Spectrum Analyzer Guide
If you deal with electronic devices design or repair (those involving radio frequency signals), spectrum analyzer is an important test equipment that you need to have under your fingertips all the time. It is a quite complicated instrument, although it takes a little while to get used to it. If you are a novice or an engineer amateur, information given in this article will turn very handy for you. Below we are going to dwell upon the basics of analyzer structure, work and practical use.
Spectrum Analyzer Principle
The analyzer may have similar features with an oscilloscope as both provide a graphical display. Although, the former gives frequency against the amplitude (power) data, while the latter displays amplitude and time.
Actually, a typical spectrum analyzer works as a common radio receiver. It is adjusted to the required range and its filters are made to accept the needed signal bandwidth. The underlying principle of the analyzer work is a superhet receiver which uses a mixer for frequency translation. In essence, these analyzers work as a superhet radios which are modified in such a way that the received waves are displayed graphically.
Different kinds of spectrum analyzers can turn into powerful tools for RF (radio frequency) equipment testing after a short familiarization. Spectrum analyzers are very practical and can be used for many different purposes, like these:
- to estimate the overall spectrum of a modulated signal (to check whether is wide enough etc);
- to detect any spurious or undesired signals;
- to check whether the signal’s frequency is right;
- to give a graphic display of the signal – sometimes you need to ‘see’ the signal to indicate the problem; in this case analyzers work as tester’s eyes;
- to measure power; spectrum analyzers are not accurate in power measuring, but can also be used for this purpose; to measure power precisely use special power meters;
- to measure frequency; it is not designed to display frequency data, but in case of need you can obtain the figures (to get the precise ones use frequency counters).
In other words, spectrum analyzers receive a signal and display it in the form of easily comprehensible data.
Different Types of Spectrum Analyzers
There is a variety of spectrum analyzers available on the market. According to the nature of signal you are going to measure, you can easily find audio, sub audio, radio frequency, microwave and optical analyzers. Besides the signal type division, there also exist analog, digital and hybrid analyzers. As a rule, digital spectrum analyzers are the most practical, as they can perform more complex analysis. These can transform electrical or optical signal into simpler and smaller components. Digital analyzers can vary in their characteristics (bandwidth range, resolution, frequency etc.) from a model to a model.
For those who are unfamiliar with testing equipment of any kind, using spectrum analyzers can be a complicated task. However, after some practice it is possible to make a good use of the device to obtain precise measurements.
Let’s take a look at the basic controls of the device:
- The display. Spectrum analyzer display shows the ratio of amplitude against frequency. It is a vital part of the device structure as it gives the plot or trace of the signal in such a way that it can be easily recognizable. The display typically has ten vertical and ten horizontal divisions for this purpose. The horizontal axis is for frequency (the right hand side indicates the higher frequency). The vertical axis shows the amplitude data. The scale is usually calibrated in dBm (decibels relative 1 milliwatt). One division of the display as a rule has 10 dBm value.
- Frequency settings. There are two options for this: center frequency or span. The former adjusts the frequency of the scale center to the desired value. Typically it must correspond to the signal that is going to be monitored. In this case the monitored signal will be in the middle of the graticule, while the rest of the minor or spurious signals will appear to the left or right. The span selection sets the extent of the signal that will be monitored and displayed.
- Gain and attenuation. It is important to adjust these specs properly in order to achieve accurate measurements. RF attenuator is used in case when the sections are overloaded and spurious signals generated by the spectrum analyzer itself can affect the readings.
- Scan characteristics. It goes without saying that the faster analyzers scan, the faster you get the measurements. The scanning ability of the spectrum analyzers is hidden in their two elements: the filter used in IF and the video filter.
- Bandwidths. IF and video filters are also very essential controls. IF filter is responsible for the resolution of the analyzer concerning frequency. Choosing a narrow bandwidth for filter, for example, will enable the analyzer to detect the signals which are very close together. Although, because of the narrow band filters cannot react quickly. Video filters are used to reduce the variations caused by noise thus detecting weaker signals. Both filters usage limits the speed of the analyzer scanning abilities.
These are the common controls used for average spectrum analyzers of different types. To learn more about your particular model specs read the manual and have some practice with the device. Spectrum analyzers are frequently used today as they serve for different purposes in equipment testing. Don’t be afraid of complicated look of the device, it is rather simple in use and reading after a while.
High quality spectrum analyzers are not cheap, that is why it is important to conduct a thorough research before buying one. First you need to figure out which particular type and model you need for your particular application. In terms of practice, you can choose from hand held, bench top, fixed and portable spectrum analyzers. Another thing to consider is a presence of additional instruments and PC based units (USB analyzers, for example). The specifications of the chosen model must correspond to the device’s characteristics. The spectrum analyzer must cover the frequencies of the appliance it will be used for.