Spectral Measurements (Part 1)

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining the theory and giving practical examples. This tutorial covers an introduction to RF, wireless, and high-frequency signals and systems.

For the complete list of tutorials, return to the NI Measurement Fundamentals Main page, or for more RF tutorials, refer to the NI RF Fundamentals main subpage. For more information about National Instruments RF products, visit www.ni.com/rf.

Several common spectral measurements in RF and communications systems include power in band, occupied bandwidth, peak search, and adjacent channel power. The NI Spectral Measurements Toolkit combined with the National Instruments vector signal analyzer can make each of these measurements. The following document describes the theory and applications related to each of these measurements.

Power in Band

Power in band measures the total power within any specified frequency range or band. Power in band is characterized by the following equation:

where X is the input power spectrum from a specified band, f_l is the low bound of the frequency band, and f_h is the high bound of the frequency band. The low and high bounds of this band can be determined from the center frequency.

Occupied Bandwidth

Occupied bandwidth is a measurement of the frequency band bandwidth that contains a specified percentage of the total power of the signal. Occupied bandwidth is the inverse of power in band.

For a specified percentage B, the upper and lower limits of the frequency band are the frequencies within which the total power is found. For example, if B is 99, then the occupied bandwidth is the bandwidth that contains 99% of the total power of the signal. Figure 1 shows a calculated occupied bandwidth of 15 MHz.

Figure 1. Occupied Bandwidth Calculation in LabVIEW

Peak Search

A spectral peak search algorithm determines the levels and frequencies of peaks in a specified band. The algorithm uses interpolation to precisely locate frequency peaks in the amplitude or power spectrum in any units or scaling. You can also specify whether to locate a single maximum peak or multiple peaks that exceed a specified threshold. Figure 2 shows an example of locating multiple peaks above a specific threshold.

Figure 2. Peak Search in LabVIEW

Adjacent Channel Power

Adjacent channel power (ACP) measures the way a particular channel and its two adjacent channels distribute power. This measurement is performed by calculating the total power in the channel and also the total power in the surrounding upper and lower channels. Figure 3 illustrates a typical ACP measurement and the center frequency, bandwidth, and spacing that describe the channels.

Many technologies allocate adjacent channels for information distribution from different providers, such as cell phones, TV, radio, and cable. In these and other applications, it is important that transmission from one channel does not cross over to an adjacent channel, which noticeably degrades the quality in the other channel.

Depending on the technology standard you are measuring, different criteria exist for adjacent channel power measurements. For example, the core division multiple access (CDMA) wireless standard requires transmissions to fit within a 4.096 MHz bandwidth. Moreover, adjacent channel power, measured at 5 MHz offsets, must be at least 70 dB below the in-channel average power.

Related Products

NI PXIe-5663 6.6 GHz RF Vector Signal Analyzer
The National Instruments PXIe-5663 is a modular 6.6 GHz RF vector signal analyzer with 50 MHz of instantaneous bandwidth optimized for automated test.

NI PXIe-5673 6.6 GHz RF Vector Signal Generator
The National Instruments PXIe-5673 is a 4-slot 6.6 GHz RF vector signal generator that delivers signal generation from 85 MHz to 6.6 GHz, 100 MHz of instantaneous bandwidth, and up to 512 MB of memory.

NI PXI-5660 2.7 GHz RF Vector Signal Analyzer
The National Instruments PXI-5660 is a modular 2.7 GHz RF vector signal analyzer with 20 MHz of instantaneous bandwidth optimized for automated test.

NI PXI-5671 2.7 GHz RF Vector Signal Generator
The National Instruments PXI-5671 module is a 3-slot RF vector signal generator that delivers signal generation from 250 kHz to 2.7 GHz, 20 MHz of instantaneous bandwidth, and up to 512 MB of memory.

NI PXI-5652 6.6 GHz RF and Microwave Signal Generator
The National Instruments PXI-5652 6.6 GHz RF and microwave signal generator is continuous-wave with modulation capability. It is excellent for setting up stimulus response applications with RF signal analyzers.

NI RF Switches
The National Instruments RF switch modules are ideal for expanding the channel count or increasing the flexibility of systems with signal bandwidths greater than 10 MHz to bandwidths as high as 26.5 GHz.

NI LabVIEW
National Instruments LabVIEW is an industry-leading graphical software tool for designing test, measurement, and automation systems.

NI Modulation Toolkit
The National Instruments Modulation Toolkit extends the built-in analysis capability of LabVIEW with functions and tools for signal generation, analysis, visualization, and processing of standard and custom digital and analog modulation formats.

Conclusion

For the complete list of tutorials, return to the NI Measurement Fundamentals main page, or for more RF tutorials, refer to the NI RF Fundamentals main subpage. For more information about National Instruments RF products, visit  www.ni.com/rf.

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