[Test-Equipment] Spectrum Analyzer BW measurement

[Dave Emery]

On Fri, Jan 30, 2004 at 01:32:56PM -0600, windy10605@juno.com wrote:
> 
> So I'm going to assume that the guys who spec'd -55dB for 20Khz BW using
> a 3Khz RBW did so 1) because that instrumentation is more readily
> available, 2) however, their BW reading is going to be approx 3Khz wider
> than actual.

	3 khz RBW is extremely standard in spectrum analyzers, one of
the most common bandwidths because it matches the bandwidth of
a standard voice SSB signal and is also a member of the 1,3,10 set
of values (each one about 3 times the last) common in many kinds
of instrumentation.

	But for most digital signals - which tend to be noiselike - and
for that matter also for spurious sideband energy (which is what you
seem to be trying to measure) what is often more important is noise
power density in dbC/hz a given distance from the carrier frequency.

	This can be specified directly (and many modern spectrum
analyzers have firmware to measure and compute this number) or in a
format slightly more conveniant for someone with a vanilla spectrum
analyzer and a transmitter to measure - but less precisely accurate.

	Your RC remote control spec seems have used the later form -
specifying how many db down from the carrier level the noise power
reading in a 3 khz RBW centered 20 khz from the carrier is supposed to
be.   This obviously is trivial to measure correctly if you have a
spectrum analyzer with a 3 khz bandwidth, but a little more complex if
you have one (as you do with only 500 hz and 5 khz bandwidth).

	In fact most (or at least many) digital (and for that matter
analog)  signals have a nose or hump that contains most all the energy.
The width of this can be most accurately viewed with a narrow resolution
bandwidth relative to the width of the signal (few percent).

	But probably your RC transmitter spec is more concerned with
how much energy lies outside this hump where it could interfere with
other users of nearby channels.

	And for measuring this broadband noiselike noise you need
to take resolution bandwidth into account since a wider resolution
bandwidth will see more noise.

> 
> 73 Kees K5BCQ
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-- 
   Dave Emery N1PRE,  die@dieconsulting.com  DIE Consulting, Weston, Mass 02493