[SI-LIST] Re: Question of taking measurement. Thanks.
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 10 Dec 2005 14:08:21 -0500
> To get accurate results, the bandwidth of the oscilloscope must be
> 2X+ of the signal. (Please correct me if I'm wrong. Thanks.)
Technically, in theory, the bandwidth only needs to be 1X of the signal
frequency.
BUT, ...
Most of us are looking at digital signals, and the "signal frequency" in
that case is not the fundamental signal switching frequency, but a harmonic
of it. How high that harmonic is (5th, 21st, etc.) depends on the edge
rates (which determine harmonic content) and how accurately you want
them reproduced on your scope. Fully accurate results require the scope's
bandwidth to equal or exceed the highest harmonic present in your signal.
Not only that, but no scope has perfect response out to its specified
bandwidth. It may be "down 3dB" at that point, making the amplitude 30%
inaccurate there. And its phase response may be nonlinear too, which
affects the shape of displayed digital signals.
So a bandwidth of 2X the highest signal frequency is a nice margin. But
even then, there is some inaccuracy.
You may be confusing the fact that a digitizer (which might be in a digital
sampling scope) must have a sampling rate of at least 2X the signal
frequency. If it isn't, aliasing results and the displayed waveform may
look totally wrong. But sampling rate is not bandwidth.
A sometimes neglected characteristic is the bandwitch of the scope probe.
It combines with the scope's own bandwidth to result in a net bandwidth that
is lower than either one alone.
And then there's the effect of connecting the scope probe to the circuit
under test.
And the effect of the ground lead. A good probe with a long ground lead is
wasted bandwidth because the loop inductance may prevent an accurate signal
from ever reaching the probe.
> So if I know that the bandwidth of my scope is not enough compared to the
> signal that I want to measure, is that any way I can still get accurate
> results?
I doubt it, unless you know exactly how your scope behaves (i.e., if you
calibrate it yourself with a good signal source). Even then, ...
Regards,
Andy
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