SAW Filter Comparison: IIDT, DMS, and Ladder Types

Introduction

SAW (Surface Acoustic Wave) filters are essential components in RF systems for signal filtering. Different types, such as IIDT (Inter-digitated Inter-Digital Transducer), DMS (Double Mode SAW), and Ladder filters, offer unique advantages. This guide compares these types to help you choose the right one for your specific application.

Initially, SAW filters were used instead of LC filters in the IF (Intermediate Frequency) stage of TVs. Over time, their applications expanded to radar, CATV (Community Antenna Television), mobile phones, pagers, satellite tuners, and more. There are three primary types of SAW filters:

• IIDT Filter
• DMS Filter
• Ladder Filter

IIDT SAW Filter

• Figure 1 depicts the structure of an IIDT SAW filter.
• It typically has an insertion loss of about 3 to 4 dB.
• It requires an external matching circuit.

To address the disadvantages of IIDT filters, DMS and Ladder type SAW filters were developed. These filters offer lower loss characteristics, similar to BAW (Bulk Acoustic Wave) filters but at lower frequency bands.

DMS SAW Filter

• Figure 2 illustrates the structure of a Double Mode SAW (DMS) filter.
• It’s constructed by arranging multiple IDTs between gratings.
• This SAW filter type exhibits wideband characteristics due to the combination of various resonant modes.
• It’s suitable for balanced input or output configurations through proper arrangement of IDTs, making it useful for connecting balanced input amplifiers.
• This filter utilizes two identical resonant modes acoustically coupled in the longitudinal direction.
• It offers low loss, lower power durability, and good out-of-band rejection.

Ladder SAW Filter

• Figure 3 depicts the structure of a Ladder SAW filter.
• It’s constructed by connecting multiple 1-port SAW resonators in a ladder-like configuration.
• It is used for unbalanced input and output.
• It utilizes two types of 1-port SAW resonators: one in the series arm and the other in the parallel arm. These resonators have different resonant frequencies and are electrically coupled.
• It offers lower loss, high power durability, and moderate out-of-band rejection.

Difference between IIDT, DMS, and Ladder Types

The following table summarizes a comparison between SAW filter types based on research conducted at Fujitsu Labs in Japan:

From the table, we can summarize the following differences between SAW filter types:

• The insertion loss of DMS and Ladder types are lower compared to IIDT.
• Attenuation levels outside the band are good across all three types.
• The bandwidth of the Ladder type is wider and generally better compared to DMS.
• High power durability is better for Ladder and IIDT types compared to DMS.
• A balanced design is not possible with Ladder type SAW filters. Lattice type filter designs are used for those applications.

Conclusion

IIDT, DMS, and Ladder filters each serve different purposes in RF systems. Selecting the right type depends on specific design requirements such as frequency, bandwidth, and insertion loss.