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> I would like to know how to use the RBW control and the VF filter in a
> spectrum analyzer. I don't understand well how such controls affects
> measurements.
The resolution bandwidth filter is like the selectivity filter in a radio;
it determines what bandwidth of signal at the analyzer's present frequency
gets fed into the power detector.
The Video Bandwidth filter is the filter *after* the power detector. it's
easiest to see the effect of the video filter by setting the analyser
to zero span with a sampling detector (not peak!) and just changing the
video bandwidth. You don't even need to have a signal going into the analyzer,
just look at the effect on the noise-floorr; it gets filtered just like
a normal audio signal would be, but the level shouldn't change.
> Is normal that amplitude measurement varies with one or other RBW selection?
Only if you are measuring a modulated signal. A pure tone only has
one frequency component, so widening or narrowing the RBW filter just couples
the same amount of signal into the detector (with a bit more or less noise).
But if you are measuring a modulated signal, then widening the RBW filter means
that you are coupling more of the signal's power into the detector, so the measurement
increases. If you narrow the RBW filter, you're coupling less of the signal's
power into the detector, and the measurement will go down.
> How should I measure for maximum accuracy?
If you're just trying to measure signal power, then you need a wide RBW, so that
all of the energy contained in the signal gets into the power detector. Note that
if the RBW is sufficiently wide, then anything you feed through it just looks
like an impulse, and what you see on the analyzer screen then looks like the
impulse response of the filter - i.e. you lose the "detail" of the signal
you're measuring.
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If you're trying to measure the modulation itself, then you need a RBW
filter thats narrow compared to the signal bandwidth, otherwise you just end
up measuring the total (wideband) power as above. Many signals are specified
in terms of a resolution bandwidth at a certain offset from the centre frequency.
> Thanks for sharing your experience.
--
Rick
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