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Classification and operation of spectrum analyzers

sohu | 18/06/2021

A spectrum analyzer is an instrument for studying the spectrum structure of electrical signals. It is used to measure signal distortion, modulation, spectral purity, frequency stability, and intermodulation distortion. It can be used to measure certain circuit systems such as amplifiers and filters. Parameter is a multi-purpose electronic measuring instrument. It can also be called frequency domain oscilloscope, tracking oscilloscope, analysis oscilloscope, harmonic analyzer, frequency characteristic analyzer or Fourier analyzer. Modern spectrum analyzers can display analysis results in analog or digital ways, and can analyze electrical signals in all radio frequency bands from very low frequency to sub-millimeter wave bands below 1 Hz. If digital circuits and microprocessors are used inside the instrument, it has storage and calculation functions; equipped with standard interfaces, it is easy to form an automatic test system.

Spectrum analyzers are divided into two types: real-time analysis type and sweep frequency type. The former can obtain all the required spectrum information and analyze and display the analysis results within the actual time when the measured signal occurs; the latter requires multiple sampling processes to complete repeated information analysis. The real-time spectrum analyzer is mainly used for non-repetitive, short-duration signal analysis. The non-real-time spectrum analyzer is mainly used for the analysis of a certain segment of continuous radio frequency signals and periodic signals from the audio frequency to the sub-millimeter wave band.

Sweep frequency spectrum analyzer

It is a swept frequency superheterodyne receiver with a display device, which is mainly used for spectrum analysis of continuous signals and periodic signals. It works from audio to sub

In the millimeter wave frequency band, only the amplitude of the signal is displayed, but the phase of the signal is not displayed. Its working principle is: the local oscillator adopts a swept frequency oscillator, its output signal and each frequency component in the measured signal undergo difference frequency conversion in sequence in the mixer, and the generated intermediate frequency signal passes through a narrowband filter. After amplification and detection, it is added to the video amplifier as the vertical deflection signal of the oscilloscope, so that the vertical display on the screen is proportional to the amplitude of each frequency component. The frequency sweep of the local oscillator is controlled by the sawtooth voltage generated by the sawtooth wave sweep generator. The sawtooth wave voltage is also used as the horizontal sweep of the oscilloscope, so that the horizontal display on the screen is proportional to the frequency.

working principle. Use the swept frequency oscillator as the local oscillator of the superheterodyne receiver. When the selector switch S is set to 1, the sawtooth sweep voltage sweeps the local oscillator I. The frequency components in the input signal are in the mixer. Perform difference frequency with the local sweep signal, they fall into the passband of the first middle-amp narrowband filter in turn, are selected by the filter, after secondary frequency conversion, detection, and amplification, they are added to the vertical deflection of the oscilloscope tube The system makes the vertical display on the screen proportional to the amplitude of each frequency component. The sweep voltage is simultaneously applied to the horizontal deflection system of the oscilloscope, so that the X coordinate of the frequency screen becomes the frequency coordinate, and the analyzed input signal spectrogram is displayed on the screen. The above-mentioned working mode is frequency-swept on the local oscillator I, which is called "scan-before-type" working mode, which has a wide analysis frequency band. When S is set to 2, it can also perform frequency sweep on the local oscillator II, which is called "sweep intermediate frequency" working mode. At this time, narrowband spectrum analysis can be performed.

Real-time spectrum analyzer

The instrument that extracts all the spectrum information of the signal for analysis and displays the results within the limited time of the measured signal is mainly used to analyze non-repetitive stable random processes and transient processes with a short duration, and can also analyze the frequency below 40 MHz Low-frequency and extremely low-frequency continuous signals can display amplitude and phase. The Fourier analyzer is a real-time spectrum analyzer. Its basic working principle is to convert the analyzed analog signal into a digital signal through an analog-to-digital conversion circuit, and then add it to the digital filter for Fourier analysis; it is controlled by the central processing unit The quadrature digital local oscillator produces the digital local oscillator signal that changes according to the sine law and the cosine law, which is also added to the digital filter and the measured signal for Fourier analysis. The quadrature digital local oscillator is a swept frequency oscillator. When its frequency is the same as the frequency of the signal under test, it will output. After integration processing, the analysis result is obtained for the oscilloscope to display the spectrum graph. The analysis result of quadrature local oscillator using sine and cosine signals is complex, which can be converted into amplitude and phase. The analysis results can also be sent to a printing plotter or connected to a computer through a standard interface.

Frequency Range

The frequency range in which the spectrum analyzer works normally. The frequency range of modern spectrum analyzers can range from less than 1 Hz up to 300 GHz.


The ability of a spectrum analyzer to distinguish the frequency interval between two adjacent spectrum lines on the display is an important technical indicator of a spectrum analyzer.

Mark. The resolution is related to the filter type, form factor, bandwidth, local oscillator stability, residual frequency modulation and sideband noise. The resolution of the swept spectrum analyzer is also related to the scanning speed. The narrower the resolution bandwidth, the better. Modern spectrum analyzers have a resolution of 10 to 100 Hz in high frequency bands.

Analysis spectrum width

Also known as frequency span. The frequency range that a spectrum analyzer can display in a measurement and analysis can be equal to or smaller than the frequency range of the instrument, and is usually adjustable.

Analysis time

The time required to complete a spectrum analysis is closely related to the analysis spectrum width and resolution. For a real-time spectrum analyzer, the analysis time cannot be less than the reciprocal of its narrow resolution bandwidth.

Sweep speed: the ratio of the analysis spectrum width to the analysis time, that is, the rate of change of the local oscillator frequency of the sweep.


The ability of a spectrum analyzer to display weak signals is limited by the internal noise of the spectrum analyzer, and the higher the sensitivity, the better. The dynamic range refers to the ratio of the strong signal to the weak signal that can be observed simultaneously on the display. The dynamic range of modern spectrum analyzers can reach 80 decibels.

Display method

The relationship between the amplitude displayed by the spectrum analyzer and the amplitude of the input signal. There are usually three modes: linear display, square law display and logarithmic display.

False response

Unwanted spectral lines appear on the display. This is inevitable for superheterodyne systems and should be kept to a minimum. Modern spectrum analyzers can achieve less than -90 decibel milliwatts.


Hard keys, soft keys and knobs

This is the basic operation method of the instrument.

1. Three big hard keys and one big knob: The function of the big knob is set by three big hard keys. Press the frequency hard key, the knob can fine-tune the center frequency displayed by the instrument; press the sweep width hard key, the knob can adjust the frequency width of the instrument sweep; press the amplitude hard key, the knob can adjust the signal amplitude. When you turn the knob, the center frequency, sweep width (start, stop frequency), and amplitude in dB are displayed on the screen at the same time.

2. Soft keys: On the right side of the screen, there is a row of longitudinally arranged unmarked keys whose functions vary with the item. What is displayed on the right side of the screen corresponding to the key is what the key is.

3. Other hard keys: There are ten hard keys in the INSTRUMNT STATE control area: RESET reset, CANFIG configuration, CAL calibration, AUX CTRL auxiliary control, COPY printing, MODE mode, SAVE storage, RECALL recall, MEAS/USER Measurement/user-defined, SGL SWP signal scanning. There are four hard keys in the MARKER area: MKR cursor, MKR cursor movement, RKR FCTN cursor function, PEAK SEARCH peak search. There are six hard keys in the control (CONTRL) area: SWEEP scan, BW bandwidth, TRIG trigger, AUTO COVPLE automatic coupling, TRACE tracking, DISPLAY display. There is a BKSP back in the numeric keypad. On the right side of the numeric keypad is a vertical row of four ENTER keys, which are also unit keys. The three hard keys above the large knob are window keys: ON, NEXT, and ZOOM. The two keys with arrows, STEP under the big knob, can be used to adjust up and down in conjunction with the big knob.

Input and output interface

Located one row below the same panel. TV IN signal input port for measuring video indicators; VOL INTEN is a set of internal and external knobs to control and adjust the volume and screen brightness of the built-in speaker; CAL OUT instrument self-test signal output; 300Mhz 29dBmv instrument standard signal output port; PROBE PWR instrument probe power supply ; IN 75Ω1M—1.8G test signal total input port.

Test preparation

1. Restrictive protection: stipulate high input RF level and high voltage value causing permanent damage: DC 25V, AC peak-to-peak 100V.

2. Warm-up: The test must wait until the OVER COLD disappears.

3. Self-calibration: Perform self-calibration before using for three months or important measurement.

4. System measurement configuration: The configuration is to input some of the measured parameters before the measurement, eliminating the need for a parameter input for each measurement. Contents: test items, signal input method (frequency or channel), display unit, format, noise measurement bandwidth and sampling point, frequency point for measuring CTB and CSO, test line gating, etc. Configuration steps: Press MODE key-CABLE TV ANALYZER soft key-Setup soft key to enter the setting state. The details are tune config tuning configuration: including frequency, channel, format, and level unit. Analyzer input input configuration: whether to add a preamplifier. Beats setup beat frequency setting, measuring CTB, CSO frequency (frequency offset CTB FRQ offset, CSO FRQ offset). GATING YES NO Whether to gate the test line. C/N setup carrier-to-noise ratio settings: frequency (frequency offset C/N FRQ offset), bandwidth.


Spectrum Analyzers

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