TECHNICAL INFORMATION
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Published in "AEI September 2008"
Daishinku Navigates on Scaling Down GPS Receiver Modules

 The Global Positioning System (GPS) is a navigation system that can be accessed 24 hours a day through satellite. The development of GPS was started by the U.S. government in the 1970s as a navigation system for military use. In time, however, the GPS was transformed into a navigation system for common use of both military and civilians. The operation of GPS officially started in 1993.
 The GPS was first adopted for car navigation systems in the 1990s, which has become a standard feature in many automobiles today. In 2000, the U.S. government abolished the Selective Availability (SA) that intentionally degrades positioning accuracy by including error information in the GPS signals used by civilians. Since then, positioning accuracy has been improved up to about 10 times without special compensation technology. As a result, car navigation units have become more simplified and less expensive, and widespread use of this system has accelerated.
 On the other hand, a basic navigation system known as the Personal Navigation Device (PND) is also available nowadays. Because of its low price, expanded use of the PND on a global scale has been forging ahead since 2005. In addition, the position data notification function, which can be used in the event of emergency situations, has become mandatory for mobile phones as typified by the Enhanced 911 (E911) service in the U.S. This regulation has served as an impetus for installing the GPS function in mobile phones.
 An extensive range of applications has been developed in this field, and at present, there are mobile phones available with GPS navigation function support equal to that of a PND. The percentage of high-end mobile phones equipped with the GPS function has significantly increased worldwide and the market is expected to grow in the future. A high-accuracy temperature compensated crystal oscillator (TCXO) is used for the GPS receiver in order to provide a circuit for receiving very weak radio signals from the GPS satellite in a stable manner.
 Daishinku Corp. supplies high-precision TCXOs for the GPS market. In response to this expanding market, the company has continued its efforts toward improving the performance of its TCXOs. At this time, Daishinku has completed the development of a TCXO specialized in the GPS-related equipment market and has started the production of the device. This article describes the technology trends of the GPS receiver, which is the core of GPS-related equipment as well as information on the features of the TCXO products newly developed by Daishinku for GPS. 

Technology Trends of GPS Receiver Modules

 The GPS receiver has to process extremely weak radio signals from a satellite. Therefore, in many cases, the receiver is designed in a module while also considering noise suppression.
 Trends of GPS receiver modules are described below.

Downsizing

 GPS receiver modules have to be reduced in size to adjust downsizing of the equipment they are being used for so that these modules can fit in the equipment. Size reduction of the modules can be achieved by the following methods: a) miniaturization of ICs that are used to configure the receiver, for example radio frequency (RF) IC and broadband IC are integrated into a single chip instead of a two-chip configuration: b) reducing the number of components to be used such as the adoption of direct conversion of frequencies instead of down conversion in order to reduce the intermediate frequency (IF) filters: and c)miniaturization of the components used, for example, the size of TCXO is reduced from the 3225 size to 2520 size. Nowadays, it has been possible to reduce the size of the modules to 10sq. mm or less. 

Low voltage, low current consumption

 Low-voltage modules have been developed for low current consumption of the receiver modules and for simplification of power supply circuits. Because the GPS does not have a transmission circuit, the development of low-voltage ICs during the early stages has been promoted.
 The voltage required for the receiver modules have been lowered in the following steps: +5V/3.3V (dual power supply) to +3.3V to +3.3V/+1.8V (dual power supply) to +1.8V. Today, all devices configuring a receiver module can operate at +1.8V, and the module can be operated on a single +1.8V power supply.

High-sensitivity

 Indoor (area of about 5m from a window) use of GPS receiver modules must be taken into consideration when they are installed on mobile phones. Therefore, modules with high sensitivity are required.
 Some of the GPS receiver modules are capable of tracking satellites with extremely weak signals of approximately –160dBm.

High precision

Improving the positioning accuracy is an important issue. It has become essential for GPS receiver modules to determine a position with even higher accuracy without special compensation technology. 

High-speed TTFF

 It is imperative to reduce time to first fix (TTFF) or the time required for a GPS receiver module to reach the state where it can determine the positions after turning the power on. A technology for achieving a shorter TTFF during hot start or when a valid navigation message is received is being required. 

Daishinku’s TCXOs for GPS

 In order to keep up with the trends of GPS receivers and to contribute to improving their performance further, Daishinku has put in a lot of effort to improve the performance of TCXOs used as reference oscillators. Consequently, the company has developed high-precision TCXOs that are optimized for the GPS receiver.
 The features and performance of Daishinku’s newly developed TCXOs called DSB221SDA with outside dimensions of 2.5 × 2.0 × 0.8mm and DSB321SDA with outside dimensions of 3.2 × 2.5 × 0.9mm are introduced below.

Miniaturization

 Daishinku has reduced the size of its TCXO in response to the downsizing trends of the GPS receiver module. In addition to its existing 3225-sized flagship products, the company has developed an even smaller product, the 2520-size (See Fig.1).

Low-phase noise design

 Daishinku has improved the phase noise characteristic of its TCXO, which is used as the reference signal for frequency and time (See Fig. 2). By improving the phase noise characteristic, the company has made it possible to respond to GPS receivers with higher sensitivity and higher precision.

Low-voltage design

 Daishinku has developed TCXOs that can operate at low voltage so that low voltage ICs can be used to configure the GPS receiver. Daishinku’s TCXOs can operate at +1.8V or at any voltage between +1.7V and +3.6V (See Fig.3).

High-precision design

 Daishinku has improved the frequency temperature compensation accuracy of its TCXO. The characteristic where frequency changes against temperature or the frequency slope characteristic has been improved by means of a higher-accuracy frequency temperature characteristic. The frequency drift against changes in the ambient temperature of the TCXO during positioning has been also improved (See Fig.4). By reducing the TCXO’s frequency deviation, a narrow carrier search range can be set for receiving a radio signal transmitted from a satellite after the TCXO is powered. As a result, the TTFF performance of a GPS receiver can be improved.
 The reception sensitivity and positioning accuracy of a GPS receiver has been improved by providing a superior drift performance of frequency against temperature changes.

Standby function

 Daishinku has provided its TCXO with a built-in function that puts the TCXO in standby state by shutting down its supply voltage using a standby control signal. Previously, when the GPS receiver module was powered down using the power down control signal, the power supply to TCXO was controlled through the low dropout regulator. As Daishinku’s TCXO is equipped with a standby function, it does not need the low dropout regulator or other power control devices. Therefore the circuits can be downsized further and the number of components required can be reduced. About four components, including the low dropout regulator and other peripheral components, can be eliminated (See Fig. 5).

 

Electrical performance

The typical electrical performance of the newly developed DSB221SDA and DSB 321SDA is presented in Table 1.

Outlook on GPS Market

 The GPS-related equipment market is expected to expand significantly in the future. Further downsizing, higher performance, and lower prices are demanded for the functional modules of GPS receivers.

 Daishinku intends to continue with its development activities in order to introduce products that meet the needs of customers timely by keeping pace with the demands of the GPS market in the future.