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

[ah7i@atl.org]

Kees-

Do you have to 'tie' your curve to the curve presented using a different
instrument or do you have to produce a reasonably accurate representation
of the bandlimited signal you are trying to measure?

The latter is simply a matter of correcting the curve on the screen for
the shape of the SA filter. Of course you have to know the shape of the
filter. The curve is the log of the convolution of the filter with the
signal. So, antilog, convolve with inverse, log.

For the former you antilog, convolve with inverse, convolve with filter
curve of the 'other' instrument, then log.

If you're only doing this once, you can do it on a piece of paper.
if you're doing it many times, you'll want to let the computer do the
work.

Is this a 'paying' job or are you just futzing around?

-bob
AH7I



On Fri, 30 Jan 2004 windy10605@juno.com wrote:

> 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: windy10605@juno.com
> To: garyschafer@comcast.net
> Cc: test-equipment@mailman.qth.net
> 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
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