Common Noise Specifications

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.

In real-world RF communications systems, characterizing noise is extremely important because of the high frequencies involved. Thus it is important to accurately represent two noise characteristics: phase noise (spectral purity) and the noise floor (noise density). These two topics are discussed below.

Phase Noise (Spectral Purity)

Phase noise refers to noise in a carrier signal due to phase and frequency modulation in the signal. Phase noise is normally very close to the carrier, and is measured in decibels relative to the carrier frequency (dBc).

Phase noise is expressed as a function of power spectral density and frequency. In a 1 Hz bandwidth, phase noise is given by the following formula:

 = 10log[0.5(S Φ(ƒ))] Φ

where SΦ(ƒ) is the spectral density of phase fluctuations.

Noise Floor (Noise Density)

The noise floor is the noise level below which signals cannot be detected, under the same measurement conditions.

For example, in an audio system the broadband noise level may be 5 µV. This means that broadband signal levels cannot be detected below this level. However, if the noise is broadband random noise, instead of consisting of sinusoidal components, you can use a narrow band filter to further investigate the noise.

Noise floor is normally specified as one or more of the following characteristics:

1. Broadband noise (referenced to full-scale deflection)
2. Spurious free dynamic range (the highest sinusoidal component referred to the full-scale deflection)
3. Noise spectral density (used to specify broadband random noise)

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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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