Measure radio frequency interference in complex RF environments

From commercial wireless networks and equipment to military communications, radar and electronic warfare (EW) systems, radio frequency (RF) interference is everywhere. Because the interference is unpredictable, it is very tricky to solve this problem. Commonly used signal analyzers use intermittent failure modes, making data collection particularly difficult. Therefore, if the root cause of a problem is not clear, it is difficult for engineers to find a measurement method to capture the failure.
Despite the difficulties, finding, identifying and analyzing interference signals in the crowded spectrum, regardless of its purpose, has become increasingly important in various applications. An RF recording technique called non-stop capture may be particularly useful for solving this problem. Using this technology, system engineers can continuously measure data over a long period of time, ensuring that all RF events are captured.
Difficulties and challenges in measurement
When analyzing the characteristics of system interference, system engineers usually rely on signal analyzers to complete long-term continuous recording and storage, as shown in Figure 1. The main limitation of long-term recording is that the memory capacity of the board in the test equipment is not large enough. The target signal first enters the RF input of the analyzer and is then processed to produce the waveform shown on the right side of Figure 1. Assuming that the system uses a fixed local oscillator, all target signals within the capture bandwidth of the instrument will undergo real-time processing before reaching the blue vertical line. Once the sampled signal enters the buffer or RAM, the instrument no longer pays attention to the newly entered digital sample. Instead, it must process the previously recorded sample data.




Figure 1: Block diagram of a typical signal analyzer
When the signal analyzer performs post-processing on the previously captured data, no new data samples will be captured, thereby effectively forming a gap between successive data collections. If a new event occurs while processing the previous event, or the duration of the new event exceeds the existing memory capacity, these events will fall into this gap and may be missed. In addition, the analyzer's trigger settings only capture events based on a set of restricted conditions. Once the analyzer misses an event, the event is gone forever.
How to find a feasible solution
Solving the problem of radio frequency interference in a complex RF environment is a tricky task. The non-stop recording method provides a feasible method to solve the measurement problems encountered by traditional signal analyzers. This technology realizes the continuous collection of data for a long period of time, and solves the problem of not knowing when and where an interference event will occur and how long the event will last. Since the recorded data will not be interrupted, it is easy to capture target signals, such as intermittent RF events.

Figure 2: This figure has been modified to record and save the signal analyzer in Figure 1 without interruption
In the example in Figure 2, the signal analyzer has been modified to record and save without interruption. The signal analyzer in Figure 2 is the same as in Figure 1, but it contains a high-speed data link or bus that allows engineers to move the collected data out of memory. It uses bypass processing and display update, and at the same time writes the collected data directly into the final storage medium using ring RAM cache to achieve uninterrupted high-bandwidth recording and storage of data. Engineers can write and read data in the ring RAM cache at the same time. If you record and store data for a long time under a large bandwidth, a redundant array of independent disks (RAID) storage system is required.
Agilent's dual-channel M9392A PXI vector signal analyzer is such a wideband continuous recording and storage solution. It can provide two independently adjustable channels, each of which can record and store hours of data in a 100-MHz bandwidth. Data (Figure 3).

Use with ordinary PC hard drive or external mass storage
Broadband recording in the RF environment has proven to be a useful feature analysis tool for long-duration radio frequency interference research. The powerful search tool can reduce the burden of searching for interference signals in the massive recorded data. For example, the Agilent M89600 vector signal analyzer software can be used with the M9392A to analyze the characteristics of the interference source and its impact on the interference signal in the continuous recording of target data. Using such software simplifies and reduces the time to find the target signal, and speeds up the process of analysis and problem solving.
It may also be helpful if the continuous recording and storage scheme uses a method similar to time stamping, which maps the recorded data to a time to generate a trigger or pre-trigger. Pre-trigger data enables engineers to read signal data and generate a specific trigger event.
Another main function is dual-channel recording. In a single-channel recording system, it is difficult to trigger only the required signals. As a result, in order to ensure the capture of interference events, more data is recorded than actually needed. These extra data need to spend extra time and resources to process. A dual-channel recording and storage system such as M9392A reduces the possibility of false triggers, and it only records the required data. Signals can be acquired and triggered on one channel, but recorded on another channel. Since these signals can be found more effectively in the RF environment, such an arbitrary trigger mechanism can save a lot of time and help engineers solve interference problems more effectively.
Another point: structured processing flow
Even if the continuous capture method is adopted, it is still difficult to solve the problem of radio frequency interference. Therefore, it is necessary to follow a systematic process. The following is an example of this processing flow, which includes:
Step: Capture
This step uses a long-term recording method to collect data to ensure that failure events are captured. Signals in the RF environment tend to last longer, so long duration is necessary. Moreover, the RF environment changes over time, and the spectrum is usually crowded. In addition, the ever-increasing bandwidth of modern communication signals means that the noise spectrum is wider, and the interaction between them is usually intermittent, subtle or short-lived.
Step 2: Search
After the data collection is completed, the recorded data will be played back in the laboratory and the necessary analysis will be performed to extract information about the source of the fault. In the case of a large amount of data, it is strongly recommended to use a signal search tool that can automatically search according to a variety of different conditions to find the source of interference. After the data search is completed, a series of qualified signals will be found, and the signal analysis application will separate these signals and play them back.
Step 3: Recapture data
After the engineer has a deeper understanding of the crux of the problem or the potential source of signal interference, it may be necessary to capture more and more specific signal data. In this optional step, the engineer triggers a recording task with a higher signal-to-noise ratio based on the knowledge of the fault. These recording tasks focus on how the interfered receiver responds to a particular source of signal interference. At this time, a dual-channel recording and storage system may be particularly useful because it can use one of the channels to trigger recording and storage after configuration.
Fourth step: analysis
, engineers can use analysis software to show the influence of signal interference sources.
Using this process, engineers can not only understand the RF environment, but also record information about the frequency band for a long duration. As a result, they can effectively use the RF recording method to record, search and analyze target signals in a complex RF environment.
It is quite difficult to solve the problem of radio frequency interference in the complex RF environment. However, with the continuous recording and storage method, engineers can continuously measure data over a long duration, ensuring that all target RF events are captured. The broadband recording and storage system specially improved for continuous data capture, especially the dual-channel system, can analyze the characteristics of system interference in the RF environment very effectively. Using this system in a structured process provides an effective way to find and analyze the target signal. In commercial wireless and EW applications where interference problems continue to emerge, such functions are becoming more and more important.