Fw: Re: [Test-Equipment] Spectrum Analyzer BW measurement

Fri, 30 Jan 2004 14:57:27 -0600

Wait a minute, that "actual BW is roughly the measured BW minus the RBW
applies only to the -3dB point". The -55dB point of the internal
oscillator, when measured with a 500Hz RBW is 3.2Khz and when measured
with a 5Khz RBW it's 30Khz. Approx a 10x relationship which is the
specified RBW ratio. So now how do I correlate to a 3Khz RBW measured on
a different SA ? 

73 Kees K5BCQ

--------- Forwarded message ----------
From: [email protected]
To: [email protected]
Cc: [email protected]
Date: Fri, 30 Jan 2004 13:32:56 -0600
Subject: Re: [Test-Equipment] Spectrum Analyzer BW measurement

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