Abstract:
Methods and devices suitable for monitoring the frequency of microwave tunable filters in real time. The frequency readout relies on the natural response of such a filter when excited by a pulse. Methods of measuring an operating frequency of a pole in a tunable filter include measuring a number of cycles in a natural response in the filter when the filter is excited by an electric current pulse, and determining a resonance frequency based on the number of cycles measured in the natural response. Such a method can provide the operating frequency information in a binary digital format, making it relatively easy to read and process. A measuring resonator may be mounted to the filter resonator and connected by a common actuator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present patent application is related to and claims the priority benefit of U.S. Provisional Patent Application Ser. No. 62/121,548, filed Feb. 27, 2015, the contents of which is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     STATEMENT REGARDING FEDERALLY FUNDED RESEARCH 
       [0002]    This invention was made with government support under Contract No. HR0011-12-C-0096 awarded by the Defense Advanced Research Projects Agency. The Government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The present application relates to microwave tunable filters and, more specifically, to methods of monitoring and tuning the frequency of high-Q microwave tunable filters in real time. 
       BACKGROUND 
       [0004]    Tunable filters are the essence of emerging reconfigurable radios and spectrum-aware systems. Their capabilities of switching bands, changing communication standards, and handling jammers, among others, make them a very attractive choice for radio frequency (RF) front ends. Yet, the flexibility of tunable filters comes at the cost of being potentially vulnerable to variations in terms of frequency drift caused by aging or environmental effects. Such frequency stability issues can be addressed with high-Q cavity filters that are tunable using equipment such as network analyzers or by monitoring other operating modes, e.g., secondary mode, in the cavity of the filter. However, these tuning methods can be costly and time-consuming processes. Accordingly, there is a need for improvements in the field. 
       SUMMARY 
       [0005]    The present invention provides methods and devices suitable for monitoring the frequency of microwave tunable filters in real time. The frequency readout relies on the natural response of such a filter when excited by a pulse. 
         [0006]    According to various aspects, an evanescent-mode RF filter is disclosed, comprising an RF filter resonator having a first membrane enclosing a first cavity, a monitoring resonator having a second membrane enclosing a second cavity, the monitoring resonator mounted opposing the filter resonator such that the first and second membranes are facing one another, a planar actuator mounted between the first and second membranes, and a power supply configured to apply a voltage bias signal to the actuator, the voltage bias signal causing the actuator to increase or decrease the operating frequency of the filter resonator. The filter may further comprise a pulse injection circuit operatively connected to an input of the monitoring resonator, the pulse injection circuit configured to supply a pulse signal to the monitoring resonator. The filter may further comprise a readout circuit connected to an output of the monitoring resonator, the readout circuit configured to determine a number of pulses from the output having a voltage greater than a predetermined threshold in a predetermined time period, the number of pulses corresponding to a natural response frequency of the filter resonator in response to the pulse signal. 
         [0007]    Methods of measuring an operating frequency of a pole in a tunable filter include measuring a number of cycles in a natural response in the filter when the filter is excited by an electric current pulse, and determining a resonance frequency based on the number of cycles measured in the natural response. Such a method can provide the operating frequency information in a binary digital format, making it relatively easy to read and process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the following description and drawings, identical reference numerals have been used, where possible, to designate identical features that are common to the drawings. 
           [0009]      FIG. 1  is a diagram showing a communication system according to various aspects. 
           [0010]      FIG. 2 a    is a perspective view diagram of an RF filter monitoring system according to various aspects. 
           [0011]      FIG. 2 b    is a cross-sectional side view of a monitoring device according to various aspects. 
           [0012]      FIG. 2 c    is a plot showing the relationship between resonator gaps for a filter resonator and monitoring resonator according to various aspects. 
           [0013]      FIG. 2 d    is a plot showing the relationship between filter resonator and monitoring resonator frequencies when the gap changes according to various aspects. 
           [0014]      FIG. 3  is a plot showing an injected pulse to a monitoring resonator according to various aspects. 
           [0015]      FIG. 4  is a schematic diagram of a filter monitoring system according to various aspets. 
           [0016]      FIG. 5  is a schematic block diagram of a feedback control circuit according to various aspects. 
           [0017]      FIG. 6  is a schematic block diagram of a power supply circuit according to various aspects. 
       
    
    
       [0018]    The attached drawings are for purposes of illustration and are not necessarily to scale. 
       DETAILED DESCRIPTION 
       [0019]    Various aspects relate to electrostatic control of an ionic environment in a droplet based platform for biological applications. The terms “I,” “we,” “our” and the like throughout this description do not refer to any specific individual or group of individuals. 
         [0020]    Throughout this description, some aspects are described in terms that would ordinarily be implemented as software programs. Those skilled in the art will readily recognize that the equivalent of such software can also be constructed in hardware, firmware, or micro-code. Because data-manipulation algorithms and systems are well known, the present description is directed in particular to algorithms and systems forming part of, or cooperating more directly with, systems and methods described herein. Other aspects of such algorithms and systems, and hardware or software for producing and otherwise processing signals or data involved therewith, not specifically shown or described herein, are selected from such systems, algorithms, components, and elements known in the art. Given the systems and methods as described herein, software not specifically shown, suggested, or described herein that is useful for implementation of any aspect is conventional and within the ordinary skill in such arts. 
         [0021]      FIG. 1  shows a communication system  100 , having an antenna  102  connected to an radio frequency (RF) cavity filter  104 , a feedback control circuit  106  connected to the filter  104 , and a receiver  108  connected to the filter  104 . In certain embodiments, the filter  104  may comprise an evanescent-mode cavity filter. In operation, the antenna  102  receives radio frequency signals and directs them to the filter  104 . The control circuit  102  tunes the filter  104  to a desired frequency or frequencies  110  and holds the filter  104  at that frequency, regardless of effects from hysteresis or creep. The control circuit  106  may operate on poles of the filter  104  independently, without interfering with the received RF signal. In certain embodiments, the control circuit  102  is configured to tune the filter  104  with a resolution of 33 MHz to 6 MHz (3.5-0.4%) in the frequency range of 0.9-1.45 GHz. In other embodiments, the resolution may be 20 MHz to 2 MHz (0.13-1.3%). The frequency range may include RF signals in the 1 GHz to 5 GHz range. The frequency range may also include microwave signals in the 300 MHz to 300 GHz range. 
         [0022]      FIG. 2( a )  shows a diagram of an evanescent-mode cavity filter  200  having a monitoring device  201  for monitoring each pole the filter  200  (illustrated here as a two pole filter). A cross-sectional side view of the monitoring device  201  is shown in  FIG. 2( b ) . As shown, the monitoring device  201  comprises a monitoring cavity resonator  204  stacked on top of each filter resonator  202  of the filter  200  in an opposing fashion. The monitoring resonator  204  comprises rigid housing  218 , a post  220  and a membrane  214  which encloses a cavity  222 . The filter resonator  202  comprises a rigid housing  228 , a post  224  and a membrane  216  which encloses a cavity  226 . The bottom side of the membrane  214  of monitoring resonator  204  is mounted to a top side of an actuator, such as piezoelectric disk  208  to which a voltage bias is applied to tune the filter  200 . The bottom side of the piezoelectric disk  208  is mounted to a top side of the membrane  216  of the filter resonator  202 . It shall be understood that separate monitoring devices  201  may be used for the different poles (filter resonators) in the filter  200 . 
         [0023]    The piezoelectric disk  208  is electrically isolated from the membrane  216  by an insulating material  210 , which in one embodiment is an electrically insulating glue. The glue is applied in a thin layer, allowing mechanical attachment without having a large impact on the tuning range of the piezoelectric disk  208 . The piezoelectric disk  208  may also be electrically insulated from the membrane  214 . 
         [0024]    Since the resonant frequency of each resonator  222  and  226  is controlled by the gap g (see  FIG. 2( b ) ) between the post and the membrane (as shown in  FIG. 2( c ) ), and since the gaps of both cavities  222  and  226  are controlled by the same actuator (e.g., piezoelectric disk  208 ), the resonant frequency of the monitoring resonator  204  (f mon ) will change whenever the resonant frequency of the bottom filter cavity  226  (f RFcav ) changes. Hence, monitoring the frequency of one cavity reveals the frequency of the other. This technique is not susceptible to hysteresis, creep or temperature effects, since any changes in one cavity will be reflected in the other. 
         [0025]    The relationship between the resonant frequency of a cavity and its gap is given by 
         [0000]    
       
         
           
             
               
                 
                   f 
                   ≈ 
                   
                     1 
                     
                       2 
                        
                       π 
                        
                       
                         LC 
                       
                     
                   
                   ≈ 
                   
                     
                       1 
                       
                         2 
                          
                         π 
                       
                     
                      
                     
                       
                         
                           g 
                           
                             
                               e 
                               0 
                             
                              
                             AL 
                           
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Where L and C are the effective inductance and capacitance of the cavity, respectively, g is the gap between the post and the membrane, and A is the area of the top of the post. The approximation in equation (1) is due to the parallel plate approximation of the capacitor. 
         [0026]    From equation (1), the relationship between the resonant frequency of a cavity and the gap between the post membrane is monotonic and bijective (one-to-one correspondence). By transitivity, the relationship between the two resonant frequencies of the filter resonator  202  and monitoring resonator  204  is also monotonic and bijective. This relationship is shown in  FIG. 2( d ) . 
         [0027]    The frequency of the monitoring cavity  222  may be equal to, greater, or smaller than the frequency of the RF filter cavity  226 . This is also due to the monotonic relationship between the two frequencies. In addition, since the monitoring and the RF paths are separated, each cavity can be optimized independently. 
         [0028]    In order to excite the monitoring resonator  204 , a pulse injection circuit  402  is provided as shown in  FIG. 4 . The pulse injection circuit  402  generates a current pulse  403  to excite the monitoring resonator  204 . In one embodiment, the pulse is generated by applying a step waveform on one input of an XOR gate  406 , and a delayed version of that step waveform to the other input of the XOR gate  406 . In one embodiment, an RC circuit consisting of a series resistance (R DELAY ) and the input capacitance of the XOR gate  406  create the delay. The output of the XOR gate then drives a transistor  412  (shown here as an npn transistor, although other types may be used) through a current-limiting resistor  414  (R LIM ). The transistor  412  generates the current-pulse  403  at the input of the monitoring resonator  204 . An example current-pulse  403  output of the pulse injection circuit  402  is shown in  FIG. 3 . 
         [0029]    The frequency of the monitoring resonator  204  can be detected from the natural response of the filter. Therefore, the natural response should be analyzed. In order to study the response of the cavity to a pulse, the cavity needs to be modeled. The monitoring cavity resonator  204  can be modeled as a parallel RLC circuit  416 , as shown in  FIG. 4 . When an RLC circuit is excited by a short current pulse (such as pulse  403 ), the natural voltage response is a damped sinusoid. The voltage across a high-Q parallel RLC circuit under natural response can be approximated as 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     RLC 
                   
                   = 
                   
                     
                       V 
                       0 
                     
                      
                     
                       e 
                       
                         
                           - 
                           t 
                         
                         
                           2 
                            
                           
                               
                           
                            
                           RC 
                         
                       
                     
                      
                     
                       
                         sin 
                          
                         
                           ( 
                           
                             
                               2 
                                
                               π 
                                
                               
                                   
                               
                                
                               
                                 f 
                                 0 
                               
                                
                               t 
                             
                             + 
                             θ 
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where V 0  is a constant, t is time, R and C are the resistance and capacitance, f 0  is the natural frequency expressed in equation (1), and θ is the phase. This has been verified by simulating an RLC model circuit (using SPICE) when excited by the measured pulse from  FIG. 3 . 
         [0030]    The current pulse from the pulse injection circuit  402  will typically exhibit jitter. Jitter can be caused by several mechanisms such as random additive noise. Additive noise can cause the logic gate (XOR  406 ) to trigger before or after the signal reaches the threshold, randomly. This causes different output pulse widths, which can change the response of the circuit. As a result, the monitoring resonator  204  should be designed such that the response is not significantly affected by jitter. The frequency of the monitoring resonator  204  is chosen such that the response is not largely affected by the jitter in the pulse, which becomes prominent at frequencies close to the inverse of the pulse width. On the other hand, the frequency of the monitoring resonator  204  cannot be too low because filter fabrication becomes problematic at low frequencies due to size requirements. As a result, the frequency of the monitoring resonator  204  should preferably be chosen between those two limits. If the aforementioned limitations on the monitoring resonator  204  define a range smaller than the tuning range (limited by the piezoelectric actuator  208 ), the upper limit can be further moved to higher frequencies by using a pulse injection circuit  402  that can provide a smaller pulse width (T pulse ). 
         [0031]    The natural frequency response of the monitoring resonator  204  is needed to determine the frequency of the filter resonator  202 . When tuning the filter  200 , the capacitance C changes, which, in turn, changes the natural response waveform in equation (2). This change can be detected by counting the number of cycles above a voltage threshold in the damped response, as shown in  FIG. 4 . This can be expressed analytically as 
         [0000]      N=f mon t 0 .   (3)
 
         [0000]    where N is the number of cycles above the threshold, to is the time it takes for the signal to go below the threshold, and f mon  is the natural resonant frequency of the monitoring resonator  204 . Given that the sinusoidal component in equation (2) has a unity maximum, t 0  can be found by solving 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       0 
                     
                      
                     
                       e 
                       
                         
                           - 
                           
                             t 
                             0 
                           
                         
                         
                           2 
                            
                           
                               
                           
                            
                           RC 
                         
                       
                     
                   
                   = 
                   
                     V 
                     T 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where V T  is the threshold voltage. From equation (4), t 0  can be found to be 
         [0000]    
       
         
           
             
               
                 
                   
                     t 
                     0 
                   
                   = 
                   
                     2 
                      
                     
                         
                     
                      
                     RC 
                      
                     
                         
                     
                      
                     
                       ln 
                        
                       
                         ( 
                         
                           
                             V 
                             0 
                           
                           
                             V 
                             T 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0032]    From equations (1), (3) and (5), the relationship between the number of cycles and natural resonant frequency of the monitoring resonator  204  is given by 
         [0000]    
       
         
           
             
               
                 
                   N 
                   = 
                   
                     
                       
                         2 
                          
                         
                             
                         
                          
                         R 
                       
                       
                         
                           
                             ( 
                             
                               2 
                                
                               π 
                             
                             ) 
                           
                           2 
                         
                          
                         
                           f 
                           mon 
                         
                          
                         L 
                       
                     
                      
                     
                       ln 
                        
                       
                         ( 
                         
                           
                             V 
                             0 
                           
                           
                             V 
                             T 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0033]    From equation (6), it can be seen that, in the natural response of a cavity, the number of cycles that are above a voltage threshold (V T ) is inversely proportional to the resonant frequency. This relationship is also monotonic and bijective, which allows it to be used for monitoring. 
         [0034]    Since the number of cycles N is inversely proportional to the monitoring resonator  204  frequency (Nα1/f mon ), and since the monitoring resonator  204  frequency is inversely proportional to the filter resonator  202  (f mon α1/f RF cav ), the number of cycles N is directly proportional to the filter resonator  202  frequency (Nαf RF cav ). 
         [0035]    As shown in  FIG. 4 , a readout circuit  430  outputs the number of cycles above a voltage threshold in the signal output from the monitoring resonator  204 . In one embodiment, the readout circuit  430  comprises a limiting amplifier  432  as the input stage. The limiting amplifier  432  outputs a signal with a constant amplitude as long as the input is larger than the set threshold. The output of the limiting amplifier  432  drives a high-speed ripple counter  434  to count the number of cycles. 
         [0036]    When a pulse  403  is injected into the monitoring resonator  204 , the counter  434  provides the number of cycles observed in the damped response. As discussed herein, the number of pulses can identify the resonant frequency of the monitoring resonator  204 . As a result, the frequency of the filter resonator  202  is determined as well. 
         [0037]    In certain embodiments, the monitoring readout circuit  430  outputs the number of pulses output from the monitoring resonator  204  in digital form, easing the integration of the readout in a control system. 
         [0038]      FIG. 5  shows one embodiment of the feedback control circuit  106  which provides tuning of each pole in the filter  200  to a desired frequency, and to maintain that tuning regardless of memory effects such as hysteresis or creep. To accomplish this, the control circuit  106  changes the power supply that generates the bias voltage of the piezoelectric disk  208  based on the frequency reading from the readout circuit  430 . 
         [0039]    As shown in  FIG. 5 , the control circuit  106  takes two inputs, the readout (N) from the readout circuit  430  and a digital number (D IN ) representing the desired operating frequency. The input received from the readout circuit  430  is first averaged (using averaging unit  502 ) to suppress any noise in the monitoring reading. Experiments show that averaging over 32k samples seems sufficient for the data to be stable and flicker free. Also, the data input (D IN ) is latched (using latch  504 ) and sent to a look-up-table (LUT unit  506 ) to generate an initially estimated control signal to the power supply (PS CT RL  est.). This speeds up the process of generating the correct control signal to the power supply. The averaged readout data are then compared with D IN  using a binary magnitude comparator  508 . If they are not equal, as desired, a counter  510  generates an error signal (positive or negative) which will be added to the estimated signal. This will change control signal (PS CTRL ) of the power supply  512  (which is connected to the piezoelectric disk  208  to supply the voltage bias to the disk  208 ). The error signal will keep increasing (or decreasing if negative) until the averaged readout data is equal to the desired input data (D IN ). At that point, the power supply control signal PS CTRL  has adjusted the power supply  512  to generate the piezoelectric bias signal V Bias  that would correspond to the desired operating frequency of the filter  200 . If the frequency of the filter resonator  202  is changed due to creep or any other environmental perturbations, it will change the readout signal (N). This will cause the control circuit  106  to change the error signal until the operating frequency of the filter  200  is corrected automatically. 
         [0040]    In certain embodiments, the control circuit  106  is fully digital can therefore be implemented in a microcontroller or a field programmable gate array (FPGA) platform. 
         [0041]    In certain embodiments, to ease the integration of the system, the power supply  512  may be controlled digitally and should be capable of generating high voltage bias for the piezoelectric disk  208 .  FIG. 6  shows one embodiment of the power supply  512  which comprises a Digital-to-Analog converter (DAC)  602  and amplifier  604 . The DAC  602  receives the power supply control signal (PS CTRL ) and converts the signal to an analog low-voltage replica of the desired voltage. The output of DAC  602  is directed to the high voltage amplifier  604 . The output of the amplifier  604  is then directed to the piezoelectric disk  208  as shown in  FIG. 6 . 
         [0042]    Steps of various methods described herein can be performed in any order except when otherwise specified, or when data from an earlier step is used in a later step. Exemplary method(s) described herein are not limited to being carried out by components particularly identified in discussions of those methods. 
         [0043]    According to various aspects, technical effects can include the capability of measuring an operating frequency of a pole of a filter in real time with relatively low cost devices. In preferred embodiments, the frequency response of each pole in a filter can be measured using simple circuitry using off-the-shelve electronics that can be embedded in a system with reduced power consumption overhead, resulting in a relatively inexpensive solution in comparison to conventional techniques tuned with lab equipment. Also, these methods preferably provide the frequency information in a digital format, and without affecting the main cavity operation. 
         [0044]    Various aspects described herein may be embodied as systems or methods. Accordingly, various aspects herein may take the form of an entirely hardware aspect, an entirely software aspect (including firmware, resident software, micro-code, etc.) run by one or more computer processors connected to electronic memory, or an aspect combining software and hardware aspects These aspects can all generally be referred to herein as a “service,” “circuit,” “circuitry,” “module,” or “system.” 
         [0045]    Furthermore, various aspects herein may be embodied as computer program products including computer readable program code (“program code”) stored on a computer readable medium, e.g., a tangible non-transitory computer storage medium or a communication medium. A computer storage medium can include tangible storage units such as volatile memory, nonvolatile memory, or other persistent or auxiliary computer storage media, removable and non-removable computer storage media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. A computer storage medium can be manufactured as is conventional for such articles, e.g., by pressing a CD-ROM or electronically writing data into a Flash memory. In contrast to computer storage media, communication media may embody computer-readable instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave or other transmission mechanism. As defined herein, “computer storage media” do not include communication media. That is, computer storage media do not include communications media consisting solely of a modulated data signal, a carrier wave, or a propagated signal, per se. 
         [0046]    The invention is inclusive of combinations of the aspects described herein. References to “a particular aspect” (or “embodiment” or “version”) and the like refer to features that are present in at least one aspect of the invention. Separate references to “an aspect” (or “embodiment”) or “particular aspects” or the like do not necessarily refer to the same aspect or aspects; however, such aspects are not mutually exclusive, unless otherwise explicitly noted. The use of singular or plural in referring to “method” or “methods” and the like is not limiting. The word “or” is used in this disclosure in a non-exclusive sense, unless otherwise explicitly noted. 
         [0047]    The invention has been described in detail with particular reference to certain preferred aspects thereof, but it will be understood that variations, combinations, and modifications can be effected within the spirit and scope of the invention.