Spectrum analyzer

Spectrum analyzers are one of the important testings which are used to measure frequencies and many other parameters.

A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most common spectrum analyzers measure is electrical; however, spectral compositions of other signals, such as acoustic pressure waves and optical light waves, can be considered through the use of an appropriate transducer. Spectrum analyzers for other types of signals also exist, such as optical spectrum analyzers which use direct optical techniques such as a monochromator to make measurements. By analyzing the spectra of electrical signals, dominant frequency, power, distortion, harmonics, bandwidth, and other spectral components of a signal can be observed that are not easily detectable in time domain waveforms. These parameters are useful in the characterization of electronic devices, such as wireless transmitters.

The display of a spectrum analyzer has frequency on the horizontal axis and the amplitude displayed on the vertical axis. To the casual observer, a spectrum analyzer looks like an oscilloscope and, in fact, some lab instruments can function either as an oscilloscope or a spectrum analyzer.

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.

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.

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.

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

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.

Uses for Spectrum analyzers

Todays electronic circuit design laboratories will use very many test instruments. Everything from simple digital multimeters to oscilloscopes, signal generators and much more. Spectrum analyzers are particularly used in electronics laboratories associated with RF design and test. In these areas they can provide a view of a signal in a way that no other form of test instrument is able. This gives insights into the operation fo the radio frequency aspects of the circuit. The spectrum analyzer can be used for a number of tasks

Looking at the frequency spectrum of a signal to see items like the following
The overall spectrum of a modulated signal to see whether it is wide enough or too narrow, etc. If it is too wide then it could cause interference to users in adjacent channels.
To investigate whether any spurious or unwanted signals are present. These signals could cause interference to users on other frequencies is signals are transmitted.
To find out whether a signal is on the right frequency, and not in another band for example.
To investigate general problems with a signal. Often it can just help looking at a signal to see what a problem is. With RF signals a spectrum analyzer can prove to be the eyes for the person investigating the problem.
Sometimes spectrum analyzers can be used to measure power, although power meters may be more applicable in some circumstances.
Sometimes spectrum analyzers can be used to measure frequency, although frequency counters may be more applicable in some circumstances.
Spectrum analysers can also be used to measure the phase noise on a signal. This can be achieved provided that the pose noise on the spectrum analyzer local oscillator is typically 10dB better than that of the oscillator under test. These test instruments are one of the best ways to measure phase noise provided the spectrum analyzer local oscillator has a sufficiently low level of phase noise.
Another application for these test instruments is that of measuring the noise figure of an item. Although the test method does involve a number of stages, it can be undertaken relatively easily.
Spectrum analysers are often used when undertaking EMI & EMI (electromagnetic interference and electromagnetic compatibility) measurements. The analyzer can be used to locate the frequency and nature of the signal that may be causing an issue.

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

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Spectrum analyzer working principle, used and applications