I wrote:

> To be fair, though, the 2230's digital mode rarely lies to me...
> Its aliasing problems are nowhere near as bad as what I've seen in
> my more-or-less limited experience with the Tek TDS220 and the HP
> 54645D.

and Oyvind Kaurstad <PICLIST@MITVMA.MIT.EDU> replied:

> I'm not sure what you mean. The [TDS220] scopes have a sample rate
> of 1 GS/s and an analog bandwidth of 100 MHz (60 MHz for the TDS
> 210).
>
> Aliasing should not occur with frequencies less than 500 MHz.
> Since the bandwith limitation would attenuate such a signal to
> virtually zero, I cannot see how you would be able to experience
> aliasing problems.

    Oyvind:

    John Payson explained it well:  The problem has to do with the
    fixed-number-of-samples-per-graticule-division sampling speed;
    just because a scope CAN sample at 1 GS/sec doesn't mean that it
    ALWAYS samples at that speed.

    If a scope takes 100 samples per division, signal pulses shorter
    than 1/100-division can occur BETWEEN samples, where they'll be
    completely missed.

    To test your scope for this behavior, program a PIC to generate
    1-microsecond pulses spaced 5 milliseconds apart, then set your
    scope to 20 milliseconds per division and see if it always shows
    exactly four pulses in each division... Unless it has an
    "envelope" mode which samples at a very high rate and displays
    the minimum and maximum sampled values for each column of pixels
    on the screen, it'll miss pulses.

    If your scope passes the above test, you can move on to testing
    for the noise-susceptibility problem to which John alluded...

    Modify your PIC program to repeatedly generate a 2-millisecond
    pulse followed by a string of 47 1-microsecond pulses spaced at
    1-millisecond intervals... The idea is that you're simulating a
    signal (a 2-millisecond pulse once every 50 milliseconds) in the
    presence of noise (a 1-microsecond pulse every millisecond).

    Set your scope to 200 milliseconds per division and see what it
    displays.

    It's a rare digital scope that passes both these tests...

> BTW, I'm really pleased with the performance of these scopes. I
> think they have an unbeatable performance/price factor.

    I agree that they're great tools for the money... My only point
    was that there's still a need, even when dealing with
    relatively-slow signals, for an true analog scope.

    -Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499