How Spectrum Analyzers Work (1)

The spectral distribution of the signal is actually the reflection of the measurement results in the frequency domain. The relationship between the frequency domain and the time domain is shown in the figure:

The oscilloscope measures an approximately square wave signal in the time domain, which is decomposed into the fundamental wave and up to 11 odd harmonics by Fourier transform. When viewed from the frequency domain with a spectrum analyzer. All frequency components can be identified. Take the above figure as an example, the fundamental wave, the 3rd harmonic, the 5th harmonic and the 11th harmonic can all be distinguished. It can be seen from this that the time domain and the frequency domain describe the same signal from different angles.

A spectrum analyzer works like a wideband receiver, and the wideband range starts from tens of kHz or tens of MHz. The function of the receiver is to convert the frequency of the input signal into a frequency band that the detection loop can handle. The wideband receiver consists of a mixer, a local oscillator (LO), and a bandpass filter. The local oscillator produces a mixing oscillator signal. The mixer mixes the input signal with the signal generated by the local oscillator. The total signal includes the sum and difference of the two signals. The difference between the two signals is called the intermediate frequency (IF), which is part of the signal used by the detection loop. Bandpass filters filter out unwanted components of the signal and then pass only the IF that remains to the detection and display unit.

A spectrum analyzer is essentially a wideband receiver and therefore requires more than one frequency conversion, the number of times determined by frequency range, frequency resolution, and RBW filter.