25Figure 16. Measurement of 500 MHz unfiltered transmitter showing second harmonic generated at the outputInterference Classifications and Measurement Examples (continued)Main signal2nd harmonic1000 MHz500 MHzCo-channel interference is one of the most common types of radio interference as system designers attempt to support a large number of wireless users within a small number of available frequency channels.The easiest way to observe co-channel interference is to turn off the transmit-ter of the desired radio and use the spectrum analyzer tuned to the channel frequency to look for other signals from the desired system. It may be necessary to set the spectrum analyzer to HiSensitivity mode and use the Max Hold display or a spectrogram to record any intermittent signals.Out-of-band interference Out-of-band interference originates from a wireless system designed to transmit in a different frequency band while also producing energy in the frequency band of the desired system. Such is the case when a poorly designed or malfunc-tioning transmitter creates harmonics that fall into a higher frequency band. Harmonics are multiples (2x, 3x, 4x, etc) of the fundamental carrier transmission. For example, Figure 16 shows the spectrum of a transmitter designed to operate at 500 MHz. The measurement, taken from the Agilent HSA, shows the funda-mental component at 500 MHz and a second harmonic transmitting at 1,000 MHz. This second harmonic signal could potentially interfere with other wireless systems operating near 1,000 MHz.