Re: Digital Osci and Logic Analyzer

Rich Webb <bbew.ar@xxxxxxxxxxxxxxxxxx> wrote:

On Fri, 07 Apr 2006 15:08:34 GMT, nico@xxxxxxxxxxx (Nico Coesel) wrote:

Rich Webb <bbew.ar@xxxxxxxxxxxxxxxxxx> wrote:

As a rule of thumb, estimate the useable bandwidth as being limited by
one-tenth of the ADC sample rate, not one-half. That lets you recover
(more or less) the fifth harmonic of the signal of interest right at
Nyquist. For some commercial examples, there's the Fluke 199C: 2.5 Gsps,
overall bandwidth 200 MHz; and Tek TDS2012: 1 Gsps and 100 MHz. So for
your

I disagree here. Shannon says a signal can be reconstructed up to half
the sampling frequency. The higher sampling rates of the examples you
mention are probably used to use less bits for sampling and make the
analog input filter easier to construct (an RC filter still won't do
the job) to prevent frequency aliasing. Digital filtering does the
rest.

Thought problem. Signal A is a 1 MHz sine wave, signal B is a 1 MHz
square wave. Sample each at 2 Msps. Based only on the samples, and not
on any a priori knowledge of the expected signal characteristics, what
would the o'scope display at 1 usec/div'n?

This situation can produce any result.

Anyway, the presumption there is no knowlegde about the input signal
is not correct. Signals can always be expressed as a sum of sine waves
with different frequencies and phase. With this in mind it easy to see
it is possible the reconstruct the spectral part of a signal which is
within the nyquist limit.

Try to display a 100MHz square wave on a 100MHz analogue scope. You'll
see a sine wave. On a 300MHz analogue scope you'll see something that
resembles a square wave. According to your 'rules', an analogue scope
would also be usefull up to 1 tenth of its bandwidth.

You don't need to capture or store more samples than you need for one
display screen.

Assume that your display area is 500 pixels x 500 pixels at ten
divisions each for horizontal and vertical. No matter how fast you
sample, you've only that area to work with. For each time step (vertical
column) you have several choices.

I disagree again, the less memory you have the more difficult it will
be to capture a problem. You'll need some pretty fancy triggering.

Ultimately, however many samples are collected at whatever resolution
and at whatever rate, they're going to end up on a relatively low-res
display (low-res at least as compared to an "infinite" resolution analog
display). The 500x500 is just an example to make the numbers easy; most
are probably based on 1/4 VGA, VGA, or XGA resolutions.

True, but it is very convenient to be able to scroll through a signal
and zoom in/out without recapturing the signal.

--
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