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Old 27th April 2017, 02:46 AM   #1
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Default Spectrum Analyzer Question

I came across this and thought it was kind of screwy.

So, we have a spectrum analyzer that displays the 20kHz sinusoid
on the 2nd vertical graticule and the 120kHz sinusoid on the
7th vertical graticule.

Then it asks to determine the bandwidth, the kHz per division,
and finally identify the the start of the left graticule and
the stop at the the right graticule.

Enlighten me, please and correct me if I'm wrong, but
I was under the impression that we use a spectrum analyzer
to look at the fundamental frequency and the harmonics from it
along with any other stuff that would show up in the given spectrum.
That is, seeing if the power supply is operating correctly, seeing
if there are odd ball frequencies that are being picked up, etc.

But maybe it's a description of where on the screen the peaks
are, but doesn't sound like much of a spectrum analyzer.

What say you?
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Old 27th April 2017, 03:55 AM   #2
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It is difficult to follow your question. What model spectrum analyzer are you using?
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Old 27th April 2017, 04:24 AM   #3
1audio is offline 1audio  United States
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OK- 20 KHz at #2 120 KHz at #7 7-2=5 so 100 KHz in 5 divisions or 20 KHz per division. And 0 Hz is at division #1 not 0. This seemingly odd situation is normal for a swept spectrum analyzer.

Just think of it as a radio with a linear dial (remember those) where the dial goes backwards below zero. (Its not quite that simple but that is good enough for this.)

Bandwidth usually means the bandwidth of the measuring filter. Not obvious from this info. Its how close two signals can be and still be separated.

If they mean how much spectrum is displayed it would be 200 KHz - the repetition of the 20 KHz at the bottom of the span (the usual term for the frequency range) In this case the top is 180 KHz and the bottom is actually 20 KHz. And there are few spectrum analyzer that could show this, most quite obsolete (like my Tek 7L5). Modern RF analyzers don't usually go quite that low and even if they do its not that useful compared to FFT analyzers.

You use a spectrum analyzer to see the distribution of energy vs. frequency.

(Isn't this cheating for your homework?)
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Last edited by 1audio; 27th April 2017 at 04:31 AM.
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Old 27th April 2017, 06:52 AM   #4
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Default OMG Demian!!!

@ Demian,

Howly Cow!

It took me about 1/2 hour to figure that out.
And yes, my bad...the left graticule IS 1, not 0.

No not homework. It was a test question. And I got the
same answers but by different means. I drew it out a couple
of times using an oscilloscope display layout then I counted.

The question wasn't that obvious to me either.
I counted out the major lines in 20 kHz steps.
which was the only way I came up with the scaling.

The other was to do the Hi Lo frequency method.
120kHz - 20kHz = 100kHz
120kHz + 20kHz = 140kHz
then I divided the low 100kHz by 2 to get the bandwidth = 50kHz.

I hope I got that one right. The other was the DC - 200kHz swept
bandwidth. But for some reason reading the question was really screwy.

Since the 20 - 180 wasn't an answer....(I hope not)
I recall the DC to 200kHz was one choice
and 30 to 300kHz was one.
There were two more I think. For some reason I perused the answer choices
but then went back drawing out the screen and trying to figure it out that way.

Demian, you've been at this a whole lot longer and the simple math
you did shows....
Quote:
Originally Posted by 1audio View Post
OK- 20 KHz at #2 120 KHz at #7 7-2=5 so 100 KHz in 5 divisions or 20 KHz per division. And 0 Hz is at division #1 not 0. This seemingly odd situation is normal for a swept spectrum analyzer.
I tried to present it the way it was written but who would say something
like that.

Next question then I guess that is how the older spectrum analyzers would
display the information? Not like the QA400, 401, or the ARTA, AP, R&S
type FFT that we plot with.

So for most folks from our "Instrumentation" class the first time they've
ever seen an FFT plot, our fearless leader had us use 2kHz and 5kHz
sine waves from two signal generators into our o-scopes.

What a mess! I had to explain to my classmates how I used FFTs to
measure THD, and look at the harmonic content and it would also
show power supply frequencies and harmonics too.

I got slammed by the prof for that use.

So I slammed him back! Shame on me! What worthless lab is this?
With two signals no one can tell what is the screens because the signals
are all mixed together. At least with ONE 1kHz tone, we can look
and see the harmonics clearly.

So, you tell me what is anyone learning with this worthless exercise?

AND, sadly he couldn't answer because he really doesn't know what
he is doing.

Funny though,when I was taking with Duke (our resident AP fixer upper)
he knew right off the bat how to teach this, as follows:

1. FFT of wall power using a voltage divider, see what it looks like.

2. FFT of a power supply showing the harmonic content of power supplies.

3. FFT of the standard 1kHz tone, showing the harmonic content of both
the power supply source for the signal generator and that of the 1kHz tone.

So I've got three weeks left, I just need to keep quiet and suck up all
the good kharma and put two more courses under my belt.

I've come a long way...have a long way yet to go.

Cheers,



and our lab where we did an FFT
was a complete fiasco.
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Last edited by SyncTronX; 27th April 2017 at 07:01 AM.
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Old 27th April 2017, 07:03 AM   #5
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Quote:
Originally Posted by Sawdust123 View Post
It is difficult to follow your question. What model spectrum analyzer are you using?
There wasn't one. Just the generic question. It wasn't related to any
of the analyzers that I have or have used. But for some of the folks
here, they have.

Cheers,
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Old 27th April 2017, 04:27 PM   #6
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Thanks for the clarification. I thought you were trying to figure out the settings on a particular instrument. It looks like 1audio had your answer.
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