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FEATURE MONITORING & METERING SPECTRUM MONITORING


Remote spectrum monitoring is typically done when a system user complains of interference from an unknown source, as Cyril Noger of Anritsu Europe explains


spectrogram graph which plots the various amplitudes in a colour-coded format across the frequency band. The spectrogram in Figure 1 shows the


frequency carrier’s activity over time, and allows the operator to spot intermittent interferers easily. This user-friendly display helps to show how amplitude changes with time (vertical scale). Interferers may appear in the frequency band as being like a carrier - modulated or not modulated, pulsed or not pulsed - or as an increased noise level bandwidth. Many possible types of interference


ll wireless applications are potentially susceptible to interference from various sources. To identify potential interferers in a particular frequency band, the owners of the frequency carrier or band (generally cellular mobile operators) typically use a spectrum analyser to display the signal amplitudes over frequency. This plot is a first step to observe the behaviour of known frequencies and to highlight if any unwanted signals might be occupying a part of the frequency bands, whether intermittently or continuously. This might not be 100 per cent effective if, for example, the interference comes from jammers, who may move into the frequency band and not be switched on when the time comes to hunt the interference source in the field. A second step could be to observe the same frequency band with a


A


Figure 1: Spectrogram graph showing the channel occupancy in the UMTS downlink band


source may be present, including PIM (passive intermodulation), which generally occurs in transmission systems when more than two carriers share the same infrastructure (jumpers, feeders, duplexers, amplifiers, antennas etc.). PIM is characterised by an increase of noise level in a given frequency band (Figure 2). Measurement tools like the PIM Master are available that link with the infrastructure to identify the PIM source location inside the transmission system or even beyond the antenna.


Figure 2: Typical PIM effect observed onto a LTE carrier inside the Rx band, shown using Anritsu’s Spectrum Master measurement tool


SPECTRUM ANALYSERS For interference hunting, the most appropriate measuring tool is the spectrum analyser. There are various types of spectrum analysers, including handheld or easily portable for use in the field (generally associated with directional or omnidirectional antennas) or benchtop size for use in a laboratory, a vehicle, or even from outdoor positions close to antennas. Handheld spectrum analysers are


commonly used by cellular operators, subcontractors, Government frequency regulators, TV broadcasters and public safety radio providers to quickly monitor a frequency span and to decide what to do next. Typically an omnidirectional antenna


will help the user to detect a signal without knowing its origin as a first step to check if there is any unwanted signal present. Then, using a directional antenna (such as a Yagi type) will help to identify the potential direction of the interference location (Figure 3).


26 APRIL 2018 | INSTRUMENTATION


Figure 3: Yagi directional antenna connected to a handheld spectrum analyser pinpointing the signal strength directions captured and seen onto the spectrum analyser’s embedded map (below)


However, this technique is still not accurate enough on its own, as this generally requires the measurement location to be geographically located on a map, which involves using the triangulation method. If these methods are not accurate enough to locate the interference source - and especially if this requires a longer survey over a few days or even weeks - other hunting tools may have to be used. Instead of using handheld spectrum


analysers and driving to a few positions on the map at specified times, very high-speed spectrum analysers (or “probes”) specifically designed for frequency surveys can be used. The Anritsu MS2710xA platform, for example, is able to sweep 24GHz in one second. The measurement process here is the


same as the triangulation task, but it is automated using software based on field data acquisitions: various probes are positioned in the field around a known disturbed area, with each probe associated with a directional antenna covering a field of around 120°. These probes are remotely controlled and monitored via Ethernet or 3G or 4G modems using the control software to


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