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 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. Analog spectrum analyzers use the superheterodyne principle. Spectrum analysers normally use a linear scale for the frequency on the horizontal or x-axis, but they normally use a logarithmic scale for the amplitude on the vertical or y-axis. By using a logarithmic or decibel scale for the amplitude scale, it is possible to see signals with large differences in amplitude.
Signals being viewed on a spectrum analyzer may differ by 60dB, 70 dB or more. Using a logarithmic scale is the only way to see these signals on the same screen. For some applications it may be necessary to use a linear amplitude scale, and often there is a switch to accomplish this.
Modern spectrum analyzers have a high degree of capability. As most use digital techniques, not only is the signal processing accomplished using digital signal processing using Fast Fourier Analysis, FFT, but also the front panel controls and display are controlled using a control processor. This enables the spectrum analyzer to incorporate a host of capabilities and include a good number of automated routines. The display can normally determine the peak signal, displaying its frequency and power level, or the it can determine the value of the signal at a particular point, or the value of a second peak, etc.
These and many functions are common place in todays test instruments. 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. Although the RF spectrum analyzer can be used for many radio frequency tests, the table below gives a summary of the different types of test instruments used for RF testing and their typical applications.
Spectrum analyzer types: There are several different types of spectrum analyzer that can be bought and used. Each type has its own characteristics: performance and cost can be balanced to give the best option for any application. Older types are normally based around the superheterodyne principle, sweeping the receiver across a band of frequencies and noting the output. More modern spectrum analysers use fast Fourier transforms, FFTs, converting the signal from an analogue to a digital format and then using Fourier analysis to monitor the sign.
There are also real time spectrum analysers. These are based upon the same concept as FFT spectrum analyzers, but these test instruments use time overlapping samples to ensure that no transients are lost. Although more complicated internally, because they need to be able to accommodate very fast and overlapping sampling, they ensure that all signals can be seen. They are often essential for resolving some of the issues with the very complicated equipment that is being produced today for the various forms of wireless communication: 5G; Wi-Fi, etc.
There are also variations in the format of the test instruments as well. Standard box instruments are widely used, and not only do they offer manual control, but they also provide provision for remote operation via a number of different interfaces often including Ethernet, USB and GPIB.
There are also options for rack mounted spectrum analyzers. Typically, you use a directional coupler to tap the power without interrupting the communications. In this way, you can verify that the frequency and signal strength of your transmitter are according to the specified values. Interference Measurements: Any large RF installations normally require site survey. A spectrum analyzer can be used to verify identify and interferences.
Any such interfering signals need to be minimized before going ahead with the site work. Interference can be created by a number of different sources, such as telecom microwave towers, TV stations, or airport guidance systems etc.
Note: The above specifications are given as an example only, and may not accurately represent the actual equipment specifications. The price is primarily determined by the frequency range i.
Some instruments have additional options such as in-build tracking generator, frequency counter, or power meter that may also add to the overall cost. Toggle navigation. What is a Spectrum Analyzer? Spectrum Analyzer Vs. Oscilloscope 3.
Simplified Block Schematic of a Heterodyne Receiver 4. Applications for Spectrum Analyzers 6. Spectrum Analyzers that you may consider buying 7. More Information 1. Oscilloscope A spectrum analyzer displays received signal strength y-axis against frequency x-axis. An Oscilloscope, displays received signal strength y-axis against time x-axis. Spectrum analyzer is useful for analyzing the amplitude response of a device against frequency. The amplitude is normally measured in dBm in Spectrum Analyzers, where as the same is measured in volts when using Oscilloscopes.
Normally, Oscilloscope can not measure very low voltage levels say, dBm and are intended for low frequency, high amplitude measurements. A spectrum analyzer can easily measure very low amplitudes as low as dBm , and high frequencies as high as GHz. The spectrum analyzer measurements are in frequency domain, whereas the oscilloscope measurements are in time domain. Also, a spectrum analyzer uses complex circuitry compared with an Oscilloscope.
As a result of this, the cost of a spectrum analyzer is usually quite high.
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