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Identify interference in complex RF environments

Posted: 14 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:signal analyser? oscillator? RF recording?

Radio frequency interference occurs in everything from commercial wireless networks and devices to military communications, radar and electronic warfare (EW) systems. Addressing this problem can be especially difficult since measuring interference is unpredictable. Additionally, the intermittent failure modes in typical signal analysers make data capture particularly challenging. Consequently, when the root cause of a problem is not yet known, it can be difficult for engineers to set up a measurement that captures the failure.

Despite the challenge, the task of finding, identifying and analysing interfering signals in a crowded spectrumwhether intentional or nothas become increasingly important in a wide array of applications. One RF recording technique that may prove particularly useful in addressing this problem is gapless capture. Using this technique, system engineers can now measure data continuously over long durations and ensure the capture of all RF events when they occur.

Understanding the measurement challenge
When characterizing system interference, system engineers have traditionally relied on a signal analyser performing continuous long-duration recordings (figure 1). The main limitation to long-duration recording is that test equipment typically has limited on-board memory. Signals-of-interest enter the analyser's RF input and are processed by subsequent stages, resulting in the displayed waveform on the right of figure 1. Up to the blue vertical line, all signals-of-interest within the instrument's capture bandwidth are processed in real-time, assuming a fixed local oscillator. However, once the samples fill the memory buffer or RAM, the instrument no longer looks at incoming digital samples. Instead, it must process previously recorded samples.

Figure 1: Shown here is a block diagram of a typical signal analyser.

The signal analyser does not capture any samples while it post-processes previously captured data, effectively creating a gap in its continuous acquisition of data. If events occur while the previous event is being processed or if the new event lasts longer than the available memory, it falls into this gap and may be missed. Moreover, the analyser's trigger setup only captures signals for one set of limited conditions. Once the analyser fails to capture the event, it is gone forever.

A viable alternative
While resolving RF interference problems in complex RF environments can be a tricky task, gapless recording offers a viable solution to the measurement challenges presented by the typical signal analyser. The technique solves the problem of not knowing when or where an interference event will occur, or how long it will last, by enabling continuous acquisition of data over long durations. Because there is no gap in the data recorded, the signal-of-interest, such as an intermittent RF event, is easily captured.

Figure 2: In this block diagram, the signal analyser in figure 1 has been modified for gapless recording.

An example of a signal analyser modified for gapless recording is illustrated in figure 2. It is the same signal analyser shown in figure 1. However, it now includes a high-speed data link or bus that allows the engineer to move data from memory as it is acquired. By bypassing processing and display updates, and writing acquired data directly to final storage using a circular Random-Access Memory (RAM) buffer, it's possible to create high-bandwidth recordings with no gaps in the data. With a circular RAM buffer, the engineer can simultaneously write to and read from it. When recording at wide bandwidths for long durations, a Redundant Array of Independent Disks (RAID) storage system is required.

One such wideband gapless recording solution is Agilent Technologies' dual-channel M9392A PXI Vector Signal Analyser, which can provide two independently tunable channelseach able to record data at 100MHz bandwidth over many hours.

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