Spectrum Analyzer Terminologies or Glossary of terms

This page of T&M World covers Spectrum Analyzer Terminologies. This Spectrum Analyzer Glossary include RBW, VBW, span, reference level, frequency control etc.

The figure-1 depicts block diagram of spectrum analyzer. As shown in the figure, RF signal to be measured is first down converted to lower IF frequency. Later this IF frequency is passed through IF amplifier and IF filter before it is given to diode detector. The detected signal is noise filtered using video filter before being displayed. Based on frequency span set for horizontal axis, appropriate sweep time is calculated and used.

RBW-Resolution Bandwidth

The RBW can be set using front panel of spectrum analyzer. This bandwidth is related to bandwidth of IF filter which is a band pass filter as shown. It sets window for detecting signals.

➨When RBW value is less, selectivity of spectrum analyzer is improved and hence we will be able to resolve two frequencies which are very close to each other. Hence Resolution Bandwidth (RBW) should be set accordingly.

VBW-Video Bandwidth

The VBW can be set using front panel of spectrum analyzer. This bandwidth is related to bandwidth of Video filter shown after diode based detector. This is useful when signal amplitude is almost equivalent to noise floor level. Video Bandwidth (VBW) smoothen out the noise floor so that low amplitude level signal can be viewed distinctly.

➨VBW does not have any effect on frequency resolution of spectrum analyzer as does by changing RBW. Hence VBW change will not have any improvement on sensitivity of the spectrum analyzer.
Note: Decrease the VBW to smoothen out the noise to view signal clearly.

Span

The span setting sets frequency start and frequency end limits at horizontal axis of the spectrum analyzer. This is done in tandem with RF center frequency set initially. There is also provision to set START Frequency and STOP Frequency in Spectrum Analyzer.

➨Higher is the span, more frequencies can be viewed.

Reference Level

As shown in the figure-1, it related to IF gain and attenuator shown after RF mixer. It helps in testing the DUT as per appropriate dynamic range. Reference level sets power level at upper most vertical line of the display. All the magnitude or amplitude readings are measured with respect to this value. There is control which allows to set this. The units can be dBm, dBV, Watts or volts.

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