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Using a spectrum analyzer to find LC resonance?

by hobbs125
Tags: analyzer, resonance, spectrum
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sophiecentaur
#19
Apr12-13, 06:23 AM
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Quote Quote by jim hardy View Post
The truth about computer produced electrical noise is leaking out !
They are a real nuisance for lots of receiving equipment. You need an antenna up in the attic or on the roof; a hand held receiver gets all sorts of old rubbish from the computers.

But is still don't see why the spectrum of that nice looking sine wave has so much apparent HF shash on it, according to the 'analyser mode'. That scope trace should, by rights, be a mush unless there's some extensive LP filtering of the signal when in 'scope mode'. We need to know more details about what those display pictures actually represent.
eq1
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Apr12-13, 06:12 PM
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Doesn't even have to be radiated noise. Or even the computer's fault. It could also be common mode noise from a poorly made wall wart.

http://www.eevblog.com/2013/03/22/ee...on-mode-noise/
sophiecentaur
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Apr12-13, 06:31 PM
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Quote Quote by eq1 View Post
Doesn't even have to be radiated noise. Or even the computer's fault. It could also be common mode noise from a poorly made wall wart.

http://www.eevblog.com/2013/03/22/ee...on-mode-noise/
So why doesn't it turn up on the Oscilloscope Mode?
vk6kro
#22
Apr13-13, 02:48 AM
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Quote Quote by sophiecentaur View Post
So why doesn't it turn up on the Oscilloscope Mode?
That's right.

The Spectrum Analyser mode and the Oscilloscope operate at different times.

Somehow, when it operates in Spectrum Analyser mode, it generates noise.

This may be a faulty device component or just a driver bug.

It can't be externally generated noise or it would show on the sine wave in the oscilloscope window.

You could try to get it repaired or replaced or try to get updated drivers from the manufacturer.
sophiecentaur
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Apr13-13, 05:02 AM
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Quote Quote by vk6kro View Post
That's right.

The Spectrum Analyser mode and the Oscilloscope operate at different times.

Somehow, when it operates in Spectrum Analyser mode, it generates noise.

This may be a faulty device component or just a driver bug.

It can't be externally generated noise or it would show on the sine wave in the oscilloscope window.

You could try to get it repaired or replaced or try to get updated drivers from the manufacturer.
The only reason I can think of for the two different apparent behaviours is that the FFT is being performed at the wrong time - i.e. instead of using the string of samples that are shown on the scope mode trace (which should produce a single spike), it must be using an unsynched string of samples. That would have to be more or less software. Looks like it's a matter of approaching the manufacturer. There could be an upgrade just waiting to be installed.
I wonder if it is possible to vary the input frequency by a small amount and produce a large effect on the arrangement of those HF spurii. Aliasing can be a real problem in digital systems and this could be a form of temporal aliasing.
AlephZero
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Apr13-13, 07:42 AM
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A basic question, unless I missed the answer somewhere in the thread already:

What is the sampling rate of your ADC?

I wonder if the "spectrum" above about 24 kHz is just nonsense (random numbers) that shouldn't be plotted at all, because you have set the range of the X axis beyond the Nyquist frequency of your sampling rate.

Everything looks OK up to about 24 KHz, and there's nothing above about 48HKz. 48 = 2 x 24. That may or may not be a coincidence.
sophiecentaur
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Apr13-13, 01:50 PM
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Quote Quote by AlephZero View Post
A basic question, unless I missed the answer somewhere in the thread already:

What is the sampling rate of your ADC?

I wonder if the "spectrum" above about 24 kHz is just nonsense (random numbers) that shouldn't be plotted at all, because you have set the range of the X axis beyond the Nyquist frequency of your sampling rate.

Everything looks OK up to about 24 KHz, and there's nothing above about 48HKz. 48 = 2 x 24. That may or may not be a coincidence.
Good point. Most 'good' equipment stops you from entering silly settings and asks what you can ask it to do. For something that is based on computer software, it may be that they just haven't been careful enough. But wouldn't you have expected them to use an appropriate Nyquist filter?


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