[Test-Equipment] Spectrum Analyzer BW measurement

[windy10605@juno.com]

OK, now we're starting to get somewhere. I appreciate the input supplied
by SEVERAL people, Jim, Bob, Tom, Gary, etc.

>from Gary:......With the 2 khz filter the band width of the signal
appears to be 12 khz wide. With the 5 khz band width filter that same
>signal appears > to be 15 khz wide.

So the Actual BW is very roughly: Measured BW minus the RBW. This is one
reason why the RBW/BW ratio should be only a few %. If you are trying to
measure a 15Khz BW, a 300Hz or 500Hz RBW is much preferred over a 3Khz or
5Khz RBW.

>from Gary ....To see the shape of the filter in the analyzer just tune
into an unmodulated carrier or the zero frequency signal (local
oscillator) >on the analyzer. With a 10 khz filter switched in you will
see a signal that looks like it is 10 khz wide. Switch to a 500 hz filter
and the signal >now looks to be 500 hz wide (at the -3 db points). A
steady unmodulated signal has zero bandwidth (unless it is noisy) so any
apparent >width that you see is the filter width. Good way to check your
internal filters!

Sure enough, the -3dB points of the internal osc signal are approx 500Hz
apart for a 500Hz RBW and approx 5Khz apart for a 5Khz RBW. Cool ! That
also implies that RBW isn't adjusted/compensated for internally. The only
"automatic" function is filter selection.

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.

73 Kees K5BCQ