single RF mode optical loop  212  and a controller  214 . The RF input is coupled through the first RF switch  202  the signal lock module  210 . The RF reference source  202  is coupled-through the second RF switch  204  to the reference clock module  208 . The reference lock module  208  and the signal lock module  210  are coupled to a single RF mode optical loop  212  to form an RF-photonic filter (combining both RF and photonic technologies into a single filter). 
     In operation, the RF-photonic filter  114  operates in two modes: a first mode is a reference lock mode for coarsely tuning the filter  114  and a second mode is a single lock mode for finely tuning the filter  114 . In the reference lock mode, the controller  214  couples the RF reference source  202  to the reference clock module  208  via the RF switch  204 . The controller  214  applies a frequency control signal to the reference lock module  208  to select an operating frequency for the filter  206 . This operating frequency is approximately equal to the expected frequency of the RF input signal to the receiver. As such, the reference lock module  208  and the single RF mode optical loop  212  form an oscillator having a resonant frequency at the frequency set by the frequency control signal. Thus, the filter  206  is coarsely tuned to the expected frequency of the RF input. Once the filter  206  is locked to the reference signal and oscillating at the selected frequency, the filter  114  switches to the signal lock mode wherein the controller opens the second RF switch  204  and closes the first RF switch  200 . In this manner, the RF input signal is coupled to the signal lock module  210  and the RF reference signal is disconnected from the reference lock module  208 . 
     In the signal lock mode, the signal lock module  210  uses the RF input signal to finely tune the filter  206  to center upon the frequency of the RF input signal. The combination of the signal lock module  210  and the single RF mode optical loop  212  form a high Q bandpass filter centered at the center frequency of the RF input signal. The filter can be rapidly retuned to another signal by switching to the reference lock mode and using the controller to change the operating frequency of the reference lock module. 
     For example, in a radar application, the radar transmission signal may be used as the reference signal to coarsely lock a RF-photonic filter to the frequency of transmission. The reflected signal from an object will have a frequency that is slightly higher or lower than the frequency of transmission. During signal lock mode, the reflected RF signal is rapidly locked and processed by the signal processor. The high Q of the RF-photonic filter facilitates high spur free dynamic range. 
       FIG. 3  is a detailed functional block diagram of the RF-photonic filter  114  in accordance with a specific embodiment of the present invention. The RF-photonic filter  114  comprises an RF reference signal source  302 , a phase-locked loop (PLL) and switch unit  304 , a first RF phase shifter  306 , an RF filter  308 , an RF coupler  310 , an electro-optical modulator  312 , a bias control unit  314 , a fiber splitter  316 , a first photo detector  318 , an RF amplifier  320 , a second RF phase shifter  324  and an RF combiner  326 . The RF reference signal source  302  provides a reference RF signal. The PLL and switch  304  select the reference frequency to apply to the phase shifter  306 . In one embodiment, the PLL  304  provides an RF signal equivalent to the carrier signal of the signal that is to be received. The RF Phase Shifter  306  provides tunability of the filter  114 . According to exemplary embodiments, the RF filter  308  is not a very narrow filter for bandwidth as it corresponds to the desired receiver system bandwidth. 
     The RF signal is coupled to the RF filter  308 . In one embodiment, the RF filter  308  is a narrow band, band pass filter, e.g., percentage (10%) of a single expected received signal center frequency. In other embodiments, the RF filter  308  has a bandwidth wide enough to pass the expected received RF signal plus any expected carrier frequency tuning. The RF coupler  310  provides a sample of the RF signal exiting the RF filter  308 . This sample of RF signal is coupled to the PLL  304  to complete a feedback loop for the PLL  304 . In this manner the reference lock module ( 208  in  FIG. 2 ) comprises the PLL  304 , RF phase shifter  306 , RF filter  308 , and RF coupler  310 . 
     The RF signal is coupled from the RF coupler  310  to the electro-optical modulator  312 . The electro-optical modulator  312  modulates the light from the laser  309  with the RF signal. The resulting optical signal is coupled to the optical filter  314  (optional) for wavelength filtering. 
     The optical signal from the filter  314  is coupled to the fiber splitter  316 . According to exemplary embodiments, the fiber splitter  316  splits the filtered optical signal into two signals, one signal is transmitted through the optical fiber link  211  (which may be 10 m to 100 m or longer and has a low Q) to a photo detector  318 . The other signal is coupled to an optical fiber  333  to carry the signal to a remote location to provide remote signal output. At the remote location, a photo detector  334  converts the optical signal into an RF signal that can be processed at point  336 . 
     The photo-detector  318  converts the optical signal to an RF signal and sends the RF signal to the RF coupler  320 . The photo-detector  318  may optionally send the RF signal to an RF amplifier  320 , which further amplifies the signal and couples it to the RF coupler  320 . In one embodiment, the photonic link provides enough gain where the amplifier  320  is not needed. The RF coupler  321  taps the RF signal to provide the first RF output signal  338 . The RF signal is coupled to the phase shifter  324 . Phase shifter  324  is electronically tunable and its output forms one input to the RF combiner  326 . 
     The second input of the RF combiner  326  is the received RF signal from the antenna ( 108  in  FIG. 1 ) into switch  340 . The RF combiner  326  combines both RF signals from the first and second inputs. The output of the combiner  326  is coupled to an RF coupler  330  for coupling a sample of the output of the combiner  326  to the injection lock servo  328 . The signal lock module ( 210  of  FIG. 2 ) comprises the phase shifter  324 , RF combiner  326 , RF coupler  330 , and Insertion lock servo  328 . This combination of components is used to fine-tune the filter  114  such that the filter locks to the center frequency of the RF input signal during the signal lock mode. 
     The bias control unit  332  applies DC bias to the electro-optical modulator  312  and to the injection lock servo  308 . DC bias is applied to end used by the electro-optical modulator  312  in a well-known manner to facilitate modulating light with an RF signal. DC bias is applied to the insertion lock servo to offset an accumulated DC bias in the feedback loop within the signal lock module. The output of RF coupler  330  is coupled to a second RF phase shifter  306 . The phase shifter is a tunable shifter which is controlled by the electronic servo  308 . 
     The single RF mode optical loop  212  comprises portions of both the reference lock module  208  and the signal lock module  210 . Specifically, the loop  212  comprises the RF combiner  326 , the phase shifter  306 , RF filter  308 , electro-optical modulator  312 , the optical filter  314 , the fiber splitter  316 , the optical fiber  311 , the photo detector  318 , the RF amplifier  320 , and the phase shifter  324 . According to an exemplary embodiment, the length of optical fiber  311  is predetermined such that only one RF mode can oscillate within the filter  114 . All other natural frequency modes of the filter  114  are suppressed, causing the loop  212  to form a single mode cavity or resonator for the RF oscillation. The filter  114  can filter out other strong signals in a close frequency range and amplify the signal of interest, providing a spur-free dynamic range for the receiver. In one example, optical fiber  311  is 100 m in length providing a bandwidth of 10 Mhz for a 10 GHz RF carrier. 
       FIG. 4  is a flow diagram of a method  400  for operating the RF-photonic filter  114  using the controller  325  in accordance with exemplary embodiments of the present invention. The method  400  begins at step  402  and proceeds to step  404  where the RF reference source coupled to the reference lock module to begin the reference lock mode. At step  406 , the method  400  uses the PLL to select a reference frequency for the reference lock mode. At step  408 , the queries whether the filter has locked to the reference signal. If the query is negatively answered, the method  400  continues in the reference lock mode (path  418 ). If the query at step  408  is affirmatively answered, the method  400  proceeds to step  410 . At step  410 , the method  400  disconnects the reference signal and at step  414  applies the RF input signal to the filter. At step  416 , the method ends. 
     The method  400  forms the reference lock mode using steps  402  through  410 . Step  414  represents the signal lock mode. If a receiver using the filter of the present invention is required to process a signal at a different frequency, the method  400  can be executed again using a different reference frequency in step  406  that facilitates receiving a different input signal. In this manner the RF-photonic filter of the present invention can be used as a flexible front-end to an RF receiver. 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the present disclosure and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as may be suited to the particular use contemplated. 
     Various elements, devices, modules and circuits are described above in associated with their respective functions. These elements, devices, modules and circuits are considered means for performing their respective functions as described herein. While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.