Identifying RF interference sources with automatic IQ capture
17 August 2017
IQ (in-phase/quadrature) capture is a feature of modern spectrum analysers that helps engineers with in-depth analysis of RF signals. However, while it allows engineers to analyse the signal type being captured, it doesn’t always easily meet user requirements for triggering the capture in an automated way. This article explains how SCPI programming can be used to capture signals that exceed specified thresholds to help identify sources of interference.
This article originally appeared in the July 2017 issue of Electronic Product Design & Test; to view the digital edition, click here – and to register to receive your own printed copy, click here.
For intermittent RF signals, it is helpful for test and measurement equipment to recognise these signals as they appear, then capture them automatically. This facility is often required by broadcasters, regulatory bodies and intelligence agencies, and is greatly eased by the remote control and programming features provided in modern handheld spectrum analysers, such as the Anritsu MS2720 (Figure 1).
Identifying interference sources
When facing the problem of an interference signal, two main tasks must be addressed as quickly as possible: locating the source, and characterising what type of signal it is. Interference location can be determined using multiple systems that help in identifying where a particular signal stems from. For example, a system that uses several MS2710xA probes can run algorithms to triangulate the origin of a particular signal to within hundreds of metres. The MA2700A handheld analyser can then help to pinpoint the source by measuring signal strength and calculating the direction using directional antennas.
For the next steps of characterising, analysing and identifying the interfering signal, IQ capture is an important tool, making it possible to obtain the raw demodulated data of a signal, and then run programs (such as Matlab) to post-process it. This feature is crucial for users such as government agencies and regulators, who usually need to go further when investigating interference problems, with the aim of precisely analysing the type of signal, its odulation and even its author.
Within the Anritsu spectrum analyser family, there are several models that can capture IQ data in different instantaneous bandwidths and at different lengths, allowing the capture and post-processing of a wide range of signals. IQ capture can then be used to produce a .wcap file, with the necessary information to process it using standard software.
One problem arises when the signal under investigation is an intermittent signal, appearing only from time to time and in short bursts. This scenario is particularly challenging for users, as it forces them to continuously monitor the spectrum as they wait for the event to occur.
The usual triggers that initiate IQ capture do not allow capture by an event. It is possible to manually capture the IQ using the screen menus, or via an external trigger, but there is no automatic option to launch the capture.
Almost all spectrum analysers, however, feature ‘save on event’ functionality (Figure 2), which allows the user to save a capture of the spectrum when certain conditions are met. This is useful, for example, to capture the signal when it exceeds a certain threshold, and can be the key to hunting down intermittent sources.
In this case, the capture of intermittent signals is achieved by creating the appropriate threshold and using the ‘save on event’ feature. It is not necessary to monitor the spectrum the whole time to catch the moment when the interference appears.
It is enough for this configuration to automatically save the signal every time it appears and exceeds the limit.
So the solution for users who want to capture IQ data in the same way that an instantaneous spectrum is saved with the ‘save on event’ feature is to integrate both these functions in some way.
Automatic capture of IQ data
At this stage, the programming capability of modern spectrum analysers plays an important role. All Anritsu spectrum analysers, for example, can be externally controlled and programmed via SCPI commands. This gives the equipment the flexibility to demodulate, nlaunch measurements via LabVIEW programs, and start and end captures.
SCPI programming can expand the spectrum analyser’s functionality to take it beyond the typical specifications. In the case shown in Figure 3, for example, a LabVIEW application was created to interact with the MS2720T unit and launch IQ capture via the ‘save on event’ feature.
Here, the application window shows the spectrum in the spectrum analyser window, and the user is able to configure the limit line that will trigger the IQ capture function when exceeded. The MS2720T is configured with the same parameters, such as IQ sampling rate and capture length, and the ‘capture on event’ button will then show the extracted IQ data of the signal on the graph underneath. The ‘save IQ data’ button can be used to provide an ASCII file which contains the IQ data in tab-separated format.
IQ data will be saved only when the threshold is exceeded, and a number of ‘save’ events can be triggered using the ‘hold off’ configuration field.
In this case, there is no need to continuously monitor the spectrum analyser screen, while waiting for the signal interference to appear and be captured. If the user has an approximate idea of how high its power level is, it is possible to configure an appropriate limit, run the application and leave it to obtain the results itself – with all captures stored in the application folder (Figure 4).
SCPI programming is a powerful tool that can enhance the functionality of modern handheld spectrum analysers. For example, the aforementioned automatic IQ capture capability has proved useful in addressing the needs of one particular intelligence agency, who wanted to capture interference sources that were appearing intermittently in a specific zone.
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