Understanding RF & Microwave Specifications - Part I


This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series, will teach you a specific topic of common measurement applications, by explaining the theory and giving practical examples. This tutorial covers the first part of RF and microwave specifications.

For the complete list of tutorials, return to the NI Measurement Fundamentals Main page.


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:

 = 10log[0.5(S Φ(ƒ))] Φ


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

Spectral Density

Spectral density is a measure of total signal power in a specified spectral bandwidth divided by the bandwidth, expressed in watts per hertz (W/Hz).

Resolution Bandwidth (RBW)

The resolution bandwidth (RBW) determines the FFT bin size or the smallest frequency that can be resolved. For a traditional swept-tuned (non-digital) spectrum analyzer, the resolution bandwidth is the bandwidth of the IF filter which determines the selectivity. For wide sweeps a wide resolution bandwidth is required to shorten acquisition times and for narrow sweeps a narrow filter is used to improve frequency resolution. In FFT-based (digital) spectrum analyzers and vector signal analyzers (VSA) the resolution bandwidth is inversely proportional to the number of samples acquired. By taking more samples in the time domain, or making the acquisition time longer while keeping the sampling rate the same, the RBW will be lowered. You will have more bins in the same span and thus improve frequency resolution.

As can be seen from the figure above, by using lower resolution bandwidths the instrument can resolve the sidebands. The penalty for high resolution is a longer acquisition time. Higher resolution bandwidths are used when the display needs to be updated rapidly or when wide modulation bandwidths are to be displayed.

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 "dig further down" into the noise.

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

  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. The noise power is normalized to 1 Hz bandwidth; hence, its units are V2/Hz or V/Ö Hz.

Dynamic Range

Dynamic range is the ratio of the highest signal level a circuit can handle to the smallest signal level   

it can handle (usually equal to the noise level), normally expressed in dB. Spurious free dynamic range(SFDR) is the dynamic range over which the frequency spectrum is free from unwanted sinusoidal frequency components.

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For the complete list of tutorials, return to the NI Measurement Fundamentals Main page.

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