When it comes to PXI and virtual instruments, many engineers may have a rough impression that virtual instruments adhere to the modular design concept, rely on the PXI bus platform, and share the PC resources, which are already testing, measuring, controlling, and monitoring. The field has achieved good results. For the specific progress of RF PXI test instruments, many engineers still don't know much about it! At the just-concluded China's 4th PXI Technology and Applications Forum, the progress made in the field of RF testing by PXI virtual instruments has indeed made many participants look impressive.
For RF test instruments, people may first think of RF modules of various sizes and sizes, as well as rigid cables that are wound around them, and even waveguide components that are difficult to install. The virtual instrument, which is characterized by the use of master and bus design and fusion with the PC, has a living digital feature that seems to be less than the above-mentioned RF features. So, how do virtual instruments on the PXI bus architecture implement RF functions?
The design idea of ​​PXI virtual instrumentThe successful breakthrough of NI's PXI virtual instrument in the RF field has benefited from a variety of new technologies, including SDR, high-speed backplane bus, DDS, and precision compensation technology.
The RF signal generator is the source device of the RF test system. According to the block diagram of Figure 1, it is not difficult to find that the series of signal generators can use internal or external clocks. When using an internal reference, DDS technology can be used to directly generate RF signals up to 50MHz. For signals above 50MHz, the frequency synthesizer module in the system and the phase tracking DDS output of the frequency synthesizer can realize the RF output up to 6GHz, and the frequency step can reach 1Hz or even lower.
In addition, according to the analysis, sophisticated temperature control and compensation (unconfirmed) measures are used in the system. Extremely high precision is achieved by monitoring the temperature and compensating the output with a correction amount pre-stored in the storage table based on the change.
In addition, National Instruments has defined its own SDR architecture. The company's analyzer or receiver series, the NI-PXI-566x series, has a defined frequency range of 9KHz to 2.7GHz, a real-time bandwidth of 20MHz, and a memory of 32-6. MB. The signal source or transmitter series is defined as: 250KHz ~ 2.7GHz, real-time bandwidth is 22MHz, and memory is 32 ~ 256MB. The instrument becomes more flexible with SDR technology.
High-speed backplane technology is undoubtedly the core technology of PXI virtual instrument and one of the main bottlenecks of PXI RF instrument. This backplane is very different from a traditional PC backplane because it handles mixed signals that include extremely high frequencies. The system contains up to 10 GHz of analog signals, including extremely high speed ADCs and DACs, which make the backplane's EMC design and power and reference plane design difficult, as well as miniaturization requirements. At present, there are nearly 10 trigger buses on the backplane, and in order to ensure reliability and reduce delay, the system adopts star trigger mode. That is, the star trigger control generates multiple trigger signals and sends them to all slots. This ensures that each slot receives the trigger signal at the same time, reducing the trigger time error to less than 1 ns.
Features of PXI-based virtual RF instrumentFor the X-band and above frequency bands covered by traditional box instruments, it may not be the strength of PXI virtual instruments, but after analysis, it is found that these instruments have their own unique characteristics and can be used as traditional box-type RF test instruments. Better to add. Key features of the PXI virtual RF instrument include:
1. Flexible synchronization and triggering
2. High-speed broadband bus function
3. Comprehensive test function with high integration
4. Flexible user development space
Figure 1: Block diagram of the NI PXI-565x series RF signal generator.
After careful analysis, the above main features are actually determined by the nature of the PXI bus and modularity. Based on these characteristics, NI PXI-based RF test instruments are suitable for the following applications:
* More complex or cutting-edge application development
In this kind of application, because of the pre-research stage, or the development stage, which has a relatively advanced stage, there are many custom protocols. Testing and analysis of such protocols requires a lot of flexible synchronization modes and trigger modes, which are often Traditional instruments are not fully available. As a result, users can flexibly build their own test verification system as needed. There are many examples of such applications, including TD-SCDMA system development testing.
* Users have unique needs
In this case, users often have many specific uses. For example, it is necessary to extract original data of a radio frequency, including amplitude information and phase information, and even modulation information within a bit. This information can be used to debug, analyze or stream the system, and of course some information can be used as a corrected loop control. In addition, in complex systems with multiple systems, flexible triggering and synchronization are often required, which can also be used to characterize such instruments.
* Requires applications with technology convergence
Because it can provide flexible multiple programming interfaces (including Matlab, NET, VB, VC, DLL, EXE, AcTIveX, etc.), and provide an open software and hardware secondary development interface, making this instrument more suitable for various technologies Converged applications.
* As part of the dynamic control field
In the field of dynamic control, the test instrument is no longer only for testing and verification purposes, but the test results are directly or indirectly used as control variables to correct or correct the controlled parameters in the system. In this application, coupled with the powerful features of the PC, this type of test instrument can perform its small and flexible function.
PXI RF test instrument exampleSo, what kind of RF function is implemented, or what kind of RF test instrument is NI and its PXI Alliance members? The RF test instruments introduced by NI can be roughly summarized into the following three categories:
1. PXI RF signal generation and analysis
In test systems, signal generation is the source of analysis and plays an important role. Using the above design ideas, NI implemented a series of RF and MW signal generators, including PXI 5650, PXI 5651 and PXI 5652 with frequencies up to 1.3 GHz, 3.3 GHz and 6.6 GHz, with various analog modulations commonly used in the RF field. Digital modulation and high phase noise and very low signal jitter. The series of signal generators achieve a phase noise of -110dBc/Hz at 1GHz with an offset of 10kHz. The instrument uses a 3U single-slot structure, which is more dexterous.
2. Universal wireless communication terminal test system
As terminal functions continue to increase and protocols become more complex, test costs continue to rise, including time costs, which poses significant challenges for testing. Around these challenges, NI has developed a flexible terminal test system. The system consists of PXI-5660 plus Labview. The testing field covers all current systems, including China's own TD-SCDMA. The system is characterized by the flexibility to extend the number of parallel channels in the test system, thereby reducing the cost of testing.
3. Wireless network analysis test system
A typical representative of such instruments is the PXI-5661. In addition to being able to perform test analysis in the field of conventional wireless communication, it also provides multi-vector analysis, such as 3D spectral analysis, I/Q quadrature modulation analysis, and the like. The system consists of a high-precision digitizer and a high-performance inverter with a frequency of 2.7 GHz. With a precision attenuator, the input dynamic range is up to 160 dB and the noise density is -140 dBm/Hz. In addition, an important feature of the system is flexibility. For example, a PRI-5690 preamplifier module can be added to achieve a noise density of up to -165 dBm/Hz.
In addition to the above categories, PXI-based RF instruments continue to advance into a wider range of specialized areas. Poly Star Instruments, a member of the PXI Alliance, recently developed an RFID RF parameter test system. It is understood that the system is the industry's first RFID professional test system. The system has complete RFID protocol support capabilities, supports various modulation methods and coding methods in the RFID standard, and can also extend support for the manufacturer's own instruction set, so that it can cover all tests from the physical layer to the protocol layer.
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