What is a swept spectrum analyzer? How does it work?

The function of the spectrum analyzer is to distinguish each frequency component in the input signal and measure the frequency and power of each frequency component. In order to complete the above functions, the superheterodyne method is used in the sweep-tuning spectrum analysis, which can provide a wide frequency coverage while allowing signal processing at the intermediate frequency (IF). Figure 1 is a structural block diagram of a superheterodyne swept spectrum analyzer.

The input signal enters the spectrum analyzer and is mixed with the local oscillator (LO). When the mixing product is equal to the intermediate frequency (IF), this signal is sent to the detector. The output video signal of the detector is amplified, sampled, and digitized to determine the CRT display signal. vertical level. The sweep oscillator controls the horizontal frequency axis of the CRT display and is synchronized with the local oscillator tuning, which simultaneously drives the horizontal CRT deflection and tuning LO.

The spectrum analyzer relies on the intermediate frequency filter to distinguish each frequency component, the detector measures the signal power, and relies on the corresponding relationship between the local oscillator and the displayed abscissa to obtain the signal frequency value.

The working principle of this scanning-tuning analyzer is just like the amplitude modulation (AM) receiver in your home, except that the local oscillator of the AM receiver is not scanned, but manually tuned with a scale knob; in addition, instead of displaying information on the display, Use speakers.

Figure 1 Simplified block diagram of a swept superheterodyne spectrum analyzer

Based on the scanning working principle, when the input signal is a single-point frequency signal, the signal needs to be mixed with the scanning local oscillator signal, so that the intermediate frequency signal is also a sweep signal with changing frequency. The sweep signal passes through the intermediate frequency filter and detection The output waveform after the filter is the frequency response shape of the intermediate frequency filter.

Figure 2 Measurement process of swept frequency spectrum analyzer