Reflective Array Compressors Research Articles

Investigations on groove design for modulating frequency response of SAW RAC chirp filter

Surface Acoustic Wave (SAW) Reflective Array Compressor (RAC) type chirp filters are used in radar compressive receivers as matched filters or compressors. The conventional way of designing the grooves in such filters is to make groove length constant throughout the length of the array. However, this does not give levelled frequency response and shows a rise toward the high frequency side. A new idea of tapering the groove length is suggested in this article. The suggested design shows that the frequency response of the chirp filter is flat in the frequency range of interest compared with a conventional one. Simulated results are shown for a chirp filter with centre frequency 450 MHz and bandwidth of 200 MHz.

A comprehensive analysis of surface acoustic wave reflections. III. Oblique incidence grooved and metallic gratings

First- and second-order reflection effects in oblique incidence surface acoustic wave gratings are examined. A general formulation usable for both metallic and same material grooved overlays is provided. The existence of several important effects, particularly a reflection effect at the second harmonic of the grating and a velocity shift at the first harmonic for grooved gratings, are demonstrated, and the behavior of these effects as a function of incidence angle is examined. For angles of incidence of interest in the production of reflective array compressors, these effects are analyzed as a function of metallization ratio.

Correction factor for strongly reflecting U-path RACs

An equation from the literature describing multiple reflections in a U-path surface-acoustic-wave (SAW) reflective array compressor (RAC) is simplified for RACs with a large time-bandwidth product, yielding a very convenient expression consisting of the relation for weakly reflecting RACs plus phase and amplitude correction factors. The quantitative effect of multiple reflections is immediately apparent, so that the designer has a better understanding of how various parameters affect the final performance. A down-chirp, which is more common in practical devices, is described. Empirical extensions to higher-order and variable weighing are deduced and verified numerically, while the possibility of variable chirp is discussed.

A high-resolution chirp transform spectrometer for microwave measurements

A microwave heterodyne spectrometer consists of a radiometer front-end and a real-time spectrum analyser back-end. Common spectrometer types are filterbanks, autocorrelators and acousto-optic spectrometers (AOS). A high-resolution chirp transform spectrometer using reflective array compressor (RAC) filters with a time bandwidth product of 6400 is presented. The spectrometer has 1600 channels within a 40 MHz input bandwidth and a linear dynamic range of 63 dB for a CW input. It seems to be very promising for future space applications.

Investigation of chirp interference on RAC demodulation

Recently, many satellite systems have started to employ reflective-array compressors (RAC) to demodulate efficiently their M-FSK communication signals. Because the RAC's time delay varies with temperature, pilot-tones are usually introduced as the operational reference. In this letter, the basic chirp Fourier transform (CFT) is briefly reviewed. Then, an investigation into possible pilot-tone interference caused by various chirp signals with the RAC's dispersive delay properties is presented and discussed.

High-accuracy post-fabrication trimming of surface acoustic wave devices by laser photochemical processing

Laser direct writing reactions for molybdenum etching and Cr/Cr2O3 cermet conductivity transformation have been developed for concurrent phase and amplitude trimming of surface acoustic wave devices on LiNbO3. The molybdenum etching reaction utilizes photolytic decomposition of Cl2 with 488 nm light as a means to achieve low process temperature and avoid substrate damage. The cermet reaction utilizes a solid-state transformation in an O2 environment to render the film highly nonattenuating to surface acoustic waves with no underlying substrate damage. These techniques have been applied to trimming of reflective array compressors and have provided order-of-magnitude improvement in phase and amplitude accuracy over devices compensated by lithographic processing.

High-performance SAW dispersive delay line using reflective thin metal dot arrays

The first U-path surface-acoustic-wave (SAW) reflective array compressor (RAC) employing reflective arrays of thin metal dots on lithium niobate is reported. The thin rectangular metal dots are made together with the transducers in a single photolithographic step. Performance comparable with that of large time-bandwidth, conventional, grooved RACs is demonstrated, in good agreement with the design.

Reflective array compressors using 180° reflecting metal dot arrays

Recently, a reflective array compressor using 180° grooved reflectors and a 3-dB multistrip coupler has been described. This has the advantage over conventional 90° reflective devices that the surface waves undergo only one reflection instead of two, and so the insertion loss can be low. This paper reports a new device, using surface acoustic waves propagating on YZ LiNbO3, in which the grooves are replaced by metal dot arrays, removing the need for ion-beam milling and—through variation of dot density—providing a simple method of weighting. The devices designed had a time-bandwidth product of 1000, dispersion time of 20 μs, bandwidth of 50 MHz centered at 150 MHz, and a 40-dB sidelobe six-term Taylor amplitude weighting. Compared with dot array devices using 90° reflectors the design is simplified since the velocity and scattering anisotropy, important for 90° reflectors, need not be considered. The surface wave velocity change under the metal dot array is potentially a source of phase error, but it is shown that a simple empirical correction in the mask design is sufficient to reduce this to around 5° rms. Deviation of the weighting characteristic from the ideal is typically of the order ±1 dB.

Amplitude Weighted Metal Strip Reflective Array Compressors

Amplitude weighted metal strip reflective array compressors (RAC's) have been developed in which the reflection coefficient of the arrays is adjusted by varying the strip width. This structure has an advantage that the reflectors and the interdigital transducers can be fabricated in a single photolithographic step. Linear and non-linear 30 MHz FM RAC's, whose time-bandwidth (TB) products are 60 and 48, respectively, are fabricated on Bi12GeO20 substrates. Time sidelobe levels of the compressed pulse are less than -27 dB in linear FM case and -28 dB in non-linear FM case. The resulting metal strip RAC's have been proved to be adequate for practical use.

Pulse Compression Results Using Metallic Reflective Array Lines

Radar pulse-compression results are presented for the first reflective-array compressor (RAC) dispersive delay lines (DDL) with both metallic reflecting arrays and phase-compensating films. The time-bandwidth product of the devices reported is approximately 400. Operation in a recirculation loop with a 37.5-dB Taylor weighting filter yielded 36-dB near-in range sidelobes. RMS phase errors less than 0.71 degrees across the band were achieved. Greater than 50-dB rejection of spurious response is achieved in the far-out range gate region. The potential for high-quality cost-effective fabrication of metallic RAC DDL for system applications is explored.

A new surface-acoustic-wave cut of quartz with orthogonal temperature-compensated propagation directions

A theoretical study of the surface-acoustic-wave (SAW) properties of quartz has produced a new cut which is temperature compensated along two orthogonal directions. This new cut is attractive for use in reflection-grating devices such as the reflective array compressor (RAC), in which orthogonal propagation paths are required. Except for a small electromechanical power flow angle, the SAW properties along the two temperature-compensated directions are similar to those of x-propagating ST-cut quartz. Because the attractive propagation paths lie in the plane of a singly rotated crystal cut, it should be relatively easy and inexpensive to manufacture.

Applications of edge-bonded transducers to SAW components

Various applications of the EBT to acoustic devices are reported. First we describe the operation of these transducers in the case of Rayleigh waves. Then we report the results we have obtained in the fabrication of: 1) a piezo-MOSFET programmable filter with a bit rate reaching 50 MHz; 2) a Love wave-dispersive delay line with a 240 compression ratio; 3) a fused silica reflective array compressor exhibiting a 30-µs dispersion over a 60-MHz bandwidth around 100 MHz and a 24-dB total insertion loss.

Suppression of Bulk Wave Radiation from Surface Acoustic Wave Devices

It is well known that interdigital transducers usually launch a substantial amount of bulk wave radiation as well as the required surface wave. This dual effect is hardly surprising, since some of the first acoustic applications of such structures were specifically as sources of bulk wave radiation. The standard interdigital structure was used for this purpose by Mortley in 1965, and the interdigitally poled version described by Thomann in 1970 had already been described as a bulk wave transducer by van der Pauw in 1966. Furthermore, arrays of grooves were used as a means of converting bulk wave signals to surface waves, and vice versa , long before their use in reflective array compressors. Bulk wave radiation is therefore an expected rather than a surprising phenomenon in all these devices.

S.A.W. fused-silica reflective-array compressor

A surface-wave reflective-array compressor using edge-bonded transducers and a fused-silica substrate is described. A flat amplitude response over a 60 MHz bandwidth about a 100 MHz centre frequency has been obtained by weighting the groove depth. This is in good agreement with the theoretical calculated transfer function involving a δ function method. Insertion loss is 25 dB and a pulse-compression loop has shown a compression ratio of nearly equal to 1.800.

Sidelobe reduction in reflective s.a.w. pulse compressors without external filtering

The letter describes the incorporation of amplitude weighting into the response characteristics of a reflective-array compressor. The reflective-array grooves, formed by ion etching, have nonuniform depth, thereby achieving a nonuniform amplitude response. The resulting sidelobe reduction is then evaluated, and the advantages of this weighting method over external and other internal weighting methods are discussed.

Mode conversion in surface-acoustic-wave reflective arrays

We have investigated the possibility of the conversion of surface acoustic waves to other modes in a reflecting-array structure, such as that employed in the reflective-array compressor (r.a.c.). As the angle of incidence on the reflectors is reduced from normal incidence, we find that the mode-conversion possibilities become more restricted. However, for a 45° inclined array of reflectors on LiNbO3, coupling to a strongly piezoelectric pseudo-surface-wave is still allowed. Using metal reflectors, conversion to this mode is observed at frequencies only 19% above the s.a.w. reflection frequency. Comments are made on the relevance of the observations to the fabrication of dispersive filters for pulse-compression systems.

The Use of Surface-Elastic-Wave Reflection Gratings in Large Time-Bandwidth Pulse-Compression Filters

A new type of surface-wave device has been developed which uses the reflection of surface elastic waves to achieve a desired transfer function. A series of experiments on the reflection of surface waves at normal and oblique incidence from periodic arrays of grooves and overlayer stripes provided guidelines for the choice of the type of reflector, the reflection angle, and the depth of grooves. A prototype pulse-compression filter with a time-bandwidth product of 1500 (T=30 /spl mu/S, /spl Delta/f=50 MHz) has been developed. The grooves were etched into LiNbO/sub 3/ by a neutralized argon ion beam in a manner which provided precise depth control and a desired amplitude response. This reflective-array compressor (RAC) has proved to be relatively free of spurious signals and second-order effects and, as a result, large capacities have been obtained. In the prototype device, rms phase errors were 3.5 deg or less and, as a result, the compressed-pulse sidelobe structure was near ideal. A compression ratio of 1500 was demonstrated. The same device, when operated over a wider bandwidth, yielded a compression ratio of about 4000 with only a modest sacrifice in the level of the time sidelobes.

Large-time-bandwidth-product surface-wave pulse compressor employing reflective gratings

A novel dispersive delay line with a time-bandwidth product in excess of 1500 has been developed. The device uses the reflection of elastic surface waves from sputter-etched gratings on LiNbO3. This reflective-array compressor is relatively free of the spurious signals which limit the performance of inter-digital transducer arrays.