A Guide to Calibrating Your Spectrum Analyzer Application Note Introduction As a technician or engineer who works with example, the test for noise sidebands that deter- electronics, you rely on your spectrum analyzer to mines whether the spectrum analyzer meets its verify that the devices you design, manufacture, phase noise specification often expresses the results and test—devices such as cell phones, TV broadcast in dBc, while analyzer specifications are typically systems, and test equipment—are generating the quoted in dBc/Hz. Consequently, the test engineer proper signals at the intended frequencies and must convert dBc to dBc/Hz as well as applying levels. For example, if you work with cellular radio several correction factors to determine whether the systems, you need to ensure that carrier signal spectrum analyzer is in compliance with specifications. harmonics won’t interfere with other systems For these reasons, spectrum analyzer calibra- operating at the same frequencies as the harmon- tion is a task best handled by skilled metrolo- ics; that intermodulation will not distort the infor- gists, who have both the necessary equipment mation modulated onto the carrier; that the device and an in-depth understanding of the procedures complies with regulatory requirements by operating involved. Still, it’s helpful for everyone who works at the assigned frequency and staying within the with spectrum analyzers to understand the value allocated channel bandwidth; and that unwanted of calibrating these instruments. This application emissions, whether radiated or conducted through note is intended both to help application engineers power lines or other wires, do not impair the opera- who work with spectrum analyzers understand the tion of other systems. importance of regular calibration and to explain to All of these measurements can be verified with calibration lab metrologists the key steps in cali- a spectrum analyzer, which displays the frequency brating a spectrum analyzer. It begins with a brief content of the signal generated by a device. But the recap of what a spectrum analyzer is and what it performance of spectrum analyzer circuits can drift does and then goes on to describe several of the over time and under changing temperature condi- most important tests needed to keep your spectrum tions. This drift affects the accuracy of the analyz- analyzer operating within specifications. er’s measurements. If the analyzer’s measurements are not accurate, the devices you are testing with it may not perform as intended. Table of Contents Since you use your spectrum analyzer to test other equipment, you need to have confidence in Introduction...............................................................................1 its measurements. Confidence, both that a device What.is.a.spectrum.analyzer?....................................................2 that tests out as good really is operating properly, and that a device that tests out as having problems What.does.a.spectrum.analyzer.measure?..................................3 really does not meet requirements. A high level Which.calibration.tests.are.needed?.........................................4 of confidence is especially important these days, Display Linearity test ......................................................................... 5 as more signals are crowded into the same space, Noise Floor test ................................................................................... 6 making even small deviations capable of causing Input Attenuator test .......................................................................... 7 problems. This is why it is important to calibrate Absolute Amplitude Accuracy and Frequency Response test ...... 8 your spectrum analyzer at the intervals specified by Resolution Bandwidth and Selectivity test .................................... 9 the manufacturer and why it is vital that all critical Resolution Bandwidth Switching test ........................................... 10 spectrum analyzer functional parameters are tested Reference Level Accuracy test ....................................................... 11 to make sure they are operating within specification. Noise Sideband test ......................................................................... 12 Spectrum analyzers are often perceived as Residual FM test ............................................................................... 13 being complex products that are time-consuming Frequency Span test ........................................................................ 14 to calibrate. It is true that the procedure can take Sweep Time Accuracy test ............................................................. 15 several hours or even days and requires an array of Harmonic Distortion test .................................................................. 16 equipment, including signal sources, sophisticated Third-Order Intermodulation Intercept test ...................................17 reference standards, and accessories. However, Gain Compression test ......................................................................17 calibration time can be significantly reduced by Manual.vs..automatic.testing...................................................18 simply automating the verification process. Another issue with spectrum analyzer calibration Conclusion...............................................................................18 is the difficulty of interpreting the test results. For References...............................................................................18 What is a spectrum analyzer? There are several types of spectrum analyz- • An IF.filter, which is a bandpass filter whose ers, ranging from low-cost entry-level handheld bandwidth is adjustable from the spectrum devices and traditional analog analyzers to modern, analyzer’s front panel. This bandwidth, referred high-performance analyzers employing digital to as IF bandwidth or resolution bandwidth, signal processing techniques. In this application determines how well input signals with small note, we concentrate on the swept-tuned, super- frequency differences can be distinguished from heterodyne spectrum analyzer. However, it should each other. be noted that real-time spectrum analyzers have a • A detector/log.amplifier, which responds to the significantly different architecture and are outside IF signal level, performing a logarithmic conver- the scope of this application note. sion to obtain a display scaled in dB per division. Figure 1 shows the key components that make up a typical swept tuned, superheterodyne spec- • A video.filter (sometimes abbreviated as VBW, trum analyzer. Its architecture resembles that of an for video bandwidth), which uses low-pass filter- AM superheterodyne receiver, in which a mixer is ing to average and smooth the displayed trace. used to down-convert the input signal to a lower, • A display, which shows the spectrum of the intermediate frequency (IF) for processing. Most measured input signal. As the local oscilla- spectrum analyzers use two or three stages of tor sweeps, the spectrum analyzer digitizes down-conversion, but a single conversion is shown the measured signal levels and stores them for here for simplicity. subsequent display as a complete spectrum. As the figure illustrates, a swept tuned, superhet- (Older analyzers without digital storage used erodyne spectrum analyzer typically consists of the long-persistence CRT displays that displayed the following components: spectrum trace as the sweep progressed.) • An.RF.input.attenuator, which reduces the • A sweep.generator, which controls the amplitude of high-level input signals to prevent frequency of the local oscillator and the refresh the mixer from being overloaded. rate on the analyzer display. • A mixer, which combines the input and local • A local.oscillator, which can be swept to oscillator frequencies to frequency shift the input generate the normal display or held constant in signal as the local oscillator sweeps, allowing a zero-span mode. With modern analyzers, which narrow band of input frequencies to pass through use frequency synthesizers as the local oscilla- the IF gain amplifier and filter for measurement. tor, the resolution of the synthesizer setting will • A variable IF.gain circuit, which amplifies the influence the accuracy of both the display and mixer output before passing it to the IF filter, the cursor frequency. which then filters out the signals of interest. It is important that this gain be variable to allow the reference level at the top of the display to correspond to the required input signal level. Input Attenuator Mixer IF Gain IF Filter Detector/ Video Log Amp Filter (RBW) (VBW) Local Display Oscillator Sweep Figure 1. Architecture of a typical swept-tuned, superheterodyne spectrum analyzer. 2 Fluke Corporation A guide to calibrating your spectrum analyzer. What does a spectrum analyzer measure? Marker readout The display also shows the marker readout, or A spectrum analyzer displays the frequency content cursor position. For spectrum analyzers with digital of a signal, with the horizontal (X) axis indicating displays, differences between the cursor accuracy the signal frequency and the vertical (Y) axis the and display accuracy are less of a concern, as amplitude, as illustrated in Figure 2. In calibrating a the displayed trace and the cursor readouts are spectrum analyzer, it’s important to verify that the produced from the same measurement data. But X