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Abstract : Introduce the application range of GPS and the working principle of its receiver and the control method of using a temperature compensated crystal oscillator DS4000 to become a precision oscillator
1 Introduction
First, an introduction to the intervening GPS (Global Positioning System) application field and its receiver
1.1 When GPS) has been widely used
The satellite (up to 27) network operates in non-synchronized, low-Earth orbit, covering the whole world and ensuring the operation of the positioning system. The GPS receiver needs to lock at least 4 stars to provide positioning information. These long series of codes (or digital combinations) broadcast or transmitted by these satellites are called pseudo-random codes. Through the known satellite pseudo-random code, speed of light and look-up table to maintain the satellite position and other parameters, the GPS receiver can calculate the satellite's transmission time, and then convert the transmission time to distance. Under the condition of multiple satellites (greater than 4), the position of the GPS receiver can be calculated by finding the trigonometric equation, and the user's position is also provided.
1.2 Application range of GPS receiver
It can be used for personal positioning and orientation of general-purpose handheld devices to navigation, aviation, exploration, and timing synchronization in telecommunications networks. Each application requires receivers with different characteristics. For example, in a universal handheld device application, the receiver will use 4 or more satellites to receive the signal and convert it to position information, which can be connected to a map database to indicate the land location. In nautical and aeronautical applications, the dynamic position data obtained from satellite reception signals are imported into the navigation system on board or on board for real-time positioning and orientation.
1.3 Another important feature and application of GPS
It provides a fairly accurate time reference, such as synchronization in telecommunication networks, calibration of test and measurement equipment, synchronization of aerospace observations and meteorological stations, earthquake monitoring, and fault recorders for public power grids. For synchronization and timing applications, the phase of the satellite signal is more important than the data carried by the signal.
In those applications that prioritize time synchronization, the phase difference of the transmitted signal is the most important. In telecommunication networks, the GPS synchronization engine provides end-to-end timing for such networks. When running a network for voice, video or time-critical data transmission, the most important thing is the quality of service requirement.
For synchronization and timing requirements, a precise frequency reference is crucial. The most precise definition of time and frequency is based on the cesium atom, and the precise frequency is generated by the cesium beam standard.
What makes the GPS satellite system enough to meet the network synchronization requirements? Each GPS satellite has a clock source based on cesium atoms. These very accurate clocks ensure that the time is accurate to within 3ns per year, and the precise time is then transmitted to the GPS receiver via microwave.
1.4 The role of temperature compensated crystal oscillator
To ensure accurate time, the GPS receiver also includes a local oscillator, such as Chengyuan, Temperature Controlled Crystal Oscillator (OCXO), or Temperature Compensated Crystal Oscillator (TCXO), as a strictly controlled clock source to maintain Long-term accuracy and stability. Because satellites cover the world, the use of GPS is an accurate, feasible, and economical way to ensure the synchronization of telecommunications networks. Most vendors provide equipment that supports timing synchronization of the system, such as telecommunications networks, base stations, or other time-critical applications.
2. GPS receiver analysis
A typical GPS receiver contains the functional blocks shown in Figure 1, which includes: a radio frequency (RF) part, a GPS signal processor, and a main processor. The RF part includes: GPS antenna, RF filter and GPS RF front end. The RF part receives the satellite signal, separates the pseudo-random code from the carrier frequency, and sends it to the GPS signal processor. In most existing receivers, the front-end part + GPS signal processor can process 4 to 12 satellites at the same time. signal. This parallel processing capability provides higher positioning accuracy and shortens the time to output data. The main processor provides data to the user, and the data can be provided to the user through a GUI (graphical user interface), display screen, or other operating systems. As for which way, it depends on the requirements of the actual application.
In the block diagram of FIG. 1, there are two oscillation sources, including a REF (reference) crystal (or oscillator) and an RTC (real-time clock) crystal. The REF crystal or oscillator can be quite accurate or inaccurate, depending on the receiver used. The frequency requirements of the vibrator depend on the special product standard (ASSP) adopted by the GPS front end. The typical range is between 13MHz and 30MHz, depending on the manufacturer. The REF oscillator can be a rubidium source, OCXO, or even TCXO. In this case, the main processor will correct any timing slip between the satellite and the receiver.
The RTC crystal provides real-time clock information for the capture process to capture different satellites in the 27 satellite constellation. Through a look-up table on satellite position information, RTC helps provide a starting point for locking all visible satellites.
3. Introduction and method of DS4000 temperature compensated crystal oscillator
The new DS4000 CNC TCXO can provide ± 1PPm accuracy, ± 6PPm traction and programmed frequency output for wireless applications, telecommunications, satellite communications and GPS is particularly ideal.
DS4000 CNC TCXO exceeds all performance standards related to control, cost, packaging and accuracy. The device can keep the frequency stable within ± 1ppm, and has a digital tuning capability and programming frequency output with a traction range of ± 6ppm. Dual channels can be provided. CMOS square wave output. The F1 output provides a fixed fundamental frequency, while the F2 output provides one of the 256 different integer frequency divisions of the fundamental frequency (see Figure 2). The device operates on a 5V ± 10% power supply and uses Ping thin 24-pin BGA package.
With the introduction of the DS4000 numerically controlled temperature compensated crystal oscillator (TCXO), a new control method for calibrating the REF oscillator came into being. DS4000 provides precision oscillators, whose unique performance is beyond the ability of many TCXOs currently on the market. First of all, the DS4000 is factory-calibrated with an accuracy of within ± 1ppm; in addition, it also provides a digital traction capability in the + 6ppm range with a typical resolution of less than 0.1PPm.
Figure 2 is a block diagram of the DS4000. The device is controlled by a 2-wire (SDA and SCL) serial data port, (SDA and SCL are both digital frequency adjustable communication ports). In this way, by using a microcontroller, the digital code is input to control the traction and Precision. A special system program can be designed to continuously adjust the accuracy of the device. In many outdated TCXO designs, this requirement can only be fulfilled by manual adjustment or by a voltage control. The DS4000's digital traction capability enables automatic calibration during production and recalibration at the application site.
The device also has a frequency output F2 (programmable temperature compensation square wave output), F2 is the fractional value of the fundamental frequency F1 (fixed frequency temperature compensation square wave output). F2 can be programmed to 1/256 to 255/256 of the fundamental frequency F1. Of course, programming is also done through a 2-wire interface (SDA, SCL). The fundamental frequency range of DS4000 is selectable from 10MHz to 20MHz. The F2 output follows the frequency accuracy of F1, and the accuracy can also be within ± 1ppm. Figure 2 DS4000 block diagram shows the digital interface for frequency pulling, frequency output and temperature detection.
Because the DS4000 uses Dallas Semicconductor's dedicated digital temperature measurement technology, the device can also be used as a temperature sensor. The accuracy of the temperature sensor is within ± 2 ℃. The device maintains its frequency accuracy (± 1ppm) over the entire industrial temperature range of -40 ° C to + 85 ° C.
4. Application of DS4000 in GPS
Due to its high flexibility, the DS4000 can be used to provide strictly controlled oscillators for REF crystals (XTAL) in GPS applications, and in some cases, 32KHz for GPS signal processors or main processors. RTC XTAL input. Figure 3 illustrates that the DS4000 performs the roles of two crystals, REF and RTC XTAL, in the GPS receiver block diagram.
Several assumptions should be made about the fundamental frequency used in Figure 3: First, it is assumed that the RTC XTAL input to the GPS front end can use a frequency of 16.384MHz. If this assumption is true, the DS4000 can generate a GPS signal processor on the F2 output The 32.768KHz frequency required by the RTC. Through the line interface SCL and SDA, the main processor can set the frequency traction amount, F2 frequency output (32.768KHz) and measure the temperature as needed.
5. Conclusion
GPS applications represent one of the many applications that require highly accurate time sources. The flexibility of the DS4000 is especially suitable for applications that pay special attention to timing accuracy and device control methods. Why is this? In general, TCXO or OCXO that allow frequency adjustment require manual adjustment or provide an external voltage adjustment, but the disadvantage of this solution is: manual adjustment is required when manually adjusting TCXO or OCXO; The designer must ensure that the control voltage is stable so that it does not affect the output characteristics of the oscillator. The adoption of DS4000 avoids the unreliability in the above two adjustments. This is because when changing or pulling the fundamental frequency, only the digital quantity needs to be transmitted to Ds4000.
In addition, because the DS4000 provides two frequency outputs (F1 and F2), one of which is programmable, if the required second frequency F2 is related to F1 after being divided by an integer, the user can save the second oscillation RTC XTAL. Also, with its digital control interface, the DS4000 can bring benefits to the device's automatic calibration process, so that during the life of the product, there is no need to periodically recalibrate the device.
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