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What is the purpose of a spectrum analyzer?

Some electronics students and junior engineers are always meet one question: what is the purpose of a spectrum analyzer?

As we all know, we can describe an electrical signal from a time perspective (time domain) or from a frequency perspective (frequency domain). The figure below clearly shows the different display of the same signal from the time domain and spectrum perspective.

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Fig.1 Different Between Time Domain and Frequency Domain

All Saluki Oscilloscopes have spectral display capabilities, this is mainly the use of Fourier Transform (FFT) for the time and frequency domain conversion, but the main purpose of the oscilloscope to observe the time-domain characteristics of the signal (that is, Voltage over time), mainly for baseband signal analysis (sine wave, square wave, bit stream and other unmodulated signal), and spectrum analyzer mainly for radio frequency signals (especially with modulated complex signals or multi-frequency signals , This signal in the time axis almost do not see any regularity) analysis. Although oscilloscopes can also display signals from frequency domain via FFT, its performance is generally not sufficient to analyze RF-modulated, modulated signals.

The spectrum analyzer is mainly used to analyze the carrier and the modulated RF signal. Therefore, the frequency range of the spectrum analyzer is very wide. For example, SALUKI's entry-level spectrum analyzer S3531A spectrum analyzer is 1.8 GHz, the upper-end S3532B spectrum analyzer is 7.5 GHz and the high-end S3503 spectrum analyzer Can reach 50 GHz. The spectrum analyzer looks at the power, frequency, distortion (harmonics and intermodulation products) of the RF signal, the bandwidth after modulation, the size of the leaked adjacent channel, noise testing, and in-depth analysis of complex modulated signals (modulation, IQ Constellation, modulation error, etc.)

2-1.8GHz-Spectrum Analyzer S3531A.png

Fig.2 Saluki S3531 Spectrum Analyzer

In many cases, the spectrum analyzer needs to measure the frequency spectrum of the transmitted signal or the radio frequency signal received from the air, and the power is often lower than a few milliwatts or even a dozen or so tenths of a millimeter. The conversion is a few microvolts or even lower. Therefore, measurement result unit of a spectrum analyzer is usually "dBm". Based on this, the spectrum analyzer can observe a large dynamic range of signals, which means that the small signal power can be one millionth, 1/10000 million, one hundred millionth of the power of a large signal. In the field of radio frequency measurement, it is often necessary to have such a large dynamic range.

3-wide dynamic range display.png


The spectrum analyzer is suitable for the measurement needs

1. Concerned about the RF equipment and RF signal testing (carrier and after modulation signal), such as testing RF transceiver

2. Wide band of unknown signals to be checked, such as the composition of the air signal and a variety of radio frequency interference, EMC testing of electronic products, etc.


3. At the same time concerned about the large signals and small signals, such as the signal to understand the transmitter distortion, intermodulation, clutter and so on.

4. Concerned about the millivolt level below the weak signal, such as the attention of the printed circuit board signal coupling, power and clock parasitic clutter.