Abstract:
A Lorentz force-driven mechanical resonator apparatus that utilizes a high-Q resonant structure as both a mixing device and a high-Q bandpass filter. Specifically, an external time varying, but quasistatic, magnetic field is applied to the resonating device while simultaneously running a time varying electrical current through the device. The resulting Lorentz force (I×B) is proportional to the vector product of the electrical current in the bar (I) and the external magnetic field (B). Integrating such a resonant device with a magnetic field coil produces the functionality of an ideal radio frequency (RF) mixer coupled with a high-Q intermediate frequency (IF) filter. Wide tunability provides the capability to scan, or even step, an array of filters having very narrow bandwidths via a common local oscillator to a desired frequency range.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 of PCT/US02/13058 filed Apr. 24, 2002 which claims the benefit of U.S. provisional application Ser. No. 60/286,431, filed Apr. 26, 2001 entitled “Mechanical Filter/Mixer for Radio Frequency Applications” which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to a Lorentz force-driven resonator, such as a xylophone bar magnetometer (XBM), based mechanical mixer/filter for radio frequency (RF) applications. More specifically, the present invention is related to an array of Lorentz force-driven mechanical filter/mixers for use in channelized RF receiver applications. 
     BACKGROUND 
     The Johns Hopkins University Applied Physics Laboratory has patented a novel device capable of high frequency magnetic field measurement, the xylophone bar magnetometer of U.S. Pat. No. 5,959,452, which makes use of the Lorentz force generated by a current in a magnetic field. Two principle advantages of the Lorentz force-driven resonant device over other mechanical designs are its ability to function as both a filter and mixer/downconverter and its implementation as a micro electromechanical system (MEMS) design. 
     There is increasing interest in the development of miniature, high frequency narrow band filters to replace existing filters in RF applications. The trend is constantly toward smaller size, lower power consumption, and lower cost for similar performance. Existing high frequency, narrow band filters are based on large, superconducting systems or utilize multiple electronic filter stages. The Lorentz force-driven mechanical resonator described herein can be used as a filter and a mixer to process signals over a broad range of frequencies. 
     SUMMARY 
     The present invention is a mechanical mixer/filter apparatus that is rooted in a resonator design described in commonly owned U.S. Pat. No. 5,959,452 which is incorporated herein by reference. The present invention also includes a system in which an integrated array of micro-fabricated, electromechanical mixer/filters may be used, inter alia, in channelized RF receiver applications. 
     The present invention is based on the Lorentz force-driven resonating bar magnetometer that utilizes a high Q resonant structure as both a mixing device and a high-Q bandpass filter. Specifically, an external time varying magnetic field (B) may be applied to the device while simultaneously running a time varying electrical current (I) through the device. The resulting Lorentz force (I×B) is proportional to the vector product of the electrical current in the bar and the external magnetic field. By adjusting the frequencies of the current and external magnetic field, the Lorentz force can be controlled to cause the bar to vibrate at its resonant frequency. 
     Integrating a Lorentz force-driven mechanical resonator with a magnetic field coil produces the functionality of an ideal RF mixer coupled with a high-Q intermediate frequency (IF) filter. Furthermore, a Lorentz force-driven mechanical resonator mixer/filter can operate in mixing mode at frequencies into the GHz range making it useful for UHF and VHF applications. This includes, but is not limited to, cellular and wireless applications, particularly those in which space, weight and power are considerations. Each of the Lorentz force-driven mechanical resonator designs presented herein may also be arrayed for use in channelized RF applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art design of a Lorentz force-driven mechanical resonator known as a xylophone bar magnetometer (XBM). 
     FIG. 2 illustrates an Lorentz force-driven mechanical resonator based component design that provides the basis for an RF-mixer/filter. 
     FIG. 3 illustrates an alternative Lorentz force-driven mechanical resonator mixer/filter component design using a coil positioned beneath the resonator. 
     FIG. 4 illustrates still another alternative Lorentz force-driven mechanical resonator mixer/filter component design using a single strip adjacent to the resonator. 
     FIG. 5 illustrates yet another alternative Lorentz force-driven mechanical resonator mixer/filter component design using a strip loop around the resonator. 
     FIG. 6 illustrates an alternative Lorentz force-driven mechanical resonator design that includes a secondary resonator. 
    
    
     DETAILED DESCRIPTION 
     A Lorentz force-driven mechanical resonator measures the deflection in a conducting bar produced by the Lorentz force as represented by the equation (F=I×B) where F is the Lorentz force, I is a current, and B is a magnetic field. FIG. 1 (prior art) illustrates an embodiment of a Lorentz force-driven mechanical resonator in the form of a xylophone bar magnetometer (XBM)  5  described in commonly owned U.S. Pat. No. 5,959,452. It is comprised of a resonator  10 , in this case a thin conductive, e.g., aluminum, bar, supported by two wires  12 ,  14 . The wires are bonded to the bar to provide low-resistance electrical contacts and positioned at the nodal points expected for a bar free at both ends and vibrating in its fundamental mode. 
     In operation, alternating currents, generated by a sinusoidal source oscillating at the fundamental transverse resonant mode, are supplied to the bar at one of two support nodes  16  and extracted at the other node  18 , and the device is placed inside a magnetic field. The Lorentz force generated by the current and the applied magnetic field causes the bar to vibrate in its fundamental mode, the amplitude being proportional to the vector component of the magnetic field parallel to the support wires in the plane of the bar. 
     The amplitude of the vibration can be measured using various techniques, including optical beam deflection, optical interferometry, differential capacitance and tunneling currents. The Lorentz force-driven mechanical resonator structure can serve as a fundamental component for numerous RF applications. 
     FIG. 2 illustrates a Lorentz force-driven mechanical resonator based mixer/filter component  20  that provides a basis for an RF-mixer/filter array design. A local oscillator (LO) input signal at frequency f LO  drives a pair of magnetic field coils  22  to create a magnetic field (B). In this design, the magnetic field coils  22  are placed lengthwise on either side of Lorentz force-driven mechanical resonator  26 . An RF input signal at frequency f RF  passes through an impedance matching network  24  and drives an electrical current (I) in the mechanical resonator  26 . If the RF frequency is equal to (f Lo +f 0 ) or to (f LO−f   0 ), where f 0  is the resonance frequency of the mechanical resonator  26 , then the mechanical resonator  26  begins to resonate. The mechanical resonator  26  is supported by a pair of support arms  28 . The ends of one support arm  28  are coupled with anchor/electrodes  30  that receive the impedance matched RF input signal while the ends of the other support arm  28  are coupled with anchor/electrodes  30  that are grounded. A readout electrode  32  is coupled with the mechanical resonator  26  to provide a Lorentz force output signal (F) for the mixer/filter component  20 . In this design, the amplitude of the vibration of the mechanical resonator  26  is determined via direct measurement of capacitance between the bar and an electrode  32  placed near the bar. Other methods or means for determining the amplitude of the vibration of the mechanical resonator  26  may be substituted as described above. 
     In its implementation as a mixer/filter, a Lorentz force-driven mechanical resonator is a component that can be fashioned into a combined mixer/IF filter for traditional superheterodyne receiver applications, as illustrated in FIG.  2 . Because of its high mechanical Q factor, the Lorentz force-driven mechanical resonator can eliminate the multiple conversion stages required in traditional superheterodyne receivers that operate in the UHF to VHF range. To achieve a narrow-IF bandwidth though traditional means, the IF frequency must be relatively low compared to the bandwidth of the signal of interest due to the limitations on the Q factors of electronic devices. For many practical applications, this necessitates the use of multiple IF stages within a receiver system. However, the Lorentz force-driven mechanical resonator allows for high IF frequencies with very high Q values reducing the requirements of the image reject filter while also supplying high compression of interfering signals. 
     For mixing/filtering implementations where the RF signal frequency content is greater than the resonant frequency of the Lorentz force-driven mechanical resonator, the Lorentz force-driven mechanical resonator resonates only when the difference between the current and magnetic field frequencies are within the bandwidth of the mechanical resonance (i.e., |f LO −f RF |=f 0 ). Thus, the Lorentz force-driven mechanical resonator behaves as a narrowband mixer, with an IF given by its mechanical resonant frequency. 
     One advantageous aspect of a Lorentz force-driven mechanical resonator based mixer/filter design is the ability to build a heterodyne receiver having a single IF stage. As stated above, the limited Q of electronic filters means that multiple IF stages are generally required to achieve a desired band selection. Utilizing an XBM based mixer/filter design, however, allows down conversion with a narrow bandwidth in a single step and requires only one local oscillator (LO). For a channelized receiver, arrays of different XBM components can be used, each with a very distinct bandwidth (i.e., different resonant frequencies f 0 ). A single local oscillator can be used to tune the array of Lorentz force-driven mechanical resonators to the band of interest, and to compensate the array for environmentally induced frequency drifts. Such a mixer/filter design principally differs from other approaches in the wide tunability of the entire array. Wide tunability provides the capability to scan, or even step, an array of Lorentz force-driven mechanical resonator filters having very narrow bandwidths to a desired frequency range via a common local oscillator. 
     The outstanding performance of a superheterodyne receiver is based on the benefits of tuning the local oscillator rather than the filter. However, an array of high Q Lorentz force-driven mechanical resonator filter/mixer components maintains this benefit and mimics the behavior of a tunable system by utilizing a fixed frequency oscillator with an array of Lorentz force-driven mechanical resonator IF filters tuned to different frequencies. Thus, an array of Lorentz force-driven mechanical resonator devices (including MEMS designs) achieves the performance of a tunable receiver without having to tune the local oscillator. 
     A characteristic of mechanical resonators is their sensitivity to temperature variations. When used as an oscillator or filter, this can lead to the need for temperature-controlled environments of the sort used for high accuracy crystal oscillators. However, because of the reduced size and thermal mass of Lorentz force-driven mechanical resonator MEMS devices, temperature-controlled environments can be reduced in size, power level, and complexity. On an array of these devices, the thermal drift of a reference resonator can be used to tune a local oscillator (LO) and maintain the resonant condition for a given RF signal. Moreover, if a resonator is used not only as a mixer and IF filter, but also as an oscillator, an integrated device in which the oscillator frequency and the IF center frequency drift together can be designed such that their combined performance is temperature-invariant. If the IF filter is followed by a traditional frequency-independent second detector as typically used in a superheterodyne system, it is of little consequence if the IF frequency drifts, so long as the oscillator drifts by a corresponding amount. 
     There are a variety of additional RF applications for the system of the present invention ranging from radios to radars to spectrum analyzers. When implemented as an array of Lorentz force-driven mechanical resonators, multiple devices can be used in many applications. For instance, a channelized radio receiver can be developed that receives and processes multiple narrowband signals simultaneously. Or, multiple devices can be combined as a demultiplexing system for stripping individual telephone calls from, for instance, a T 1  carrier. Another application can be a multi-channel spectrum analyzer in which the simultaneous use of parallel channels provided by a resonator array eliminates the constraint between the resolution bandwidth and sweep speed that currently exists for traditional spectrum analyzers. This constraint represents a serious signal-processing bottleneck in conventional systems. 
     FIG. 3 illustrates an alternative Lorentz force-driven mechanical resonator mixer/filter design  34  using a magnetic field coil positioned lengthwise beneath the Lorentz force-driven mechanical resonator  26  between the support arms  28 . In this mixer/filter design, a local oscillator (LO) input signal (X) drives a magnetic field coil  22  to create a magnetic field (B). An RF input signal (Y) drives an electrical current (I) in the Lorentz force-driven mechanical resonator  26  via a pair of support arms  28 . As described earlier, the amplitude of the vibration of the resonator  26  can be measured using a variety of techniques, including optical beam deflection, optical interferometry, differential capacitance and tunneling currents. 
     FIG. 4 illustrates still another alternative Lorentz force-driven mechanical resonator mixer/filter design  36  using a single strip  38  positioned lengthwise beneath the Lorentz force-driven mechanical resonator  26 . In this mixer/filter design, a local oscillator (LO) input signal (X) drives the strip  38  to create a magnetic field (B). An RF input signal (Y) drives an electrical current (I) in the Lorentz force-driven mechanical resonator  26  via a pair of support arms  28 . Again, the amplitude of the vibration of the Lorentz force-driven mechanical resonator  26  can be measured using a variety of techniques, as previously described. 
     FIG. 5 illustrates yet another alternative Lorentz force-driven mechanical resonator mixer/filter design  40  using a strip loop  42  positioned lengthwise about the Lorentz force-driven mechanical resonator  26 . In this mixer/filter design, a local oscillator (LO) input signal (X) drives the strip loop  42  to create a magnetic field (B). An RF input signal (Y) drives an electrical current (I) in the Lorentz force-driven mechanical resonator  26  via a pair of support anus  28 . Again, the amplitude of the vibration of the Lorentz force-driven mechanical resonator  26  can be measured using a variety of techniques as previously described. 
     FIG. 6 illustrates an alternative Lorentz force-driven mechanical resonator design  44  that includes a secondary resonator. It is comprised of a first mechanical resonator  26 , supported by two support arms  28 . The support arms  28  are bonded to the mechanical resonator  26  to provide low-resistance electrical contacts, and positioned at the nodal points expected for a bar free at both ends and vibrating in its fundamental mode. A secondary mechanical resonator  46  is bonded to one of the support arms  28 . The secondary resonator  46  is driven mechanically by the motion of the primary resonator  26 . Relative to the primary resonator  26 , the more compliant secondary resonator  46  exhibits increased vibrational amplitude, hence increased sensitivity. 
     In operation, alternating currents, generated by a sinusoidal source oscillating at the fundamental transverse resonant mode, are supplied to the first resonator  26  at one of the support arms  28  and extracted at the other support arm  28 , and the device is placed inside a set of Helmholtz coils. The Lorentz force generated by the current and the applied magnetic field causes the bar to vibrate in its fundamental mode, the amplitude being proportional to the vector component of the field in the plane of the bar and parallel to the support wires. 
     The Lorentz force-driven mechanical resonator designs of FIGS. 2-5 are interchangeable with respect to one another. It is also noted that for each of the mixer/filter designs, the RF input signal can be applied across the coil/strip and the local oscillator (LO) signal applied across the Lorentz force-driven mechanical resonator  26  or, the RF input signal can be applied across the Lorentz force-driven mechanical resonator  26  and the local oscillator (LO) signal applied across the coil/strip. That is, the X and the Y in FIGS. 3-5 are interchangeable. In addition, each of the Lorentz force-driven mechanical resonator mixer/filter designs described herein are capable of being arrayed together to produce the same functional results as described with respect to FIG.  2 . Moreover, the Lorentz force-driven mechanical resonator design of FIG. 6 which includes a secondary resonator may also be substituted into the Lorentz force-driven mechanical resonator mixer/filter designs. 
     In the following claims, any means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.