Today, there are many signal sources that transmit or broadcast signals over different fre...

Today, there are many signal sources that transmit or broadcast signals over different frequencies. To explore available frequencies in the air, a dipole antenna can be connected to a spectrum analyzer and the output signal can be observed in the frequency domain. It is interesting to know that when tuning a radio station via a spectrum analyzer via connecting an antenna to its input port, the received signal is not a pure sinusoid. The main reason (see Example) is the fact that the transmitted signals are modulated. Thus, they are a combination of multiple sinusoids. If you observe the signal using a spectrum analyzer, you will see a broad range of frequencies.

APPLICATION EXAMPLE

Communication Systems

An oscilloscope and a spectrum analyzer can be used together in order to simultaneously investigate the behavior of signals (with applications in communications) in the time and frequency domains. The output of a function generator can be connected to an oscilloscope

FIGURE Time and frequency analysis using an oscilloscope and a spectrum analyzer: (a) general structure and (b) practical implementation of the diagram of (a).

FIGURE Frequency and time domain for a sinusoid, AM modulation and FM modulation.

and spectrum analyzer, as shown in Figure, and the time and frequency domains can then be observed and compared. Note that many signal generators can generate amplitude modulated (AM) and frequency modulated (FM) signals (see Section).

The time and frequency domains of these signals are shown and compared in Figure. Using a spectrum analyzer, it is possible to determine the bandwidth (BW) of these signals as shown in the figure. The bandwidth of a communication signal represents the frequency band required to transmit the signal as discussed in Equations. Comparing the AM and FM signal in Figure, it is observed that the FM signal needs a higher bandwidth.