SAW Technology

A Surface Acoustic Wave (SAW) propagating at the surface of a piezoelectric crystal can be used as a carrier of information. Since the acoustic energy is concentrated on the surface of the crystal, information is accessible for signal processing. The technique is extremely general: any linear bandpass filter may be synthesized, with arbitrary amplitude and phase, limited only by photolithographic line width and crystal size. The devices are small, rugged, stable, and capable of high volume low cost production.

SAW Filters are of two types: Transversal Resonator
Conceptually modeled as: tapped delay line LC resonator network
Impulse response: FIR (finite) IIR (infinite)
Transfer function: all zeros, no poles poles and zeros
Passband phase: linear minimum (non-linear)

SAW Transversal Filters

A basic SAW transversal filter is composed of two electromechanical transducers, which transmit and receive acoustic waves. Each transducer is composed of a planar set of periodic interdigital electrodes connected to two bus bars. The bus bars are connected to an electric generator or load. A single interdigital electrode acts as an elementary acoustic source or detector with amplitude given by electrode length and phase given by electrode position. A filter may have other electrodes in addition to the transducers: a Multi-Strip Coupler which gives added design flexibility and performance, or grating reflectors for added impulse response length or triple transit suppression or lower loss.

SAW Resonator Filters

A basic SAW resonator consists of an interdigital transducer between two grating reflectors. The reflectors form a resonant cavity, and the transducer couples the cavity to the external circuit. The equivalent circuit of a SAW resonator has the same form as a crystal resonator, and SAW resonator filters can be designed using crystal filter techniques. A simple SAW resonator may have Q>10000 and be used for oscillator stabilization. SAW resonators may also be multi-mode with multiple transducers and multiple reflectors, which allows a rich filter design palette. SAW resonator filters are generally much narrower band than transversal filters, and have much lower insertion loss, but have non-linear phase and are more limited in shape factor.

SAW Dispersive Filters

To realize a dispersive FM response, a transversal filter can use long non-periodic interdigital electrodes, and long non-periodic resonant grating reflectors can be added to further extend the FM response length.

Transducer Design

Adjustment of electrode length and position allows the SAW designer to synthesize any finite impulse response. For filters specified in the frequency domain, FIR digital filter design techniques are used to find the initial optimum, i.e. shortest, time response. Filters may also be specified directly in the time domain. The time response sampling frequency is often chosen equal to 4 times the center frequency. The electrode width is generally half the period, thus generally 1/8 of an acoustic wavelength. This structure has a low reflection coefficient in the passband, which reduces spurious.

The optimum electrode lengths are determined by an iterative process, which begins with the initial time response: 

  • find electrode lengths and positions from sampled time response
  • analyze
  • test for spec compliance, if not:
  • add estimated error correction and time truncate
  • reiterate

A number of secondary effects must be modeled in the analysis, including:

  • reflections inside the transducers
  • mass, topography, and conductivity of the metal electrodes
  • propagation diffraction, attenuation, and dispersion
  • source and load impedance
  • parasitic impedances
  • bus bar electromagnetic transmission line effects
  • reflections between transducers

Substrate and package secondary effects that must also be considered:

  • spurious surface and bulk waves, reflections from substrate edges and bottom
  • electromagnetic leakage in the substrate, package, and external connections

General Performance

Typical Limit Typical Limit
Filter Type Transversal Resonator
Center frequency (Fo, MHz) 20-1500 10-2500 100-1500 50-2500
Relative bandwidth (%):
                   for quartz: .5-5 .1-20 .05-.1
     for lithium tantalate: 5-10 1-40 .5-1
       for lithium niobate: 10-30 5-150 5-10
Min. insertion loss (dB) 20 10 6 3
Min. return loss (dB) 3 15 10 15
Max. response length (us) 10 100

 

Insertion loss and relative bandwidth are closely coupled. As the relative bandwidth increases, the insertion loss increases at 12 dB per octave. The insertion loss is typically 20 to 40 dB without tuning. Tuning the filter will reduce the insertion loss and increase the return loss, but also increase triple-transit spurious and passband ripple.

Triple Transit

Triple transit in transversal filters is caused by unwanted acoustic wave reflections from the transducers. The reflection can be visualized as an electrical reflection from the source or load resistance. The triple transit is down from the main response by approximately twice the insertion loss, and trails the main response by exactly twice the insertion delay. A -40 dB triple transit will cause .17 dB p-p and 1.15 deg p-p fast passband ripple. A SPUDT (Single Phase Uni-Directional Transducer) purposely introduces distributed mechanical reflections within the transducer to cancel the electrical reflections, which allows lower insertion loss. A resonator filter has no triple transit.

Feedthru

Feedthru is the unwanted direct leakage from the input circuit to the output circuit. It is sometimes called zero time spurious because it is undelayed and leads the main response. It causes a fast passband ripple like triple transit and degrades stopband rejection. It can be caused by internal capacitive or inductive coupling, but is usually due to insufficient grounding. It is often a serious problem and is critically dependent on circuit board mounting practice.

Material

The choice of piezoelectric substrate material is most fundamental, involving a trade off between relative bandwidth, insertion loss, and temperature stability.

Quartz (SiO2) Lithium Tantalate (LiTaO3) Lithium Niobate (LiNbO3)
Cut and Propogation Direction 36YX X112Y YZ
SAW Velocity (m/s) 3158 3296 3488
dVelocity (ppm) vs X = (temperature-25C) -.03*X^2 -18*X -90*X
SAW Piezoelectric Coupling k^2 .00116 .0075 .043
SAW Propagation Attenuation dB/us*GHz^2 3.44 .94 .88

Impedance Matching

A SAW transducer can be approximately modeled as a parallel RC circuit with reactance near 50 ohms and resistance much greater than 50 ohms. Generally, though not always, SAW filters must be impedance matched. The transducer is sometimes shunt resistor loaded to provide broadbanding and triple transit suppression, then matched to 50 ohms using a simple LC or autotransformer circuit.

Power Handling

SAW filters are passive linear devices, but input power is ultimately limited by dielectric break-down and material strength. The input -1dB compression level IP-1 is approximately 15-20*log10(Fo/1GHz) dBm for  LiNbO3 & LiTaO3, and ~7dB higher for SiO2. The 3rd order input intercept IIP3 is ~10dB above IP-1.

Temperature

SAW filters operate without damage from -55C to 125C, but the response’s frequency axis scales because the SAW substrate velocity is temperature dependent. Bandpass filters’ designed pass band width must increase, and designed stop band width must decrease, by the total center frequency shift. Dispersive filters’ chirp slope must match the input signal with time-bandwidth-error <.1.  All filters’ propagation attenuation increases with absolute temperature squared, which can become significant for long filters. Some designs may become unrealizable without ovenization.

Package

The package may be TO, DIP, flatpack, LCC or custom connectorized. All packages are hermetic, and 100% fine and gross leak tested. Impedance matching components may be integrated in the package or externally mounted. Amplifiers may be integrated in the package for unity gain. A proportionally controlled oven may be integrated in the package for temperature stabilization.

Mounting

Mounting must consider feedthru and impedance matching. Use a dense array of ground vias to establish a solid ground plane and to prevent leakage in the circuit board dielectric. Use direct grounds from the case to the circuit board: direct solder, short soldered tabs, mechanical stud or flange, and no gap. External input and output impedance matching inductors should be separated as much as possible and positioned to minimize mutual inductance. Phonon generally supplies a test fixture with the first prototype device, which incorporates these measures.

Test

Phonon measures the 2-port frequency domain s-parameters of all products at low, room & hot temperatures. Phase data is presented with the least mean squared polynomial fit removed and the polynomial coefficients given in the plot heading. Time domain responses are calculated by FFT from the measured frequency domain data. A Final Test Report with Acceptance Test Data Sheet and supporting plots is shipped with each product.