HANDBOOK OF ACOUSTIC SIGNAL PROCESSING BAW Delay Lines
Introduction: Andersen Bulk Acoustic Wave (BAW) delay lines offer a very simple yet reliable means of time delaying a video or RF signal with more precise storage times over wider bandwidths than virtually any other techno l- ogy. The numerous advantages of BAW delay lines include : Wide Range of Time Delay: These devices offer delays ranging from 0.25 microseconds to over 5 milliseconds in a single device with accuracies to within 1 nanosecond. Broad Range of Frequencies and Bandwidths: BAW delay lines offer a range of center frequencies from 5 MHz to over 150 MHz with bandwidths of up to 90%. High Reliability/Durability: BAW delay lines are passive devices requiring only a few fixed components for tuning. They are inherently radiation resistant, capable of operating in hostile environments, and able to perform over a wide temperature range; all without ever requiring recalibration. No Power Required: Because of their passive design, BAW delay lines do not require system power (except where heated packages are used for extreme temper- ature stability over a wide temperature range). Flexibility: These devices can be easily tailored to meet the electrical and physical constraints of individual system requirements. As a result, Andersen BAW delay lines are used across a broad spectrum of military and commercial applications ranging from missile and radar systems to video enhancement equipment, TV cameras and test bench delay lines. Description The operation of a BAW delay line is based on two distinct physical properties of materials. The first property is that of the velocity of sound waves in a solid medium. The transmission of sound, or acoustic, waves in a solid medium is in fact the transmission of mechanical displacements, or stresses and strains. Acoustic waves therefore have a velocity significantly slower than that of electromagnetic waves by a factor of 10-s. Because of this property, it is possible to achieve extremely long signal delays in a very small volume. The second property is the piezoelectric effect. When
certain asymmetric crystals or polycrystalline materials are subjected to an electric field, mechanical stress or strain is produced in the material; conversely, when subjected to mechanical stress or strain, electrical charges will collect on specific surfaces of the piezoelectric material. Piezoelectric materials are therefore used to both produce and detect mechanical, or acoustic, vibration in a given medium. A basic BAW delay line is comprised of two piezoelectric transducers bonded to a low velocity medium such as quartz or glass. The time delay is determined by both the acoustic velocity and the length of the path to be traversed by the acoustic wave. The medium, therefore, can have any number of geometrical configurations employing internal reflections from surface interfaces to increase the path length and minimize the volume. The simplest geometry is that of a rectangular bar (Figure 1A) with piezoelectric transducers bonded to each end. An electrical signal is applied to the input transducer and converted to an acoustic signal. This acoustic signal is then transmitted through the solid medium to the output transducer where it is detected and converted back to an electrical signal. Figure 1B depicts a typical double bounce beam pat- tern and placement of acoustic damping material to attenuate unwanted spurious signals. In general, delays over 50 microseconds are accomplished through means of multiple internal reflections as shown in Figure 1C. Types of BAW Delay Lines As shown in Figure 2, three types of Andersen BAW delay lines are available: Standard Delay Lines -This configuration is used for applications requiring a specific signal delay with broadband frequency characteristics and good suppression of unwanted time responses. Typical applications include TV broadcast and receiving equipment, airborne transponders, image processing, radar MTI and video integrator circuits. Echo Delay Lines -These devices yield multiple, precisely delayed outputs from a single input pulse. Typical applications include radar range calibration and amplifier linearity calibration. Digital Delay Lines -This configuration is used in digital circuits to store and delay bits of unmodulated video signals.
Typical Performance Range The practical "standard-device" performance range for each key parameter is shown in Table I. These performance ranges are presented here only as general selection guidelines and do not necessarily constitute design limitations. The type, configuration and performance of a BAW device are affected by many interrelated factors. For performance requirements beyond those shown in Table I, consult Andersen Laboratories. Delay Delay can be measured at fₒ or across the entire band. While the delay will vary across the band, these variations (otherwise known as Group Delay) can be kept to within a few nanoseconds of the average value across a wide band as shown in Fig. 3. Delay Media The propagation velocities of the delay media primarily used in Andersen designs -fused quartz, crystalline quartz and zero-temperature coefficient glass - are shown in Table II. The velocity at which an acoustic wave travels through a medium depends on the manner in which the wave is generated by the transducer (shear mode or compressional mode). A shear mode wave travels at a slower velocity than a compressional mode wave and therefore requires a shorter beam path for a given delay. In addition, compressional mode waves cannot always be reflected satisfactorily within the medium. Therefore, shear mode transducers are generally used for most SAW delay lines. Fused quartz is the delay medium most frequently used for standard BAW delay lines. Its acoustic loss constant is low, enabling large storages to be obtained with good insertion loss performance (see Fig. 5). By using multiple internal reflections, delays of up to 5,000 microseconds can be achieved with a single device. This is a practical upper limit for individual devices since it is difficult and costly to obtain satisfactory fused quartz large enough to accommodate internally reflected signal paths longer than 745 inches. For delays in excess of 5,000 microseconds, individual devices can be cascaded. Zero-temperature coefficient glass is another widely used delay medium. A shear mode wave travels at a slower velocity in glass than in fused quartz, which
allows for shorter beam paths in glass. The other advantages of this medium are better bandwidth characteristics at low frequencies and extreme temperature stability (see Fig. 4). Glass. however. has a higher insertion loss than fused quartz (see Fig. 5) and is therefore used primarily for delays up to 200 µs. Z-A xis crystalline quartz is used primarily for Echo Delay Lines. Its neutral polarization and temperature coefficient of +80ppm/ C make it an optimum medium for delayed multiple echo applications. A pulse train is achieved by launching a compressional mode wave from one end of a crystalline quartz bar to the other end. The highly reflective end facets initiate internal reflections which support numerous back-and-forth cycles of the beam, creating the output echos. The low acoustic loss constant of crystalline quartz (see Fig. 5) supports minimum echo-to-echo attenuation while maintaining a linear amplitude degradation. Fused Quartz Crystalline Quartz Zero Temperature Coefficient Glass TABLE II Propagation Velocities of Various Delay Media (at +25 C) Shear Mode Wave Approx. 6.7 µ.sec/in. Not Applicable Approx. 9.8 µ.sec/in. shear mode waves cannot be propagated satisfactorily in crystalllne quartz. Compressional Mode Wave Approx. 4.3 µ.sec/in. Approx. 4.3 µ.sec/in. Approx. 5.8 µ.sec/in.
Temperature and Delay Stability Figure 4 shows how delay time changes in glass with changes in ambient temperature. Glass has a TCDT (Temperature Coefficient of Delay Time) of +0.05 ppm/ C 2. This equates to a parabolic function with the zero temperature coefficient intersect at approximately +55 C. Fused quartz has a TCDT of approximately -75 ppm/ C. This means that for a delay of 1000 µs there is a variance of.075 µs per C. When extreme loss and delay stability is required, the temperature of the fused quartz delay medium is controlled by incorporating the medium in a single or double oven. This packaging technique can maintain the internal temperature to within ± 0.01 C over an ambient range of greater than 100 c. Center Frequency The fundamental center frequency of a BAW delay line is governed by the thickness of the transducers as follows: A center frequency of 150 MHz represents a practical upper limit for most devices because of the difficulty in fabricating transducers of sufficient thinness for higher frequency operation. However, devices can be tuned and optimized for a third harmonic output with the same bandwidth as the fundamentaloutput.
Bandwidth A common generalization is that the overall bandwidth referenced to 3dB points will approximate 50% of the center frequency. In certain cases, 90% bandwidths have been achieved. Bandwidth is affected by transducer performance and the effects of the delay medium on the ultrasonic wave. For short delays, the medium effects are negligible due to the short beam path. While it is possible to achieve long delays with wide bandwidths, loss can become an issue because of "beam spreading" (diffraction) for long beam paths. Insertion Loss Figure 5 shows the medium loss for fused quartz, glass and crystalline quartz as a function of db/µs as related to frequency. The graphed results represent the friction loss of the medium only and are independent of all other effects. It should be noted that the loss constant significantly decreases with heat. Medium loss is a major component of total insertion loss. Losses due to beam spreading (diffraction losses) are only a concern in lines that combine long delays and low frequencies. Transducer loss is generally not a factor either, since transducer systems with essentially zero loss can be provided using computer-aided design techniques. While there is no concise formula that fully defines the relationship between medium loss, center frequency and delay, the following empirical formulas closely approximate this relationship. Hence, a fused quartz 300 µs delay line with a center frequency of 30 MHz would have a medium loss of 3.4 db (excluding any diffraction or transducer loss). The same 300 µs delay line at 150 MHz would have a medium loss of 52.11 db. Generally speaking, increases in loss occur with increases in delay time and/or frequency.
Transducer Match A critical factor in the performance of a BAW device is the design of the transducer assembly. The better the match between transducers, medium, and external electrical circuitry, the better the performance characteristics of the delay line, particularly for insertion loss and triple travel. Transducer materials used include crystalline quartz, titanates and niobates. Andersen has developed proprietary methods to insure good impedance match between the transducer, delay medium and external electrical circuitry to achieve maximum signal transfer with minimum distortion and unwanted time responses. Unwanted Time Responses Unwanted time responses which appear at the delay line output may originate from any of a variety of causes. Maximum permissible levels for each type of response are referenced to the output signal and generally specified by the customer. The four most common types of unwanted responses are discussed below. Feedthrough (FT)...FT occurs when the input electrical signal is capacitively transferred from the input transducer directly to the output transducer. FT is minimized by shielding and grounding techniques and is readily suppressed to greater than 50 db. Triple Travel (TT)..TI is caused by a slight impedance mismatch between the transducer, delay medium and external electrical circuitry. A small portion of the ultrasonic wave striking the output transducer is reflected. This reflection reverses its path, strikes the input transducer, and is reflected back to the output transducer once more. The result is a signal at three times the delay. Andersen has developed improved impedance matching techniques and acoustic damping methods to reduce TI. For short delay lines, TI is suppressed to greater than 40 db, and greater than 50 db for longer lines. Figure 6 shows the effect of TI on amplitude, phase and delay ripple.
Random Signals (RS) When a transducer launches an ultrasonic wave, side lobes are generated which may follow different reflection paths through the delay medium. These "random" signals will arrive at the output transducer at delays substantially different from the specified delay. Through acoustic damping methods, RS is suppressed to greater than 50 db. Cross-talk (CT) CT is electromagnetic in nature and occurs in multi-line packages where an unwanted signal from one line is displayed at the output of another line. Through careful shielding, CT is suppressed to greater than 60 db in standard devices. Power Handling Power handling is limited by the amount of heat the transducer is able to dissipate. A typical value is + 21 dbm, but depending upon design, this may be increased to over + 30 dbm. Phase Linearity Utilizing computerized test equipment, the phase response of a given delay line is referenced to zero and displayed linearly across the frequency band of interest. Any unwanted time responses will create slight variations in the phase shape. Phase linearities of 3 peak-to-peak are typical and phase linearities of 0.75 peak-to-peak have been achieved.
Representative Applications for BAW Delay Lines Illustrated on the following pages are just a few of the typical applications for which Andersen has provided SAW delay lines. While some lines are high volume catalog-type products, most were designed and built to optimize a specific systems' performance.
Design Flexibility Andersen Laboratories has the design and production capabilities to provide not only individual SAW delay lines but complete IF signal processing subsystems as well (e.g. pulse stretchers, moving target indicators, unity gain modules, etc.). Incorporating Bulk Acoustic Wave delay lines with other electronic functions, these subsystems optimize the performance potential of this technology for specific system requirements. Whether you require components or subsystems, Andersen SAW technology lends itself to a wide range of packaging techniques to meet special physical or electrical system constraints. Packaging for individual delay lines ranges from PCboard pin modules to units with RF connectors (and DC power connectors where ovens are involved). Subsystem packaging can range from simple boxes with RF and DC connectors to rack mounted drawers. Hopefully, this volume has given you a fundamental overview of BAW delay lines, but in no way does it reflect the full potential of this technology, a technology uniquely capable of fulfilling many of today s and tomorrow s requirements with a simplicity unmatched by other signal processing methods.