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

Thu, 29 Jan 2004 21:58:04 -0600

Thanks Gary, best answer yet although I have not dug through the HP info
suggested by several.

What I'm trying to do is measure the BW of a radio control transmitter.
The FCC specifications are: For a 20Khz signal frequency spacing, 
various BWs at -25dB, -45dB and -55dB. Lets take the -55dB at 20Khz using
a 3Khz RBW as specified by the AMA. My SA has automatic filter selection
of 5Hz, 50Hz, 500Hz, 5Khz, 50Khz. For this measurement I select a scale
of 5Khz /Div which gives a "normal" -3dB RBW of 500Hz. Sounds good to me
and gives a great 2.5% RBW to BW ratio. The SA also allows selection of
10x (5Khz) or 1/10x (50Hz) RBW. 50Hz is, of course, way too slow, but 10x
gives a MUCH WIDER BW reading (2x ???). So I'm wondering if by using the
500Hz RBW and if the test spec used a 3Khz RBW are we supposed to measure
the same BW ? I don't think so.

Question:
So how do I correlate between the two BWs, never mind that I think I have
the more accurate BW representation using the 500Hz RBW.

All BW measurements are made in "peak detect" mode.

--------- Forwarded message ----------
From: Gary Schafer <[email protected]>
To: [email protected]
Cc: [email protected]
Date: Thu, 29 Jan 2004 21:49:14 -0500
Subject: Re: [Test-Equipment] Spectrum Analyzer BW measurement
Message-ID: <[email protected]>
References: <[email protected]>

Kees,

Not sure exactly what you are trying to measure?
I think what you are reading is related to the minimum bandwidth filter 
that can be used when looking at a given spectrum width. It has to do 
with how fast you are moving the filter across the signal.
The more spectrum you try to see with a given bandwidth filter the 
slower you have to sweep it.

Think of it as how long the particular signal remains in the filter band 
pass. The wider the spectrum you try to look at the faster you need to 
sweep it so the display does not blink from the sweep. If you use a very 
narrow filter when sweeping a lot of spectrum then a particular signal 
does not spend much time in the filter band pass. Not enough energy gets 
through. Using a wider filter will allow the signal more time in the 
band pass and thus more energy to get through.

Using manual modes it is easy to see. Look at a signal with a rather 
narrow filter. Then speed up the sweep. When the sweep speed starts to 
get too fast the amplitude of the signal will start to drop.
If you slow the sweep down to the point just before the signal starts to 
drop in amplitude and switch in a narrower filter you will again see the 
signal amplitude drop. You would then need to slow down the sweep speed 
further to bring the amplitude back to normal.

Also narrowing the sweep width (look at a narrower part of the spectrum) 
will allow a narrower filter to be used. Again, as you look at a smaller 
  part of the spectrum that allows more time for the signal to remain in 
the filter band pass.

Most analyzers automatically switch filters and sweep speed as you 
change sweep widths. The wider the sweep width you select the wider the 
filter that is switched in. They do this so that the minimum filter 
width is used. If you go below that minimum width you will not get a 
true representation on the screen. You will not see the full amplitude 
that is really there.

Most have a manual mode that allows any filter at any sweep width and 
any sweep speed as I described above. You have to determine how far you 
can go in the manual mode and still have a true picture.

Hope this is what you are referring to.

73
Gary  K4FMX

[email protected] wrote:
> I'm trying to measure some 10-30Khz bandwidths. As I understand, the
> correct method is to use the instrument's -3dB resolution BW (RBW) of
> approx 1% ?? of what you are trying to measure. This means a RBW of
> approx
> 0.1-0.3Khz. This instrument has and RBW of 5Hz, 50Hz, 500Hz, 5Khz, and
> 50Khz so I use 500Hz. If I use 5Khz the BW shown nearly doubles. I
expect
> it to get wider but why twice ?. And if it's twice, it seems like I'd
get
> widely variant BW measurements depending on if I'm able to select
500Hz,
> 1Khz or 3Khz or ? bandwidths. Or the BW would always have to be
specified
> with a RBW ...which begs the question "What do you do if the spec is at
> 3Khz RBW and the closest you can get is 500Hz and 5Khz ? Do you assume
> the narrower RBW (500Hz) is more accurate anyway ?
> 
> Please send the resonse to me directly since I'm set to receive daily
> summary notes.
> 
> 73 Kees K5BCQ
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