February 25, 2016
Wireless networks are being impacted by interference that degrades performance. For wireless service providers, issues such as dropped calls, blocked calls, and data throughput are all affected negatively by harmful interference. Operators need to utilize the full capacity of their networks to keep up with consumer demand. One solution is a mobile interference hunting system that can deliver a fast and simple method to locate and correct interfering signals.
Using the proper hunting system, field technicians and engineers can locate many interferers over a wide range of power levels in a variety of RF environments. Adding mobility to direction finding enables more measurements to be collected in a shorter time span. This enables special averaging techniques to be employed to mitigate multipath and quickly identify the location of the interferer.
One fundamental idea used in a mobile interference hunter system is that when a measurement is being taken at a fixed position of a CW emitter, the measured power should not fluctuate. If it does, there are variations in the path loss. By averaging several measurements, the change in power level generally will be reduced. Selecting the right amount of averaging can sufficiently reduce the variation to make interference hunting fruitful. Labels on the sensitivity numbers are intended to guide the user in selecting the proper amount of averaging for different usage situations, but they are just guides, not fixed usage rules.
The number is shown in the status bar at the bottom of the display.
The purpose of this setting is to reduce the sensitivity of the software to rapid power variations caused by multipath propagation. This is done by taking multiple measurements for each data point and averaging the measurements.
There may be situations in which the sensitivity setting isn’t sufficient to smooth out variations. In such cases, settings in the spectrum analyzer are employed to further clean up the measurements. Generally, the first thing a field technician should do is to go to a narrower VBW. That is the suggested initial step because it slows down the measurement the least. A good starting point is a video bandwidth of 300 Hz. Narrower bandwidth can be used, if needed, to obtain a reasonably stable reading. Be aware that sweep speed can be significantly impacted with VBW less than 100 Hz.
Trace averaging in the handheld spectrum analyzer can be used to reduce variations. This isn’t the first choice since measurement speed is significantly reduced, but it can be employed, if necessary. Field technicians should experiment with the number of traces to be averaged to get a reasonably stable value while not slowing down the measurement process too much.
Changing resolution bandwidth (RBW) doesn’t reduce variations in signal level, it just changes the noise floor for measurements. You may say, “Wait a minute, when I reduce RBW on my spectrum analyzer the noise variation goes down.” That happens because, by default, the VBW is coupled to the RBW and as the RBW is reduced, the VBW is also lowered to maintain a 3:1 RBW/VBW ratio, or whatever ratio you have chosen. If you decouple the VBW from the RBW by turning Auto VBW off, you will see that changing RBW by itself doesn’t affect the signal level variations.
To learn more on interference hunting, you can download an application note that outlines a variety of techniques to help locate interference sources.