11Hint:Figure 5. Over-the-air measurement of a 915 MHz RF signal in an office environment with the lighting turned offSources of Interference (continued)Center frequencyFrequency spanReference levelMarker10 dBThe analyzer center frequency was set to “915MHz” using the [FREQ] button and the displayed frequency span was set to “500kHz” using the [SPAN] but-ton. The top line of the graph is the reference level and is adjustable using the [AMPTD] button. The reference level is adjusted to optimize the measurement display; in this case, the reference level was set to “-40dBm”. Using the default scaling, each vertical box represents an amplitude difference of 10 dB, shown as “10dB/” on the screen. Therefore with a total of 10 boxes, the bottom line on the graph represents “-140dBm”.The measured trace in Figure 5 shows a single RF carrier without modulation. The maximum amplitude level and frequency of the signal was measured using a marker placed at the peak of the signal. The marker functions, including “peak search”, are found on the Agilent HSAs but similar functions are available on most commercially-available spectrum analyzers. Using the marker, the peak amplitude is measured to be -49.84 dBm at the expected frequency of 915 MHz. For this measurement, without the fluorescent lights, the spectrum appears relatively clean of any spurious or sideband modulation.Next, the lights were switched on and the spectrum was measured for a second time. Figure 6 shows the measured spectrum with the fluorescent lighting and now the spectrum includes undesired sidebands modulated onto the RF carrier. This interference has been introduced into the RF signal by the electronic ballast of the fluorescent light fixtures.It is important to properly adjust the reference level so that the signal does not appear to be clipped off the top of the graph.