Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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3. Sharing LO: RF local oscillator (LO) signals can be shared using either a star configuration or a daisy- chained configuration. In the daisy-chained configura- tion, the LO signal presented to the various downcon- verters is not at the same phase for each channel. In this scenario, a phase-aligned CW signal, with synchronized clocks and triggers, can be provided to each input channel. However, a phase offset between the chan- nels still exists, since the LO input to each channel is at a different relative phase. This problem persists even if an aligned LO using an (optional) amplifier and splitter is provided, as slight differences in cable lengths and the splitter's phase-tracking performance leave some residual phase offset. It is important that the instrumen- tation has the ability to measure and compensate for this phase offset using onboard signal processing. This signal processing allows the flexibility of providing a phase offset value to the numerically controlled oscilla- tor, so that a corresponding phase offset can be applied to each channel. 4. Repeatability Of Synchronization: ADC clock align- ment, triggering, and LO shift compensation all factor into repeatability, a prerequisite for any type of multi- channel phase calibration. If you cannot achieve the same clock alignment every time across acquisition initializations and across power cycles, you cannot achieve a tight relative phase calibration. You need a mechanism with phase digital-to-analog converters (DACs) that provide mechanisms to reproduce exactly a previously performed clock alignment. 5. Onboard Signal Processing: Particularly in military and defense systems, it often is necessary to iden- tify interference from systems attempting to obstruct a communications channel. Frequently called "jamming" signals, this type of interference can jam a communica- tions signal by producing unwanted power within the band of interest. Various types of signals are commonly used as jamming signals, including single tones, ran- dom white noise, pulsed signals, frequency hopping signals, and modulated "fake" communications signals. Each produces trade-offs among effectiveness, power requirements, ease of generation, and difficulty of detection. The difficulty in identifying jamming signals lies in the need to capture both time and frequency information regarding the signal of interest. As a result, systems with onboard signal processing commonly are used to capture and analyze a dedicated portion of RF bandwidth for several hours. A second type of interference is a pirating or a pig- gybacking communication signal. In this application, the interferer attempts to use the existing telecom- munications infrastructure to transmit an illegal com- munications channel. For example, an illegal transmit- ter attempts to use a repeater tower to rebroadcast a custom communications channel. Because the repeater simply amplifies a specified band of spectrum, interfer- ers can use it to amplify their signals as well as the intended signals. Again, onboard signal processing can be used to demodulate the incoming signal and identify the source of interference. Military and defense systems rely increasingly on commer- cially available technologies to incorporate multichannel phase synchronization. A traditional instrumentation approach does not meet many of the requirements listed above. Instead, a platform-based modular approach is needed. Figure 2 shows a four-channel RF/microwave analysis system with multiple stages of downconversion. Each stage of downconversion consists of mixing, filtering, and amplifi- cation. To drive the tight synchronization among these four channels, this system needs to share a common LO signal and it needs a mechanism to align the baseband clocks. One example of a commercially available system that meets these requirements is the NI PXIe-5668R. This software-designed instrument is a super-heterodyne vector signal analyzer with frequency support of up to 26.5 GHz, bandwidth of up to 765 MHz, and an onboard, user-programmable Xilinx Kintex-7 FPGA for inline signal processing. Military and defense systems have to overcome key technical challenges related to phase, time, and frequency synchronization to coherently receive and process the data acquired from multiple channels. The underlying challenge is not just achieving precise synchronization among channels but also sustaining the drift in the phase, magnitude, and fre- quency relationship among the multiple channels over time. Temperature drifts can cause a considerable change in phase, due to thermal expansion-caused changes in the lengths of cables and in hardware components. Such conditions become even more predominant in multichannel systems that are run- ning field tests over long periods of time. Thus, next-generation test systems must offer enhanced support and maintenance by providing a dynamic feature for software-based recalibration, eliminating the burden of bring- ing the test system back to the labs for hardware recalibration. To accomplish this goal, engineers must rely on a platform- based modular system architecture and meet the five key requirements described in this article. Feature 22 Electronic Military & Defense Annual Resource, 5th Edition Figure 2: This four-channel super-heterodyne receiver system uses a platform- based modular architecture. Abhay Samant is section manager of RF and wireless communications at National Instruments. He is a senior member of IEEE and has over 18 years of experience in the areas of RF, wireless communications, and signal processing. Samant has multiple patents in the areas of GPS, WLAN, and signal intelligence. He is co-author of the book LabVIEW for Signal Processing and has published numerous conference and journal papers.