SAW Filters in GPS Applications
Surface Acoustic Wave (SAW) filters provide improved environment for Global Positioning System (GPS) applications.Max Hasegwa, AVX Corporation
Since the earliest days of civilization, man has sought to find a reliable way to determine accurate distances and exact position in relationship to other objects. The ancient Greeks used the sun and moon to measure both direction and distance. Others relied on the sextant for angular measurements of the stars' positions to determine latitude at sea. In the mid to late 1700s, John Harrison, a cabinetmaker by trade, invented the marine chronometer, which determined lunar tables within one minute of arc, establishing the most accurate and reliable way to predict ones' position at sea.
Although the chronometer served seamen well for many years, with the dawn of the 20th century came the development of a new class of navigation aids. These new aids included the use of radio transmitters as beacons, and eventually included the implementation of satellite technology by means of line of sight signals for navigation and position determination. Today, the most well known of these satellite systems is called the NAVSTAR Global Positioning System.
NAVSTAR Global Positioning System (GPS) consists of 24 orbiting satellites, land-based handheld or mobile receivers, and five controller ground stations located around the world. These three segments; space, user and controller, make up the complete Global Positioning System. The satellites orbit the earth once every 12 hours, sending signals to the ground-based receivers.
The time difference between when the signal is sent and when it is received, multiplied by the speed of light determines the distance to the satellite. Longitude, latitude and altitude are calculated using other satellites in the system. GPS can determine precise location within a few hundred feet.
Growth in GPS Applications
GPS technology was originally developed for military needs, although it has quickly found many commercial applications for everyday life. GPS applications include land surveying, navigation assistance for trucking fleets, delivery trucks and courier services, as well as police, fire and emergency-based service units. Automobile manufacturers are now putting GPS units in cars as standard equipment features. It is estimated that within a few years every newly manufactured car will have some basic GPS function. Telematics technology is an example of one such use of GPS capabilities. Telematics combines wireless voice and data communications with GPS location capabilities to deliver location-specific security and information. Although GPS integration into automotive applications will help fuel the growth of GPS technology over the next few years, another one of the largest areas of growth for GPS can be found in the mobile phone or handset markets.
The current mobile phone market is estimated to be about 450 million units in 2001 and is poised for substantial growth over the next few years as next generation 3G infrastructure and services come online.
Recent U.S. government E-911 emergency location legislation requires all phones, including mobile handsets, to be able to accurately locate vehicles and individuals in case of emergency. Coupled with the interest of global service providers to offer customers enhanced location-based services, the result is the rapid development of integrated GPS transceivers for wireless terminals and handsets. In consideration of these factors, it is not surprising that several electronic component manufacturers have started to produce board level devices that are specifically designed to operate in GPS applications.
Miniaturization of GPS Technology
The need to embed GPS technology in products such as cars and cell phones has driven the need for miniaturization. Manufacturers have constantly reduced the number of components needed to build a GPS receiver, until today, a basic GPS receiver can be fabricated using only two or three major integrated circuits. These GPS receiver units include a radio frequency and intermediate frequency "front end" section, which serves to translate the frequency signals arriving at the antenna to a lower frequency, called an intermediate frequency (IF), which is more easily managed by the rest of the receiver.
These "front end" sections require filters to extract the satellite signal from the interference present in the environment. Without filtering, it would be impossible to accurately detect the correct frequency from the other noise found in the signal. In the case of GPS applications, a Surface Acoustic Wave (SAW) filter is utilized to extract the passband frequencies from 1573.92 MHz to 1576.92 MHz.
Figure 1. Dual Band (CDMA)/GPS Phone Block Diagram
SAW filters use a fine-pitch comb pattern of electrode material deposited at 0.5 micron pitch and width on a piezo ceramic substrate, allowing for filtering of high frequencies above 1 GHz, such as those found in GPS wireless terminal applications. A typical SAW filter such as the SF30 Series offered by AVX Corp. for GPS applications will have the following characteristics:
A center passband frequency of 1.575 GHz
1.8 dB maximum insertion loss
40 dB rejection in the cellband
Operating temperature range from -30 ° C to +85 ° C
Small 3 mm 3 3 mm size for lightweight space saving designs.
This type of filter is designed to improve insertion loss on the front end, improve attenuation for interference rejection at the interstage and create wide-stopband attenuation against other AMPS, PCS or Bluetooth systems.
Figure 2. GPS Front End & Interstage 2 in 1
When considering that many handsets are becoming smaller and lighter, while at the same time integrating more and more functionality into the device, it is reasonable that board space and component count are becoming a very important consideration for designers. To meet this need, a first of its kind 2-in-1 SAW filter was recently introduced to the market by AVX Corporation. This unique device combines the low loss front-end filter and the high selectivity of the interstage filter in a single package.
Typical characteristics include:
A center passband frequency of 1.575 GHz
1.8 dB maximum insertion loss on the Front End and 3.0 maximum insertion loss on the Interstage
1.0 dB maximum for passband variation
2.0 dB maximum for passband VSWR
Small 3.8 mm × 3.8 mm package size for lightweight space saving designs.
Single front end and interstage SAW filters, as well as 2-in-1 packages such as those offered by AVX Corp., are the most commonly used types of filtering devices for GPS applications today. However, new technologies such as LTCC (Low Temperature Co-fired Ceramics) are emerging from companies such as Kyocera Corp. that will provide for the complete integration of the front end circuit with the SAW filters and other embedded discrete passive components into one single package. These multilayer ceramic technologies will continue to offer wireless handset manufacturers further advantages in size reduction and cost efficiencies in the very near future to meet the ever-increasing needs of the 21st century consumer.
Max Hasegawa is FAE manager for AVX Corporation located in Myrtle Beach, SC. Max can be reached at email@example.com.