Design of aviation navigation VOR signal comprehensive tester based on ADLINK PCI-9846

• Application Fields
   Aviation navigation equipment test application, signal generation and acquisition.
• Challenges
   The test of avionics equipment requires the use of limited resources to build a multifunctional automatic test system. The signals of airborne electronic equipment are numerous and complex, covering low-frequency and high-frequency signals, continuous and discrete signals, and also some non-electrical signals. Traditional test systems are built with discrete instruments, which is costly, has low measurement automation, and poor scalability. With the development of the civil aviation transportation industry, most airborne flight electronic equipment is highly digitalized and integrated, and it is impossible to test and inspect them manually, and traditional instruments are difficult to meet the needs. In contrast, the signal processing mechanism based on software radio has outstanding advantages and is more adaptable to needs due to its flexibility and openness. At the same time, this will also facilitate related teaching and research.
• Solution
   Take the aviation navigation VOR signal as an example. Study the VOR navigation principle and analyze and model its synthetic signal. The time domain data of the waveform can be easily expressed and calculated using a public expression. The obtained data is input into the DAC to generate an analog waveform, which is then up-converted to the required frequency. For the existing analog signals, the PCI-9846 high-speed digitizer is used for acquisition. The acquired data can be displayed in real time or stored as files for later use and analysis. It has been proven that the application of software radio in automatic test systems can greatly save costs, simplify the system, and improve efficiency. The performance indicators of the digitizer can meet the needs.

Design of aviation navigation VOR signal comprehensive tester based on ADLINK PCI-9846

Abstract: Taking aviation navigation VOR (Very High Frequency Omnidirectional Range) signals as an example, ADLINK PCI-9846 high-speed digitizer is used to collect and analyze the time and frequency domains. After demodulation, the azimuth information can be restored to quickly verify the accuracy of the signal. It has been verified that the application of software radio in automatic test systems can greatly save costs, simplify the system, and improve efficiency. The relevant performance indicators of the digitizer can meet the needs.

Introduction: Testing of avionics equipment requires the use of limited resources to build a multifunctional automatic test system. The signals of airborne electronic equipment are numerous and complex, covering low-frequency and high-frequency signals, continuous and discrete signals, and also some non-electrical signals. Traditional test systems are built with discrete instruments. This method is costly, has a low degree of measurement automation, and poor scalability. With the development of the civil aviation transportation industry, most airborne flight electronic equipment has become highly digitalized and integrated, and it is no longer possible to manually test and inspect them. Therefore, developed countries around the world currently use automatic test equipment to complete this type of work. [1][2]

   The basic idea of ​​software radio is to use a universal, standard, modular hardware platform as a basis, and to implement various functions of the radio station through software programming, thus freeing the radio station design method based on hardware and application [3]. The automatic test system has put forward higher requirements on the flexibility and comprehensiveness of the signal source, and traditional signal generators are difficult to meet the requirements [4]. In comparison, the signal generator based on software radio has outstanding advantages and is more adaptable to the needs due to its flexibility and openness. At the same time, this will also provide convenience for related teaching and research.

   ADLINK PCI-9846H is used in the test and calibration of the signal source of the automatic test system. Taking the aviation navigation VOR signal as an example, the signal is collected and processed to restore the basic information. It proves that the performance indicators of the digitizer can meet the needs.

1. VOR signal

   The basic function of a VOR is to provide a complex radio signal to an airborne VOR receiver. After demodulation by the airborne VOR receiver, the magnetic bearing of the ground VOR station relative to the aircraft, i.e., the VOR bearing, is measured [5]. The spatially synthesized VOR signal received by the airborne receiver includes a reference phase signal and a variable phase signal. Directionality is achieved by comparing the phases of the two signals. The VOR operating frequency range is 108 MHz to 117.95 MHz, with a channel spacing of 0.05 MHz.

1.1 VOR reference phase signal

   The VOR reference phase signal (Reference Phase Signal) includes an RF carrier and a 9960Hz subcarrier. The frequency range of the RF carrier is from 108MHz to 117.95MHz. The 9960Hz subcarrier is modulated by the 30Hz reference signal with a modulation factor of 16, expressed as:

   Where is the amplitude modulation coefficient of the subcarrier to the RF carrier, is the subcarrier frequency, is the modulation coefficient of the subcarrier, is the baseband signal frequency, and is the RF frequency.

   The intensity of the radiation signal and the phase of the 30Hz signal contained in the reference phase signal remain unchanged in each radial direction from 0° to 360° in space, and the horizontal directivity diagram of the radiation field is a circle.

1.2 VOR variable phase signal

   The variable phase signal only contains a simple RF carrier with a frequency range of 108MHz~117.95MHz. Two pairs of orthogonal sideband antennas radiate sine modulated sideband waves and cosine modulated sideband waves respectively, and the field strength changes according to the 30Hz rule. In this way, an amplitude modulated wave with a 30Hz sinusoidal change is generated in space, and the expression is:

                （2）

Where is the current VOR radial azimuth.

1.3 Synthetic Signal

   The composite signal (Composite Signal) received in the space includes the superposition of the reference phase signal and the variable phase signal, as shown in Figure 1, and the expression is:

   （3）

The receiver obtains direction information by demodulating and comparing the phase difference between the two. [5,6]

       Figure 1 Spatial synthesis signal

2. System Implementation

   The system architecture based on GPP (General-Purpose Processor) is adopted, and the industrial computer is used directly for digital signal processing. For this kind of radio system, a real radio station cannot be observed physically, and it completely solves the radio communication problem from the software perspective. Since the general machine is not a real-time synchronous system, it is not suitable for real-time processing of strictly timed sampling signals, and can only maintain a certain degree of synchronization through interrupts. However, due to its advantages in openness, flexibility, programmability and human-computer interface, it is the closest to the ideal software radio and is more suitable for testing, teaching and research.

   The system uses a pipeline connection, which is consistent with the direction of the signal flow. It has high efficiency, short delay, and high processing rate, which can make up for the slow processing speed of GPP signals to a certain extent. However, since the modules are interconnected by actual circuits, the modules are tightly coupled and the degree of independence is not high. If the system function changes, it is necessary to add, remove or modify a module, which involves corresponding module changes and even changes in the overall structure. Because the design purpose is to test and calibrate the signal source of the test system, the signal is relatively fixed and does not need to be changed frequently, so a pipeline structure is selected. The structural block diagram is shown in Figure 2 [7]. Signal generation and processing are completed by the industrial computer, and the arbitrary waveform generator (AWG) module realizes waveform output (a digital up-conversion card is required for high frequencies). If wireless transmission and reception are required, additional antennas and RF amplifiers are required. Signal acquisition and digital-to-analog conversion are completed by ADLINK PCI-9846 high-speed digitizer. The conversion results can be displayed in real time or saved as waveform files for subsequent processing.

Figure 2 System Block Diagram

2.1 Signal Source

   The intermediate frequency signal is generated by PXI-5421. This is an arbitrary waveform generator with onboard signal processing (OSP) that has 16-bit resolution and -91 dBc closed spurious-free dynamic range (SFDR), providing instrument quality standards for applications requiring digital up-conversion and baseband interpolation. As a full-featured AWG, PXI-5421 can also generate general electronic test signals with a maximum output range of 12 Vpp, 50 Ω resistive load, and a maximum frequency of 43 MHz[8]. The up-conversion card uses NI PXI-5610, which has a 2.7 GHz up-converter with high real-time bandwidth and stable time base, and its accuracy can reach ±50 ppb. In RF generation applications, it is tightly integrated with a modular function generator to generate signals with a frequency range of 50 kHz to 2.7 GHz and an adjustable gain range of 130 dB[9]. The high-frequency VOR signal generated by PXI-5421 is sent to PXI-5610 for up-conversion processing to the required VHF band.

   The PXI boards are installed in the NI PXI-1402 control box. With the NI PXI-PCI833x kit, the PXI modules can be controlled from a computer using a fully transparent MXI-4 network connected via copper cables. MXI-4 bridges the PCI-PCI high-bandwidth connection to allow remote control of the PXI system through the computer's PCI interface.

2.2 Data Collection

   Data acquisition was completed using the ADLINK PCI-9846 high-speed digitizer. The ADLINK PCI-9846 is a 16-bit, 4-channel digitizer with a 40MHz sampling frequency, designed for high-frequency, large dynamic range signals, with a maximum input frequency of 20MHz. The analog input range can be set to ±1V or ±0.2V via software, and a 50-ohm input impedance can be selected to accommodate high-speed, high-frequency signals. Equipped with a 4-channel, high-linearity 16-bit A/D converter, it is ideal for large dynamic range signals such as radar, ultrasonic and software radio.

   With up to 512MB of onboard memory, the PCI-9846 can record waveforms for longer periods of time without being limited by the transfer rate of the PCI bus. The digitized signal data is stored in the onboard memory before being transferred to the main memory. The data transfer uses SG-DMA (Scatter-gather Direct Memory Access), which can provide higher data transfer rates and more efficient use of system memory. If the data transfer rate of the digitizer is lower than the available PCI bus bandwidth, the PCI-9846 also has an onboard sampling point first-in-first-out memory to bypass the onboard memory and transfer the data directly to the host memory in real time.

   The PCI-9846 has flexible triggering options, including software triggering, external digital triggering, analog triggering of any analog channel, and PXI bus triggering. A variety of triggering methods make it more adaptable to needs. Post-triggering, delayed triggering, pre-triggering, and mid-triggering modes can collect data near the trigger event. The PCI-9846 can also trigger acquisition repeatedly to collect multiple data segments with very short time intervals. The various triggering options provided by the PXI backplane make it easy for the PCI-9846 to synchronize multiple modules. Using the PXI trigger bus, the PCI-9846 can output trigger or timebase signals to the PXI trigger bus when set to "master", and receive trigger or timebase signals from the PXI trigger control slot when set to "slave". The PXI backplane provides a precise 10MHz signal that can also be used as a timebase signal source.

   The PCI-9846 includes an accurate, low temperature drift onboard reference. This not only provides a stable calibration source, but also ensures data acquisition stability over a wide temperature range. The automatic calibration process is completed via software and does not require any manual adjustments. Once the calibration process is complete, the calibration information will be stored in the onboard EEPROM (Electrically Erasable Programmable Read-Only Memory) and the calibration values ​​can be loaded from the board when needed. [10] [page]                        

2.3 Software

   LabVIEW is an innovative software product of NI (National Instruments). Its full name is Laboratory Virtual Instrument Engineering Workbench. It is a test system software development platform based on G language (Graphics Language) [11]. Signal generation, digitizer calling and digital signal processing are carried out in the LabVIEW2010 environment.

   Due to the limited performance of the machine, the program is divided into three parts: generation, acquisition, and processing. According to the results of signal modeling (refer to "1.3 Synthetic Signal"), the waveform data is calculated and saved to the file. In the waveform generation program, the waveform data is first read out and written into the arbitrary waveform generator, which is called to generate the required signal. The DAQPilot related module is called to control the digitizer to collect signals and store them in the file for subsequent calls. The demodulation program calls this waveform file, performs relevant demodulation and calculations, and completes the signal analysis. The signal generation and acquisition program block diagram is shown in Figure 3, and the demodulation and calculation program block diagram is shown in Figure 4.

Figure 3 Signal generation and acquisition procedures

Figure 4 Signal demodulation procedure

3. Operation results

   Connect the hardware according to the designed hardware structure, set the waveform information, the baseband signal is a 30Hz sine wave, the FM subcarrier is 9960Hz, the frequency deviation is 480Hz, and the modulation coefficient is 0.3. Considering the machine performance and running time, the VOR signal is taken as 1MHz. The calculated waveform is stored in a file so that it can be called in the waveform generation program. In the waveform generation program, first initialize the device and adjust the parameters, set the board address, power to -10dBm, center frequency to 1MHz, and mode to "Arb Waveform". When calling, IQ modulation is used, the I channel is the modulation signal, and the Q channel is 0. When writing data, select the same sampling rate as when generating to ensure that the spectrum of the generated signal is correct. [12]

   The signal generation module runs continuously and calls the digitizer for acquisition. It is also necessary to set the virtual channel, the range is ±1V, the signal type is "AI Voltage", the sampling frequency must meet the Nyquist theorem, here it is selected as 8MHz, the sampling clock is set to "Continuous Samples", and the duration is 1 second. The sampled data can be displayed in real time in the waveform chart and saved in a file through "Write waveform data to file.vi" for subsequent calls.

   In order to measure and verify the signal, it is necessary to demodulate the collected waveform and restore the corresponding information. The collected VOR spatial synthesis signal is coherently demodulated to obtain the outer envelope of the spatial synthesis signal, including a 30Hz variable phase signal and a 9960Hz frequency modulation subcarrier. The 30Hz variable phase signal is directly obtained through a 30Hz filter, and its time domain, frequency domain waveforms, frequency, phase, amplitude and other information can be seen on the front panel; the 9960Hz subcarrier is filtered and demodulated to obtain a 30Hz reference phase signal, and its corresponding parameters can be seen on the front panel. The phase difference between the reference phase signal and the variable phase signal can indicate the current azimuth information, and the reading is the VOR azimuth. The front panel after running is shown in Figure 5. When the program is running, the various tabs can be displayed in turn, and pressing the "Pause" button on the lower right can lock the current tab.

Figure 5 Front panel of the comprehensive tester

4. Conclusion

   Software radio technology has been widely used in military and civilian communications. Test equipment based on this technology has a broader application prospect than traditional equipment due to its openness and flexibility. The digitizer is the key to achieving the transition from analog signals to digital signals. It has been verified that ADLINK PCI-9846 high-speed digitizer is easy to install, has a user-friendly interface, can be easily called in the LabVIEW environment, is competent for accurate acquisition and analog-to-digital conversion of complex analog signals, and can be stored as waveform files for subsequent calls. Due to the limitations of technical level and computer performance, it is not possible to perform higher frequency sampling and signal processing experiments, which needs to be further studied and improved in future work.

References:

[1] Dong Qinpeng, Xiong Huagang, Design and implementation of automatic test system based on certain avionics equipment, Modern Electronic Technology[J], 2008, 21, 146-149;

[2] Lv Yiqing, A new generation of avionics automatic test equipment, Foreign Electronic Measurement Technology [J], 2000, 1, 8-11;

[3] Baidu Encyclopedia. Software Radio[OL]. [2010.11.27]. http://goo.gl/lKi0D.

[4] Dong Qinpeng, Xiong Huagang, Design and implementation of automatic test system based on certain avionics equipment, Modern Electronic Technology[J], 2008, 21: 146-149.

[5] Zheng Lianxing, Ni Yude. DVOR VRB-51D Doppler Omnidirectional Beacon[M]. Beijing: China Civil Aviation Press, 1997: 8-15.

[6] Rockwell Collins Company. VOR-700 Receiver Component Maintenance Manual. 17th Revision. Printed the United States of America, 2006: 71-76.

[7] Xiang Xin. Software Radio Principles and Technologies[M]. Xi'an: Xi'an University of Electronic Science and Technology Press, 2008:14-19.

[8] National Instruments Corporation, NI PXI-5441 Specifications, National Instruments Corporation,2010:1-5.

[9] National Instruments Corporation,NI PXI-5610 2.7 GHz Upconverter [OL]. [2010.12.20]. http://sine.ni.com/nips/cds/view/p/lang/en/nid/13478.

[10] ADLINK Technology Inc. PCI/PXI-9816/26/46 4-CH 16-Bit 10/20/40 MS/s Digitizer with 512 MB SDRAM User’s Manual. ADLINK Technology Inc, 2009: 11-12;

[11] Dai Pengfei, Wang Shengkai, Wang Gefang. Test Engineering and LabVIEW Application[M]. Beijing: Publishing House of Electronics Industry, 2006: 13-15.

[12] Song Guan, Guangbo Wu. Study on a VOR Signal Generator Based on Software[C]. ICTIS2011, Wuhan.