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Ralph Mowery

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says...

In article t,

It's important not to conflate the scope's analog bandwidth, with its
sampling rate... or to confuse a "real-time" sampling rate with an
"equivalent time" sampling rate!

Some of the less-expensive digital scopes may quote a high analog
bandwidth, but sample at a fairly poor rate... e.g. a 100 MHz scope
which samples at only 150 or 200 megasamples/second. A scope of this
sort can only let you look at fast waveforms well, using "equivalent
time" sampling, and this requires a "repetitive" waveform that doesn't
vary significantly between one scan-sweep and the next.

On the other hand, a "100 MHz" scope with a 2-gigasample rate can do a
very decent job on fast, nonrepetitive (i.e. modulated) signals.
It'll also cost more, due to the need for faster ADCs and memory.

Other tidbits:

There are a number of things that can influence how well the scope can
show you what you want to see.

If your scope is simply displaying the individual samples, and drawing
straight lines between the points, then you probably need a sampling
rate of at least 20x the carrier frequency (or more) in order to avoid
having the display confused by the "chopping off" of the tops of the
waveforms when the sampling time doesn't happen to fall right on the
peak of the waveform. 50x is even better.

Some digital scopes have the ability to interpolate... they'll do some
sort of polynomial or sin(x)/x curve fitting or filtering, and will
show you a display of what they *think* the actual waveform would have
looked like on a continuous (i.e. analog) display.

You can also run into problems where a scope deliberately reduces its
sampling rate, because it's been configured to capture a large amount
of the waveform "before" and "after" the trigger point, and let you
scroll through it. The scope has only so much memory available, and
if you tell it you want to capture a long interval of the signal, it
has to reduce the number of points it captures per (micro or
milli)second in order to fit the data in memory. This can drop the
*actual* sampling rate well below the Nyquist point, and results in
all sorts of confusing-looking aliasing of the signal on the display.
It might be worth turning down your scope's "capture width" to be no
more than can fit on the screen... and checking the display to see if
you can confirm that the *actual* data-capture rate is high enough.

Some digital scopes drop their sampling rate in half if you use both
channels. The popular Rigol DS1052E/1102E can sample at 1
gigasample/second on one channel, but only 500 gigasamples/second if
both inputs are active. If you don't need the second channel, turn it
off!

And, part of the problem is sometimes that the scope's display just
doesn't have enough horizontal resolution. If it only has, say, 640
pixels in each horizontal line (VGA resolution), or worse yet only
half that many, you're not going to see a terribly smooth waveform no
matter how fast you sample it. In addition, most inexpensive digital
scopes are sorta "colored monochrome" - they either light up a pixel
in a color, or don't, but have little or no ability to vary the
*brightness* of a pixel.

You may get better results if you use the scope to sample, then upload
the waveform data to a PC via USB or GPIB and view it on a
higher-resolution display (and/or with a plotting program that can
implement an anti-aliased display with at least 8 bits of intensity).

So, there are often things you can do to make a digital scope more
usable for signals like this... but I agree with you that an analog
scope is often a much more satisfying solution. I have both types,
and haul out the Tek 2232 (or wheel out the 7904) as often as I do any
of the digital scopes.

If I did not post it, the scope is a Hantek dso5202p which is a 200 MHz
scope and a 1 GB sample rate if only one Chanel is use. It does the sin
(x)/x thing to smooth the curve between points. The scope may blow up
but for the money it seems to be a very nice hobby scope. Bought it new
for only $ 299 from a company in the US. It does have a 3 year warranty
I feel safe with it.
I am retired now and just thought it would be nice to play with a
digital scope. I am now finding out some of the limitations of them.
Glad I still have an old Techtronics 465B analog. Seems that going to
the X-Y display is not very good either if I want to compare 2 signals
that are very close in frequency. Never could get the circle to show up
and rotate slowly. It will show up if I hook both probes up to the same
signal and an elips due the the slight delay between the signal.