Application of high-precision automatic clock calibration in medium wave broadcasting stations

introduction

Time plays a role in our lives all the time. The accuracy of time is a major issue concerning the national economy and people's livelihood, and medium wave relay stations have special requirements for the accuracy of time. As medium wave relay stations move towards digitalization and automation, the accuracy of time has an increasingly greater impact on the automatic control system of medium wave relay stations. With the advancement of science and technology, more and more new technologies are applied to medium wave relay stations. The high-precision automatic clock using GPS technology receives and demodulates the reference time signal of the global positioning system (GPS), calibrates and synchronizes the time of the medium wave relay station, and is applied to the automatic control system.

High-precision automatic clock calibration GPS system

High-precision automatic clock calibration is mainly composed of two parts: GPS system and satellite clock system. In high-precision automatic clock calibration, the GPS system plays a leading role, which is the key to synchronous calibration. The full name of the navigation global positioning system is "Global Positioning System for Timing and Ranging Navigation", abbreviated as "Global Positioning System (GPS)".

The GPS system consists of three parts: the space part, the ground monitoring part and the user part. The GPS space part is mainly composed of a constellation of 24 satellites. The satellites are distributed on six orbital planes with ascending nodes 60° apart and an orbital inclination of 55°. There are four satellites distributed on each orbit, and the satellites on two adjacent orbits are 40° apart. At least four satellites can be seen at any place on the earth at the same time. The satellite's transmission signal can cover 38% of the ground area. At any position in the orbit, its distance to the ground and the beam coverage area are basically unchanged. At the same time, within the beam coverage area, the satellite signal strength received by the user is approximately equal, which is very beneficial to improving the accuracy of synchronous calibration.

The ground monitoring part includes 1 master control station, 3 injection stations and 5 monitoring stations. The main task of the monitoring station is to observe each satellite, accurately determine the position of the satellite in space, and provide observation data to the master control station. Each monitoring station is also equipped with a GPS receiver to continuously observe each satellite, perform pseudo-range measurement and integrated Doppler observation every 6 seconds, and collect relevant data. The monitoring station is controlled by the master control station and sends the observation data to the master control station at a regular interval. The master control station has a large computer and is the main equipment for data collection, calculation, transmission, diagnosis, editing and other functions. The master control station transmits the edited satellite signal to the three injection stations located in the three oceans, and the injection stations regularly inject the signal into each satellite through the S-band microwave link, and then send the GPS satellite signal to the user.

The user part mainly includes a GPS receiver, which is a satellite signal receiver used to receive signals sent by GPS satellites and thereby obtain a reference time signal.

As a key part, the GPS system can continuously provide users with accurate reference time signals, and the amount of information provided is large. At the same time, relevant data can be collected through pseudo-range measurement and integrated Doppler observation. The satellite clock of the GPS system is compared with the master clock on the ground to keep the satellite clock and the master clock precisely synchronized. The GPS receiver processes the signals transmitted by the GPS satellite and, after a set of rigorous error correction, makes the output reference time signal reach a high accuracy. The GPS receiver is an important part of the satellite clock system.

High-precision automatic clock calibration satellite clock system

The satellite clock system is an important part of the automatic clock calibration, and the time calibration function of the automatic clock calibration is mainly completed by it. The satellite clock system is mainly composed of five parts: GPS receiver, steady-state clock, encoder driver, synchronous time calibration circuit, and software program. The satellite clock system uses a special GPS receiver to receive satellite signals and demodulate the AC code to obtain high-precision time code and second pulse as the reference time signal for high-precision automatic clock calibration. The functional block diagram of the satellite clock system is shown in Figure 1.

The steady-state clock uses a high-stability crystal oscillator inside. Even if the calibration source reference time signal is interrupted, it can still maintain stable operation and output the calibration time signal. The high-stability crystal oscillator outputs a sine signal, which is shaped, amplified, and sent to the frequency division network to generate a standard second pulse signal. After the microcontroller converts the second pulse, the time code is serially output following the second pulse to form a complete serial compressed BCD code. The external input reference time code is read into the microcontroller, and after it is judged to be correct, the reference time code is read in to calibrate the steady-state clock. The microcontroller controls the electronic switch to enable, and then the output time code is input into the encoding driver to synchronize the phase of the local clock with the calibration source.

The encoder driver is mainly used to convert one time code input into multiple time code outputs, and is used for multiple time code distribution of high-precision automatic clock calibration time code. Its characteristics are: if the input is high impedance, the waveform is output after shaping; if the input is low impedance, each channel is isolated.

The core of the synchronous timing circuit is a high-performance computer, which can perform complex signal processing and data calculation in conjunction with high-performance programmable gate array logic ICs. The synchronous timing circuit is divided into main modules such as timing interface, signal processing, single-chip control circuit, signal conversion, decoding and display. The timing module outputs a unified serial reference time code, which is sent to the decoding and display unit after shaping, and then sent to the computer through the RS-232 and RS-422 output ports for time calibration.

The calibration of computer time can be realized through computer software programs, and the software programs are mainly composed of Clock.exe and NetTime.exe software. Clock.exe is mainly used to obtain GPS reference time from RS-232 output port to calibrate the host (local time calibration), and NetTime.exe is mainly used to obtain GPS reference time from RS-422 output port to calibrate other computers in the local area network (network time calibration).

When using the Clock.exe software to calibrate the time, you need to remove the "read-only" in the properties of the LocalParam.ini file in the folder Clock1.3. And set the "time calibration serial port" through the "system settings" to achieve the calibration and synchronization of GPS reference time, software time, and computer time. When calibrating other computers in the LAN, you need to select the Properties item of the NetTime.exe software to set the interface. After selecting the Settings option, enter the Net Time Options interface and enter the host IP of the Clock.exe software in the Time Server (IP or Host name) option to calibrate the time of the computers in the LAN. The network time calibration interface is shown in Figure 2.

Conclusion

This paper mainly studies the high-precision automatic calibration clock and its main system. Its overall structure is relatively simple, and it is relatively stable, reliable and accurate during operation. Especially in the automatic control system of medium wave relay stations, the high-precision automatic calibration clock can make the reference time signal more accurate and stable, and truly calibrate the automatic control system, which plays an important role in the daily maintenance of medium wave relay stations.