Spectrum Analyzer RBW&VBW

The structure of frequency sweep spectrum analyzer

Effects of RBW (Resolution Bandwidth)

The RBW dictates the resolution bandwidth, which is related to the minimum bandwidth detectable by the instrument. The smaller the bandwidth, the more spectral resolution. However, there is a trade-off between what quickly the display can update the full frequency span under consideration and T He frequency resolution, which is relevant for distinguishing frequency, which is close together. For a swept-tuned architecture, this relation for sweep time are useful:where ST is sweep time in seconds, K is Proportiona Lity constant, Span is the frequency range under consideration in Hertz, and RBW are the resolution bandwidth in Hertz.

It determines the RF noise floor and how close of the signals can be and still is resolved by the analyzer into both separate Peaks. Decreasing the bandwidth of a RBW filter decreases the measured noise floor and vice versa. This was due to higher RBW filters passing more frequency components through to the envelope detector than lower bandwidth RBW filters, therefore a higher RBW causes a higher measured noise floor.

VBW (Video Band wdth)

Averaging or peak detection then refers to how the digital storage portion of the device records samples-it takes several Samples per time step and stores only one sample, either the average of the samples or the highest one. The video bandwidth determines the capability to discriminate between the different power levels. This is because a narrower VBW would remove noise in the detector output. This filter was used to "smooth" the display by removing noise from the envelope. Similar to the RBW, the VBW affects the sweep time of the the display if the VBW are less than the RBW. If VBW is less than RBW, this relation for sweep time is useful:

Displayed average Noise level

The Displayed Average Noise level (DANL) are just what it says it is-the Average Noise level Displayed on the anal Yzer. This can either is with a specific resolution bandwidth (e.g. -120 dBm @1 kHz RBW), or normalized to 1 Hz (usually in dbm/ Hz) e.g. -170 dBm (Hz)

The modern spectrum meter uses the all-digital if structure, such as

Reference 1 Spectrum and Signal Analyzer measurements of Agilet

Spectrum Analyzer RBW&VBW