To test a cell phone, base station, or DTV transmitter, you need to measure the amount of power that leaks from one channel to adjacent channels.
Transmitters use filters to limit the power that leaks into adjacent channels.
You can measure a transmitter's adjacent-channel power with a spectrum analyzer. If your instrument lacks that feature, you can still make the measurements, but you'll have to calculate the instrument's noise and compensate for it. If you increase adjacent-channel power by 10 dB, then the spectrum analyzer's noise will contribute less than 0.5 dB of measurement error. The range of signal power that lets you measure adjacent-channel power depends on the spectrum analyzer's phase noise, thermal noise, and distortion products. Phase noise is wideband noise caused by jitter in the spectrum analyzer's local oscillators.