What is a Spectrum Analyzer : Working & Its Applications

Spectrum analyzers are one of the important testings which are used to measure frequencies and many other parameters. Interestingly, spectrum analyzers are used to measure signals which we know and find signals which we don’t know. Due to its accuracy, the spectrum analyzer has gained a lot of applications in the field of electrical and electronic measurements. It is used to test many circuits and systems. These circuits and systems operate at radio frequency levels.


With its different model configurations, this device has its own versatility in the instrumentation and measurement field. It comes with different specifications, sizes, and even available based on specific applications. The use of the device in an even high-frequency range at the level of ultra-frequency is presently in research. It can be even connected to a computer system and the measurements can be recorded on the digital platform.

What is Spectrum Analyzer?

Spectrum Analyzer is fundamentally a testing instrument that measures various parameters in a circuit or in a system at radio frequency range. A piece of normal testing equipment would measure the quantity based on its amplitude with respect to time. For example, a voltmeter would measure the voltage amplitude based on the time domain. So we will get a sinusoidal curve of AC voltage or a straight line for DC voltage. But a spectrum analyzer would measure the quantity in terms of amplitude versus frequency.


Frequency domain response
Frequency domain response

As shown in the diagram, the spectrum analyzer measures the amplitude in the frequency domain. The high peak signals represent the magnitude, and in between, we have noise signals also. We can use the spectrum analyzer to eliminate the noise signals and make the system more efficient. Signal to noise cancellations factors (SNR) is one of the important features nowadays for electronic applications. For example, headphones come with a noise cancellation aspect. For testing such equipment, spectrum analyzers are used.

Analyzer Block Diagram

Block Diagram
Block Diagram

The block diagram of the spectrum analyzer is shown above. It consists of an input attenuator, which attenuates the input radio frequency signal. The attenuated signal is fed to a low pass filter to eliminate the ripple content.

The filtered signal is mixed with a voltage tuned oscillator, and fed to an amplifier. The amplifier is fed to the cathode ray oscilloscope. On the other side, we have a sweep generator also. Both are fed to the CRO for vertical and horizontal deflections.


Spectrum Analyzer Working Principle

The spectrum analyzer fundamentally measures the spectrum content of the signal i.e. fed to the analyzer. For example, if we are measuring the output of a filter, let us say low pass filter, then the spectrum analyzer would measure the spectrum content of the output filter in the frequency domain. In this process, it would also measure the noise content and display it in the CRO,

As displayed in the block diagram, the working of the spectrum analyzer can be fundamentally categorized as producing a vertical and a horizontal sweep on the cathode ray oscilloscope. We know that the horizontal sweep of the measured signal would be with respect to frequency and the vertical sweep would be with respect to its amplitude.

Working
Working

To produce the horizontal sweep of the measured signal, the signal at the radio frequency level is fed to the input attenuator, which attenuates the signal at the radio frequency level. The output of the attenuator is fed to the low pass filter to eliminate any ripple content in the signal. Then it is fed to an amplifier, which amplifies the magnitude of the signal to a certain level.

In this process, it is also mixed with the output of the oscillator which is tuned at a certain frequency. The oscillator helps to generate an alternating nature of the fed waveform. After getting mixed with the oscillator and amplified, the signal is fed to the horizontal detector, which converts the signal into the frequency domain. Here in the spectrum analyzer, the spectral quantity of the signal is represented in the frequency domain.

For the vertical sweep, the amplitude is required. To get the amplitude, the signal is fed to the voltage tuned oscillator. The voltage tuned oscillator is tuned at the radio frequency level. Generally, resistors and capacitors combination is used to obtain the oscillator circuits. This is known as RC oscillators. At the oscillator level, the signal gets phase shifted by 360 degrees. For this phase shifting, different levels of RC circuits are used. Usually, we have 3 levels.

Sometimes even transformers are also used for phase-shifting purposes. In most cases, the frequency of the oscillators is also controlled using a ramp generator. The ramp generator is also sometimes connected to a pulse width modulator to obtain a ramp of pulses. The output of the oscillator is fed to the vertical sweep circuit. Which provides amplitude on the cathode ray oscilloscope.

Types of Spectrum Analyzer

Spectrum analyzers can be classified into two categories. Analog and Digital

Analog Spectrum Analyzer

Analog spectrum analyzers use the superheterodyne principle. They are also called swept or sweep analyzers. As shown in the block diagram, the analyzer will have different horizontal and vertical sweep circuits. To show the output in decibels, a logarithmic amplifier is also used before the horizontal sweep circuit. A video filter is also provided to filter the video content. Using a ramp generator provides each frequency a unique location on the display, by which it can display the frequency response.

Digital Spectrum Analyzer

The digital spectrum analyzer consists of fast Fourier transform (FFT) blocks and analog to digital converters (ADC) blocks to convert the analog signal to a digital signal. By the block diagram representation

Digital Spectrum Analyzer
Digital Spectrum Analyzer

As shown by the block diagram representation, the signal is fed to the attenuator, which attenuates the level of the signal, and then fed to LPF for eliminating the ripple content. Then the signal is fed to an analog to digital converter (ADC) which converts the signal to the digital domain. The digital signal is fed to the FFT analyzer which converts the signal into the frequency domain. It helps to measure the frequency spectral of the signal. Finally, it is displayed using the CRO.

Advantages and Disadvantages of the Analyzer

It has many advantages, as it measures the spectral quantity in the signal on the radio frequency range. It also provides a number of measurements. The only disadvantage is its cost, which is higher as compared to the usual conventional meters.

Applications of Analyzer

A spectrum analyzer which is fundamentally used for the testing purpose can be used to measure a variety of quantities. All these measurements are made at the radio frequency level. Frequently measured quantities using spectrum analyzer are-

  •  Signal levels– The amplitude of the signal based on the frequency domain can be measured using the spectrum analyzer
  • Phase Noise – As the measurements are done on the frequency domain and the spectral content is measured, the phase noise can be easily measured. It appears as ripples in the output of the cathode ray oscilloscope.
  • Harmonic distortion – This is a major factor to be determined for the quality of the signal. Based on harmonic distortion, the total harmonic distortion (THD) is calculated to evaluate the power quality of the signal. The signal must be saved from sags and swells. Reduction in harmonic distortion levels is even important to avoid unnecessary losses.
  • Intermodulation distortion– While modulating the signal, based on the amplitude (Amplitude modulations) or frequency (frequency modulation) distortions are caused in the intermediate level. This distortion must be avoided to have a processed signal. For this, a spectrum analyzer is used to measure the intermodulation distortion. Once the distortion is reduced using external circuits, the signal can be processed.
  • Spurious Signals– These are unwanted signals to be detected and eliminated. These signals cant be measured directly. They are unknown signal which needs to be measured.
  • Signal Frequency– This is also an important factor to be evaluated. Since we used the analyzer at the radio frequency level, the band of frequencies is very high, and it becomes important to measure the frequency content of each and every signal. For this spectrum, analyzers are specifically used.
  • Spectral Masks – Spectrum analyzers are also helpful to analyze the spectral masks

Hence we have seen the working principle, design, advantages, and application of spectrum analyzer. One must think, how to store the data that is being measured in a spectrum analyzer? And how to transfer it to other mediums like the computer for further measuring.

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