digital scopes with FFT function

[Jamie Morken]

Hi,

If a digital scope has FFT function, does that mean you can view
the real time spectrum of voltage over frequency up to the scopes
bandwidth? Also can this be zoomed into at a desired frequency?

Is a digital scope with FFT able to fully replace a spectrum
analyzer up to the same bandwidth?

cheers,
Jamie

 

[Joerg]

Hi,

If a digital scope has FFT function, does that mean you can view
the real time spectrum of voltage over frequency up to the scopes
bandwidth? Also can this be zoomed into at a desired frequency?

On mine it pretty much works that way.

Is a digital scope with FFT able to fully replace a spectrum
analyzer up to the same bandwidth?

Absolutely not. A scope has a very paltry amplitude resolution. The
effective number of bits could be around six. If you are lucky.

 

[Fred Bartoli]

Joerg a écrit :

On mine it pretty much works that way.



Absolutely not. A scope has a very paltry amplitude resolution. The
effective number of bits could be around six. If you are lucky.

But this can allow you to see signals buried in the scope time domain
noise floor.
They are also subject to aliasing since there's no... BW limiting.
This can be a real nightmare and windowing do anything against this.

 

[Paul Mathews]

Joerg a écrit :








But this can allow you to see signals buried in the scope time domain
noise floor.
They are also subject to aliasing since there's no... BW limiting.
This can be a real nightmare and windowing do anything against this.

--
Thanks,
Fred.- Hide quoted text -

- Show quoted text -

I agree that the FFT function on a scope is no match for a good
spectrum analyzer.
However, on a good scope with a comparatively high sampling rate, the
FFT can be quite useful. It is the nature of the FFT that, provided
certain assumptions are met, it can provide considerably more
resolution and dynamic range than is implied by the number of bits. I
believe that this subject was covered in an earlier thread.
Most scopes do provide some BW limiting ranges, although the selection
is very limited.
Whether you use an SA or scope FFT, it's important to understand how
windowing and sampling work, since otherwise you're likely to
underestimate the energy in any given band.
Paul Mathews

 

[Ben Jackson]

Is a digital scope with FFT able to fully replace a spectrum
analyzer up to the same bandwidth?

No. The FFT is always going to have a resolution determined by the
sampling rate and the depth of the buffer. The left edge will always
be DC. You could zoom in as much as you wanted at DC, but the most
you can zoom in (the finest fft bin granularity) at 100MHz won't be
nearly as good as a spectrum analyzer.

Still a very useful feature in a scope.