Swept spectrum analyzer is to use frequency sweep principle to complete the signal frequency domain test.
The function of spectrum analyzer is to distinguish each frequency component in the input signal and measure the frequency and power of each frequency component. To accomplish these functions, the super-heterodyne is used in scan-tune spectrum analysis. This can provide wide frequency coverage while allowing signal processing at the intermediate frequency (IF). Figure 1 is a super-heterodyne sweep spectrum analyzer block diagram.
Figure 1 Structure of super-heterodyne sweep spectrum analyzer
Input signal is mixed with local oscillator (LO) signal, output signal pass through the IF filter, other frequency components will be filtered and only left the IF signal. Then IF signal is sent to the detector, the detector output video signal then passes through the amplification, sampling, digital CRT display signal vertical level. Sweep controller controls the horizontal frequency axis displayed by the CRT is synchronized with the local oscillator tuning, which simultaneously drives the horizontal CRT to deflect and tune the LO.
Note: Usually, LO frequency ranges from Max. Instrument frequency to Max Instrument frequency + Instrument Span. For example, Saluki S3531B, max frequency is 3GHz, span is 9kHz -3GHz, so the LO sweeps frequency ranges 3GHz – (6GHz – 9kHz). The IF filter will be set at around 3GHz. This ensures mixer output component (FLO – FInput) can fall into the IF filter.