Hi Bill,

Welcome to the wonderful world of posting.

I'd expect noise levels to be specified in (nano)volts per sqrt(Hz),
or (femto)watts per Hertz. This can be experessed in dBm, too, of
course. If you know the noise bandwidth of the filter the SA is
putting the signal through, then you just take the measured noise
floor and adjust it by the ratio of that bandwidth to 1 Hz. So if it
were a 10Hz noise bandwidth, the noise per Hz would be 10dB below what
you see. If the filter is 100Hz, it would be 20dB below. Now, is the
noise bandwidth of the filter the same as the resolution bandwidth?
Not necessarily, but it should be fairly close, generally within a few
percent. We build FFT-based spectral analysis equipment, and have to
document the noise bandwidth of the effective filters, but we also
generally have a way to directly display "noise density" which is
already in v/rt(Hz) or w/Hz or dBm -- so it's worth looking to see if
the analyzer MIGHT have such a function. -- It's worthwhile because
of the variation from measurement to measurement to use as much
averaging as is convenient. This might be called "video filtering",
but is done after the signal filtering and detection. Lessee...what
else. Normally, the base level noise of the analyzer would be
measured with the input terminated in its input resistance, usually 50
ohms.

Footnoote about the bandwidths: the noise bandwidth (or noise power
bandwidth) says if you have an input signal of, say, 1.000 picowatts
per Hz, and it's uniform power density per unit bandwidth (a flat,
broadband noise), and you measure 12.345 picowatts coming through your
filter, then the noise bandwidth is 12.345Hz. That may differ from
the -3dB bandwidth of the filter, though generally not by a huge
amount. With analog filters, it's probably expecting a lot for it to
be really the same from analyzer to analyzer of the same model, but
with digital filters (not necessarily even FFT-based), it should be
spot-on the same from one to the next.

Cheers,
Tom

(Bill B.) wrote in message . com...
Hello,
I need some help in figuring out how to properly use a Spec. Analyzer.
I have done several hours of research online now, and have somewhat of
an idea of what I am doing, but I just can't get down one piece of the
puzzle. In my line of work, we use simple devices that give us the
noise floor level in dB. I (stupidly) expected to be able to get this
same information easily out of a SA. As you may have guessed by now, I
ran into a problem when switching the RBW value. After much... much
reading, I fully understand *WHY* it changes (Filters increasing in
size cause a greater internal noise level, etc.) but I can't figure
the best way to get a "base" noise level reading. And as much as I
would like to fully understand the theory, what I really need is a few
more examples... IE: If you are looking a 100 MHz span and your RBW is
1MHz simply ___Fill in the Blank____ to find the base noise floor. To
further explain what I am trying to do, we are setting up a link that
requires a SNR of at least 6dB... I need to get the noise floor level
to compare with my projected signal strength for this link.

Please be nice... This is my first post grin

Thank you! Thank you!

Bill B. - N1SNI