频谱分析仪是在其频率范围内测量输入信号的频谱（幅值-频率关系）的仪器。它的主要作用是测量信号的功率谱。频谱分析仪的输入信号是电信号。但是若配合合适的传感器，也可以测量声波、光波等其他信号的频谱。也有专门的光谱分析仪，能够用单色器之类的光学技术直接测量光波的频谱。

通过分析电子信号，可以在频谱中观察到其主要频率，功率，失真，谐波，带宽，和其它频谱分量。这些，在时域波形中是不容易被探测到的。这些参数在描述无线发射器等电子设备的特性时是有用的。

频谱分析仪显示的是在水平轴上的频率和垂直轴上的幅度。不经意地看，一台频谱分析仪像是一个示波器。事实上，一些实验室仪器在功能上既可以作为示波器，又可以作为频谱分析仪。

A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that a spectrum analyzer measures is electrical, however, spectral compositions of other signals, such as acoustic pressure waves and optical light waves, can be considered through the use of an appropriate transducer. Optical spectrum analyzers also exist, which use direct optical techniques such as a monochromator to make measurements.