Spectrum Analyzer RBW and VBW

I'd like to add my input.
VBW is not the same as averaging.  RBW, VBW, and sweep speed, are all related.  As the video showed (great video BTW), RBW allows separation of closely spaced signals.  There is a relationship between RBW, VBW and sweep speed.  You must have a sufficiently SLOW sweep speed to allow the detectors to property charge up and output the correct amplitude.  If you uncouple RBW or go manual, and you select a sweep speed that is too fast, most analyzers will alert you with a "uncal" notification.  The same will happen with VBW.  The VBW is a low pass filter, if you have a certain sweep speed, the video applied to the screen has certain high frequency components.  If you turn down the VBW, you will start to filter out those high frequency components. 

The video may be somewhat misleading in stating the VBW will not change noise amplitude.  It does not change the noise floor, but it does filter the noise and it will change signal amplitude.  Both the RBW and VBW will change signal amplitude if not set properly.  If you independantly change, and go too far in turning down (narrowing) the RBW, VBW, or sweep speed, you will start to filter out the sharp peaks.  You don't care about the noise, but if there is a very narrow signal peak, whether the signal of interest or another signal off to the side, the VBW will prevent the signal from reaching it's maximum peak.  As in the RBW, the analyzer will notify you when your VBW setting is compromising the amplitude accuracy.  This applies whether it's an analog or digital analyzer.  Leaving the RBW, VBW, and sweep speed in "auto" or "coupled will keep you out of trouble.

Now for averaging.  Averaging does not "filter out" the noise, it mathematically adds them together, and as we all know, "true" noise is random, and thus mathematically they eventually disappear after a sufficient number of sweeps.  But what does it do to the signal of interest?  Nothing.  If the signal is stable and there continuously, it will be mathematically reinforced.  This includes the most thinnest and sharpest peaks.  So averaging does not remove the high frequency components that are really there, only the ones that are random and not there all the time.  It's very good at looking at the signal of interest, but it can mask intermittent interfering signals.  One note of caution, the analyzer's frequency drift puts a limitation of the amount of averaging that is possible.  If the swept bandwidth is moving due to high oscillator drift (as in the older HP8590), the averaging will destroy your signal.  This happens by adding sweep copies that are not in the same place; the peak in the current sweep, is moving relative to the location it occupied in the previous sweeps.  Note that the analyzer will probably NOT alert you of this condition.  Most likely in analyzing a signal, you will be changing all of these parameters at one time or another, this is why you need to know how to operate your instrument, as our video author is trying to convey. Again, well done on the video.

Bob

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Setting Bandwidth Parameters

Bandwidth parameters are set using the BANDWIDTH Menu

Resolution Bandwidth

Resolution Bandwidth (RBW) determines frequency selectivity. The spectrum analyzer traces the shape of the RBW filter as it tunes past a signal. The choice of resolution bandwidth depends on several factors. Filters take time to settle. The output of the filter will take some time to settle to the correct value so that it can be measured. The narrower the filter bandwidth (resolution bandwidth), the longer the settling time needs to be, and therefore, the slower the sweep speed.

The choice of resolution bandwidth will depend upon the signal being measured. If two closely‑spaced signals are to be measured individually, then a narrow bandwidth is required. If a wider bandwidth is used, then the energy of both signals will be included in the measurement. Thus, the wider bandwidth does not have the ability to look at frequencies selectively, but instead simultaneously measures all signals falling within the resolution bandwidth. Therefore, a broadband measurement would include all signals and noise within the measurement bandwidth into a single measurement.

On the other hand, a narrow‑band measurement will separate the frequency components, resulting in a measurement that includes separate peaks for each signal. There are advantages to each. The ultimate decision will depend upon the type of measurement required.

There is always some amount of noise present in a measurement. Noise is often broadband in nature; that is, it exists at a broad range of frequencies. If the noise is included in the measurement, the measured value could be in error (too large) depending upon the noise level. With a wide bandwidth, more noise is included in the measurement. With a narrow bandwidth, less noise enters the resolution bandwidth filter, and the measurement is more accurate. If the resolution bandwidth is narrower, the noise floor will drop on the spectrum analyzer display. As the measured noise level drops, smaller signals that were previously obscured by the noise might now be measurable. Zero span is used for noise and noise-like measurements that are usually wider than the RBW. The RBW is ideally set to be as wide as the bandwidth of the signal you are measuring.

Video Bandwidth

Spectrum analyzers typically use another type of filtering after the detector that is called video filtering. This filter also affects the noise on the display, but in a different manner than the resolution bandwidth. In video filtering, the average level of the noise remains the same, but the variation in the noise is reduced. Therefore, the effect of video filtering is a “smoothing” of the signal noise. The resultant effect on the analyzer’s display is that the noise floor compresses into a thinner trace, while the average position of the trace remains the same.

Changing the video bandwidth (VBW) does not improve sensitivity, but it does improve discernibility and repeatability when making low‑level measurements. As a general rule, most field spectrum analyzer measurements are made at a video bandwidth that is a factor of 10 to 100 less than the resolution bandwidth. Using this ratio, with a resolution bandwidth of 30 kHz, typically, the video bandwidth is set between 300 Hz and 3 kHz, although it can be set anywhere from 1 Hz to 10 MHz.

Setting Frequency Bandwidth

1.

Select BANDWIDTH on the main menu.

2.

Toggle AUTO

RBW

or AUTO

VBW

(or both) off to manually change values. If using Auto, refer to the following sections.

3.

Set the RBW and VBW to achieve the desired resolution and sweep characteristics. Lower values increase resolution and reduce noise, but at the expense of measurement (sweep) speed.

4.

Set the VBW TYPE to Logarithmic (geometric mean) or Linear (arithmetic mean).

Setting Bandwidth Auto Coupling

Both resolution bandwidth and video bandwidth can be coupled to the frequency span automatically, or set manually. When set to Auto RBW, the instrument automatically adjusts the RBW in proportion to the frequency span. The default ratio of the span width to the resolution bandwidth is 100:1 and can be changed as follows:

1.

Select BANDWIDTH on the main menu.

2.

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Getting StartedSelect SPAN:RBW and change the coupling value, and then press ACCEPT to enter the value.

When auto‑coupling between the span and RBW is selected (AUTO RBW is toggled on), the bandwidth parameter is displayed normally at the bottom of the graph. If manual RBW is selected (AUTO RBW is toggled off), the bandwidth label at the bottom of the graph is prefixed with the ‘#’ symbol, and resolution bandwidth is set independently of the span.

Auto coupling VBW links the video bandwidth to the resolution bandwidth so that VBW varies in proportion to RBW. If manual VBW coupling is selected, the VBW label at the bottom of the graph is prefixed with the “#” symbol and video bandwidth is set independently of resolution bandwidth.

By default, the RBW/VBW ratio is set to 3 and can be changed as follows:

1.

Select BANDWIDTH on the main menu.

2.

Select

RBW/VBW

and enter the desired value.

The RBW range varies with instrument features. Refer to BANDWIDTH Menu and check your technical data sheet for the bandwidth range of your instrument.

 

BANDWIDTH Menu

 

BANDWIDTH Menu

 

AUTO RBW

When toggled on, the instrument selects the resolution bandwidth based on the current span width. The ratio of span width to RBW can be specified using the SPAN:RBW button. When toggled off (manual), the RBW label at the left edge of the x-axis will be preceded by the “#” symbol.

RBW

The current resolution bandwidth is displayed under the RBW button. Once auto RBW is toggled off, the RBW can be changed using the keypad or the slider controls. Bandwidth values increment in a 1:3:10 sequence, from 1 Hz to 3 Hz to 10 Hz or from 10 Hz to 30 Hz to 100 Hz, for example. Refer to your instrument technical data sheet for the resolution bandwidth range.

AUTO VBW:

When toggled on, the instrument selects the video bandwidth based on the resolution bandwidth. The ratio of video bandwidth to resolution bandwidth can be set using the RBW:VBW button. When toggled off (manual), the VBW label at the left edge of the x-axis will be preceded by the “#” symbol.

VBW

The current video bandwidth is displayed under the VBW button. Once auto VBW is toggled off, the VBW can be changed using the keypad or slider controls. Bandwidth values increment in a 1:3:10 sequence, from 1 Hz to 3 Hz to 10 Hz or from 10 Hz to 30 Hz to 100 Hz, for example. Refer to your instrument technical data sheet for the video bandwidth range.

VBW TYPE

Toggles between linear averaging (arithmetic mean) and logarithmic averaging (geometric mean).

RBW:VBW

This parameter displays the ratio between resolution bandwidth and video bandwidth. To change the ratio, press this button and use the keypad or the slider controls.

Displays the ratio between the span and the resolution bandwidth. The default value is 100, meaning that the span will be 100 times the resolution bandwidth. To change the ratio, press this button and use the keypad or slider controls.

Note

When Quasi-Peak marker is set, RBW and VBW are automatically set to meet the CISPR specification. See Marker Functions

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