How the latest spectrum analysers help engineers to troubleshoot radio interference problems
Added to this mix are military, aeronautical, and emergency services, plus low-power unlicensed signals, such as Wi-Fi and wireless video cameras. All of this traffic in a crowded RF spectrum is reason enough for interference problems, but when you add the fact many of these services are expanding, being modified, aging or failing, interference can become an epidemic.
A spectrum analyzer, such as the MS2720T (figure 1) from Anritsu, is typically used to find and locate causes of interference. The first and best place to start looking for interference is at the antenna input to the receiver. If the receiver has a pre-filter, it’s best to measure the signal after the pre-filter.
If it is a cellular issue and the base station has a high noise floor, the uplink channels should be checked. However, if the issue is device reception in a given area, then the downlink frequencies need to be measured. Once an interfering signal is present at the receiver input, it affects the receiver’s front end, causing a reduction in sensitivity. This will lower the effective carrier-to-interference ratio (C/I) and result in all the symptoms of a weak signal (noisy, waterfall effect, low data rate), except that the received signal strength measurements will be strong due to the high noise floor.
Characterizing the signal
Once an interfering signal is spotted, it should be characterized before disconnecting the spectrum analyzer from the receiver’s signal. To characterize the signal, the user should adjust the spectrum analyzer to best view the signal by using the pre-amp, reference level, span, and resolution bandwidth controls. The signal’s shape, bandwidth, and behavior should be observed. Users may also want to look for frequency drift, amplitude changes, and frequency hopping.
If the signal is intermittent, the Max-Hold feature of the spectrum analyzer can be used to create an envelope. If the spectrum analyzer has spectrogram capability, it can be used to check for periodicity. For signals that are intermittent with a long time between appearances, it can be helpful to use a “Save on Event” capability. This capability uses a mask automatically generated from the “normal” signal and only saves a trace when something unusual appears. Once saved, the traces can be examined for time-of-appearance and signal characteristics. The burst detect feature in many Anritsu handheld spectrum analyzers is very useful when hunting for bursty signals, especially ones that occur with a low duty cycle.
While looking for signals that don’t belong on the input to a receiver, it’s important to know what signals are typically present in the bands, as well as what other legitimate signals may be present. This can save much time when hunting signals. If this is not possible, the signal can be demodulated so the user can listen for the station ID call sign.
Many interfering signals are not so easy to identify, so field engineers must find them by hunting. Possible interference causes to seek are on-channel interference, in-band interference, impulse noise, harmonics, passive intermodulation (PIM), or intentional interference caused by a jammer.
Locating the source of interference
Once an interfering signal has been spotted and characterized using the tower’s antenna, the next task is to find the same signal using a ground level antenna. This will allow the field engineer to search for the signal, either by direction finding or seeking areas of higher signal strength. One issue is that signals that may be strong atop the tower may be weak at ground level.
Initially, it must be determined if the signal is visible near the tower base. If it is, the signal has been spotted at ground level and it’s time to move to the next task, locating the source. If not, there are several things to attempt:
- Check other sectors for the interfering signal;
- Look for the interfering signal from a nearby rooftop or top floor. In an urban area, this may be the best way to direction find;
- Move to higher ground;
- Investigate nearby valleys, swales, or other low spots;
- Use in-instrument mapping techniques to plot signal strength versus location.
Once the signal has been spotted at ground level, the RF source needs to be located using the following method:
- Import a geo-referenced map onto your spectrum analyzer;
- Select an antenna;
- Setup the spectrum analyzer;
- Go to mapping mode;
- Find the signal direction;
- Repeat the direction finding process from several locations.
A Geo-referenced map has GPS latitude and longitude information embedded in it. This allows a GPS-enabled spectrum analyzer to locate the user’s current position when plotting signals on the map.
Traditionally, a Yagi antenna is used for direction finding because it has good directivity, good front-to-back ratio, and generally low side lobes. Its biggest disadvantage is it usually has a fairly narrow frequency band. Other options include a Log Periodic antenna, which has broad frequency coverage but less directivity, a panel antenna (best to ward off reflections), or omni-directional (appropriate when seeking the strongest signal).
Anritsu has developed the MA2700A Handheld InterferenceHunter (figure 2) that accepts many types of antennas with standard N-(f) connectors. The handle also contains a magnetic compass and a GPS receiver, which report position and direction to the spectrum analyzer. This simplifies taking directional bearings. The InterferenceHunter also has a built in pre-amp to ease the process of getting a sufficiently strong signal to the spectrum analyzer.
Users then load the map into the spectrum analyzer and enter mapping mode. Next, the InterferenceHunter is installed and enabled. The spectrum analyzer will show the current location on the map and the direction that the antenna is pointing.
Once set up, rotate the antenna to find the direction of the strongest signal. When the direction of the strongest signal is located, the user presses the trigger on the antenna handle to place a record of the direction on the map. Next, the user moves to a new location and repeats the direction-finding process. It helps the triangulation effort if the moves are made at a right angle to the direction last plotted.
Sometimes a map is not required for signal hunting. In the simplest cases, it can be faster to take direction finding readings with a signal strength meter, use the tried-and-true Max-Hold method, or simply travel until the signal strength readings increase. A signal strength meter is available on many spectrum analyzers.
Selecting the right spectrum analyzer
Some spectrum analyzers are more capable than others when looking for interference. Handheld spectrum analyzers clearly have an edge over bench instruments, since they can easily go to where the signal is located. If you are going to be spending hours away from power sources, long battery life is helpful.
The ability to see small signals in the presence of large signals that may be nearby in the RF spectrum is important, as well. A spectrum analyzer with a dynamic range of >106 dB in 1 Hz RBW allows users to see a small signal 90 or 100 dB below a strong signal, while both signals are present.
Another key capability is a fast sweep speed with a low resolution bandwidth, so the spectrum analyzer can sweep fast while resolving sufficient detail to see the interfering signal. For many interference hunts, a 1 MHz span is useful. A good spectrum analyzer can use a 1 kHz resolution bandwidth to create a noise floor at –126 dBm, with an update rate of 3 sweeps per second. Figure 3 shows a display of a handheld spectrum analyzer with a fast sweep speed and high resolution.
Conclusion
Tracking interfering signals is becoming increasingly difficult due to the proliferation of wireless services in the finite RF spectrum. Using the proper testing tools, such as handheld spectrum analyzers with wide dynamic range, fast sweep speed, and low resolution bandwidth, will help locate interfering signals faster and more efficiently.
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
