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