The Spectrum Analyzer
The spectrum analyzer, also referred to as a “spec an”, is an essential piece of test equipment for the engineer’s bench. The spectrum analyzer displays frequency vs. amplitude as opposed to time vs. amplitude as with an oscilloscope. Since spectrum analyzers display frequency vs. amplitude they are essentially an AC input device. This means that they cannot display DC values like an oscilloscope can. However, slowly varying DC is a perfectly valid input signal. The use of a spectrum analyzer allows the visualization of the harmonic content of any signal. An absolutely pure sine wave will have no harmonics (multiples of the fundamental frequency). Since this is impossible in the real world a spectrum analyzer will display the entire harmonic content of any signal .
Spectrum Analyzer Technologies
There are two types of technologies used to produce a display, though they are combined in the latest instruments. One technology is the swept local oscillator type. It compares the input signal against an internal swept frequency and uses a mixer to display a trace. The second type uses digital signal processing and fast Fourier analysis, FFT, to produce a trace . FFT analyzers are the most common type in low frequency analysis. By using a numerical analysis such as FFT, results can be displayed more rapidly than by other techniques.
Spectrum Analyzer Types
Spectrum analyzers come is basically three types. Analyzers that display frequencies above 100kHz are the most common and are used in the RF realm. They can display frequencies into the 10’s of GHz. The Keysight N9000 series is representative of this type. Another type of spectrum analyzer deals with audio frequencies and is used for sound analysis and highly precise distortion measurements. An example of this type would be the Audio Precision model 2700, an industry standard. The third type of spectrum analyzer deals with extremely low frequency inputs and is known as a dynamic signal analyzer. These are used for sub Hz signal analysis such as measuring slowly varying phenomena. Stanford Research model SR785 is an example of this type.
Dynamic Range Considerations
Besides frequency range another consideration with spectrum analyzers is dynamic range. It is limited by a parameter called “noise floor” typically measured in units of dBm, the noise floor will vary between spectrum analyzer types with the lowest noise floor associated with lower frequencies across all types of analyzers. If you need more dynamic range, i.e. a lower noise floor, then you can add optional LNAs. These are low noise amplifiers that have been optimized for low noise figure. All RF spectrum analyzers have 50 ohm inputs, however the lower frequency type analyzers will have selectable high impedance 1 Meg ohm inputs as well.
Applications of the Spectrum Analyzer
As mentioned earlier in this article, some analyzers have DSP capability. Modern spectrum analyzers with this option installed allows them to demodulate many complex RF signals. The most basic are AM and FM modulation. In fact you can use them to listen to local radio or TV station audio by setting the analyzer to zero frequency span and tuning it to the center frequency of the channel of interest. (A good way to listen to Monday night football for those late nights in the lab.) In reality a modern spectrum analyzer with demodulation capability can be thought of as a universal radio. As long as a particular demodulation option is installed, the analyzer can recover that modulation signal anywhere within it’s frequency span. This can include all of the various cellular standards in 3G, 4G, and LTE. Another use for these analyzers is radar analysis. Due to the extremely high frequencies of some radar, a device known as a downconverter will need to be used in conjunction with the analyzer. (Downconverters are available from the major spectrum analyzer manufacturers.) Another major application is in the field of EMI, or electro-magnetic interference. Spectrum analyzers, because basically they are precision radio receivers, are essential for the testing of electrical interference, coming from or induced into, equipment under test. Every piece of electronic equipment, from hearing aids, to cell phones, to rack size systems, to automobiles, to aircraft will need to be tested for electro-magnetic interference. EMI testing is a worldwide requirement. EMI is another facet of world wide EMC, or electro-magnetic compatibility.
Features of the Spectrum Analyzer
Some analyzers have provisions to drive noise sources. This extends the analyzer capability to enable noise figure analysis. It is important to note the many types of displays that a modern analyzer can provide as well. Cumulative power density plots, histograms, IQ modulation diagrams for QAM signals, variable persistence eye diagrams, inverse Fourier plots showing the waveform in terms of time, and many others. Connectivity is also a feature of modern spectrum analyzers. No need for a floppy drive anymore; serial, GPIB, parallel, multiple USB, even Centronics ports to drive an external display are also a feature on some models. They can be used to drive an LCD projector for design reviews. Direct drive of many types of printers for hardcopy is universally available. It really is amazing what a modern spectrum analyzer can measure and display.
In choosing which type spectrum analyzer you need, you need to determine what the upper and lower frequency range is of the signal you will be measuring, as well as its maximum amplitude. Take into consideration the harmonic content as well. Remember, you can’t go wrong if you extend the frequency range you think you need. If the signal amplitude exceeds the input range of the analyzer you can always use external attenuators to reduce the signal level to proper input amplitude.
Since 2005, Axiom Test Equipment has been providing a variety of spectrum analyzers. Additionally we offer repair on most brands and types of spectrum analyzers. Major manufacturers we carry include Anritsu, Teledyne/LeCroy, Rhode & Schwarz, and Keysight (formerly Agilent). Axiom Test Equipment offers a complete range of spectrum analyzers for your needs, with frequencies ranging from 0.01 Hz up to more than 50 GHz as well as stocking handheld units. View these online at, http://www.axiomtest.com/Spectrum-Analyzer.
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